Inner sphere and outer sphere electron transfer to methyl iodide, deuterium and C-13 kinetic isotope effects

Deuterium and C-13 kinetic isotope effects (KIEs) have been determined for the conversion of methyl iodide into methyl radical via inner sphere ET (electron transfer) and via outer sphere ET. The alpha-deuterium KIE was found to be very high for inner sphere ET and very low for outer sphere ET, while the C-13 KIE was high for inner sphere ET, but also significant for outer sphere ET. The use of carbon KIEs for distinguishing between inner and outer sphere ET reactions to methyl iodide is therefore not appropriate, while alpha-D KIEs are well suited. The reason for the high alpha-D KIEs in inner sphere ET is the 'stiffer' C-H bonds in methyl iodide as compared with the more pliable C-H bonds in methyl radical. Formation of methyl radical by ET from methyllithium and methylmagnesium halide has very low alpha-D KIEs since the organometal reagents, like methyl radical, have low force constants in their carbon-hydrogen bonds.