Page 100 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

H 79
H
TOPIC 1.1
C C
C C
The Origin of the
H Rotational (Torsional)
H Barrier in Ethane and
Other Small Molecules
The repulsive electronic interactions were emphasized in early efforts to under-
stand the origin of the rotational barrier. 104 In particular the character of the ,
z

, , and (see Figure 1.34) was emphasized. 105 The repulsive interactions among
z
y
y
these orbitals are maximized in the eclipsed conformation.
Efforts have been made to dissect the contributing factors within an MO
framework. The NPA method was applied to ethane. Hyperconjugation was found to
contribute nearly 5 kcal/mol of stabilization to the staggered conformation, whereas
electron-electron repulsion destabilized the eclipsed conformation by 2 kcal/mol. 106
These two factors, which favor the staggered conformation, are partially canceled by
other effects. The problem is complicated by adjustments in bond lengths and bond
angles that minimize repulsive interactions. These deformations affect the shapes and
energies of the orbitals. When the effects of molecular relaxation are incorporated into

π '
π ' y
z

σ
x

π π y
z

σ '

σ

Fig. 1.34. Molecular orbitals of ethane

revealing character of z , y , , and
z y
orbitals. Only filled orbitals are shown.
104 J. P. Lowe, J. Am. Chem. Soc., 92, 3799 (1970); J. P. Lowe, Science, 179, 527 (1973).
105 E. T. Knight and L. C. Allen, J. Am. Chem. Soc., 117, 4401 (1995).
106
J. K. Badenhoop and F. Weinhold, Int. J. Quantum Chem., 72, 269 (1999).