Highly correlated ab initio molecular orbital calculations have been used to study the energetics and mechanism governing the reaction between the radical 1CH2 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.

Highly correlated ab initio molecular orbital methodologies have been used in the study of substituent effects on the singlet−triplet gaps of a series of nitrenium ions and their corresponding isoelectronic carbenes. Calculations in solution were carried out with the isodensity polarizable continuum models, IPCM. The results show a net stabilization of the singlet species as a result of electronic density donation of the substituents to a vacant p orbital located in the central atom. It is shown that this “π-donor−π-acceptor” dependence of the singlet−triplet gap is more significant in the case of the nitrenium ions, due to the presence of a positive charge. In addition, it was found that the singlet−triplet gaps decrease with the polarity of the solvent, most likely due to stabilizing electrostatic interactions between the solvent and the charge distribution of the singlet.