Inner Sphere and Outer Sphere Electron Transfer to Methyl Iodide. Deuterium and 13C Kinetic Isotope Effects

Deuterium and ¹³C 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 α-deuterium KIE was found to be very high for inner sphere ET and very low for outer sphere ET, while the ¹³C 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 α-D KIEs are well suited. The reason for the high α-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 α-D KIEs since the organometal reagents, like methyl radical, have low force constants in their carbon-hydrogen bonds.