Water assisted dehalogenation of thionyl halides in the presence of water molecules

A second-order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT) investigation of the dehalogenation reactions of thionyl halides (SOF2 and SOBr2) are reported in which water molecules (up to seven for some reaction steps) were explicitly considered in the reaction complex. The dehalogenation processes of thionyl halides were observed to be substantially catalyzed by the presence of water molecules in the reaction system. The reaction rate became faster as more water molecules became involved in the reaction complex. The dehalogenation processes can be reasonably simulated by the gas phase water cluster models and the results here indicate that water molecules can help to solvate the thionyl halide molecules so as to activate the release of a halide (F− or Br−) leaving group. Kinetic rate constants of proposed reaction pathways were estimated so as to compare with results from a previous theoretical study of the dehalogenation of SOCl2. The proposed reaction pathways show a decreasing barrier from SOF2 to SOCl2 to SOBr2 and this trend is briefly discussed.