Page 92 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 1.19. AIM Charge Distribution and Bond Order in Planar and 71
90 Formamide a

SECTION 1.4
A. Charge Distribution
Representation of
Planar 90
Electron Density
Distribution
Total Total
O 5 682 1 710 9 394 5 736 1 604 9 344
N 4 626 1 850 8 476 4 852 1 370 8 222
C 1 632 1 710 4 020 1 854 0 390 4 240
B. Bond Order
Planar 90
C=O 0 458 0 668 1 127 0 571 0 677 1 248
C−N 0 229 0 655 0 884 0 046 0 844 0 891
a
K. B. Wiberg and K. E. Laidig, J. Am. Chem. Soc., 109, 5935 (1987). K. B. Wiberg and C. M.
Breneman, J. Am. Chem. Soc., 114, 831 (1992).

evidently reflecting the increasing ability of S, Se, and Te to accept electron density
by polarization. As indicated earlier in Scheme 1.3, formamide shows a considerable
contribution from the dipolar resonance structure when analyzed by natural resonance
∗
theory. At the MP2/6-31G+ level, the weightings of the two dominant structures are
58.6 and 28.6%. The C=N bond order is 1.34 and the C=O bond order is 1.72. The
specific numbers depend on the computational method, but this analysis corresponds
closely to the traditional description of amide resonance.
An AIM electron distribution analysis was also performed by comparing the planar

structure with a 90 rotated structure that would preclude resonance. The charges, ,
and total bond orders are given in Table 1.19. Although the O charge does not change
much, the C–N bond order does, consistent with the resonance formulation. The charge
buildup on oxygen suggested by the dipolar resonance structure is largely neutralized
by a compensating shift of electron density from carbon to nitrogen.
O
H
H
H N
90°

Table 1.20 gives data by which we can compare the output of several methods
of charge assignment for substituted ethenes. If we compare ethene and ethenamine
as an example of a double bond with an electron-releasing group (ERG) substituent,
we see evidence of the conjugation (resonance) effects discussed on p. 22. The MPA
and NPA methods show an increase in negative charge at the terminal CH group,
2
as indicated by resonance. There is a smaller, although still negative, charge on this
carbon in the compounds with electron-withdrawing groups (EWG), e.g., CH=O,
CF , CN, and NO . The AIM charges present a quite different picture, with inductive
2
3
effects resulting from differences in electronegativity playing a dominant role. The
shift of electron density to more electronegative atoms overwhelms other factors.
Compare, for example, the fluoro, hydroxy, amino, cyano, and methyl substituents.
The dominant factor in the charge distribution here is accumulation of negative charge
on the more electronegative atoms. Note that the calculated charge at the terminal
C is the same for NH and CN substituents, which have opposing influences on
2
reactivity.