Gas-phase identity S(N)2 reactions of halide anions and methyl halides with retention of configuration

High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out on the identity front-side nucleophilic substitution reactions with retention of configuration, X(-)+CH(3)X, for X=F, Cl, Br, and I. Overall gas-phase barrier heights do not show a strong variation with halogen atom and are calculated to be 184.5 (X=F), 193.8 (X=Cl), 178.9(X=Br), and 171.4 kJ mol(-1) (X=I), substantially higher than the corresponding barriers for back-side attack (-8.0 for X=F, 11.5 for X=Cl, 5.8 for X=Br, and 6.5 kJ mol(-1) for X=I). The difference between the overall barrier for back-side attack and front-side attack is smallest for X=I (164.9 kJ mol(-1)). Central barrier heights for front-side attack decrease in the following order: 241.0 (X=F), 237.8 (X=Cl), 220.0 (X=Br), and 207.4 kJ mol(-1) (X=I). The minimum energy pathways for both back-side and front-side S(N)2 reactions are found to involve the same ion-molecule complex (X(-)... H(3)CX), with the front-side pathway becoming feasible at higher energies. Indeed, our results suggest that the chloride exchange in CH3Cl, which has been found in gas-phase experiments at high energies, may be the first example of a front-side S(N)2 reaction with retention of configuration at saturated carbon. Analysis of our computational data in terms of frontier orbital theory suggests that elongation of the bond between the central atom and X could be a significant factor in decreasing the unfavorable nature of the front-side S(N)2 reaction with retention of configuration in going from X=F to X=I. Ion-molecule complexes CH3-X ... X(-), which may be pre-reaction complexes in direct collinear halophilic attack, were found for X=Br and I but not for X=F and Cl. The calculated complexation energies (Delta H-comp) for halophilic complexes are considerably smaller (7.3 and 19.4 kJ mol(-1) for X=Br and I, respectively) than those for the corresponding pre-reaction complexes for S(N)2 attack at carbon (41.1 and 36.0 kJ mol(-1) for X=Br and I, respectively). Nucleophilic substitution reactions at the halogen atom in CH(3)X (X=F-I) (halophilic reactions) are highly endothermic and appear to represent an unlikely mechanistic pathway for identity halide exchange.