Page 91 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

70 1.4.4. Comparison and Interpretation of Atomic Charge Calculations

CHAPTER 1 While the total electron density is a real physical property that can be measured or
Chemical Bonding calculated, atomic charge distribution is not a physically observable quantity. Rather,
and Molecular Structure the values depend on the definition and procedures used to define atoms and assign
charges. Moreover, with some methods, the charge found is dependent on the basis set
orbitals that are used. For these reasons, the actual numerical values of charge should
not be taken literally, but useful information about the trends in electron distribution
87
within a molecule and qualitative comparisons can be made. For example, Table 1.18
gives the total charge on carbon and oxygen in formaldehyde using several different
basis sets by the MPA, NPA, and AIM methods. Each of the methods shows the
expected shift of electron density from carbon to oxygen, but the shift is considerably
more pronounced for the AIM analysis.
Atomic charges have been used to analyze the nature of the interaction between
the nitrogen and carbon groups in amides. In VB language this interaction is described
in terms of resonance. These resonance structures account for the most characteristic
properties of amides. They are quite polar and react with protons and Lewis acids at
oxygen, but not at nitrogen. The partial C=N double bond character also accounts for
the observed rotational barrier of about 18 kcal/mol.
O O –
+ O
C
RC NH 2 R C
R NH 2 N
H
H
88
There has been a NPA analysis of the delocalization in amides. Both the planar
and rotated forms of formamide and its =S, =Se, and =Te analogs were studied
∗
by NPA and natural resonance theory. HF/6-31+G , MP2/6-31+G , and B3LYP/6-
∗
31+G calculations were employed. At the MP2/6-31+G level, the transfer of charge
∗
∗
noted on going from the planar to rotated form of formamide was +0 105 at N, −0 088
at O, and −0 033 at C. This charge transfer is consistent with the resonance formulation.
The shifts were in the same direction but somewhat larger for the heavier elements,
Table 1.18. Atomic Populations in Formaldehyde Calculated Using Various Methods
and Basis Sets a

Basis set Oxygen population Carbon population
MPA NPA AIM MPA NPA AIM
STO-3G 8 188 8 187 8 935 5 925 5 833 4 999
4-31G 8 485 8 534 8 994 5 824 5 778 5 069
3-21G 8 482 8 496 8 935 5 869 5 782 5 124
6-31G ∗ 8 416 8 578 9 295 5 865 5 668 4 742
6-31G ∗∗ 8 432 8 577 9 270 5 755 5 676 4 701
6-311+G ∗∗ 8 298 8 563 9 240 5 892 5 606 4 755
a From S. M. Bachrach, Rev. Comp. Chem., 5, 171 (1994).

87 S. M. Bachrach, Rev. Comp. Chem., 5, 171 (1994).
88
E. D. Glendening and J. A. Hrabal, II, J. Am. Chem. Soc., 119, 12940 (1997).