ASYMMETRY IN METHYL-GROUP OF ETHANE DURING INTERNAL-ROTATION - AB-INITIO STUDY

Computational studies of the minimum energy pathway for internal rotat ion of a methyl group are often made by constraining one dihedral angl e at a sequence of values and optimizing all other parameters. When th is is done, the methyl group adopts an asymmetric configuration at int ermediate values of the torsion angle, with unequal bond lengths, bond angles, and torsion angles, even though the moiety against which it i s rotating is another methyl group. The potential surface leading to t his phenomenon is investigated using Hartree-Fock SCF calculations at the 6-31G and 6-311G** levels and the detailed structural behavior of the methyl group during the course of the internal rotation is examin ed. It is shown that the nature of the constraint governs the resultin g deformation of the methyl group symmetry. (C) 1998 John Wiley & Sons , Inc.