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					                                                                                                   THE MAGNETIC ANISOTROPY OF THE CARBONYL BOND

                                                                                                                                           AND R. KAISER
                                                                                                                                D. L. HOOPER
                                                                                                                                                                Fredericton, New Brzinszaick
                                                                                           Departnlents of CJzenzistry and Physics, University of New Brz~nswick,
                                                                                                                              Received September 21, 19G4

                                                                                                                                       ABSTRACT
                                                                                               The magnetic susceptibility components of the carbonyl bond have been determined from
                                                                                             chemical shift differences in the n.m.r. spectra of amides. Correction of the experimental
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                                                                                             chemical shift differences for the effect of internal electric fields is found to overestimate the
                                                                                             shielding of groups close to the carbonyl bond and it is shown that the sets of susceptibility
                                                                                             components determined without this correction give a satisfactory correlatioii of structure
                                                                                             and chemical shifts in amides, a,@-unsaturatedketones, and exo-methylene cycloketones.
                                                                                               Four sets of susceptibility components are derived from formamide or acetarnide spectra a n d
                                                                                             depending on whether the effect of the carbonyl bond is assumed to be shielding or deshielding.
                                                                                             These sets are, in units of 10-"cm3   mole-':
                                                                                                                                           xT,C=O       xvuC=O           x:,c-o
                                                                                                        From acetamide spectra
                                                                                                          Shielding assumed                  18         - 14               - 19
                                                                                                           Deshielding assumed             -38              0.6               23
                                                                                                        From formamide spectra
                                                                                                          Shielding assumed                   0.5        -9                 -7
                                                                                                           Deshielding assumed             - 12          -0.7               -3
                                                                                               Spectra of 1-ethylpyrrolidinone-2 are interpreted as showing t h a t the effect of the carbonyl
                                                                                             bond is to shield distant protons in the plane of the carbonyl group and its substituents.
                               For personal use only.




                                                                                                                                           INTRODUCTION
                                                                                         A knowledge of the anisotropy of the inagnetic susceptibility of the carbonyl bond is
                                                                                      of iinportance in the interpretation of the nuclear inagnetic resonance (n.1n.r.) spectra of
                                                                                      large molecules (1). If the bond lengths and bond angles of a molecule are l;no~\-n,the
                                                                                      contribution of each magnetically anisotropic bond t o the chemical shift of protons in
                                                                                      the molecule 111aj7 be calculated (2). Conversely, Narasimhan and Rogers (3) determined
                                                                                      the magnetic susceptibility components of the carbonyl bond from measured chemical
                                                                                      shift differences in the n.il1.r. spectra of amides.
                                                                                         When the susceptibility components determined by Narasimhan and Rogers are used
                                                                                      t o calculate cheinical shift differences in soine of the substituted acetamides discussed
                                                                                      below, the results differ considerably from experimental values. A similar discrepancy
                                                                                      between calculated and observed chenlical shifts has been reported for the exo-methylene
                                                                                      protons of a-methylene cycloketones (4, 5). We wish t o report the results of an investiga-
                                                                                      tion ~vhich  had as its aim the deterinination of susceptibility components suitable for the
                                                                                      calculation of chemical shift differences in a variety of molecules containing carbonyl
                                                                                      bonds.
                                                                                                                                             THEORY
                                                                                        T h e effect of the magnetic anisotropy of the carbonyl bond on the screening coilstants
                                                                                      o(A) and a(B) of protons HA and 1-1, of a substituted acetamide (Fig. 1) is given by the
                                                                                      theory of Narasimhan and Rogers in the for111




                                                                                               z are the axes of a right-handed coordiilate system with its origin a t the point in
                                                                                      x , y, and
                                                                                      the carbonyl bond a t which a magnetic dipole is assunled t o be induced. T h e system is
                                                                                      Canadian Journal of   Chemistry. Volume 43 (1965.)
                                                                                                                                               2363
                                                                                                                  C A S A D I A N JOURNAL O F CHEIMISTRY.    O.
                                                                                                                                                            V L 43,   1965




                                                                                                                    FIG.1. Symmetrically disubstituted acetamide.

                                                                                      chosen such that the susceptibility tensor is diagonal, with z axis directed along the bond
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                                                                                      toward the oxygen atom, the y axis in the NC'O plane and directed away from the
                                                                                      C'-CI-13 bond, and the x axis perpendicular to the NC'O plane. cp, and cp, are the rotation
                                                                                      angles of the N-alltyl bonds with respect to the plane of the N-acetyl group. For a given
                                                                                      choice of the origin of the coordinate system, the functions f l , f2, f 3 , and f 4 are easily
                                                                                      calculated from the ltnown molecular geolnetry (6).
                                                                                         One of the four equations necessary to solve for the four un1;nowns x,zC=O, x,,C=O,
                                                                                      XzzC=O, and AxCpC was obtained froill a recent quantum mechanical calculation (7) of
                                                                                      the value of x,vC=O,the mean bond susceptibility. For a carbonyl bond with bond
                                                                                      order bet~veen1 and 2, a reasonable value is x,vC=O = -5.0 X lO-"cm3           mole-' which
                                                                                      leads to the equation
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                                                                                         Further siillplification is obtained if the nuinber of unltnowns is reduced to three by
                                                                                      treating AXC-' (or AXC-" for formamides) as a parameter with one of the many reported
                                                                                      values (8-13). The remaining two equations must be obtained from experimental chemical
                                                                                      shift differences between protons with known 9, and y, values, i.e. from the spectra of
                                                                                      molecules of ltnown configuration. Such spectra norn~ally   show a pair of signals that may
                                                                                      be assigned to hydrogen atoms A and B, but a decision on which signal arises from I-I,
                                                                                      and 11rhic11 fro111 I-IB is frequently ambiguous. Only the absolute value of the chemical
                                                                                      shift diifereilce can then be determined and thus the question of the sign of (u(A) - u(B))
                                                                                      arises. This is equivalent to the question of whether the effect of the carbonyl bond is
                                                                                      shielding or deshielding. In Part A of the Results both possibilities are considered, while
                                                                                      Part B discusses an experiment designed to permit a choice to be made.

                                                                                                                                  EXPERIMENTAL
                                                                                        The n.m.r. spectra were obtained on a Varian V-4302 spectron~eter     operating a t 56.4 Mc/s and equipped
                                                                                      with a Varian temperature control assembly. T h e temperature a t the sample was monitored by a copper-
                                                                                      constantan therl~~ocouple.
                                                                                        Five factors known to affect the chemical shifts of ainide protons were initially taken into account in the
                                                                                      planning and evaluation of the n.m.r. measurements: (i) interaction between amide molecules in solution
                                                                                      (14), (ii) amide-solvent interactions (14), (iii) the averaging of chemical shifts caused by rotation of the
                                                                                      central C'-N     partial double bond (15), (iv) the possibility of structural differences between acetamides
                                                                                      and formamides which is suggested by the differences in barriers to internal rotation (16), (v) the effect of
                                                                                      internal electric fields arising from the polar structure of amides.
                                                                                        The first three factors were minimized by the extrapolation of chemical shift differences to the freezing
                                                                                      point a t infinite dilution in CClr. Factor iv was taken into account by considering separately the d a t a for
                                                                                      acetaillides and formamides, and the electric field shift was estimated by means of the relation



                                                                                      derived by Bucl<inghan~(17) where on is in p.p.m. and E , is the coinpoilent of the field a t the H atom
                                                                                      parallel to the C-I-I or N-H bond in electrostatic units.
                                                                                                       I-IOOPER A N D KAISER: MAGh-ETIC AXISOTROPY O F THE CARBONYL BOND                                     2365

                                                                                                                                                RESULTS
                                                                                      Part A. Determination of the Szlsceptibility Conzponents
                                                                                         The conlponents of the carbonyl bond susceptibility in acetamides were determined
                                                                                      fro111the chemical shift difference between the N-CH3 groups in N,N-dimethylacetamide
                                                                                      and that between the two sets of CEI2 protons a to the acetyl group in N-acetylpyrrolidine.
                                                                                      Similarly, the carbonj~l  bond susceptibility components in formamides were determined
                                                                                      froill the corresponding chemical shift differences in N,N-dimethylformamide and
                                                                                      N-formylp~~rrolidine.   The experimental chemical shift differences between the two
                                                                                      N-substituents of these compounds are given in the second column of Table I. The values
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                                                                                      result from extrapolation of a series of measurements to low temperature and infinite
                                                                                      dilution in CCld. The third column gives the electric field shift difference estimated by
                                                                                      means of eq. [3]. E , was assumed to be the sum of contributions originating from a n
                                                                                      electric dipole of i~lonlent D located 1.20 A from C' on the axis of the C1=O bond, and
                                                                                                                  2.3
                                                                                      from a point charge of -0.1 e located on the oxygen atom (18). Columns 4 and 5 give
                                                                                      the difference between the experimental values in column 2 and the calculated electric
                                                                                      field contribution in column 3, and this differential shift is assumed t o be caused by the
                                                                                      bond magnetic anisotropy. The anisotropy shift difference derived in this manner is seen
                                                                                      to depend on whether the experimental values are talten as positive or negative, i.e.
                                                                                      whether the signal of the A substituent is a t higher or lower field respectively than that
                                                                                      of the B substituent. The following con~putations   were carried out for both choices.
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                                                                                                                                        TABLE I
                                                                                            Experilnental chemical shift differences of amides in CCI,, solution and electric field correction.
                                                                                                                                All values are in p.p.m.

                                                                                                                                                                                   Anisotropy shift difference
                                                                                                                                              Experimental
                                                                                                                                                  shift                            A a t higher   B a t higher
                                                                                                Compound                                       difference          UE(A) - UE(B)      tield           field
                                                                                                       -                      -                      -


                                                                                      1.   N,X-Di~l~ethylacetamide*               ((333)          f 0.150               -0.024       +0.174        -0.126
                                                                                      2.   N-.%cet\-lpyrrolidine*                 (CHd            f 0.090               -0.088       +0.17S        -0.002
                                                                                      3.   N,N-Di~~iethylformamide*               (CH3)           f0.155                -0.024       +O. 179       -0.131
                                                                                      4.   N-Formylpyrrolidine*                   (CH2)           f0.165                -0.088       +0.253        -0.077
                                                                                      5.   N-:lcetylnortropine                    (CHI            f 0 . 510             -0.152       +0.662        -0.358
                                                                                      6.   Formanlidet                            (NH)            f 0.214               -0.195       +0.409        -0.019
                                                                                      7.   1-Ethylpyrrolidinone-2                 (CH2)           +O. 108               -0.002       +O. 110
                                                                                       "ISrtrapolated to low temperature and infinite dilution.
                                                                                       t B . Sunners. L. 13. Piette, and W. G . Sclineider. Can. J. Chem. 38, 681 (1960).

                                                                                                                                        TABLE I1
                                                                                                        Values of the orientatio~Cfunctions substituted amides in units loz2cm-"
                                                                                                                                          for

                                                                                              Compound                                               fl                     fz       fa                f 4

                                                                                      K,K-Dimethylacetamide                       (CH3)             1.72             -6.82          4.96            -7.09
                                                                                      N-rZcet!~lpyrrolidine                       (CH2)             1.64             -8.08          6.33            -9.29
                                                                                      K,S-Di111etl1ylforn1amide                   (CHa)             1.72             -6.82          4.96            -6.70
                                                                                      K-Formylp)~rrolidi~le                       (CHd              1.64             -8.08          6.33            -8.90
                                                                                      K-=lcetylnortropine                         (CHI              5.22            -15.78         10.54           -12.51
                                                                                      Formnmicle                                  (NH)              2.99             -7.82          4.84           -16.77

                                                                                        Table I1 shows the values of the orientation functions used in eq. [I]. The magnetic
                                                                                      dipole \\;as assumed to be induced on the C'=O bond a t a point 1.20 A from the carbon
                                                                                      atom. This location \\;as chosen because it is believed that the strong electronegativity of
                                                                                      2366                         CANADIAN JOURNAL O F        CHEMISTRY. VOL.   43, 19G5

                                                                                      oxygen places the electronic center of charge very near the oxygen nucleus. Free rotation
                                                                                      of the methyl group was assumed for the N,N-clin~eth\.lanlides,  while for the p) rrolidine
                                                                                      derivatives the values p = p = 60' were substituted. Equation [ I ] \\-as coinbined lvith
                                                                                                              ,     ,
                                                                                      eq. [2] for coinpounds 1 and 2 and separately for coinpounds 3 and 4 to solve for the three
                                                                                      susceptibility components of acetainides and formamides separately, \vith A x as a para-
                                                                                      meter. Selected values of A x (8-13) were then substituted to obtain the results listed in
                                                                                      columns 2, 3, and 4 of Tables I11 and IV.

                                                                                                                                     TABLE I11
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                                                                                                  Susceptibility components in units       cm3/mole for acetamides and calculated
                                                                                                      anisotropy shift difference in p.p.m. for N-acetylnortropine for several
                                                                                                                  values of AxC-C. Electric field correction applied

                                                                                                                      A   ~    ~   x-      ~
                                                                                                                                           ~    ~ xuUC-~
                                                                                                                                                   ~  =      ~ X,,~=O       s ( ~ - s(n)
                                                                                                                                                                                  )
                                                                                              A a t higher field
                                                                                                                        1.21        +2.0          - 10           -6.7       $0.48
                                                                                                                        4.0         -0.4          - 11           -3.3       +0.50
                                                                                                                       10.0         -5.4          - 14           +4.0       +0.56     n
                                                                                              B a t higher field
                                                                                                                        1.21       -44             +6.9          +22        -0.68
                                                                                                                        4.0
                                                                                                                       10.0
                                                                                                                                   - 46
                                                                                                                                   -51
                                                                                                                                                   +5.6
                                                                                                                                                   13.6
                                                                                                                                                                 +25
                                                                                                                                                                 +32
                                                                                                                                                                            -0.65
                                                                                                                                                                            -0.59     n
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                                                                                                                                     TABLE IV
                                                                                                  Susceptibility componellts in units 10-%cm3/mole for formamides and calculated
                                                                                                           anisotropy shift difference in p.p.m. for formamide for several
                                                                                                                   values of AxC-Fl Electric field correction applied

                                                                                                                      AXc-H        ~ r r           xuuc=O        xi:c=o     6(A)   - 6(B)
                                                                                              A a t higher field


                                                                                              B a t higher field




                                                                                         In order to select a n optimuin set of values froin those given in Tables 111 and IV, the
                                                                                      anisotropy difference was calculated by means of eq. [l]for N-acetylnortropine (Fig. 2)
                                                                                      and for formainide. Hydrogen atoins A and B in N-acetylnortropine have rotation
                                                                                      angles c p = p = 0 and the corresponding values of the orientation functions are included
                                                                                                 ~ ,
                                                                                                                                              5
                                                                                      in Table 11. When these calculated values in colun~n of Tables I11 and IV are compared
                                                                                      with the corresponding anisotropy shift differences in Table I , it is seen that only a very
                                                                                      poor agreement results. T h e optimum values are those marked n in the last colun~n.
                                                                                                    1-IOOPER XKD KAISER: MAGNETIC ANISOTROPY O F THE CARBONYL BOND                       2367

                                                                                         I t appears that the principal source of error is an overestimation of the electric field
                                                                                      effect, especially a t sillall values of (, and ( p when the protons are close to the charges.
                                                                                                                                p         ~
                                                                                      The deterinination of the electric field shift by ineans of eq. [3] is apparently not very
                                                                                      accurate a t sillall distances although the field strengths are still small enough to make the
                                                                                      quadratic term in Buclringham's equation (17) negligibly small in comparison with the
                                                                                      linear term.
                                                                                         Better agreement between calculated and observed shift differences for N-acetyl-
                                                                                      nortropine and forinamide is obtained when the electric field effect is disregarded entirely,
                                                                                      as was done by Narasimhan and Rogers (3). The entire experimental shift difference in
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                                                                                      columil 1 of Table I is then assumed to be caused by the bond magnetic anisotropy. The
                                                                                      resulting susceptibility con~ponentsare listed in Tables V and VI, and the optimum
                                                                                      values are inarlred    u.  I t will be shown in the Discussion that these values are successful
                                                                                      in the interpretation of the spectra of a variety of molecules.

                                                                                                                                     TABLE V
                                                                                                  Susceptibility components in units 10-6 cm3/mole for acetamides and calculated
                                                                                                      anisotropy shift difference in p.p.m. for N-acetylnortropine for several
                                                                                                                 values of        Electric field correction neglected



                                                                                              A a t higher field
                                                                                                                     1.21        $-IS        - 14          - 19            0.59     U
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                                                                                                                     4.0         +15         - 15          - 15            0.62
                                                                                                                    10.0         $10         - 17           -7.9           0.68
                                                                                              B a t hieher field




                                                                                                                                     T A B L E VI
                                                                                                  Susceptibility components in units       cm3/mole for formamides and calculated
                                                                                                           anisotropy shift difference in p.p.m. for formamide for several
                                                                                                                  values of AxC-n. Electric field correction neglected

                                                                                                                    AxC-E       Xzz C-O       xuuc-O       x       ~   ~ 6(A) - 6(B) ~
                                                                                                                                                                            ~ -
                                                                                              A a t higher field
                                                                                                                    -0.74        +1.2         -8.6          -7.6           0.31
                                                                                                                      0          +0.5         -8.8          -6.7                J
                                                                                                                                                                           0.21 (
                                                                                                                    +0.48         0.0         -9.0          -6.0           0.18
                                                                                              B a t higher field
                                                                                                                    -0.74       -11           -0.5          -3.5         -0.19
                                                                                                                      0         - 12          -0.7          -2.7         -0.23     (J
                                                                                                                    +0.48       -12           -0.9          -1.9         -0.26


                                                                                      Part B. Determination of the Sign of the Experimental Shift Dzference
                                                                                        In Part A, both choices a(B) > a(A) and a(A) > o(B) were used in calculating
                                                                                      susceptibility components. There is evidence for each choice. The argument of Narasimhan
                                                                                      and Rogers (3) leads to the acceptance of only those sets which were calculated with the
                                                                                      assuinption that B is a t higher field. I t has been shown (19) that the largest contribution
                                                                                      to the diamagnetic susceptibility of urea is directed perpendicular to the carbonyl bond,
                                                                                      i.e. in the x direction. Pople (20) has postulated paramagnetic circulations in the plane
                                                                                      of a carbonyl bond and its substituents. These ought to give a positive contribution to
                                                                                      XvvC=Oand x,,C=O, while the results observed for urea indicate that x,,C=O should be
                                                                                      2368                    CANADIAN JOURNAL O F CI-IEMISTRY.    O.
                                                                                                                                                  V L 43,   10G5

                                                                                      negative. Only the sets of con~ponents   derived with the assuillption that B is more shielded
                                                                                      than A satisfy these requirements.
                                                                                        On the other hand, the opposite choice is favored by consideration of the nlagnitude
                                                                                      of the long range spin couplings through the C-N partial double bond. The analogous
                                                                                      long range coupling H-C=C-C-H              has been analyzed (21) as the sun1 of a positive
                                                                                      contribution J(u) through the u-bond frame and a negative contribution J ( T ) through
                                                                                      the .rr electrons of the C=C double bond. The first part is different for the cis and trans
                                                                                      configurations with J(u)         > J(u) ,is, while for the second part J(T) t T a , l S = J(T)
                                                                                      The magnitude of the total coupling constants (J,i,land JJt,,,,l        thus depends on the
                                                                                      magnitude of J(T), and if I J(a)l < &(J(u),is       +
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                                                                                                                                             J(u) t,,,,) then (Jt,,,,( 1 JCi,l.
                                                                                                                                                                       >           The
                                                                                      magnitude of J(T) may be expected to depend on the double bond character of the central
                                                                                      bond, and this dependence may be the cause of the observed variation of the total coupling
                                                                                      constants of amides in solvents of high dielectric constant (22). I t appears perinissible to
                                                                                      assume that the double bond character of the central bond in amides is sufficientl>rsillall
                                                                                                                                  >
                                                                                      to warrant the conclusion that IJt,,,,J (JCi,l. conclusion is confirnled 11). recent
                                                                                                                                          This
                                                                                      work on unsymmetrically disubstituted amides (23) in which the spectral assignnlents
                                                                                                                          > (Jcisl
                                                                                      based on the assumption JJtrans( were corroborated by the differential solvent
                                                                                      shift in benzene (24). In all cases that allow observation of the long range coupling i t is
                                                                                      the signal a t higher field that shows the stronger coupling. If IJt,,,,l > JJci,vl signal
                                                                                                                                                                              this
                                                                                      must be assigned to the group cis t o the carbonyl group, i.e. the A substituent must be
                                                                                      more shielded than the B substituent.
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                                                                                         We have attempted t o reach a decision between the two alternatives by illeans of the
                                                                                      following experiment. In 1-ethylpyrrolidinone-2 (Fig. 3) the -CI-12-        protons of the
                                                                                      ethyl group and those on Cg may be compared to the -CH2-           protons of ethyl groups
                                                                                      cis and trans to the carbonyl bond of an acetamide. In the acetamide the two -CH2-
                                                                                      groups can be distinguished only by inaking an assumption about the relative magnitude
                                                                                      of cis and trans coupling across the C-N partial double bond. However, in l-ethyl-
                                                                                      pyrrolidinone-2 they can be distinguished by the coupling patterns of their signals, a
                                                                                      quadruplet being expected for -CH2--       protons of an ethyl group and a triplet for those
                                                                                      on Cs. The relative positions of the centers of these multiplet signals enables us to decide
                                                                                      which CH2 group is more shielded.
                                                                                        The low field multiplets in the spectra of dilute solutions of 1-ethylpyrrolidinone-2 in
                                                                                      CC14and benzene are shown in Figs. 4a and 4b respectively. Figure 4a shows the expected
                                                                                      seven peaks, but it is impossible to decide which four peaks are those of the quadruplet
                                                                                      and which belong to the triplet. Figure 4b shows the strong separation of the quadruplet
                                                                                      and triplet structures produced by benzene dilution of the compound. Since all four peaks
                                                                                      of the quadruplet must be shifted identically by the change of solvent, as must the
                                                                                      three peaks of the triplet, the peaks in the spectrum of the CCl4 solution were assigned
                                                                                      by the measurement of spectral positions of individual peaks in each spectrun~.  The peals
                                                                                      are labelled in order of increasing field strength in the spectrum of the benzene solution.
                                                                                                     IIOOPER A N D Kr\ISER: MAGNETIC :\??ISOTROPY          017 TIlE   C.-IRBONYL BOKD    2360




                                                                                                                                              4
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                                                                                                                               H o   i   o
                                                                                                                              6 6


                                                                                                                                H   d

                                                                                                                                             0
                                                                                                                                              1



                                                                                        FIG.4. (a) LOWfield peaks in the n.1n.r. spectrum of 1-ethylpyrrolidinone-2 in CC1.1. ( b ) Low field
                                                                                      pealcs in the n.m.r. spectrum of 1-ethylpyrrolidinone-2 in CsHs.

                                                                                      Table VII suminarizes the assignment of the peaks of the CCl,, solution and sho\vs agree-
                               For personal use only.




                                                                                      ment with the rule developed by Hatton and Richards (24) in that the group farthest
                                                                                      from the carbonyl group (-C5H2-)   experiences additional shielding in benzene solution.

                                                                                                                                   TABLE VII
                                                                                                                 Room temperature spectral positions of low field
                                                                                                                   1-ethylpyrrolidinone-2 peaks with respect to
                                                                                                                           benzene as internal standard

                                                                                                                       -6 (c.P.s.)           -6 (c.P.s.)          6cc14 - ~ c   ~ H G
                                                                                                          Peak         CCI, soln.                  G
                                                                                                                                             C ~ Hsoln.              (c.P.s.)
                                                                                                                            220.3                 218.3                -2.0
                                                                                                                            227.1                 225.3                -1.8
                                                                                                                            234.2                 232.3                -1.9
                                                                                                                            241.4                 230.3                -2.1
                                                                                                                            218.1                 247.0                 29.8
                                                                                                                            224.8                 254.9                 30.1
                                                                                                                            231.1                 261.9                 30.8


                                                                                         A double resollance experinlent in which the quadruplet was collapsed b?- irradiation
                                                                                      of the high field methyl triplet confirmed that the pealts a, b, c, d are the signal of the
                                                                                      -CH2-     protons of the ethyl group.
                                                                                         Table VII shows that in CCl.&   solution the signal of the A substituent (center of the
                                                                                      quadruplet) is located a t higher field than the signal of the B substituent (center of the
                                                                                                                                         the
                                                                                      triplet) by 6.08 c.p.s. = 0.108 p.p.in., coi~firi~~ing assunlption made for the long range
                                                                                      spin coupling.
                                                                                                                                     DISCUSSION
                                                                                         The results presented in Tables 111-VI must be criticized because of the failure t o
                                                                                      achieve consistency when the shift caused by the electric field is taken into account.
                                                                                      The discrepancy between calculated and observed shift differences is greatest for protons
                                                                                      close t o the carbonyl bond. This suggests that a t distances of 1 t o 2 A the approxiination
                                                                                      2370                     CANADIAN JOURWAL OF                  O.
                                                                                                                                      CI-IEMISTRY. V L -13,   19G5

                                                                                      of a continuous electronic distribution by localized charges and dipoles is inadequate.
                                                                                       Indeed, values from 0 ( 2 5 ) to 0.4 e (6) have been suggested for tlze charge on the oxygen
                                                                                      atom in amides, and the decoinposition of the electric field into a polar and dipolar part
                                                                                      is some\\-hat ambiguous. The same criticism, of course, applies also t o the description of
                                                                                      the magnetic effect of the carbonyl bond in terins of an induced magnetic dipole. A calcu-
                                                                                      lation for the C-C bond (26) indicates the complexity of the actual physical situation.
                                                                                         The reaction field caused by polarization of tlze solvent surrounding the amide molecule
                                                                                      has beell neglected in the calculation of the electric field shift, and the values given in
                                                                                      Table I must therefore be too large. A detailed consideration of this effect is probably of
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                                                                                                                                                                          of
                                                                                      little significance in view of the use of a single inodel for a variety of n~olecules different
                                                                                      shapes and sizes.
                                                                                         I t does appear, however, that the susceptibility components derived by neglecting the
                                                                                      effect of the electric field are useful for correlating chemical shift differences with the
                                                                                      structure of lnolecules containing a carbonyl bond. For example, the optimum results
                                                                                      given in Table V have been used t o explain the dependence of the chemical shift difference
                                                                                      for the exo-methylene protons of a-inethylenecycloltetones on ring size (5). I t was shown
                                                                                      that the s-trans conformation is preferred unless the s-cis conformation is required by
                                                                                      sinall ring size. Similar results are obtained for the open chain a,@-unsaturatedketones
                                                                                      CH3CO-C=CRRf. The calculated and observed shift differences 16(R) - 6(R')I in
                                                                                      p.p.111. are for
                                                                                                                                            CALCULATED
                               For personal use only.




                                                                                                                                         s-cis    s-trans            OBSERVED
                                                                                             methyl vinyl ltetone (R, R')   =   H        0.63      0.15                0.13
                                                                                             inesityl oxide       (R, R')   =   CH3      0.55      0.33                0.27
                                                                                        T o give another application, the spectra of a series of disubstituted acetamides may be
                                                                                      considered. The observed shift differences inay be rationalized by distinguishing two
                                                                                      groups of conipounds:
                                                                                      Group I 11-ith N-CI-12R substituents show
                                                                                                        16(HA)- 6(HB) = 0 and b(RA) - b(RB)/ cu 0.09 p.p.111.;
                                                                                      Group I1 with N-CHRR'         substituents sho\v
                                                                                                        (6(E~A) 6(FIn)/ cu 0.58 p.p.111. and 16(RA)- 6(RD)/cu 0.
                                                                                                              -

                                                                                      The shift differences calculated with the optimum susceptibility components of Table V
                                                                                      agree \\-it11 these observations if it is assumed that the two groups of conlpounds exist
                                                                                      lllaiilly in the following conforn~ations:




                                                                                                                  Group I                         Group I1

                                                                                      We have not been able t o prove the existence of distinct rotational isomers, but a similar
                                                                                      proposal (27) for the closely related nitrosainines lends some support to this hypothesis.
                                                                                        The magnitude of the calculated shift difference is in these cases nearly independent
                                                                                                     HOOPER A X D KAISER: MAGNETIC AXISOTROPY O F THE CARBOSYL BOND                         2371

                                                                                      of whether the susceptibility components derived for A a t higher field or those for B a t
                                                                                      higher field are used. The results reported in Part 13,however, indicate that the assuillptioil
                                                                                      t h a t A is a t higher field is the better choice. Although this conclusion is in agreeinent
                                                                                      with deductions made froin long range spin coupling patterns and also with results for
                                                                                      the isoelectronic nitroso group (28), it does contradict Narasiinhan and Rogers' argument
                                                                                      based on the sign of the individual susceptibility components. I t is also coiltrary t o the
                                                                                      einpirical rule that in coinplex inolecules protons are deshielded by the influence of neigh-                 '

                                                                                      boring carbonyl bonds (1).
                                                                                          The susceptibility components derived for formainides differ considerably from those
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                                                                                      derived for acetamides. I t appears possible t h a t some fundamental difference in the
                                                                                      electronic structure bet~veen     formamides and acetamides is reflected in both the suscepti-
                                                                                      bilities of the carbonyl bond and in the barriers for internal rotation (1G).

                                                                                                                              ACKNOWLEDGMENTS
                                                                                        We \\-is11 t o gratefully acknowledge the assistance of the National Research Co~ui~cilof
                                                                                      Canada in the form of a grant in aid of research and the arvard of a studentship t o one of
                                                                                      us (D. L. I-I.).
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                                                                                       2. Michel Rouillard, Serge Geribaldi, Marcel Azzaro. 1981. 1H NMR utilization of through-space effects. II—conformation of
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                                                                                       4. Anita H. Lewin, Melvin Frucht. 1975. Restricted rotation in amides. VII—methods of resonance assignment in tertiary amides
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                                                                                          —an evaluation. Organic Magnetic Resonance 7:5, 206-225. [CrossRef]
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                                                                                          Resonance (1969) 10:3, 243-252. [CrossRef]
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                                                                                          alcoylée et de la stereochimie sur les vibrations de valence vC#Oet vC#C des cétones #-éthyléniques aliphatiques et
                                                                                          alicycliques à 6 chaînons et plus. Spectrochimica Acta Part A: Molecular Spectroscopy 25:6, 1081-1104. [CrossRef]
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                               For personal use only.

				
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