Semiempirical Treatment of Hindered Rotation. II. Molecules Possessing Double Bonds

Abstract

A method using electrostatic models, presented earlier for ethanelike molecules, is extended to include systems containing double bonds. Models are developed for the prototype molecules acetaldehyde, propylene, and methyl ketene. The principal barrier-producing features in these molecules, exclusive of the hydrogen adjacent to the methyl group, are taken to be, respectively, nonbonding, sp2 hybridized electron pairs on oxygen, the cis C–H bonding electrons, and the pi electrons in the C=O bond. These models are shown to predict reasonably well the barriers in vinyl silane and N-methylmethylenimine, and to be consistent with observed barriers for methyl isocyanate and methyl allene. They also are shown to be capable of rationalizing observed trends in barrier values upon halosubstitution for methine or carbonyl hydrogen in propylene and acetaldehyde when inductive and resonance effects are considered. Implications of the models for molecules with double bonds more removed from the methyl group, such as propionaldehyde, are considered, and conditions for steric repulsion are shown to be present in cases where the models fail. The connection between the models and the detailed shapes of the barriers is discussed, and some values of V6 as computed from the models, are given. Rough predictions are made of barriers for molecules derived from prototypes by halo substitution or by substitution of a methyl group by a silyl group. Problems involved in extending the method to molecules with onefold rotors are considered, and illustrative calculations are made for nitrous acid and formic acid.