Dynamics of internal rotation in 2-(cycloalkenyl)ethyl radicals
Abstract
2-(Cycloalkenyl)ethyl radicals having C4 to C7 rings were observed by e.s.r. spectroscopy. The barriers to rotation about the Ċα–Cβ bonds were determined from the temperature variation of the β-hydrogen hyperfine splittings and shown to decrease with increasing ring size. In all cases, the preferred conformation about the Cβ–Cγ bonds was found to be the one in which the Ċα–Cβ bond makes an angle of 90° with the plane of the ring. Exchange broadening due to restricted rotation about the Cβ–Cγ bond was observed and the rotation barriers were determined for the radicals with C6 and C7 rings. The radicals were studied using the semi-empirical MINDO/3 and MNDO methods and the corresponding hydrocarbons were investigated by the MM2 force field approach. The variations in the rotation barrier heights were attributed to steric effects.