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.