Pairwise trapping of radicals in single crystals of n-decane irradiated at 1.5 and 4.2°K

Abstract

The electron spin resonance spectra of single crystals of n‐decane‐d₂₂, n‐decane‐h₂₂, and their mixtures irradiated and measured at 1.5 and 4.2°K have been studied to shed light on the mechanisms of alkyl radical formation in saturated hydrocarbons. From the analyses of low temperature spectra and of the spectral changes caused by thermal annealing, it is suggested that almost all alkyl radicals are inherently formed in pairs and that the initial radical pair formation is fairly random especially for distant radical pairs, although there is some preference in the short range reactions for forming very close radical pairs with separation shorter than 6 Å. The predominant radical pairs in deuterated crystals irradiated at 1.5 and 4.2°K have separations ranging 6–25 Å. The spatial distribution of the generation of paired radicals depends upon irradiation temperature. Radical site migration takes place even at 35°K. There is a remarkable mass effect on the spatial distribution of the paired radicals. The deuterated crystals give much smaller spatial distribution. Random scission of the C–H bonds at any carbon atom seems to take place at 4.2°K.