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					                                                                                                  PROTON MAGNETIC RESONANCE SHIFTS AND THE
                                                                                              ELECTRON DENSITY DISTRIBUTION IN AROMATIC SYSTEMS1

                                                                                                                        T. SCHAEFER~ W. G. SCHNEIDEK
                                                                                                                                  AND
                                                                                                           Division of Pure Chemistry, National Research Council, Ottawa, Canada
                                                                                                                                Received October 22, 1962

                                                                                                                                       ABSTRACT
                                                                                                 The quantitative nature of the empirical linear correlation between the proton resonance
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                                                                                              shifts in aromatic molecules and the local electron density on the carbon atom to which the
                                                                                              proton is bonded has been investigated. The application of this relationship t o the deter-
                                                                                              mination of the electron density distribution of a variety of aromatic molecules, as well as the
                                                                                              main limitations inherent in this application, are discussed. Electron density distributions
                                                                                              have been derived for aniline, anisole, azulene, acepleiadylene, pyridine, pyridinium ion, and
                                                                                              for the aromatic ions pentalenyl, indenyl, fluorenyl, triphenylmethyl cation, and azulene
                                                                                              dianion, and are compared with those calculated by molecular orbital methods. In general
                                                                                              there is a fairly good correspondence between the experimental and calculated density values,
                                                                                              although for the aromatic ions the excess charge tends t o be somewhat more uniformly distri-
                                                                                              buted over the molecule than would be indicated by simple MO calculations.

                                                                                                                  I. T H E CORRELATION B E T W E E N T-ELECTRON
                                                                                                                 D E N S I T Y A N D T H E PROTON RESONANCE S H I F T
                                                                                          I t has been recognized for some time that the proton resonance shift in aromatic mole-
                                                                                       cules tends to reflect the n-electron density on the carbon atom to which the proton is
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                                                                                       bonded. For example in substituted benzenes, a strong electron-withdrawing substituent
                                                                                       decreases the shielding of ring protons, whereas electron-releasing substituents cause
                                                                                      -increased shielding (1, 2 , 3). The non-alternant hydrocarbon azulene provides another
                                                                                       example; it is not possible to account for the observed proton resonance spectrum unless
                                                                                       one invokes a contribution due to the unequal electron density a t different carbon atoms
                                                                                       (4). I t is therefore of some interest to examine the possible quantitative aspects of such
                                                                                       correlations and t o what extent proton resonance shifts can provide useful information
                                                                                       about the electron density distribution in aromatic systems.
                                                                                          Previous worlc (5, 6, 7) has provided evidence for a simple linear correlation between
                                                                                       the proton chemical shift, 6, and the local "excess" charge, Ap (located on the carbon
                                                                                       atom), in aromatic systems; thus,


                                                                                      where the constant k was found el~lpirically have a value near 10 p.p.m./electron. In
                                                                                                                                       to
                                                                                      the present work attempts have been made to examine more carefully the experimental
                                                                                      basis of this relationship, as well as its application to a wide variety of aromatic systems.
                                                                                      The main limitations and sources of error in evaluating n-electron densities in individual
                                                                                      cases have also been considered.
                                                                                         For convenience benzene has been chosen as the reference base, both for the proton
                                                                                      chemical shifts, 6, and for the excess charge densities, Ap. Thus since the n-electron density
                                                                                      on the carbon atoms in benzene is unity, Ap = 0. We adopt the further convention that
                                                                                      6 is positive for proton resonances appearing to higher field of the benzene resonance, and
                                                                                        'Issued as AT.R.C. No. 7258.
                                                                                        2Research Associate, National Research Council, Summer, 1962. Permanent address: Chemistry Department,
                                                                                       University of Manitoba, Winnipeg, Manitoba.
                                                                                       Canadian Journal of Chemistry. Volume 41 (1963)
                                                                                                        SCHAEFER AND SCHNEIDER: PROTON SHIFT A N D ELECTRON DENSITY                                       967

                                                                                      negative for resonances a t lower field. On this basis k is found experimentally to be a
                                                                                      positive constant. Accordingly a negative 6, associated with decreased proton screening,
                                                                                      corresponds t o a negative Ap on the carbon atom, that is, an electron deficiency relative
                                                                                      to the carbon atoms in benzene.
                                                                                        The constant k can be evaluated empirically in either of two ways:
                                                                                          (i) by measuring 6 for systems in which Ap is known, and
                                                                                         (ii) by forming positive or negative aromatic ions
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                                                                                      The total T-electron density has now been altered by exactly one unit charge. If the
                                                                                      shifts of all protons are measured relative t o benzene, the sum of these shifts must corre-
                                                                                      spond t o Ap = 1, whence z , = k. This is essentially a normalization condition, which
                                                                                                                      6
                                                                                      can also be adapted to neutral molecules.*
                                                                                         In applying eq. [l] to the valuation of local 9-electron densities several perturbing
                                                                                      effects are frequently encountered. These arise from contributions to the measured proton
                                                                                      resonance shifts in addition to those caused by the altered electron density. These addi-
                                                                                      tional contributions are due to:
                                                                                          ( a ) the magnetic anisotropy of substituents or of hetero atoms in the aromatic ring,
                                                                                          (b) ring current effects of neighbor rings in polycyclic aromatic compounds,
                                                                                          (c) ion association effects of aromatic ions, and
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                                                                                          (d) solvent effects.
                                                                                      Large contributions arising from any one of these effects seriousl~~     limit the reliability
                                                                                      with which the electron density can be evaluated. Frequently, however, adequate cor-
                                                                                      rections can be made to the measured chemical shifts, or the perturbing effect can be
                                                                                      avoided by an appropriate choice of the experimental conditions.
                                                                                         The magnitude of the proton resonance shift arising from the magnetic anisotropy of
                                                                                      substituent groups in individual molecules cannot be estimated with sufficient accuracy
                                                                                      a t present t o permit a satisfactory correction to the measured shifts. Since this contribu-
                                                                                      tion has a l/R3    dependence, it will affect primarily those protons in the molecule close
                                                                                      to the anisotropic center. For example, in nitrobenzene there is a substantial contribution
                                                                                      to the chemical shift of the ortho protons arising from the magnetic anisotropy of the
                                                                                      nitro group (3). At the meta and para positions the contribution is greatly attenuated
                                                                                      and, for the present purpose, it can be neglected. Other substituents, such as -HCO,
                                                                                      -CH3C0, -COCl, -SO3, -C1, -Br, and -I, also give rise to magnetic anisotropy
                                                                                      effects. Hetero atoms in aromatic rings may give rise to similar effects. For example the
                                                                                      magnetic anisotropy of the N atom in pyridine gives rise to a slight unshielding contri-
                                                                                      bution to the resonance shift of the a-protons (8). An empirical evaluation of this contri-
                                                                                      bution, which is undoubtedly possible, would permit extension of the present methods to
                                                                                      a considerable number of heterocyclic aromatic molecules.
                                                                                         The ring current effects (9) can be dealt with more readily. Since benzene has been
                                                                                      chosen as the basic reference, no correction is required for monocyclic (benzenoid)
                                                                                      systems. The corrections required for 5- or 7-membered rings are small and introduce
                                                                                      little error. For a polycyclic ring system the resonance shift of a particular ring proton
                                                                                      must be corrected for the ring current of neighbor rings to bring the shift in line with the
                                                                                      benzene reference. For this purpose the dipole model due to Pople (10) has been employed.
                                                                                        * T h e additiaity of chemical shifts and electvon charges inzplied here depends on the validity of eq. [ I ] .
                                                                                      968                       CANADIAN JOURNAL OF CHEMISTRY. VOL. 41, 1963

                                                                                      For benzene itself this gives a contribution t o the proton shift of magnitude



                                                                                      Here a is the radius of the benzene ring and R the distance of the proton from the center
                                                                                      of the ring. The simple dipole model evidently slightly overestimates the ring current
                                                                                      contribution in benzene (11). An improved value of 1.5 p.p.m., used for the present work,
                                                                                      was obtained by measuring the difference between the resonance shift of benzene and the
                                                                                      shift of the olefinic protons in 1,3-cyclohexadiene.* With this adjustment the correction
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                                                                                      for a particular proton due to the ring current in a neighbor ring, designated 6l, becomes



                                                                                      Here 6l is expressed in p.p.m.; a and R (expressed in A units) are respectively the radius
                                                                                      of the neighbor ring, and the distance of the proton being considered from the center of
                                                                                      the neighbor ring. For bicyclic systems this correction is fairly reliable and the uncertainty
                                                                                      is probably no greater than about 0.1 p.p.m. For larger molecules containing several fused
                                                                                      ring systems the corrections, and hence also the errors, begin t o mount. For such molecules
                                                                                      a greater portion of the electron current circulates around the periphery of the molecule
                                                                                      and the ring current within any one ring may differ somewhat from that of benzene.
                                                                                         With aromatic positive or negative ions a n association with counterions in solution
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                                                                                      may be anticipated. Considered as a perturbing electric field, the counterion will cause a
                                                                                      shift of the proton resonance of the aromatic ion being measured (12, 13, 14). With small
                                                                                      aromatic ions, and more particularly in ions where the charge is largely concentrated a t
                                                                                      one atom in the molecule, as for example in the anilinium ion, association with the counter-
                                                                                      ion occurs selectively a t this point and involves a close approach, on the average, t o the
                                                                                      highly charged atom. The result is a non-averaged contribution t o the resonance shifts
                                                                                      of the ring protons, which differs in magnitude a t each non-equivalent proton. In the case
                                                                                      of anilinium ion, it has been observed (15) t h a t these contributions to the proton resonance
                                                                                      shift are reduced by the substitution of bulky alkyl groups on the nitrogen atom. By this
                                                                                      means the close approach of the counterion is prevented, thus reducing the strength of
                                                                                      the perturbing ionic field. In larger aromatic ions, and particularly those in which the
                                                                                      excess charge is well distributed over the molecule, the effects of counterion association
                                                                                      may be expected to be averaged out and t o be much smaller in magnitude. In general such
                                                                                      effects may be expected t o be further minimized by a high degree of dilution of the ionic
                                                                                      species with a solvent of high dielectric constant.
                                                                                         Closely related is the problem of residual solvent effects, which may also give rise to
                                                                                      small contributions t o the proton shifts (16). With neutral aromatic molecules this can
                                                                                      usually be avoided by an appropriate choice of solvent, and by using a dilute solution
                                                                                      together with a suitable internal reference. These conditions must necessarily be somewhat
                                                                                      compromised for aromatic ions which require polar solvents. If the charge distribution
                                                                                      in the ion is highly asymmetric, the resulting "reaction field" in the solvent may contribute
                                                                                      t o the proton shift. I t is difficult t o estimate the magnitude of this effect in individual
                                                                                      cases, although when it is non-negligible it can certainly be detected and investigated by
                                                                                      direct experimentation.
                                                                                         Returning now t o the question of the quantitative basis for eq. [I], probably the most
                                                                                      direct evaluation of the proportionality constant k is based on the proton resonance
                                                                                        *Both compounds were measured as a 3 mole% solution i n cyclohexane.
                                                                                                    SCIlAEFER AND SCHNEIDER: PROTON SHIFT AND ELECTRON DENSITY                                  969

                                                                                      shifts of the symmetrical species C6H5-, CsHB, C7H7+, and C8H8'. Although the measure-
                                                                                      ments were originally made by Leto, Cotton, and Waugh (17), the most extensive ones
                                                                                      of the first three members of the series have been carried out by Fraenkel, Carter,
                                                                                      McLachlan, and Richards (5). The proton resonance of cyclooctatetraene dianion has
                                                                                      been measured previously by Katz (18) and by Fritz and Keller (19) and the C13resonance
                                                                                      by Spiesecke and Schneider (7). Because of the importance of the quantitative evaluation
                                                                                      of the constant k it was considered desirable to confirm these measurements and, in
                                                                                      particular, to investigate the possible effects of solvent and ion association. Table I shows
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                                                                                                                                     TABLE I
                                                                                                                 Proton resonance shifts of monocyclic aromatic ions

                                                                                                                                                     Concentration                  aa
                                                                                                                         Solvent                       (mole%)              (C/S a t 60 Mc/s)




                                                                                                                                                    0.5                           95.4z0.1
                                                                                                                                                    0 (extrapolated)              94.5
                                                                                                   (CsH5)-Li+            CHSCN                      4                            109.610.4
                                                                                                                                                    2                            109.7Zt0.5
                                                                                                   (C7H7)+Br-            CHaCN
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                                                                                                   (CsH8)'2Na+           CH3CN
                                                                                                                         THF
                                                                                                   aRelative to an internal benzene   reference.
                                                                                                   b~etrahydrofuran.

                                                                                      the measured proton shifts of (C6H5)-Na+ as a function of concentration in tetrahydro-
                                                                                      furan and of (C6H6)-Li+ in acetonitrile. The results for a 0.2 mole% solution of tropylium
                                                                                      bromide in CH3CN and of (CsH8)=2Na+ in tetrahydrofuran and acetonitrile are also
                                                                                      shown.
                                                                                        I t is apparent from the results of Table I that there is a measurable solvent dilution
                                                                                      shift for (C6H6)-Na+ in tetrahydrofuran. I t will be noted that the direction of this dilution
                                                                                      shift is toward lower applied (magnetic) field, contrary to that expected for decreasing
                                                                                      ion association with dilution. No explanation can be offered for this behavior. However,
                                                                                      one suggestion worth considering is the possibility of complex formation of the sodium
                                                                                      cyclopentadienide with tetrahydrofuran. A specific etherate complex, K2CsHs.THF, has
                                                                                      been reported (19). On the other hand (C6H6)-Li+ in cH~c1U'      shows a negligible dilution
                                                                                      shift and, moreover, the proton shift itself is larger (relative to benzene) than that of
                                                                                      (C5HS)-Na+ in tetrahydrofuran. Table I1 shows a comparison of some of the present
                                                                                      measurements with the corresponding results reported by Fraenkel, Carter, McLachlan,

                                                                                                                                            TABLE I1

                                                                                                                                                                       6 (p.p.m. relative to
                                                                                                                                                                             benzene)
                                                                                                                                                   Concentration                    Present
                                                                                                                            Solvent                  (mole%)            Ref. 5      results
                                                                                                    (CSH6)-Na+              THF                          2               1.72            1.63
                                                                                                    (C5Hs)-Naf              CHICN                        2               1.785
                                                                                                    (CsH5)-Li+              CHICN                        2                               1.83
                                                                                          970                      CANADIAN JOURNAL O F CHEMISTRY.     O.
                                                                                                                                                      V L 41,   1963

                                                                                          and Richards (5), who quote a measurement error in 6 of f 0.004 p.p.m. On this basis the
                                                                                          discrepancy in the two sets of measurements for (CSH5)-Na+ in T H F is unacceptably
                                                                                          large. The difference between the measurements in CHsCN is about 0.04 p.p.m., which is
                                                                                          probably within a more realistic margin of error.
                                                                                             On the basis of the solvent dilution measurements shown in Table I , the results for
                                                                                          (C5H5)-Li+ in CH&N are considered the more reliable. These have been combined with
                                                                                          the (CTHY)+Br- measurement (also shown in Table I) for the purpose of evaluating the
                                                                                          constant k. Since the ions CSH5- and C7H7f also have six a-electrons, the aromatic ring
                                                                                          current is taken to be the same as in benzene, and corrections, amounting to -0.10 and
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                                                                                           +0.07 p.p.m., respectively, were applied t o allow for the effect of different ring size on
                                                                                          the proton chemical shift. The final shifts are -1.98, 0, and $1.73 p.p.m. respectively
                                                                                          for C7H7+, CeHC, and C5H5-, to be compared with differences in n-electron densities,
                                                                                          Ap, of -0.143, 0, and +0.20. The best straight line through these points (with equal
                                                                                          residuals) results in a value of k = 10.7~k0.2       p.p.m./electron. Here f 0 . 2 p.p.m. refers
                                                                                          to the actual deviation of the individual points and accordingly a more realistic "error"
                                                                                          may be in the neighborhood of f 1 . 0 p.p.m.
                                                                                             I t is difficult t o give a reliable assessment of the actual precision of the above deter-
                                                                                          mination. In the first place the validity of eq. [l]has been assumed, which neglects possible
                                                                                          contributions of higher-order terms. Inclusion of a square term has been suggested by
                                                                                          Musher (20), but lacking further confirmation the use of the simple first-order term is to
                                                                                          be preferred for the present. Moreover the application of this expression to a variety of
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                                                                                          aromatic systems, as shown in the results following, leads to a fairly satisfactory internal
                                                                                      _   consistency. I t should be pointed out that the measurements for (C*Hg)=2Na+were not
                                                                                          included in the above evaluation of k. The much larger ring size and four additional
                                                                                          a-electrons as compared to benzene would involve additional uncertainties with respect
                                                                                          to the actual contribution of the ring current effect t o the proton shift. (The measured
                                                                                          shift relative to benzene, as shown in Table I , is actually less than that of C5H5- in
                                                                                          CH3CN.) Finally there is the question of small differences in the degree of hybridization
                                                                                          of the carbon orbitals in the 5 - , 6-, 7-, and 8-membered aromatic rings. This will un-
                                                                                          doubtedly make some contribution t o the measured proton shifts, but the magnitude of
                                                                                          this contribution cannot be reliably estimated. In any event, relative to benzene, it may
                                                                                          be expected to be largest for the C8H8 dianion.
                                                                                             Method (ii) for the evaluation of k, mentioned earlier, involves the measurement of
                                                                                          the proton shifts of aromatic positive or negative ions relative to those of the corresponding
                                                                                          neutral molecule. Unfortunately there are not Inany compounds suitable for this purpose,
                                                                                          but the method does provide a useful "self-consistency" check of the value of k determined
                                                                                          by the methods already described. In their study of benzene carbonium ions MacLean
                                                                                          and llackor (6) derived an independent value of k equal to 13.5 p.p.m./electron. This
                                                                                          was based on the assun~ptionthat the contribution of the "ring current" effect to the
                                                                                          proton shift in benzene is 1.88 p.p.m. With the value 1.5 p.p.m. for this contribution
                                                                                          used here, which we regard as a more appropriate value, the value of k based on the
                                                                                          benzene carboniunl ion shifts reported by the above authors becomes 11.8 p.p.m./electron.
                                                                                          While this value is somewhat higher than that derived above, it nevertheless represents a
                                                                                          satisfactory confirmation by an independent method which involves only six-membered
                                                                                          aromatic rings. A further iiself-consistency" check based on aromatic ions can be illustrated
                                                                                          with reference t o the indenyl ion as an example. (See Table 111 below.) This involves two
                                                                                          aromatic rings with seven protons. The two carbon atoms joining the two rings do not
                                                                                          contain a bonded proton, and hence their n-electron charge cannot be determined by the
                                                                                                    SCHAEFER AiYD SCHNEIDER: PROTON SHIFT A N D ELECTRON DENSITY                 971

                                                                                      present methods. However, since the ion contains unit excess charge, an approximate
                                                                                      value for the density a t the &%carbon atoms can be derived by application of the
                                                                                      normalization condition mentioned earlier. Thus we have



                                                                                      where 61 represents the shifts of individual protons bonded to the other carbon atoms,
                                                                                      and k = 10.7 p.p.m./electron. If the indenyl negative ion had a uniform charge distribu-
                                                                                      tion, the n-electron density on each carbon atom would be 1.11. The value a t the 8,9-
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                                                                                      positions would not be expected to differ froin this by an "unreasonable" amount. The
                                                                                      actual value, 1.17, determined in the above manner is necessarily approximate since it
                                                                                      contains the accuinulated errors of all 6,, but the result nevertheless provides a further
                                                                                      internal self-consistency test to the value of k.
                                                                                         One rather important point, however, which emerges from the experimental d a t a
                                                                                      presented in the next section, is t h a t in all cases the measured proton shifts, when used
                                                                                      in eq. [I], give the correct sign of the excess charge, Ap. This has also been noted in con-
                                                                                      nection with measurements of benzene carbonium ions (6) and indicates t h a t higher-order
                                                                                      terms (which have been neglected in eq. [I]) can make but a relatively minor contribution.
                                                                                         The complete expression for the change in the proton screening due to a perturbing
                                                                                      field E (here regarded as arising from the excess charge concentrated on the carbon atom)
                                                                                      may be written (12, 14)
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                                                                                       he-inclusion of the square term on the right-hand side t o represent the proton shift
                                                                                      measurements of CbHb- and C7H7+ has been suggested (20) and its magnitude has been
                                                                                      estimated.
                                                                                          In this connection i t is of some interest t o compare the proton shifts of H30+ and OH-
                                                                                      relative t o HzO. In both these ions, though oppositely charged, the protons are less screened
                                                                                      relative to Hz0 by 10-11 p.p.m. (21, 22). T o account for this it has been proposed (23)
                                                                                      t h a t only the square term in eq. [3] is involved. In contrast, a s already mentioned, in the
                                                                                      aromatic carbonium ions and carbanions the linear term is definitely dominant. This
                                                                                      rather curious difference remains t o be intrepreted, although the comparison may not be
                                                                                      wholly justified. For example, the ions H 3 0 +and OH-, when measured in aqueous solution,
                                                                                      form hydrogen bonds and are known t o undergo very rapid proton exchange. Accordingly
                                                                                      the direct application of equation [3] to their "proton averaged" species in solution may
                                                                                      not be appropriate.

                                                                                                              11. APPLICATION TO ELECTRON DESSITY
                                                                                                              DETERMINATION IN AROMATIC SYSTEMS
                                                                                         In order t o investigate further the quantitative nature of the correlation of proton
                                                                                      resonance shifts and T-electron densities, the electron densities have been evaluated on
                                                                                      the basis of the methods described in the preceding section, for a variety of aromatic
                                                                                      systems. These include both neutral nlolecules and positive or negative aromatic ions.
                                                                                      The value of k (cf eq. [I]) employed throughout is 10.7 p.p.m./electron. T h e results are
                                                                                      summarized in Table 111, where the notation


                                                                                      has been employed, 6 being given by eq. [2], and p ( = l + A p ) represents the derived
                                                                                                         '
                                                                                      972                                CANADIAN JOURNAL OF CHEMISTRY.                O.
                                                                                                                                                                      V L 41.   1963

                                                                                      total n-electron density. The experimental conditions employed for the proton resonance
                                                                                      shift measurements are outlined below.
                                                                                         There is, unfortun'ately, a t present no wholly reliable information on charge densities
                                                                                      for comparison with the present derived values. The only recourse is to the various
                                                                                      molecular orbital calculations. Depending on the type of approximation employed these
                                                                                      often differ considerably in the individual electron density values. However, the general
                                                                                      trend of the electron density distribution and the alternation of the densities within the
                                                                                      molecule is broadly similar in the various approximations. (A comparison of the electron
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                                                                                                                                              TABLE 111
                                                                                                                           Proton chemical shifts, derived electron densities,
                                                                                                                         and calculated electron densities of aromatic systems

                                                                                                                            6          6'           6~07,
                                                                                                                                                  )~
                                                                                                               Position (p.p.m.)" ( ~ . p . r n . (p.p.m.)    p,,,                          Pcalc


                                                                                                  Aniline                                                                     HMO (M). HMO (M)d HMOz
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                                                                                                 Anisole




                                                                                             Azulene                                                                            HMO, CIe    SCFf    VESCFo
                                                                                                                                                                                                    --a


                                                                                                      2
                                                                                                                  2         -0   5Sa   +O 29     -0 29       0 97     1 047      0 979     0 997    0 988
                                                                                                                  1,3       -0   067   +0.55     4-0 49      1 05     1 173      1 096     1 049    1 059
                                                                                                                  4,8       -0   945   +O 52
                                                                                                                  5,7       +O   28    +O 17
                                                                                                                  6         -0   17    +O 12
                                                                                                                  9,lO




                                                                                        :(,yj
                                                                                       Acepleiadylene                                                                            HMOh
                                                                                                                                                                                --
                                                                                                                  7,8       -0.50      +O. 74    +0.24       1.02                1.095
                                                                                                                  6,9       -0.96      +0.94     -0.02       1.00                0.943
                                                                                                                  5,lO      -0.58      +1.05     4-0.63      1.06                1.055
                                                                                            12            11      1,4       -0.42      +0.96     $0.54       1.05                0.934
                                                                                                                 2,3      +0.48        +0.43     $0.91        1.09                0.938
                                                                                            1             1
                                                                                                  3   2        11 --t 16 (Av.)                               (0.93)              (1.010)

                                                                                                 Pyridine                                                                       SCF, CIi   VESCFj
                                                                                                                                                                                --
                                                                                                      SCHAEFER AND SCHNEIDER: PROTON SHIFT AND ELECTRON DENSITY              973

                                                                                                                               TABLE 111 (Continued)

                                                                                                                     6          '
                                                                                                                                6         Gorr
                                                                                                                                         )~
                                                                                                        Position (p.p.m.)" ( ~ . p . m .(p.p.m.)    p,,,             Poelc


                                                                                      Pyridinium ion                                                        VESCFJ




                                                                                       b.1
                                                                                                                    -6.11                          (1.60)    1.520
                                                                                                                    -1.56                           0.854    0.899
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                                                                                                           P        -0.91                           0.915    0.927
                                                                                                           Y        -1.48                           0.862    0.829



                                                                                        Pentalenyl                                                          HMO
                                                                                          dianion


                                                                                                          1,3,4,6   $2.29     $0.55     $2.84       1.27     1.314
                                                                                                           2,5      $1.54     $0.29     +1.83       1.17     1.173
                                                                                                           7,8                                     (1.29)    1.198
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                                                                                        Fluorenyl                                                           HMO
                                                                                           ion




                                                                                       Triphenyl
                                                                                      methyl cation
                                                                                      974                             CASADIAN JOURNAL O F CHEMISTRY. VOL. 41, 1963


                                                                                                                                       T A B L E 111 (Concluded)

                                                                                                                             6           6'        &orr
                                                                                                            Position (p.p.m.)" ( ~ . p . m . ()p~ p . m . )
                                                                                                                                                .             p,,,                            PC& IC



                                                                                       Azuletle dianion
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                                                                                        NOTE:Abbreviations: HMO, Hiickel molecular orbitals; (&I), CI, configurational interaction incl uded; SCF, self
                                                                                                                                                       modified;
                                                                                      consistent field molecular orbitals; VESCF, variable electronegativity self-consistent field molecular orbitals.
                                                                                        aProton resonance shifts in p.p.m. from benzene.
                                                                                         bCorrection due to induced currents in neighbor rings.
                                                                                        CH. Baba and S. Suzuki. J. Chem. Phys. 32. 1706 (1960).
                                                                                        dC. Sandorfy. Bull. Soc. C h m . France, 16, 615 (1949).
                                                                                         eR Pariser. J. Chem. Phys. 25, 1112 (1956).
                                                                                        f ~ . ' ~ u l g J. Chim. Phys. 52, 377 (1955).
                                                                                                           .
                                                                                        gR,D. Brown and M. L. Heffernan. Australian J. Chem. 13. 38 (1960).
                                                                                        h ~ Pullman, A. Pullman, G. Berthier, and J. Pontes. J . Chim. Phys. 49, 20 (1952).
                                                                                                  .
                                                                                                  .
                                                                                         f ~ McWeeny and T . E. Peacock. Proc. Phys. Soc. (London), Sect. A, 70, 41 (1957).
                                                                                        T D. Brown and M. L. Heffernan. Australian J. Chem. 12, 554 (1959).
                                                                                           R
                                                                                        k ~ . ' ~ . Pople. J. Phys. Chem. 61, 6 (1957).
                                                                                                           24,
                                                                                         ' ~ e f e r e n c e p. 353.
                               For personal use only.




                                                                                      density values calculated by various approximations for azulene, for example, is given by
                                                                                      Streitwieser (24, p. 456).) The calculated densities included in Table I11 for comparison
                                                                                      with the densities derived from n.m.r. data were either taken from the literature or else
                                                                                      were calculated for the present purpose using simple Hiickel MO methods.
                                                                                      ( I ) Aniline, Anisole
                                                                                          As mentioned above the main difficulty in the employment of proton resonance shifts
                                                                                      t o evaluate the electron density in substituted aromatics arises from the magnetic aniso-
                                                                                      tropy of substituent groups. With substituents such as CHH, NH2, OH, and OCHB,such
                                                                                      effects are relatively small. Table I11 shows the a-electron densities for aniline and anisole
                                                                                      derived from the proton resonance shifts previously reported (3). The results exhibit t h e
                                                                                      anticipated excess density a t the ortho and para positions and an almost negligible
                                                                                      contribution a t the meta positions.* The MO calculations for substituted benzenes are
                                                                                      difficult, and the results depend largely on the assumptions made to take account of the
                                                                                      electronic effects of the substituent. Three such calculations are shown for aniline. While a
                                                                                      quantitative comparison with the n.m.r. data may not be too meaningful, the general
                                                                                      trend of the density distributions is very similar.
                                                                                      ( 2 ) Non-alternant Hydrocarbons Azulene, Acepleiadylene
                                                                                         The electron density distribution of azulene derived from proton resonance shifts has
                                                                                      been reported previously (25). A re-evalution of the densities of azulene is included in
                                                                                      Table 111, employing the present revised values for k and for the aromatic ring current.
                                                                                      The proton resonance shifts employed were measured for a 3 mole% solution of azulene
                                                                                      in cyclohexane. Accordingly solvent effects are not in question, and the main source of
                                                                                      error in evaluating the electron densities arises from the corrections to the proton shifts,
                                                                                      6', due t o current circulations in the "other" ring. These corrections, evaluated by eq. [2],
                                                                                        * T h e density values previously reported in ref. 5 for a n i l i n e dz$er significantly at the para position. T h i s i s
                                                                                      doubtless d u e to a diffevent oalzre of the proton resonance shift employed.
                                                                                                    SCHAEFER A K D SCHNEIDER: PROTON SHIFT .4KD ELECTRON DENSITY                975

                                                                                      are shown in the fourth column in the table. I t was assumed that the currents in the -5- and
                                                                                      7-membered rings are equal t o those in benzene, an assumption which derives some
                                                                                      justification from a theoretical calculation due t o Pople (26). The corrections are largest
                                                                                      for the 1,3-positions and the 4,s-positions. A rather liberal estimate of the possible error
                                                                                      in the electron density due to uncertainties in the "other" ring corrections might be
                                                                                      f0.02 for the 1,3- and 4.8-positions, and probably less than fO.O1 a t the other positions.
                                                                                      The value given in parenthesis in the table for the 9,lO-positions was obtained by diffe-
                                                                                      rence, using the normalization condition referred to earlier. Since this value is subject to
                                                                                      cumulative errors, it will be somewhat less reliable than the other density values. I t does,
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                                                                                      however, provide a useful internal self-consistency check. Of the various i\?O calculations
                                                                                      for azulene included in Table I11 for comparison, those of Brown and Heffernan (27)
                                                                                      (VESCF) have been adjusted to give the best agreement with the measured dipole
                                                                                      moment of azulene. In general, the electron densities derived from n.m.r. data show a
                                                                                      somewhat sinaller variation than those evaluated by MO methods, but the density
                                                                                      alternations around the two rings are well reproduced.
                                                                                         The non-alternant hydrocarbon acepleiadylene, which contains four aromatic rings in
                                                                                      a pyrene-type structure, is less satisfactory for the present purposes than azulene. Each
                                                                                      proton resonance shift must be corrected for current circulation in three other rings.
                                                                                      These corrections become rather large, and no doubt also involve proportionately greater
                                                                                      errors. The average density evaluated for carbons 11, 12, 13, 14, 15, and 16 was found to
                                                                                      be 0.93. This was again obtained by taking the difference of the sum of the densities
                               For personal use only.




                                                                                      derived for all the other carbon atoms which have a bonded proton, and the total number
                                                                                      of a-electrons (16). The low value of 0.93 for the inner carbon atoms might be interpreted
                                                                                      to inean that the excess charge tends to be concentrated mainly on the outer perimeter
                                                                                      of the molecule. However, in view of the possibility of accumulated errors in the "other
                                                                                      ring'' corrections this cannot be substantiated on the basis of the present results. Com-
                                                                                      parison of the charge densities derived for the remaining carbon atoms indicates a prefe-
                                                                                      rence for the excess charge t o reside in the 7-membered ring, rather than in the 5-mem-
                                                                                      bered ring a t the opposite end of the ~noleculeas indicated by simple Hiickel MO
                                                                                      calculations (28). This discrepancy appears t o be real and outside the probable limits of
                                                                                      the experimental uncertainties. I t would also imply a direction of the molecular dipole
                                                                                      moment opposite to that indicated by the MO calculations. The actual direction of the
                                                                                      molecular dipole moment is not known.

                                                                                      ( 3 ) Pyridine, Pyridinium Ion
                                                                                         Pyridine and the pyridinium ion illustrate some features common to many heterocyclic
                                                                                      aromatic systems. As already mentioned, the N atom in pyridine gives rise to a magnetic
                                                                                      anisotropy. The presence of this anisotropy, which has been demonstrated by the nitrogen
                                                                                      resonance measurements of Baldeschwieler and Randall (8), gives a non-negligible
                                                                                      contribution t o the screening of the a-protons in pyridine. In order to evaluate the electron
                                                                                      density a t the a-position this contribution must be subtracted out. At present a quantita-
                                                                                      tive estimate of such contributions is lacking. They are very much attenuated a t the
                                                                                      p- and a-positions, and accordingly the electron densities a t these positions, derived
                                                                                      from the measured proton shifts, should be fairly reliable. They are listed in Table I11
                                                                                      together with two sets of calculated densities, with which they show a rather good cor-
                                                                                      respondence.
                                                                                         In the pyridiniunl ion the magnetic anisotropy of the N atom, which now contains a
                                                                                      bonded proton, is greatly reduced. The proton resonance shifts of the pyridinium ion have
                                                                                      976                      CANADIAN JOURNAL O F CHEMISTRY. VOL. 41, 1963

                                                                                      been reported previously (29). The derived electron densities are listed in Table 111. The
                                                                                      density shown for the N atom is again obtained by difference. If the shift of the N-H
                                                                                      proton is employed in conjunction with that in pyrrole as a reference state, and with the
                                                                                      same value of k as employed here for aromatic C-H bonds, the value of the density on
                                                                                      the N atom is found to be 1.57. However, this latter procedure is very rough. The calcu-
                                                                                      lated results of Brown and Heffernan (30) for the pyridinium ion (Table 111) show a fair
                                                                                      correspondence with the present results.
                                                                                         The proton resonance measurements for pyridine were carried out in a dilute solution
                                                                                      of cyclohexane, and solvent effects may be assumed to be absent. The pyridinium ion
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                                                                                      shifts were measured in a dilute solution (-5 moleyo) in trifluoroacetic acid. Accordingly
                                                                                      the measured proton shifts may be affected somewhat by solvent interaction and ion
                                                                                      association.
                                                                                      ( 4 ) The Carbanions Pentalenyl, Indenyl, and Fluorenyl
                                                                                         The pentalenyl dianion has been prepared by Katz and Rosenberger (31). The proton
                                                                                      resonance shifts (measured in a dilute solution of tetrahydrofuran) reported by these
                                                                                      authors have been employed to evaluate the T-electron densities. These are shown in
                                                                                      Table I11 together with the density values calculated by simple Hiickel MO methods.
                                                                                      There is a fairly good correspondence between the two sets of results. The experimental
                                                                                      value in parenthesis shown for the 7,8-positions was obtained by difference and may be
                                                                                      somewhat less reliable.
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                                                                                         The indenyl and fluorenyl ions were prepared by reacting dilute solutions (2-5 mole%)
                                                                                      of the parent hydrocarbons, indene and fluorene, in tetrahydrofuran with sodium. Figure 1




                                                                                        FIG.1. Proton resonance'spectrum at 60 Mc/s for i~ldeneand indenyl ion. The reference scale is in
                                                                                      p.p.m. relative to internal cyclohexane.
                                                                                                     SCHAEFER A N D SCHNEIDER: PROTON SHIFT AND ELECTRON DENSITY                   977

                                                                                      shows the proton resonance spectra (at 60 Mc/s) of both indene and the indenyl ion, on
                                                                                      a common scale. The proton resonance of benzene on this scale is indicated by the broken
                                                                                      line. The proton resonance spectrum of indene has been previously assigned and analyzed
                                                                                      by Elleman and Manatt (32). Starting from the right, the signals a t - 1.8 p.p.m. (from
                                                                                      cyclohexane) arise from the methylene protons a t the 1-position; those centered near
                                                                                       -4.9 from proton 2, and those centered near -5.3 from proton 3. The latter signals
                                                                                      show additional splitting due to long-range spin-coupling with proton 4. Finally the com-
                                                                                      plex pattern of signals centered near -5.7 is due to the four protons in the &membered
                                                                                      ring. The assignment of the proton spectrum of the more symmetrical indenyl ion is
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                                                                                      straightforward. The three protons in the 5-membered ring give rise to an AB2 system;
                                                                                      the doublet a t -4.4 p.p.m. is accordingly assigned to the 1,3-protons, and the triplet
                                                                                      centered near -5.1 to the 2-proton. The four protons in the 6-membered ring form an
                                                                                      A2B2system; the lower-field half of the spectrum, centered near -5.8, has broader lines
                                                                                      than the higher-field half centered near -4.9 p.p.m. Assuming that the broader lines are
                                                                                      due to long-range spin-coupling with the 1,3-protons, they are accordingly assigned to
                                                                                      the 4,7-protons. Thus, in the ion the 5,6-proton signals are shifted up field to a greater
                                                                                      extent by the excess charge. The signals of protons 1, 2, and 3 also show a large up-field
                                                                                      shift, but this is partly compensated by the appearance of a ring current in the 5-membered
                                                                                      ring.
                                                                                         The density distribution evaluated from the proton resonance shifts (Table 111) shows
                                                                                      the largest density a t the 1,3-positions and the smallest a t the 4,7-positions. In this respect
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                                                                                      there is a parallel with the density distribution in azulene. The density a t the 8,9-position
                                                                                      was obtained by difference as explained above, and is accordingly subject to greater error.
                                                                                      The densities calculated by HMO methods show a remarkably good correspondence
                                                                                      with the n.m.r. values, and the density alternation around the rings is identical.
                                                                                         The proton resonance shifts in the fluorenyl ion were generally larger than those in
                                                                                      the indenyl ion and a first-order analysis of the spectrum was adequate for the present
                                                                                      purpose. The proton assignment was made on the basis of the spin-coupling patterns,
                                                                                      but there remains an ambiguity, as between protons 2,7 and 3,6 and between protons
                                                                                      1,8 and 4,6. Of the various possible combinations of assignments, only that shown in
                                                                                      Table I11 gives a reasonable pattern for the density distribution and compares favorably
                                                                                      with that calculated by HMO. As in a number of other examples, the spread between
                                                                                      maximum and minimum density is smaller than that in the calculated density distribution.
                                                                                      This difference is particularly apparent a t the 9-position.

                                                                                      ( 5 ) The Triphenyl Methyl Cation
                                                                                         As a n example of a carbonium ion, the triphenylmethyl cation has been investigated.
                                                                                      The proton resonance spectrum of the ion has been reported previously (33), and was
                                                                                      repeated for the present study under conditions of higher resolution. The main problem
                                                                                      encountered in evaluating the electron density is the assessment of the contribution to
                                                                                      the proton shifts due to neighboring ring currents. The most reasonable assumption to
                                                                                      make is that the molecular framework is essentially planar (i.e. carbon 1 trigonally
                                                                                      hybridized) but with the phenyl groups tending to be tilted out of the plane like the
                                                                                      blades of an air fan. A completely planar structure would appear to be ruled out because
                                                                                      of strong repulsions between protons in ortho positions. If in addition the phenyl rings
                                                                                      are assumed to librate or rotate, the contribution of the ring current to the proton shifts
                                                                                      of a neighboring phenyl group will be largely averaged out. In any event the effect a t
                                                                                      the meta and para protons will be negligibly small, although some average residual
                                                                                      978                          CANADIAN JOURNAL O F                     O.
                                                                                                                                                CHEMISTRY. V L 41.   1963

                                                                                      contribution could be present a t the ortho protons. Accordingly in evaluating the electron
                                                                                      densities shown in Table 111 no corrections for neighbor ring effects were made. Com-
                                                                                      parison with the theoretically calculated values shown indicates a much closer corres-
                                                                                      pondence with the SCFMO results than with those from simple Hiickel molecular
                                                                                      orbitals. The value 0.95 shown in parenthesis may be regarded as a rough indication of
                                                                                      the average density of the 1- and 2-positions. I t represents one third of the total electron
                                                                                      density a t positions 1, 2, 2', and 2", obtained by subtracting the sum of the density a t all
                                                                                      other positions from 18, the total number of a-electrons.
                                                                                      ( 6 ) The Azulene Dianiofz
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                                                                                         Aromatic negative ions of a large number of compounds are readily formed by treating
                                                                                      a dilute solution of the parent aromatic hydrocarbon in a suitable solvent with alkali
                                                                                      metals. With most hydrocarbons the formation of a paramagnetic mononegative ion,
                                                                                      which has the added electron in the lowest antibonding molecular orbital, is favored.
                                                                                      Azulene on the other hand readily forms the dinegative ion on treatment with excess
                                                                                      lithium.* Since the two electrons are paired in the same orbital the ion is diamagnetic.
                                                                                      Its proton resonance spectrum bears a close resemblance to that of azulene itself, except
                                                                                      that the whole spectrum is considerably displaced toward higher applied field and the
                                                                                      signals of the I-, 2-, and 3-protons are coalesced t o a single line. This assignment of the
                                                                                      ion was confirmed by measuring 1,3-dideuteroazulene. As an additional confirmatory
                                                                                      check, deuterated dimethoxyethane was used as solvent. This was desirable because
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                                                                                      the spectrum of the ion, occurring a t higher field, was partially overlain by the C13satellite
                                                                                      resonance peaks of the protons in the undeuterated solvent.
                                                                                          In evaluating the electron density distribution the assumption was made that the ring
                                                                                      current induced by the magnetic field in the dinegative ion is similar in magnitude to
                                                                                      that in azulene itself. Addition of two electrons to the first antibonding orbital will not
                                                                                      increase the aromatic ring current; in fact the ring current is probably somewhat de-
                                                                                      creased (26). Certainly if all five antibonding orbitals were completely filled the induced
                                                                                      ring current would be reduced to zero. Since the magnitude of the reduction in the ring
                                                                                      current for the dinegative ion is not readily estimated, the above assu~nption probably
                                                                                                                                                                         is
                                                                                      the best one can make under the circumstances. That this assumption cannot be far
                                                                                      wrong is borne out by the density values derived on this basis (Table 111); the value 1.29
                                                                                      obtained for the 9,lO-positions by difference, in the usual way, is not unreasonable in
                                                                                      the context of the overall charge distribution, particularly in view of the fact that this
                                                                                      value will involve some accumulation of errors. Had the ring current in the dianion been
                                                                                      very substantially reduced relative to that in azulene, the resulting value a t the 9,lO-
                                                                                      positions may have been expected to deviate rather widely.
                                                                                          Comparison of the experimentally derived charge distribution for the dianion with
                                                                                      that calculated from simple Hiickel orbitals shows the excess charge t o be more uniformly
                                                                                      distributed, i.e. more delocalized, than would be anticipated from the calculated results.
                                                                                      This point is perhaps more clearly demonstrated by comparing the density distribution
                                                                                      of the dianion with auzlene itself. Table IV shows the density increments a t each position
                                                                                      for the process [azulene] -+ [azulenel-. In other words, these are the density increments
                                                                                      at each position resulting from the addition of two electrons to the lowest antibonding
                                                                                      orbital in azulene. Discounting those a t the 9,lO-positions (which contain the compounded
                                                                                      errors of both density distributions), the increments derived experimentally are moderately
                                                                                      uniform a t each position. On the other hand the calculated increments are large a t the
                                                                                        *We are indebted to Professor J . Hozj'tink for pointing this out to u s .
                                                                                                    SCHAEFER A N D SCHNEIDER: PROTON SHIFT A N D ELECTRON DENSITY              979

                                                                                                                               T A B L E IV
                                                                                                                       Charge density increments for
                                                                                                                          [azulene] -+ [azulene]=

                                                                                                                                           AP
                                                                                                                 Position     Experimental      Calc. (HMO)
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                                                                                      2-, 4-, and 6-positions and negligible a t the 3- and 5-positions. These results, based on
                                                                                      HMO calculations, represent, of course, a highly idealized system. In any real system
                                                                                      it may be anticipated that an excess charge of 0.5 electron placed on any one carbon
                                                                                      atom, as a t the 6-position, would cause the molecular system to relax and part of this
                                                                                      charge would be distributed over the neighboring carbon atoms, the 5- and '?-positions.
                                                                                         The greater degree of charge delocalization indicated in the charge distributions
                                                                                      derived from n.m.r. data as compared to the MO results appears t o be common also to
                                                                                      the other aromatic ions investigated (Table 111). A similar observation has also been
                               For personal use only.




                                                                                      made previously in relation to the benzene carbonium ion (6). Colpa, MacLean, and
                                                                                      Mackor (34) have given an interpretation of this behavior in terms of T-u interaction.
                                                                                      As discussed above, the proton resonance responds t o an altered T-electron density on a
                                                                                      carbon atom due to an essentially electrostatic polarization of the C-H signia bond. I t
                                                                                      is thus t o be anticipated that the sigma bonds to neighboring carbon atoms will be similarly
                                                                                      polarized, resulting in some contribution to the resonance shift of their bonded protons.
                                                                                      Consequently the densities derived from proton resonance shifts give a measure of the
                                                                                      combined local density alteration in both the T- and u-electron systems. In contrast the
                                                                                      densities calculated by simple MO theory yield an "idealized" local density for the
                                                                                      pelectron system only. In neutral molecules, -where the T-electron densities differ but
                                                                                      slightly from unity this basic difference in the results of the two methods is not very
                                                                                      pronounced. However, in the aromatic ions, and particularly double-charged ions, where
                                                                                      the excess charges are greater, the disparity in the resulting density distributions is in-
                                                                                      creased.
                                                                                                                            111. CONCLUSIONS
                                                                                        The present investigations provide strong support for a simple empirical correlation
                                                                                      between the proton resonance shift and the local electron density on the bonded carbon
                                                                                      atom in aromatic systems. When this relationship is applied to a variety of suitable
                                                                                      aromatic systems a satisfactory reproduction of the electron density distribution, as
                                                                                      indicated by chemical and theoretical methods, is obtained. The assumption that the
                                                                                      proton screening depends linearly (to a good approximation) on the excess electron
                                                                                      density of the bonded carbon atom appears t o be well borne out. In all cases this gives
                                                                                      the correct sign of the proton shifts, and accordingly, if square terms or higher-order
                                                                                      interactions are involved, they must be relatively small. I n favorable cases, therefore,
                                                                                      the charge densities determined from proton shifts can be expected to be quite reliable;
                                                                                      they are probably more accurate than those calculated by simple MO methods and com-
                                                                                      pare favorably with those obtained by more refined calculations.
                                                                                      980                         CAXADIAN JOURNAL O F                   O.
                                                                                                                                             CHEMISTRY. V L 41.      1963

                                                                                         On the other hand many aromatic systems represent "unfavorable" cases and the deter-
                                                                                      mination of the electron density distribution by the present methods has serious limita-
                                                                                      tions. Among these are molecules with magnetically anisotropic substituents, heterocyclic
                                                                                      molecules, and large polycyclic molecules, for which the effects due to induced ring
                                                                                      currents are proportionately large. The extent to which useful information on charge
                                                                                      densities can be derived in these cases will ultimately depend on the extent to which the
                                                                                      perturbing effects can be treated, either theoretically or empirically, in a more quantitative
                                                                                      manner. In some cases solvent effects could give rise to additional uncertainties although
                                                                                      these are usually less serious.
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                                                                                         For aromatic positive or negative ions, which involve much larger excess charges than
                                                                                      neutral molecules, the charge density distributions derived from n.m.r. data generally
                                                                                      indicate a somewhat more uniform distribution of the excess charge than those calculated
                                                                                      by M 0 methods. This may be indicative of T-u interaction, the proton screening being thus
                                                                                      altered by the polarized a-orbitals as well as by the excess charge in the r-orbitals.

                                                                                                                                  EXPERIMENTAL
                                                                                      General
                                                                                         The carbanions whose cations mere sodium were prepared by shaking with sodium in a solvent vapor
                                                                                      atmosphere. They were transferred t o sample cells through a sintered-glass filter. The apparatus was self-
                                                                                      contained so t h a t contact was never made with air or water. Tetrahydrofuran ( T H F ) had been purified by
                                                                                      vacuum distillation after shaking with sodium and anthracene. Acetonitrile was dried over PzO, and was
                                                                                      vacuum distilled.
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                                                                                         The carbanions whose cations were lithium were prepared from a solution of n-butyl lithium in hexane
                                                                                      After precipitation of the salt the hexane was removed by vacuum distillation and the appropriate solvent
                                                                                      was added. Again no contact with air or water was allowed, all transfers of solvent being done on a vacuum
                                                                                      line.
                                                                                         The proton resonance measurements were made a t 60 Mc/s. Internal references were either benzene or
                                                                                      cyclohexane, present in the purified solvent a t a concentration between 0.5 and 1 m 0 l e 7 ~ .
                                                                                                                                                                                     The internal shift
                                                                                      between benzene and cyclohexane was 351.4 c/s in T H F . The same shift was found in a T H F solution which
                                                                                      had been shaken with sodium for 1 day, demonstrating the reliability of the reference positions.

                                                                                      Cyclopentadzenyl Sodzftm
                                                                                         Separate solutions (series I ) of varying concentrations were prepared from measured amounts of reactants
                                                                                      and solvent a s described above. Dicyclopentadiene was freshly distilled. A second series of dilution studies
                                                                                      was made on a n 8 mole% solution by the addition of known volurnes of solvent to the initial solution in a
                                                                                      self-contained apparatus. T h i s arrangement precluded spinning the sample b u t as a check the 8 mole%
                                                                                      solution, retrieved by vacuum removal of the correct amount of solvent, was spun, and the measurement
                                                                                      was found to agree exactly with the original one. Table I gives the results. Y the higher concentration
                                                                                                                                                                          n
                                                                                      regions (6-10 mole%) the difference in shifts between the txvo types of measurement differ by as much as
                                                                                      1.0 C/S. This is probably due to the difficulty of getting accurate concentrations with such small amounts
                                                                                      of material (the solutions in the first series of dilution experiments were made from 1 cc of solvent). However,
                                                                                      plots of the two series of points extrapolate to the same point a t zero concentration. The shifts for the
                                                                                      10 mole% and 6 m 0 l e 7 ~ solutions (series I) gave virtually the same peak intensities and shifts, so that a
                                                                                      limited solubility in T H F is indicated.

                                                                                      Cyclopentadienyl Lithiztm
                                                                                        Dilution studies were carried out from 4 mole% down to 1 1nole7~ acetonitrile. The dilution shift was
                                                                                                                                                            in
                                                                                      zero to within experimental error and the shift relative to benzene was 1.83 p.p.m.

                                                                                      Tropylium Bromide
                                                                                        A 0.2 mole% solution in CH3CN had a strong peak a t -1.91 p.p.m. relative to benzene. The salt was
                                                                                      insufficiently soluble in T H F to give a resonance.

                                                                                      CsHs'.ZNa+
                                                                                         Solutions, 3 moleyo, of this compound were prepared from freshly distilled cyclooctatetraene in both
                                                                                      T H F and CH3CN, as described above. The T H F solution threw down a dark brown precipitate which par-
                                                                                      tially redissolved to give a greenish-yellow solution. The CH3CN solution was brown. T h e shifts were 1.60
                                                                                      and 1.59 p.p.m. relative to C&6, respectively. A previous measurement of this ion, reported by Fritz and
                                                                                      Keller (19), differs excessively. Our values agree with those of Katz (18).
                                                                                                        SCHAEFER AND SCHNEIDER: PROTON SHIFT AND ELECTRON DENSITY                                        981
                                                                                      Triphenylcarbonium Ions
                                                                                         Both the chloride and the carbinol of triphenylmethane dissolved to form 5 mole% orange solutions in
                                                                                      trifluoroacetic acid. The ring proton spectra, of the AgB2C type, were virtually identical for the two solutions.
                                                                                      They were analyzed to give approximate shifts (Table 111). The spectra were very similar to those of Connor
                                                                                      et al. for the carbinol a t 40 hlc/s, allowing for the difference in spectrometer frequencies.
                                                                                         Crystal violet, the chloride of the symmetrically trisubstituted p-N,N-dimethyltriphenylmethane, gave
                                                                                      a single broad line for the ring protons in CF3COOH, about -0.3 p.p.m. from internal CcH6. Since a t least
                                                                                      partial protonation of the N(CH3)g groups occurs, no conclusions can be drawn. Solutions in various solvents
                                                                                      of dielectric constants as high as 65 all gave nearly the same A2B2 spectra for the ring protons, all a few
                                                                                      tenths of a p.p.m, to high field of internal benzene. I t was concluded that ionization was not occurring.
                                                                                         Experiments with AlC18 and the chloride as well as crystal violet, in an attempt t o get the carbonium ion,
                                                                                      were unsuccessful.
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                                                                                         Triphenylmethyl sodium in T H F gave a broad line a t about 0.2 p.p.m. to high field of internal benzene,
                                                                                      indicating no appreciable ionization and possibly some radical species present.
                                                                                      Indenyl Sodium
                                                                                        Freshly distilled indene gave a light orange or yellowish 5 mole% solution in T H F after shaking for about
                                                                                      a day. The spectrum of the molecule and the ion are shown in Fig. 1. The experiment was repeated a t a
                                                                                      lower concentration (-3 mole%). The shifts obtained agreed to within 0.04 p.p.m.
                                                                                      Fluorenyl S o d i u m
                                                                                         Solutions, 5 mole% and 3 mole%, in T H F gave orange solutions after shaking with sodium for a few days.
                                                                                      The 5 mole% solution was not completely converted to the ion but conve~sion        was very nearly complete
                                                                                      in the 3 moleyo solution. The shifts relative to benzene in the two solutions agreed to within 0.05 p.p.m. a t
                                                                                      worst and to within 0.01 p.p.m. a t one position.
                                                                                      Azulene Dianion
                                                                                        Solutions, 4 mole%, were prepared by shaking with lithium i11 dimethoxyethane and in deuterated
                               For personal use only.




                                                                                      dimethoxyethane, both solvents having been purified with sodium and anthracene. The solution was deep
                                                                                      purple in color.

                                                                                                                                  ACKNOIVLEDGMENTS
                                                                                          We g r a t e f u l l y a c k n o w l e d g e our indebtedness to Mr. Y. L u p i e n for t h e preparation of
                                                                                      s a m p l e s , to Mr. J. N i c h o l s o n for t e c h n i c a l a s s i s t a n c e , and a l s o to Dr. E. Mackor for
                                                                                      providing us with his r e s u l t s prior to p u b l i c a t i o n and to Dr. J . M a r t i n for i n f o r m a t i v e
                                                                                      discussions.
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