Reversible conformation change of free radicals in X-irradiated glutarimide single crystals studied by ENDOR

X-ray irradiation of glutarimide single crystal yields radicals by detachment of a hydrogen atom from the carbon atom at meta position with respect to the nitrogen of the glutarimide ring. Two chemically similar but crystallographically different radicals present in the ratio 3:1 were identified using ESR, ENDOR and ENDOR-induced ESR (EIE). The hyperfine couplings (hfc) of the two protons in β position with respect to the radical site of the abundant radical, changed reversibly between 51−38 G and 12–22 G in the temperature range 100–375 K. The α-proton hyperfine couplings of the two radicals were not affected. Chemical exchange, libration motion of the β protons and conformation changes of the glutarimide ring were considered as possible sources of the temperature effects. The latter models are both plausible but cannot be differentiated in the absence of crystallographic data at low temperature.