Theoretical studies on thiocarbonyl group transfer reactions

Abstract

The gas-phase thiocarbonyl transfer reactions X−  + RC(S)Z ⇌ RC(S)X + Z− have been investigated with X, Z = Cl, Br and R = H, CH3 at the MP2 and G2(+) levels using the MP2/6-311+G** optimized geometries. The thiocarbonyl transfers proceed by a stepwise mechanism through a triple-well potential energy surface involving a tetrahedral intermediate, T−. The strong proclivity toward the stepwise path is caused mainly by the low-lying πCS* level coupled with the high energy σC–Z* orbital. The reaction barriers, ΔG≠, are higher than the corresponding values for the carbonyl transfers, excepting for the X = Z = Cl case at the G2(+) level. The major factors for these elevated barriers, despite the low deformation energies (ΔEdef), are lower proximate σ–σ* charge transfer energies (δΔEσ–σ*(2)) and the relatively high electrostatic repulsion (ΔEes > 0) in the transition state due to the highly polarized structure with a large negative charge on the S atom. The occurrence of a well-defined intermediate, T−, in the reaction coordinate leads to a relatively early transition state with a low degree of bond making and breaking in the transition state.