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- + CH3X, 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 SN2 reactions are found to involve the same ion−molecule complex (X-···H3CX), with the front-side pathway...