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 sph ere ET (electron transfer) and via outer sphere ET. The alpha-deuteriu m KIE was found to be very high for inner sphere ET and very low for o uter sphere ET, while the C-13 KIE was high for inner sphere ET, but a lso significant for outer sphere ET. The use of carbon KIEs for distin guishing 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 methyllithiu m and methylmagnesium halide has very low alpha-D KIEs since the organ ometal reagents, like methyl radical, have low force constants in thei r carbon-hydrogen bonds.