Sterically hindered free radicals. Part 20. EPR and ENDOR spectroscopy of α-carbonyl radicals

Abstract

The hyperfine coupling constants and g values of 11 α-carbonyl radicals ˙CR¹R²(CRO)(1) and ˙CR³(CN)(CRO)(2)(R = H, Me, Ph, α-Naph; R¹, R², R³= H, CO₂Et, Ph, 2,4-Me₂Ph, Mes) have been determined by EPR and, partially, ENDOR spectroscopy. The g values lie between 2.0025 and 2.0051. Except for ˙CH(CO₂Et)(CMeO), the g values are smaller than expected because of the steric hindrance of the substituents R¹, R² and R³. The CRO group is twisted out of the nodal plane of the radical which diminishes the delocalisation of the unpaired electron over the carboxyl group. The proton splittings in the α-aryl substituents differ significantly from those found in ˙CH₂Ph because of distortions. Proton splittings in benzoyl groups have been observed for the first time.