Page 98 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

77
r (au) (au)
2 906 0 241 Chapter Summary
C 2 H 6
2 646 0 291
C 6 H 6
2 468 0 329
C 2 H 4
2 207 0 368
C 2 H 2
This linear relationship exists for a variety of other C−C bonds, including those in
strained ring molecules and even in carbocations. 98 There is also a high-precision
correlation r =−0 998 for a series of bond lengths in aromatic compounds having
differing bond orders, 99 but it does not appear to hold for C–OorC–N bonds. 100
Bond order in the AIM context has also been defined as: 101

" AB = 2 i
i/i
i/i B (1.29)
A
For hydrocarbons, this treatment gives bond orders closely corresponding to the Lewis
structures, but there is a reduction of the bond order for polar bonds, owing to the
ionic portion of the bond.

0.982 1.013 1.015 0.922
H H H H
H
H C C C C H C C H N N H C N – C O +
H H H H
1.018 1.918 2.897 3.045 2.232 1.524

Chapter Summary

In this chapter we have reviewed the basic concepts of chemical bonding and
their relationship to molecular structure. We have also introduced the two major
computational approaches based on both molecular orbital (MO) and density
functional theory (DFT) methods. These computational methods are powerful comple-
ments to experimental methods for describing molecular structure and properties.
The orbital and electron density representations these computations provide can
help interpret structure, properties, and reactivity. We must, however, remember
to distinguish between the parts of this information that represent physically
measurable properties (e.g., molecular dimensions and total electron distribution)
and those that depend on definition (e.g., individual orbital shapes, atomic charge
assignments). Our goal is to grasp the fundamental structural consequences of
nuclear positions and electron distribution. Three key concepts, electronegativity,
delocalization, and polarizability, allow us to make qualitative judgments about
structure and translate them into a first approximation of expected properties and
reactivity.

98
R. F. W. Bader, T. H. Tang, Y. Tal, and F. W. Biegler-König, J. Am. Chem. Soc., 104, 946 (1982).
99
S. T. Howard and T. M. Krygowski, Can. J. Chem., 75, 1174 (1997).
100 S. T. Howard and O. Lamarcke, J. Phys. Org. Chem., 16, 133 (2003).
101
J. Cioslowski and S. T. Mixon, J. Am. Chem. Soc., 113, 4142 (1991).