Page 78 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 1.14. Gas Phase Ionization Energies in kcal/mol for 57
Some Strong Acids a
SECTION 1.4
Acid DFT Experimental DFT
Representation of
306 302 2 ClSO 3 H 292 6 Electron Density
H 2 SO 4
Distribution
FSO 3 H 295 9 299 8 HClO 4 298 5
CF 3 SO 3 H 297 7 299 5 HBF 4 293 1
CH 3 SO 3 H 317 0 315 0 H 2 S 2 O 7 280 2
CF 3 CO 3 H 316 0 316 3 HSbF 6 262 4
a. From B3LYP/6-311+G ∗∗ computations. Ref. 75.

agreement with experimental values is quite good. The large differences associated with
hydridization changes are well reproduced. The increased acidity of strained hydro-
carbons such as cyclopropane, bicyclo[1.1.1]butane, and cubane is also reproduced.
For acyclic alkanes, the acidity order is tert-H > sec-H > pri-H, but methane is more
acidic than ethane. We discuss the issue of hydrocarbon acidity further in Topic 3.1.
DFT computations can be extended to considerably larger molecules than
advanced ab initio methods and are being used extensively in the prediction and
calculation of molecular properties. A recent study, for example, examined the energy
75
required for ionization of very strong acids in the gas phase. Good correlations with
experimental values were observed and predictions were made for several cases that
have not been measured experimentally, as shown in Table 1.14.
Apart from its computational application, the fundamental premises of DFT lead
to a theoretical foundation for important chemical concepts such as electronegativity
and hardness-softness. The electron density distribution should also be capable of
describing the structure, properties, and reactivity of a molecule. We explore this
aspect of DFT in Topic 1.5.

1.4. Representation of Electron Density Distribution

The total electron density distribution is a physical property of molecules. It can
be approached experimentally by a number of methods. Electron density of solids can
be derived from X-ray crystallographic data. 76 However, specialized high-precision
measurements are needed to obtain information that is relevant to understanding
chemical reactivity. Gas phase electron diffraction can also provide electron density
data. 77 The electron density is usually depicted as a comparison of the observed
electron density with that predicted by spherical models of the atoms and is called
deformation electron density. For example, Figure 1.24 is the result of a high-precision
determination of the electron density in the plane of the benzene ring. 78 It shows an
accumulation of electron density in the region between adjacent atoms and depletion
of electron density in the center and immediately outside of the ring. Figure 1.25

75 I. A. Koppel, P. Burk, I. Koppel, I. Leito, T. Sonoda, and M. Mishima, J. Am. Chem. Soc., 122, 5114
(2000).
76
P. Coppens, X-ray Charge Densities and Chemical Bonding, Oxford University Press, Oxford, 1997.
77 S. Shibata and F. Hirota, in Stereochemical Applications of Gas-Phase Electron Diffraction, I. Hargittai
and M. Hargittai, eds., VCH Publishers, New York, 1988, Chap. 4.
78
H.-B. Burgi, S. C. Capelli, A. E. Goeta, J. A. K. Howard, M. A. Sparkman, and D. S. Yufit, Chem.
Eur. J., 8, 3512 (2002).