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 Ċ_α–C_β 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 C_β–C_γ bonds was found to be the one in which the Ċ_α–C_β bond makes an angle of 90° with the plane of the ring. Exchange broadening due to restricted rotation about the C_β–C_γ 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.