Bond–antibond analysis of internal rotation barriers in glyoxal and related molecules: Where INDO fails

Abstract

AbstractAb initio and INDO LCBO–MO calculations are carried out on glyoxal, 1,3‐butadiene, and acrolein in order to analyze the qualitative failure of INDO‐like methods to describe conformation energies in these molecules. Following the method of Brunck and Weinhold for ethanelike systems we identify the principal bond–antibond interactions contributing to the glyoxal barrier, their dependence on dihedral angle and substituent, and their relative magnitudes as calculated by ab initio and INDO SCF–MO theory. We find a gross disparity in the INDO representation of π* interactions which leads to a grossly exaggerated estimate of the stability of gauche conformers in these molecules. These findings appear to have serious implications for the applicability of INDO‐like theories to conformational problems in π‐bonded molecules, including those of biological interest.