Intramolecular hydrogen transfer of excited alkyl radicals in the solid phase

Photo-induced isomerization of hexyl radicals in 77 K crystalline hexanes was studied by ESR spectroscopy. The efficiency of the isomerization from secondary to primary hexyl radicals in perdeuterated hexane is nearly the same as that in protiated hexane. The partially deuterated alkyl radical of 2-hexyl-2,2,5-d3 isomerizes to 1-hexyl-2,2,5-d3, whereas 3-hexyl-2,2,5,5-d4, isomerizes to 1-hexyl-2,2,5,5-d4. These results indicate that the isomerization predominantly occurs through intramolecular reactions: a hydrogen atom of a methyl group directly transfers to the radical site in a radical. A mechanism via a valence excited state will explain the experimental results.