Collision energy dependence on the microsolvated S(N)2 reaction of F-(H2O)with CH3Cl: A full dimensional ab initio direct dynamics study

Collision energy dependence on the microsolvated S(N)2 reaction of F-(H2O) with CH3Cl has been investigated by means of direct ab initio dynamics meth od. A full dimensional ab initio potential energy surface including all deg rees of freedom was used in the dynamics calculations; i.e., total energies of the reaction system and gradient of all atoms were calculated at each t ime step. Three energies, E-coll = 10.0, 17.7, and 25.0 kcal/mol were chose n as center of mass collision energies between F-(H2O) and CH3Cl. The prese nt dynamics calculations indicated that in all collision energies three rea ction channels were open as products. These are expressed by F-(H2O) + CH3C l --> CH3F + H2O + Cl- (channel I); F-(H2O) + CH3Cl --> CH3F + Cl-(H2O) (ch annel II); and F-(H2O) + CH3Cl --> CH3F-H2O + Cl- (channel III). Channel I is three-body dissociation of each product, whereas Cl- and CH3F are solvat ed by H2O in channels II and LU, respectively. The dynamics calculations al so showed that branching rations of channels I:II:III at E-coll = 10.0, 17. 7, and 25.0 kcal/mol are calculated to be 0.55:0.04:0.41, 0.46:0.18:0.36, a nd 0.35:0.43:0.22, respectively. These results indicate that channels I and III are more favored at lower collision energies and channel II is minor, whereas channel II becomes dominant at higher collision energies. The mecha nism of the reaction and the effects of water were discussed on the basis o f the present calculations.