Secondary Isotope Effect on Photoinduced Isomerization of Alkyl Radicals in Low-Temperature Solids

Abstract

Isotope effects on photoinduced isomerization of alkyl radicals in 77 K solid alkanes were studied by ESR spectroscopy. Quantum yield of the isomerization from secondary to primary radicals was compared for 2-hexyl-h13, 2-hexyl-1,6-d2, 2-hexyl-1,1,6,6-d4, 2-hexyl-1,1,1,6,6,6-d6, and 2-hexyl-2,5,5-d3 radicals. The quantum yield for 2-hexyl-1,6-d2, 2-hexyl-1,1,6,6-d4, and 2-hexyl-1,1,1,6,6,6-d6 is less than 1/10 of that for 2-hexyl-h13. The low quantum yield for 2-hexyl-1,6-d2 and 2-hexyl-1,1,6,6-d4 is due to a secondary isotope effect, which is an isotope effect on atoms not participating in bond breaking or forming in the reaction. In contrast, the deuteration of the 2,5-hydrogens causes a much smaller effect than the deuteration of the end methyl groups; the quantum yield for 2-hexyl-2,5,5-d3 radical is 4/5 of that for 2-hexyl-h13. These results support our mechanism of the photoinduced isomerization: a photoexcited alkyl radical is converted to a valence excited state in which two hydrogen atoms of the...