Ab initio study of the stability of the ylide-like intermediate methyleneoxonium in the reaction between singlet methylene and water

Highly correlated ab initio molecular orbital calculations have been used to study the energetics and mechanism governing the reaction between the radical (CH2)-C-1 and H2O in gas phase and in solution. It was found that methylene reacts in a barrierless fashion to produce the ylide-like intermediate methyleneoxonium, H2C-OH2, which in turn undergoes a 1,2-hydrogen shift to produce CH3OH. Results obtained at the QCISD(T)/6-311++G**/ /QCISD/6-311++G** level indicate that in the gas phase, the ylide and the transition state are located 6.4 and 4.9 kcal/mol below reactants, respectively, with an intrinsic barrier for the 1,2-hydrogen shift of 1,4 kcal/mol. In the presence of the solvent, the ylide remains more stable than reactants by 5.5 kcal/mol, while the energy of the transition state is now 1.96 kcal/mol higher than reactants giving a barrier of 7.49 kcal/mol for the 1,2-hydrogen shift.