Page 77 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

56 Bond Distances in Carbonyl Compounds

CHAPTER 1 Basis set CH 2 =O CH 3 CH=O CH 3 2 C=O
Chemical Bonding C−H C=O C−H C=O C−C C=O C−C
and Molecular Structure
Experiment 1.108 1.206 1.106 1.213 1.504 1.222 1.507
311++G ∗∗ a 1.105 1.201 1.109 1.205 1.502 1.211 1.514
aug-CCPVDZ b 1.114 1.207
a. W. O. George, B. F. Jones, R. Lewis, and J. M. Price, J. Molec. Struct., 550/551, 281 (2000).
b. B. J. Wang, B. G. Johnson, R. J. Boyd, and L. A. Eriksson, J. Phys. Chem., 100, 6317 (1996).

In Chapter 3, we compare the results of DFT calculations on the relative thermody-
namic stability of hydrocarbons with those from MO methods. There is some indication
that B3LYP calculations tend to underestimate the stability of hydrocarbons as the
size of the molecule increases. For example, with the 6-311 ++G 3df 2p basis set,
the error calculated for propane −1 5kcal/mol , hexane −9 3 , and octane −14 0
increased systematically. 72 Similarly, when the effect of successive substitution of
methyl groups on ethane on the C−C bond energy was examined, the error increased
from 8.7 kcal/mol for ethane to 21.1 kcal/mol for 2,2,3,3-tetramethylbutane (addition
of six methyl groups, B3LYP/6-311 ++G d p . The trend for the MP2/6-311 ++G
d p was in the same direction, but those were considerably closer to the experimental
value. 73 The difficulty is attributed to underestimation of the C–C bond strength. As
we study reactions, we will encounter a number of cases where DFT calculations have
provided informative descriptions of both intermediates and transition structures. 74 In
these cases, there is presumably cancellation of these kinds of systematic errors, because
the comparisons that are made among reactants, intermediates, and product compare
systems of similar size. Use of isodesmic reactions schemes should also address this
problem.
DFT calculations have been used to compute the gas phase acidity of hydro-
carbons and compare them with experimental values, as shown in Table 1.13. The

Table 1.13. Gas Phase Enthalpy of Ionization of Hydrocarbons
in kcal/mol by B3LYP/6-311++G ∗∗ Computation

Compound
H calc
H exp
a
CH 4 416.8 416.7
a
C 2 H 6 419.4 420.1
CH 3 CH 2 CH 3 (pri) a 416.5 419.4
CH 3 CH 2 CH 3 (sec) a 414.4 415.6
CH 3 3 CH (tert) a 410.2 413.1
Cyclopropane b 411.5 411.5
Bicyclo[1.1.0]butane b 396.7 399.2
Bicyclo[1.1.1]pentane b 407.7 –
Cubane b 406.7 404.0
a
CH 2 =CH 2 405.8 407.5
HC≡CH a 375.4 378.8
a. P. Burk and K. Sillar, Theochem, 535, 49 (2001).
b. I. Alkorta and J. Elguero, Tetrahedron, 53, 9741 (1997).

72
L. A. Curtiss, K. Ragahavachari, P. C. Redfern and J. A. Pople, J. Chem. Phys., 112, 7374 (2000).
73 C. E. Check and T. M. Gilbert, J. Org. Chem., 70, 9828 (2005).
74
T. Ziegler, Chem. Rev., 91, 651 (1991).