v65-373 by anmh


									                                                                                        THE AMINO PROTON SHIFTS O F SOME SUBSTITUTED ANILINES IN

                                                                                                           W. F.       H.
                                                                                                 T. YONE~~OTO,' REYNOLDS,~R'I. HUTTON, T . SCH~IEFER~
                                                                                                         Department of Cltenzistry, University of Manitoba, Winnipeg, Ma?titoba
                                                                                                                                 Received M a y 13, 1965

                                                                                                The amino proton shifts of 41 ortho, meta, and para substituted anilines a t low concen-
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                                                                                             trations in cyclohexane are reported. T h e shifts for meta and para substituents correlate with
                                                                                             Hammett u constants and it is shown t h a t they are probably proportional to the r-electron
                                                                                             density on the nitrogen atom. These shifts also correlate with the pK, values of the corre-
                                                                                             sponding anilinium ions in aqueous acid. The amino proton shifts for the ortho substituents,
                                                                                             apart from methyl groups, follow a correlation line which lies about 0.5 p.p.m. t o low field
                                                                                             from that for the meta a n d para substituents. This negative deviation is discussed in terms of a
                                                                                             weak hydrogen bonding interaction with the ortho substituents and the I-I-N-H         bond angles
                                                                                             a s derived from infrared data.

                                                                                         In recent years, correlations have been noted between Hammett a constants and the
                                                                                      chemical shift of the ring protons in p-substituted benzenes (I), formyl protons in nz- and
                                                                                      p-substituted benzaldehydes (2), hydroxyl protons in m- and $-substituted phenols (3),
                                                                                      methoxy protons of substituted anisoles (4), and amino protons in m- and $-substituted
                               For personal use only.

                                                                                      anilines (5, 6). T h e anilines were measured in carbon tetrachloride (5, 6) and acetonitrile
                                                                                      (G), where hydrogen bonding between solvent and amino protons is probable (see below).
                                                                                        The ring proton shifts a t the para position of substituted benzenes are proportional to
                                                                                      the calculated a-electron densities a t the carbon atom (7). We report 11ere the amino
                                                                                      proton shifts of 41 o-, m-,and p-substituted anilines in cyclohexane solution, where
                                                                                      interactions with the solvent are a t a minimunl, and discuss the results in terms of
                                                                                      Hanlnlett a constants of the substituents, the a-electron density a t the nitrogen atom, the
                                                                                      PI<, values of the corresponding aniliniuln ions, and the H-N-H         angles derived from
                                                                                      high resolution infrared measurements. Low solubility in cyclohexane precluded measure-
                                                                                      ments on a number of compounds, among them the nitroanilines, aminophenols, and o-
                                                                                      and 9-phenylenediamines.
                                                                                      1. Sarrzple Preparation
                                                                                         Solid anilines were sublimed before use, except for o-bromoaniline and p-iodoaniline which were recrystal-
                                                                                      lized from cyclohexane. T h e liquid haloanilines were distilled in vacuum over fresh potassium hydroxide.
                                                                                      Spectroscopy-grade cyclohexane was used a s a solvent without further purification. As long a s solubility
                                                                                      allowed, samples of 5,2, and 1 mole % concentration were prepared by weighing.
                                                                                      2. Spectra
                                                                                         T h e proton resonance spectrum of the amino group was recorded on a DP60 spectrometer. T h e solvent
                                                                                      resonance peak provided a convenient reference signal, sidebands of which were used t o find the shift of the
                                                                                      amino proton resonance. T h e line width of the latter, a broad band due to exchange and relaxation phenomena
                                                                                      involving the ",U quadrupole, was u s ~ ~ a labout 3-5 c/s a t 5 mole % a n d tended t o increase with dilution. In
                                                                                      some cases the linewidth approached 25 c/s. A dozen recordings were averaged for each solution. T h e
                                                                                      reproducibility of the shifts was about 1 c/s in most cases but for very broad or weak signals it decreased t o
                                                                                      3 c/s. T h e shifts obtained a t the three concentrations were extrapolated to infinite dilution. Most of the
                                                                                      solutions showed no significant dilution shift in this concentration range. Low solubility in some cases
                                                                                        liVationa1 Research Coz~ncil Postdoctorate Fellow, 1962-1964.
                                                                                        2Natio?zalResearch Coz~ncil!              Bolder. 1962-1965.
                                                                                        3iVational Research Corrncil ~ e n i oReharclt Fellow, 1964-1966.
                                                                                      Canadian Journal of Chemistry. Volume 43 (1DG6)

                                                                                                                            YOhTEMOTO ET AL.: PROTON SHIFTS                                             2669
                                                                                      demanded the use of the shift of the 1 mole % solution only. These are given the superscript a in Table I. In
                                                                                                                                            only allowed the measurement of 2 or 5 mole % solutions.
                                                                                      other cases large line widths a t lower conce~ltrations
                                                                                      These are given the superscript b in Table I.

                                                                                      3. Effect of Pz~rificatiorton Spectra
                                                                                         An investigation of the effect of impurity on the shift was made with aniline solutions. I n Fig. 1, line 1 was
                                                                                      obtained for aniline distilled in vacuulu over potassium hydroxide. Line 2 shows the results for the distilled
                                                                                      aniline if the solutions were prepared 1 d after opening the distillation vessel, while line 3 was obtained for
                                                                                      undistilled deeply colored aniline. T h e resonance peaks obtained for the solutions prepared from unpurified
                                                                                      aniline were narrower than for purified aniline, probably because of faster proton exchange catalyzed b y
                                                                                      water or other impurities. For this reason, the error in measurement was larger for the purified aniline solu-
                                                                                      tions. Consequently, although the solutions of unpurified aniline appear to show both a smaller dilution shift
                                                                                      and a high-field shift conlpared t o the solutions of purified aniline, the experimental error is such that little
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                                                                                      was gained by purification.

                               For personal use only.

                                                                                                                     0         1          2        3         4          5
                                                                                                                                              MOLE OI. in C H
                                                                                                                                                           6 12
                                                                                        FIG.I. Line I (open circles) shows the chemical shift in c/s t o low field from cyclohexane of the amino
                                                                                      protons of vacuum-distilled aniline a t three concentrations in cyclohexane. Line 2 (half-filled circles) is for
                                                                                      aniline which was used one day after vacuum distillation. Line 3 (filled circles) is for unpurified, deeply colored

                                                                                                                               RESULTS AND DISCUSSION
                                                                                         In Table I the amino proton shifts, Av, of 41 o-, m-, and p-substituted anilines are given
                                                                                      relative to aniline itself. Prior to the discussion of these shifts in terms of lnolecular para-
                                                                                      meters, the state of the aniline molecules in solution must be considered. If our Av values
                                                                                      represented the true shift a t infinite dilution, they would naturally be those of aniline
                                                                                      nlolecules isolated from each other by solvent molecules. Because of signal-to-noise
                                                                                      problems, however, the extrapolation to infinite dilution is probably solnewhat unreliable.*
                                                                                      There may be a substantial, a t present undetectable, change in shift between 1 and 0 mole
                                                                                      yo, especially if self-association through hydrogen bonding occurs.
                                                                                         The intensity of the first overtone of the N-1-1 stretching band of aniline in cyclohexane
                                                                                      has beell interpreted in terms of hydrogen-bonding association ( 8 ) . A dilnerization model
                                                                                      is consistent with the data up to 0.2 M. The equilibrium constant for dimer fornlation is
                                                                                      about 0.4 a t room temperature ( 8 ) . For a 0.2 M solution, which happens to be about 2
                                                                                      mole %, only about 5% of the aniline nlolecules are therefore in the form of dimers. On an
                                                                                      n.1n.r. time scale the dilners will break up and reform rapidly, so that the observed
                                                                                      shift represents the weighted average of the nlonolner and dinler shifts. Unfortunately,
                                                                                      the difference in shift between dimer and monomer is not known. However, if we assume
                                                                                        * T h e viable, i f expensive, procedure to follow Itere woz~ldbe the zrse of l5X sszlbstituted co~rapoz~nds eliminate
                                                                                      qz~adrz~pole                                                                                              or2
                                                                                                    relnsatiofr broadeni~tgand a conapz~terof average transieizts to allow ~neasz~reiiaents very dilzlte
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                                                                                                                                                                   TABLE I
                                                                                                                    Chemical shifts, Au, in p.p.m. of the amino-protons of some substituted anilines in cyclohexane solutions
                               For personal use only.

                                                                                      No.               Substituent                  AuC                                     No.           Substituent              Auc           ud     ApKac
                                                                                                                                                                                            3,4-diCI              -0.02           0.60    -      u
                                                                                                                                                                                            3,5-diCla             -0.12           0.74    -      2
                                                                                                                                                                                            2,4-diCI              -0.45           0.43    -      2:
                                                                                                                                                                                            2,5-diCI              -0.53           0.57    -      u
                                                                                                                                                                                            2,4-diCH3               0.20        -0.34     -      >
                                                                                                                                                                                            2,3-diCH3               0.13        -0.24    -0.01   r
                                                                                                                                                                                            2,6-diCH3               0.10        -0.34     -      o
                                                                                                                                                                                            2,5-diCH3               0.13        -0.24     -
                                                                                                                                                                                            3,s-diCHaa              0.20        -0.14     -      rC
                                                                                                                                                                                            3.4-diCH2               0.22        -0.24     0.67   F?

                                                                                      a b~eeesperimental    procedure.
                                                                                       =Relativeto aniline.
                                                                                      d~-Iamrnettn-constants. See discussion for choice of values.
                                                                                       CTakenfrom refs. 35 and 36.
                                                                                      ,As recornniended in ref. 14.
                                                                                                                          YONEMOTO ET AL.: PROTON SHIFTS                                      267 1

                                                                                      that the substituted anilines have very similar association constants to aniline then the
                                                                                      discussions below should still be valid. Certainly, the absortivity-concentration relation-
                                                                                      ships of 7n-methylaniline and m-cl~loroanilineare qualitatively similar to the ones for
                                                                                      aniline, although they are solnewhat different for o-chloroaniline (9). A reasonable expecta-
                                                                                      tion is that o-substituents would hinder self-association and that self-association through
                                                                                      hydrogen bonding leads to low-field shifts. Therefore the observed large low-field shifts for
                                                                                      o-substituents can also reasonably be discussed in terms of the properties of the isolated
                                                                                      molecules, as is done below.
                                                                                      i. Correlation of Av with a Constants
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                                                                                        In Fig. 2, AV is plotted versus the Hainmett a constants. The effect of more than one
                                                                                      substituent is taken as additive (10) and the recent a values froin the analysis of McDaniel
                                                                                      and Brown (11) were used when available. For o-substituents the a values derived by
                                                                                      T a f t (12) were used since these are on the same scale as the Haminett a values for m- and
                                                                                         For solubility reasons, the range of a is only 1.2 units as compared to 1.6 units available
                                                                                      in acetonitrile solution (6). Disregarding the o-substituents, except for methyl groups, the
                                                                                      range of AV is very small and we feel there is little to be gained f r o ~ n least squares treat-
                                                                                      ment. Instead, the dashed line has been drawn as shown in Fig. 2. As expected, the
                                                                                      electron-withdrawing substituents (positive a) lead to a low-field shift. The average
                                                                                      deviation for 7n- and p-substituents (but including o-methyl) is just over 0.03 p.p.m. which
                               For personal use only.

                                                                                      is probably within experimental error.

                                                                                                   ppm. -0.4
                                                                                                                   o para
                                                                                                                   m   meta
                                                                                                         -0.8 -

                                                                                                         -1.0 -
                                                                                                                   A   ortho
                                                                                                                                                         26-CI2     A 2.CN

                                                                                                               L                     I           I            I              I
                                                                                                             -0.50      -0.25        0         0.25          0.50        0.75
                                                                                        FIG.2. T h e chemical shift in p.p.m. relative to aniline of t h e amino protons in substituted anilines is
                                                                                      plotted versus the Hamlnett u-constants of the substituents. The unidentified ortho points clustered about
                                                                                      the top lineare those with methyl groups in the ortho position.

                                                                                         Per unit change in a, Av changes by 0.35 p.p.m. which compares with 0.53 p.p.m. in
                                                                                      carbon tetrachloride and 0.97 p.p.m. in acetonitrile solutions (6). The larger values in the
                                                                                      latter solutions can be understood in terms of hydrogen bonding to the solvent. As u
                                                                                      increases algebraically the N-H bond increases in acidity, and hydrogen bonding with an
                                                                                      acceptor solvent is favored, causing larger shifts. That carbon tetrachloride should hydrogen
                                                                                      bond to N-1-1 is perhaps unexpected, yet there is evidence that it does so even t o C11  --
                                                                                      bonds. As an example, proton shifts of many haloalkanes (13) in cyclohexane and carbon
                                                                                      2672                    CANADIAN JOURAT.4L O F               O.
                                                                                                                                       CHEMISTRY. V L   43. 196.5

                                                                                      tetrachloride solution indicate that the latter offers chlorine atoms for hydrogen bonding
                                                                                      to the C-H bonds of the solute. Further, it is known that some substituted anilines react
                                                                                      rapidly with carbon tetrachloride (14, 15). For the discussioil in the next subsection,
                                                                                      measurement of ainino proton shifts in solvents like cyclohexane is therefore essential.
                                                                                      2. AVand T-Electron Density on the Nitrogen Atom
                                                                                         In p-substituted benzenes, the p-proton shift changes by 1 p.p.111. when the I-Iammett u
                                                                                      constant changes by 1.4 units (1). For this u range, the calculated a-electron density on the
                                                                                      p-carbon atoll1 varies by 0.1 electron (7), indicating a proton shift of about 10 p.p.in./
                                                                                      electron. The electron density on the nitrogen atoin of substituted anilines is expected to
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                                                                                      be much less sensitive than the p-carbon atom t o the influence of a substituent on the
                                                                                      ring. Thus, the calculated T-density on the nitrogen atom in N,N-dimethylaniline is only
                                                                                      0.007 electrons less than in N,N-dimethyl-p-anisidine (16). This compares with 0.031
                                                                                      electron more a t the 9-carbon atom in anisole than in benzene (7) (these two calculations
                                                                                      used the same bond and core parameters for the oxygen atom in the nlethoxy group).
                                                                                      In this way an approximate estimate may be made of the expected change in AV of the
                                                                                      amino protons for a change of one unit in u in the substituted anilines. I t is Av = 0.007/1.4
                                                                                       X 0.031 = 0.16 p.p.rn., about half the observed value. However, the N-1-1 bond is much
                                                                                      more polarizable than the C-H bond. From chemical shift data, Musher (17) deduces
                                                                                      that it is roughly 5/3 illore polarizable, leading to a calculated Av of 0.27 p.p.m.
                                                                                         The electrostatic field of the excess charge on the nitrogen atoin produces a potential
                               For personal use only.

                                                                                      differencebetween the two ends of the N-I3 bond which drives electrons into the hydrogen
                                                                                      1s orbital. However, the hybridization of the nitrogen atom changes when the ring is
                                                                                      substituted (see below) and this will change the excitation energies of the orbitals involved
                                                                                      and therefore change any assumed proportionality between a-density and proton shielding.
                                                                                         In view of the approximations and experimental errors involved, it seems reasonable to
                                                                                      conclude that the ainino proton shift in m- and p-substituted anilines is primarily dependent
                                                                                      on the a-electron density a t the nitrogen atom. In this connection, it is interesting to note
                                                                                      a shift of the zero point of about 0.05 p.p.m. to high field in Fig. 2. Such a shift also occurs
                                                                                      for the proton shifts in p-substituted benzenes and is evidence for an additional factor in
                                                                                      determining the u constants. The latter depend on the electronic structure of a reaction
                                                                                      intermediate related to the substituted benzoic acids.
                                                                                      3. Correlation o AVwith PI<, Valzies
                                                                                         In Fig. 3, AV is plotted versus ApK, of the protonated anilines in aqueous acid. The
                                                                                      average deviation from the straight line is less than 0.03 p.p.in. However, because of the
                                                                                      slight slope of the line a measurement of the anlino proton shift in cyclohexane allows the
                                                                                      prediction of the PI<, a substituted aniline with an accuracy of only about 1t0.25 units.
                                                                                      For this purpose, it would be better to use a polar solvent like acetonitrile for which the
                                                                                      correlation line has a much steeper slope. As in Fig. 2, there is a zero point shift of the
                                                                                      correlation line. Also, the point for m-aminoaniline shows a rather large negative deviation,
                                                                                      a possible result of increased hydrogen bonding for this compound in cyclohexane solution.
                                                                                         The points for o-substituted anilines (apart froin o-fluoro and o-methyl anilines) show
                                                                                      large negative deviations but they appear to parallel the correlation line for the m- and
                                                                                      p-substituents. The reason for this will become clearer from the discussion below.
                                                                                      4.Ortho-S,zibstitutedA nilines
                                                                                        In Fig. 2, the o-substituted anilines fall, on an average, about 0.50 p.p.111. below the
                                                                                      correlation line for the m- and p-substituted anilines. Although the scatter is considerable,
                                                                                                                         YONEMOTO ET AL.: PROTON SHIFTS                                        2673
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                                                                                      F^"FIG.3. The chemical shift in p.p.m. relative to aniline of the amino protons in substituted anilines is
                                                                                      plotted versus the available ApK, values of the corresponding anilinium ions in aqueous acid; ApK, is relative
                                                                                      t o t h e pK, value of aniliniu~n

                                                                                      we consider that two parallel lines can be drawn, one for m- and 9-substituents and one for
                                                                                      the o-substituents (with the exceptions of o-methyl and o-fluoro which are discussed later).
                                                                                         A number of possible reasons for this low-field shift suggest themselves. These are a
                                                                                      steric hindrance to n-overlap of the amino group with the ring, the magnetic anisotropy of
                               For personal use only.

                                                                                      the substituent (IS, 19), the electric field of the substituent dipole (20), and an intra-
                                                                                      molecular van der Waals or dispersion interaction (21). These will now be discussed in
                                                                                         (a) Steric Hindrance to n-overlap
                                                                                         Electronic absorption spectra (22) and 13Cchemical shifts (23) of substituted anilines
                                                                                      agree that even bulkier substituents than those under consideration do not cause steric
                                                                                      hindrance to n-overlap of the amino group with the ring. In any event, steric hindrance of
                                                                                      this nature would lead to high-field shifts of the amino protons since less charge would be
                                                                                      delocalized from the amino group into the aromatic nucleus.
                                                                                         (b) DIagnetic Anisotropy and Electric Field Efects
                                                                                         We have carried out calculations for both these effects, using the point dipole approxi-
                                                                                      mation. The geometry of the systenl was assumed to be planar with H-N-H            angles of
                                                                                      120°, and the mean of the calculated shifts for the two protons was examined. Because of
                                                                                      the unfavorable angle between the internuclear vector and the C-X bond, reasonable
                                                                                      values of the magnetic anisotropy of the C-X bond lead to quite small shifts (but see the
                                                                                      discussion of the cyano group below). In the dipole approximation, the electric field effect
                                                                                      of the bond dipole moments (24) dominates. Using the recommended coefficients for the
                                                                                      N-H       bond (17) in Buckingham's equation (20), low-field shifts are calculated which
                                                                                      decrease in the order F > C1 > Br > I, opposite to the observed order of shifts. This is
                                                                                      so whether the dipole is placed midway along the C-X bond or on the at0111 X . We do
                                                                                      not reproduce a table of the calculations here since they do not account for the observed
                                                                                         I t is perhaps possible that a judicious choice of anisotropy and bond dipole moments,
                                                                                      together with a variation in the location of the dipoles, ~vould lead to agreement with the
                                                                                      observed shifts for some of the substituents (see cyano group below). The significance of
                                                                                      such an agreement is doubtful. Martin and Dailey (25) point out similar difficulties in
                                                                                      accounting for the o-proton shifts in substituted benzenes. A further relevant point these
                                                                                      3674                    CANADIAN JOURNAL O F               O.
                                                                                                                                     CHEMISTRY. V L 43,   1965

                                                                                      authors make is that the electric dipole field of the substituent does not contribute
                                                                                      significantly to m- or @-protonchemical shifts in these molecules.
                                                                                         (c) Dispersion Interactions
                                                                                        There is considerable overlap of the van der Waals radii of the aillino proton and the
                                                                                      heavier halogen substituents. Such an overlap would be expected to lead to low-field
                                                                                      shifts of the amino protons because of the fluctuating electric fields arising froin the
                                                                                      instantaneous dipoles in the halogen atoms (effect proportional to the mean square of
                                                                                      these fields (26)). However, the methoxy group causes much the same effect as the heavier
                                                                                      halogens and, furthermore, the latter cause nearly the same shift. The similar shielding
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                                                                                      for broino and chloro substituents could possibly be rationalized as follows. The hydrogen-
                                                                                      iodine distance is so small (about 2.5 A compared to the suill of the van der FVaals radii of
                                                                                      1.2 +  2.15 = 3.35 A) that a compensating high-field shift arises from the magnetic
                                                                                      anisotropy of the iodine, just as the proton in hydrogen iodide is more shielded than in
                                                                                      hydrogen bromide (27). This argument not only serves to show the unreliability of the
                                                                                      point dipole approxiination in calculating inagnetic anisotropy effects a t short distances,
                                                                                      but also illustrates the tenuousness of any explanation of such low-field shifts in terins of
                                                                                      long-range models of electric field and magnetic anisotropy; or dispersion effects. A recent
                                                                                      paper from this laboratory (28) concerning the o-proton and o-fluorine shifts in ethylenes,
                                                                                      propylenes, benzenes, and perfluorobenzenes throws soine doubt on their explanation in
                                                                                      terms of a single one of these effects.
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                                                                                         (d) Weak Hydrogen Bonding
                                                                                         I t is well known that hydrogen bonding causes a shift to low field of the proton involved.
                                                                                      As originally suggested (27) it is probably a combination of the electric field and inagnetic
                                                                                      anisotropy effects of the proton acceptor. A recent calculation of the shift of the chloro-
                                                                                      form proton caused by weak hydrogen bonds to nitrogen bases illustrates the difficulties
                                                                                      involved in the calculation (27a). I t turns out that the nitrogen lone pair is responsible
                                                                                      for a t least 95% of the electric field a t the proton in a linear intermolecular hydrogen bond.
                                                                                      The point dipole approxi~nation therefore useless. The calculation also illustrates that
                                                                                      the magnetic anisotropy effect is much less than the electric field effect, even for the cyano
                                                                                         In our compounds, we consider it most sensible to discuss the ortho shifts qualitatively
                                                                                      in terins of an interaction of the N-H bond with the substituent lone pairs, which we
                                                                                      prefer to call weak hydrogen bonding as opposed to the strong hydrogen bond found in
                                                                                      2-amino-5-chloro-benzophenone (see below). The plausibility of this approach is supported
                                                                                      by the fact that the low-field shift for the o-substituents in Fig. 2 increases as a becomes
                                                                                      more positive, i.e., as the N-H           bond becomes more acidic its interaction with the
                                                                                      substituent increases; as would be expected for hydrogen bonding.
                                                                                         Further support comes froin the H-N-H            bond angles calculated from the frequencies
                                                                                      of the symmetric and antisymmetric stretching bonds of these coinpounds (14). In Fig. 4,
                                                                                      Av is plotted versus the H-N-H           angle. With the exception of the o-trifluoromethyl and
                                                                                      o-cyano groups (discussed below) there is a roughly parallel change in the expected
                                                                                      direction of Av with the H-N-H             angle. The enhanced H-N-H         angle on ortho sub-
                                                                                      stitution has been attributed to intramolecular hydrogen bonding (14).
                                                                                         Finally, the splitting of the first overtone of the N-H symmetric stretching band in
                                                                                      o-substituted anilines has been interpreted in terms of a double-minimum potential for the
                                                                                      hydrogen bonded proton (29). This is further support for the above interpretation of the
                                                                                      low-field shifts.
                                                                                                                            YONEMOTO ET AL.: PROTON SHIFTS                                        2675

                                                                                                                                                                \ A   A
                                                                                                                   o para
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                                                                                                                   o meta      or meta,para     A   ,-,5                            \
                                                                                                                   A   ortho                                                            \

                                                                                                           . l


                                                                                                                                  Ill0    12
                                                                                                                                           1'         13



                                                                                                                                         H-N-H ANGLE
                                                                                         FIG.4. The chemical shift in p.p.m. relative to aniline of the amino protons in substituted anilines is
                                                                                      plotted versus the H-N-H    angle derived from the symmetric and antisymmetric stretching frequencies
                                                                                      in ref. 14.

                                                                                         From the viewpoint of T-electron density on the nitrogen atom it is important to realize
                                                                                      that the range in A V of about 1 p.p.m. in Fig. 4 should not be taken as representing a
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                                                                                      proportionate change in the former. The electron density changes, as discussed above,
                                                                                      \vould be reasonably expected to account only for about 0.3 p.p.m. in Av. I t is true that as
                                                                                      the H-N-H        angle increases towards 120" the T-overlap between the amino group and
                                                                                      the ring will increase but not to the extent that a naive interpretation of the Av values
                                                                                      u~ould  indicate. The excess low-field shift for the o-substituted anilines is probably caused
                                                                                      by the perturbation of the electron density near the proton by the substituent lone pairs.
                                                                                         (e) The o-Fl~roro o-Methyl Szlbstitztents
                                                                                         In Fig. 2, the amino proton shift of o-fluoroaniline does not deviate far from the straight
                                                                                      line. This is to be expected if, as indicated by infrared data (14), the small size of the
                                                                                      fluorine atom prevents the close approach of the hydrogen atom to its lone pair orbitals.
                                                                                      The amino proton shifts of the compounds containing an o-methyl group do not indicate a
                                                                                      large anisotropy of the C-C bond (30), unless the anisotropy of the C-H bonds leads
                                                                                      to a fortuitous cancellation of the C-C bond anisotropy.
                                                                                         (fl o-Cyanoaniline
                                                                                         In Figs. 2 and 4, the Av value deviates strongly to low field from all regular trends. In
                                                                                      this particular case, the magnetic anisotropy of the C-X bond is rather large. If an
                                                                                      experimental (31) and theoretical value (32) for the triple bond of about 20 X
                                                                                      cin3/mole is used, one calculates a low-field shift of about 0.33 p.p.m. for the planar
                                                                                      geoilletry assumed above. This agrees roughly with the excess low-field shifts in Figs. 2
                                                                                      and 4, i.e., from the lines drawn for o-substituents in Fig. 2 and the line in Fig. 4.
                                                                                         (g) 0-Trifiz~oronzelhylaniline
                                                                                         The Av value falls roughly on the lower line in Fig. 2. This is not unexpected since the
                                                                                      lone pairs of the fluorine atoms are now considerably closer to the hydrogen atom than in
                                                                                      o-fluoroaniline. In Fig. 4, however, the Av value deviates strongly from the expected value.
                                                                                      The deviation is easily rationalized in terins of the geometry of the molecule. The inter-
                                                                                      action of the hydrogen atom and the fluorine atom in the trifluoromethyl group, although
                                                                                      stronger than in o-fluoroaniline, need not, because of their relative positions lead to an
                                                                                      2676                       CANADIAN JOURNAL O F                 O.
                                                                                                                                          CHEMISTRY. V L 43, 1965
                                                                                      enhancement of the H-N-H           angle; but the interaction can lead t o a considerably
                                                                                      increased low-field shift because the lone pairs are now closer to the hydrogen atom.
                                                                                         (h) 2,6-Dichloro- and Dibromoanilines
                                                                                        The low-field shift in these two coinpounds is roughly twice that of the mono-substituted
                                                                                      compounds.This is reasonable in that both amino protons now interact with the substituent,
                                                                                      whereas proton exchange leads to an averaging of hydrogen bonded and unbonded shifts
                                                                                      for the monosubstituted compounds.
                                                                                      5. Ring Current Shifts
                                                                                         As the H-N-H       angle increases the possibility arises of a change in ring current con-
Can. J. Chem. Downloaded from www.nrcresearchpress.com by on 07/09/12

                                                                                      tributions to the amino proton shift (33). T o estimate this change we have calculated the
                                                                                      ring current contributions to the N-H proton shift for two extreme orientations of the
                                                                                      amino group with respect to the plane of the ring: planar, and N-H bonds perpendicular
                                                                                      to the ring. Using the procedure recommended by Dailey (34) we find that, a t most, the
                                                                                      protons in the perpendicular form would be shifted -0.1 p.p.in. relative to protons in the
                                                                                      planar form (the adverse angle is more than compensated for by the decrease in ring-
                                                                                      proton distance). However, since the H-N-H          angle appears to vary between 109" and
                                                                                      115" in our compounds the changes in ring current shifts will amount to less than 0.05
                                                                                      p.p.in., which can be disregarded in our discussion.
                                                                                      6. Strong Hydrogen Bonding
                                                                                         The one compound in Table I expected to form a strong intramolecular hydrogen bond
                               For personal use only.

                                                                                      is 2-amino-5-chlorobenzophenone. Here the hydrogen bond to the carbonyl group forins
                                                                                      part of a six-membered ring and would be expected to be strong. In agreement, we find a
                                                                                      Av of -2.65 p.p.m. The H-N-H          angle in this compound has not been calculated froin
                                                                                      the vibrational bands (14), but the splitting of the overtone of the syminetric stretching
                                                                                      band has been measured for 2-aminobenzophenone (29). From a plot of this splitting versus
                                                                                      H-N-H       angle in other coinpounds we deduce an approximate value of 127" for 2-amino-
                                                                                      5-chlorobenzophenone.* Froin Fig. 4 a predicted shift of 2.50 f 0.33 p.p.n~.follows,
                                                                                      assuming the linearity of the plot. I t is, however, to be noted t h a t the carbonyl bond and
                                                                                      the other ring may contribute t o this shift to an unknown extent.

                                                                                        The small range of the chemical shift of the amino protons of m- and p-substituted
                                                                                      anilines in cyclohexane can be interpreted in terms of small changes in the T-electron
                                                                                      density on the nitrogen atom. The much larger shifts in the o-substituted anilines are a t
                                                                                      present best interpreted in terms of weak hydrogen bonding of the amino protons to the
                                                                                      substituent. The shifts can be used to predict pR, values of the anilinium ions but for
                                                                                      this purpose it is best to use a polar solvent.

                                                                                         We are very grateful to the National Research Council for financial support.

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                                                                                         *2-Aminoacetopheno?ze has a n H-N-H                                                            i
                                                                                                                               angle of 133" (14) and the splitting of itsfirst overto~ze s greater than
                                                                                      for 2-aminobenzopltenone (19).
                                                                                                                         YONEMOTO ET AL.: PKOTON SHIFTS

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