An electron spin resonance study of the methylcyclopentadienyl radicals, MenH5 –nC5·(n= 1–5)

The radicals Men C5H5 –n· have been generated by photolysis of the corresponding hydrocarbons (n= 3–5) or mercury derivatives (n= 0–5) and their e.s.r. spectra have been recorded. The spectra are interpreted in terms of thermal population of the ψA and ψS molecular orbitals, for which the predicted hyperfine coupling constants are calculated by the McLachlan equation, taking λ 0.75 and Q(C5H5·)–30 and Q(Me5C5·) 31.85 G. The energy separations between the ψA and ψS MOs can be rationalised on the assumption that a methyl substituent on a carbon atom of the cyclopentadienyl carrying a unit π-electron has a destablising interaction, within a factor of two, of 10 kJ mol–1.