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

Molecular dynamics has been used to investigate the reaction of a series of ketyl anion radicals an 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 Å 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.