Localized orbitals and short-range molecular interactions. III. Rotational barriers in C2H6 and H2O2

Ab-initio calculations for eclipsed and staggered C2H6 and for several conformers of H2O2 are performed using a single determinant of doubly occupied nonorthogonal localized bond orbitals constructed from a minimal set of Slater AO's. The energies calculated with optimized BO's closely follow the pattern of the full SCF calculations. A component analysis of the rotation barriers is given along the lines suggested in Paper I [V. Magnasco and G. F. Musso, J. Chem. Phys. 60, 3744 (1974)]. The barrier in ethane (2.67 kcal) is primarily due to the penetration of nonorthogonal electron pairs belonging to vicinal C–H bonds into the core provided by nuclei and unperturbed electrons of all other groups. Although the minimal set is not suited to account for the small trans barrier in H2O2 both in BO and full SCF calculations, the calculated cis barrier in H2O2 (13.79 kcal) is seen to have a nature similar to that found in ethane, except for an enhanced importance of the Coulombic contribution. Delocalization effects are found to be of little importance in determining the conformational behavior of both molecules.