The gas-phase acyl transfer reactions X- + RCOY ↔ RCOX + Y- with R = H, CH3 and X, Y = Cl, Br have been investigated with MO theory at the G2(+)MP2 level. Attempts have been made to locate two types of adducts, a tetrahedral adduct formed by an out-of-plane π-attack (π-adduct) and an adduct formed by an in-plane σ-attack (σ-adduct). In all cases, the σ-type adducts are nonexistent. Careful examination of the energies (ΔE) at the MP2/6-311+G** level shows that the transition structure region is very flat with a very low barrier for decomposition of the intermediate. This small barrier becomes inverted (i.e., the intermediate level is higher than the transition state) as the zero-point and thermal energy corrections are applied, which is eventually restored to the normal barrier (δΔG = ΔGTS − ΔGInt ≅ −0.9 kcal mol-1) when entropy effect is accounted for. The π-adducts are stabilized mainly by the proximate second-order σ-σ* type charge transfer interactions. The solvent effect evaluated in acetonitrile by t...