Dynamics of internal rotation in 2-(cycloalkenyl)ethyl radicals

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 ail cases, the preferred conformation about the Cβ-Cγ bond 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 rotational barrier heights were attributed to steric effects.