Cycloheptatrienyl radicals: An EPR study of substituent effects

Cycloheptatrienyl radicals with donor and acceptor substituents were generated by hydrogen abstraction from the corresponding cycloheptatrienes and studied by EPR spectroscopy. The barrier to rotation about the C‐1CO bond in ethoxycarbonylcycloheptatrienyl radicals was found to be ca. 7.5 kcal mol⁻¹ (31.4 kJ mol⁻¹). The observed hyperfine splittings of the ring hydrogens were interpreted in terms of a model in which the substituent causes an energy separation ΔE between the frontier orbitals. Populations of the symmetric and antisymmetric orbitals were deduced, and ΔE values were estimated. A similar analysis was carried out from EPR data for substituted benzene radical anions. These energy separations show a linear correlation with Hammett constants. The p‐values from plots of this type for [n]annulene radicals decrease in magnitude as the size of the ring increases.