A single transition state serves two mechanisms: an ab initio classical trajectory study of the electron transfer and substitution mechanisms in reactions of ketyl radical anions with alkyl halides.

Abstract

Molecular dynamics has been used to investigate the reaction of a series of ketyl anion radicals and alkyl halides, CH2O(*)(-) + CH3X (X = F, Cl, Br) and NCCHO(*)(-) + CH3Cl. In addition to a floppy outer-sphere transition state which leads directly to ET products, there is a strongly bound transition state that yields both electron transfer (ET) and C-alkylated (SUB(C)) products. This common transition state has significant C-- C bonding and gives ET and SUB(C) products via a bifurcation on a single potential energy surface. Branching ratios have been estimated from ab initio classical trajectory calculations. The SUB(C) products are favored for transition states with short C--C bonds and ET for long C--C bonds. ET reactivity can be observed even at short distances of r(C)(-)(C) = ca. 2.4 A as in the transition state for the reaction NCCHO(*)(-) + CH3Cl. Therefore, the ET/SUB(C) reactivity is entangled over a significant range of the C--C distance. The mechanistic significance of the molecular dynamics study is discussed.