Pairwise Trapping of Radicals in Irradiated n-Hydrocarbons and Related Compounds as Studied by Electron Spin Resonance

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Abstract
The ESR signals due to the radical pairs were found in n‐hydrocarbons γ irradiated at 77°K. Besides the spectra due to the ΔM s = 2 transition, ΔM s = 1 spectra were also found on the tail of the predominant spectra of isolated radicals. The ΔM s = 1 spectra of the radical pairs show nearly the same hyperfine splitting as that of the ΔM s = 2 spectra. From the analysis of the ΔM s = 1 spectra, the separation d ‖ due to the zero‐field splitting was determined to be 292 G from which the distance of the paired radicals was estimated to be 5.75 Å. The radical pairs are probably in the form of two alkyl radicals on the adjacent hydrocarbon chains. The value of 5.75 Å is consistent with the intercarbon distance between the adjacent chains aligned along the direction of the crystalline b axis. A glassy hydrocarbon such as squalane also gave the ΔM s = 2 spectrum but did not give the hyperfine lines on the tail of the ΔM s = 1 spectrum as is the case of high polymers. In addition, the effect of aditives and substituents was also investigated together with some other related compounds and the mechanism of the pairwise formation of radicals was discussed.