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 and alkyl halides, CH(2)O(.-) + CH(3)X (X = F, Cl, Br) and NCCHO(.-) + CH(3)Cl. 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 Angstrom as in the transition state for the reaction NCCHO(.-) + CH(3)Cl. 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.