Internal rotation barriers of carbonium ions

Abstract

The rotation barriers of allyl-, benzyl-, ethyl-, n-propyl-, and isobutyl-ium ions have been calculated by use of the MINDO/2 SCF–MO method. Barrier heights of 14–17 kcal mol–1 for the conjugated ions were obtained. For the ethylium ion a negligible barrier was calculated. The n-propylium ion shows a relatively high barrier (7·64 kcal mol–1) compared with recent ab initio calculations. For the non-classical phenylethylium ion estimation has been made for the rotation barrier of the phenyl ring (or the ethylene residue) around the principal axis of the molecule.