The conformations of cycloalkylmethyl radicals and barriers to internal rotation

Cycloalkylmethyl radicals with C₄, C₅, C₆, C₈, and C₁₁ rings have been studied by e.s.r. spectroscopy. The preferred conformation for cyclobutylmethyl and cyclopentylmethyl radicals is the bisected form with H_β in the nodal plane of the semi-occupied orbital; the radicals with larger rings adopt the eclipsed conformation. It is suggested that the conformational preferences of the radicals are governed by steric factors, except for cyclobutylmethyl where the bisected conformation is favoured by C-C hyperconjugation involving C and the two C_α-C_β bonds. The cycloundecylmethyl radical exists in two different conformations at low temperatures but at T > 230 K interconversion is fast and a single e.s.r. spectrum is observed. The barriers to internal rotation in the cycloalkylmethyl radicals were calculated from the temperature dependence of the β-hyperfine splitting using the classical limit procedure. The internal rotation about the C-C_α bond was also studied by the MNDO and INDO semi-empirical SCF MO methods.