| Abstract
| - Isotope effects on photoinduced isomerization of alkyl radicals in 77 K solid alkanes were studied by ESRspectroscopy. Quantum yield of the isomerization from secondary to primary radicals was compared for2-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. Thequantum 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 for2-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 isotopeeffect, which is an isotope effect on atoms not participating in bond breaking or forming in the reaction. Incontrast, the deuteration of the 2,5-hydrogens causes a much smaller effect than the deuteration of the endmethyl groups; the quantum yield for 2-hexyl-2,5,5-d3 radical is 4/5 of that for 2-hexyl-h13. These resultssupport our mechanism of the photoinduced isomerization: a photoexcited alkyl radical is converted to avalence excited state in which two hydrogen atoms of the methyl groups are positively charged and theradical site is negatively charged, and one of the two hydrogen atoms transfers to the radical site. ESR spectraindicate that deuterium atoms of the CH2D groups of 2-hexyl-1,6-d2 radical and of the CHD2 groups of2-hexyl-1,1,6,6-d4 radicals preferentially occupies the positions in the plane of the C−C−C bonds, where thetwo hydrogen atoms in the valence excited state are positively charged. Deuteration of the atom of the transfercauses a large isotope effect, and the partial deuteration of the end methyl groups hence causes the largeisotope effect.
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