Collision energy dependence on the microsolvated SN2 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 method. A full dimensional ab initio potential energy surface including all degrees 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 time step. Three energies, E-coll = 10.0, 17.7, and 25.0 kcal/mol were chosen as center of mass collision energies between F-(H2O) and CH3Cl. The present dynamics calculations indicated that in all collision energies three reaction channels were open as products. These are expressed by F-(H2O) + CH3Cl --> CH3F + H2O + Cl- (channel I); F-(H2O) + CH3Cl --> CH3F + Cl-(H2O) (channel II); and F-(H2O) + CH3Cl --> CH3F-H2O + Cl- (channel III). Channel I is three-body dissociation of each product, whereas Cl- and CH3F are solvated by H2O in channels II and LU, respectively. The dynamics calculations also 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, and 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 mechanism of the reaction and the effects of water were discussed on the basis of the present calculations.