Ab initio study of the X- + RCOY displacement reactions with R = H, CH3 and X,Y = Cl, Br

It has been established that the gas-phase transfer reactions X- + RCOY - RCOX + Y- with R = H, CH3 and X,Y = Cl, Br proceed concertedly through tetrahedral transition states created by an out-of-plane pi-attack. The sigma-adduct formed by an in-plane sigma-attack is neither an intermediate nor a transition state.