Direct ab initio dynamics study on a gas phase microsolvated S(N)2 reaction of F-(H2O) with CH3Cl

A microsolvated S(N)2 reaction of F-(H2O) with CH3Cl has been investigated by means of direct ab initio dynamics calculations in order to elucidate a detailed reaction mechanism. A full dimensional ab initio potential energy surface including all degrees of freedom was used in the dynamics calculati ons. Total energies and gradients were calculated at each time step. The vi brational phase of CH3Cl was generated classically so as to take a temperat ure of 10 K. The dynamics calculations showed that three reaction channels are concerned with the reaction at a fixed collision energy (E-coll = 4.42 kcal/mol). These are expressed by F- + CH3Cl --> CH3F + Cl- + H2O (channel I), F- + CH3Cl --> CH3F + Cl- (H2O) (channel II), and F- + CH3Cl --> CH3F(H 2O) + Cl- (channel III). Channel I is three-body dissociation of each produ ct. In channels II and III, Cl- and CH3F, respectively, are solvated by a w ater molecule. It was found that the main reaction pathway is channels I an d III, while channel II is significantly minor at E-coll = 4.42 kcal/mol. I n all channels, the halogen exchange occurs rapidly with very short lifetim es of early and late complexes (i.e., the reaction proceeds via direct mech anism), which is similar to the nonsolvated reaction F- + CH3Cl. The prefer ence of the reaction channels was discussed on the basis of theoretical res ults.