Secondary isotope effect on photoinduced isomerization of alkyl radicals in low-temperature solids

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-h(13), 2-hexyl-1,6-d(2), 2-hexyl-1,1,6,6-d(4), 2-hexyl-1,1,1,6,6,6-d(6), and 2-hexyl-2,5,5-d(3) radicals. The quantum yield for Zhexyl-1,6-d(2), Zhexyl-1,1,6,6-d(4), and 2-hexyl-l,l,l,6,6,6-d(6) is less than 1/10 of that for 2-hexyl-h(13). The low quantum yield for 2-hexyl-1,6-d(2) and 2-hexyl-1,1,6,6-d(4) 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-d(3) radical is 4/5 of that for 2-hexyl-h(13). 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 methyl groups are positively charged and the radical site is negatively charged, and one of the two hydrogen atoms transfers to the radical site. ESR spectra indicate that deuterium atoms of the CH2D groups of 2-hexyl-1,6-d(2) radical and of the CHD2 groups of 2-hexyl-1,1,6,6-d(4) radicals preferentially occupies the positions in the plane of the C-C-C bonds, where the two hydrogen atoms in the valence excited state are positively charged. Deuteration of the atom of the transfer causes a large isotope effect, and the partial deuteration of the end methyl groups hence causes the large isotope effect.