Page 82 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

of electron density are more meaningful. The charge distribution of formaldehyde is in 61
accord with its fundamental chemical reactivity, that is, susceptibility to reactions with
nucleophiles at carbon and with Lewis acids at oxygen. SECTION 1.4
Representation of
0.7765 1.2235 +0.1756 Electron Density
Distribution
H
C O +0.1311 C O – 0.4822
H

1.4.2. Natural Bond Orbitals and Natural Population Analysis

Anotherapproachforassignmentofatomiccharges,knownasthenaturalpopulation
82
analysis (NPA) method, developed by F. Weinhold and collaborators, involves formu-
lating a series of hybrid orbitals at each atom. Natural bond orbitals (NBO) describe the
molecule by a series of localized bonding orbitals corresponding to a Lewis structure.
Another set of orbitals describes combinations in which electron density is transferred
from filled to antibonding orbitals. These interactions correspond to hyperconjugation
in VB terminology. The total energy of the molecule is given by the sum of these two
components:

E = E +E (1.23)
∗
Typically the E ∗ term accounts for only a small percentage of the total binding
energy; however, as it represents a perturbation on the localized structure, it may be
particularly informative at the level of chemical structure and reactivity. The charges
foundbyNPAareillustratedbelowbythemethylderivativesofthesecond-rowelements.
Note that the hydrogens are assigned quite substantial positive charges ∼ 0 2e , even
in methane and ethane. The total positive charge on the hydrogen decreases somewhat
as the substituent becomes more electronegative. The carbon atom shows a greater shift
of electron density to the substituent as electronegativity increases, but remains slightly
negative, even for fluoromethane. The protocol for the NPA method is incorporated into
MO computations and is used frequently to represent electron distribution.

∗ a
NPA Populations for CH -X 6-31G
3
X &C &H 3 b &X
H −0 867 +0 650 +0 217
Li −1 380 +0 576 +0 805
BeH −1 429 +0 689 +0 740
−1 045 +0 712 +0 333
BH 2
−0 634 +0 643 0.000
CH 3
−0 408 +0 586 −0 178
NH 2
OH −0 225 +0 547 −0 322
F −0 086 +0 513 −0 427
a. From A. E. Reed, R. B. Weinstock and F. Weinhold, J. Chem. Phys., 83, 735
(1985).
b. Total charge on the three hydrogens of the methyl group.
82
A. E. Reed, R. B. Weinstock, and F. Weinhold, J. Chem. Phys., 83, 735 (1985); A. E. Reed, L. A. Curtiss,
and F. Weinhold, Chem. Rev., 88, 899 (1988); F. Weinhold and J. Carpenter, in R. Naaman and Z Vager,
eds., The Structure of Small Molecules and Ions, Plenum Press, New York, 1988.