Single transition state serves two mechanisms.: Ab initio classical trajectory calculations of the substitution-electron transfer branching ratio in CH2O•-+CH3Cl

The reaction of a formaldehyde radical anion with methyl chloride is an example of a reaction in which a single transition state serves two mechanisms: substitution at carbon (Sub(C)) and electron transfer (ET). This reaction has been studied by ab initio molecular dynamics at the HF/6-31G(d) level of theory. Initial conditions were sampled from thermal distributions at the transition state, and ca. 200 trajectories were calculated at each of four different temperatures. Some trajectories go directly to ET products, but most go to the Sub(C) valley. Analysis of the initial conditions did not reveal any definitive factors that consistently favored one channel over the others. About half of the molecules in the Sub(C) valley subsequently dissociate to ET products within the 800-1200-fs simulation time of the present calculations. These molecules showed unimolecular kinetics for dissociation consistent with a chemically activated species. The ratio of ET to Sub(C) products varied from 1.02 to 1.43 over the temperature range 148-598 K. In a kinetic investigation of the reaction mechanism, such a temperature dependence would give the semblance of two competing transition states having different structures. However, since one transition state serves both mechanisms, the temperature dependence of the branching ratio is a reflection of the shape of the potential energy surface, and not an indicator of separate transition states.