Nonbonded interactions and the barrier heights to internal rotation of the methyl group: Part I. Cis- and trans-substituted propenes

Strain energies and barrier heights for rotation of the methyl group in several cis- and trans-substituted propenes have been calculated by means of the Wiberg method. The calculations reveal that the lowering of the barriers in cis-substituted propenes, as compared with the values in trans-compounds and in propene itself, can be ascribed to steric factors. The proximity of the barriers in trans-propenes and in the parent molecule is also considered to be due to nonbonded interactions.