Theoretical analysis of the energy barriers for the rotational isomerization of the allyl and the 1-cyano-, 1-hydroxy- and 1-cyano-1-hydroxyallyl radicals

Abstract

An ab initio computational study was carried out on the ground- and transition-state structures for the rotational isomerization of the allyl and the 1-cyano, 1-hydroxy- and 1-cyano-1-hydroxyallyl radical systems in an attempt to gain an understanding of the factors affecting the relative energy barriers for rotational isomerization. The results of ESR rate measurements have indicated that there exists the presence of an ‘extra’ lowering of the energy barrier for the rotational isomerization of the 1-cyano-1-hydroxylallyl radical relative to the sum of the lowerings of the energy barriers for the rotational isomerization of the 1-cyano- and 1-methoxylallyl radical compared with the energy barrier for the rotational isomerization of the allyl radical. The ‘extra’ lowering of the energy barrier for the rotational isomerization of the 1-cyano-1-hydroxyallyl radical has been attributed to captodative stabilization of the transition structure for rotational isomerization. Prior calculational studies in the author's and other laboratories have resulted in the suggestion that captodative substitution does not always lead to ‘extra’ stabilization to a radical center. Recent calculations in the author's laboratories and the results reported in the present paper strongly suggest that ground-state effects can be dominant in determining the rate of a radical-forming reaction. The present paper describes the results of theoretical calculations on the allyl and substituted allyl radical systems that indicate that ground-state effects appear to dominate the relative rates of the rotational isomerization of the substituted allyl radicals. © 1997 John Wiley & Sons, Ltd.