Quantum-chemical study of sulfoxide decomposition

The decomposition of sulfoxides to sulfinic acid and olefin was studied by quantum-chemical calculations. Enthalpies of formation of reactants, transition state and products of decomposition of the sulfoxides were calculated by DFT, ab initio, and semiempirical methods. It was found that the MP2 level of theory is required for the correct description of this type of chemical reactions. The activation energies obtained by this method are in a better correlation with experimental data than those calculated by other compared methods. The calculations confirm experimental evidence that the activation energies of decomposition of sulfoxides with the general formula CH₃-S(O)-CH₂-R increase in a range from R = CH₃ (56.4 kJ mol^-1) to R = CH₂CH₃ (71.5 kJ mol^-1) and R = CH(CH₃)₂ (85 kJ mol^-1). Experimental data on 17 reactions of sulfoxide decomposition were examined using the parabolic model of reactions with hydrogen atom abstraction. Thermoneutral analogs E_e0 of activation energies of the sulfoxide decomposition reactions were calculated. The E_e0 was found to be 101.4 kJ mol^-1 for the sulfoxide with the CH₃ group and sufficiently higher (E_e0 = 110.5 kJ mol^-1) for sulfoxides with CH₂R′ group. This difference was attributed to the destabilization of the transition state arising from the adjacent R′ group.