G2 Theory and Experiment in Concert: Enthalpy of Formation of CH3O−CO+ and Its Isomers Revisited

Ab initio molecular orbital calculations at the Gaussian-2 level of theory on a set of isodesmic, atomization, and substitution type reactions have been used to deduce the enthalpy of formation of the methoxycarbonyl ion as ΔHf298[CH3O−CO+] = 130 ± 2 kcal/mol. From the G2 computed ionization energy (IEa = 7.32 eV) and ΔHf298 (−40 kcal/mol) of the parent radical CH3O−C•O, we arrive at 129 kcal/mol for its ionic counterpart. Combining these theoretical findings with a reevaluation of existing experimental data (appearance energy measurements) yields 129 ± 2 kcal/mol as our recommended value for ΔHf298[CH3O−CO+], a large upward revision of the current literature value of 120 kcal/mol. By use of the new value as the anchor point, G2 derived ΔHf298 values for the isomers HOCH2−CO+, +CH2−H···OCO, +CH(OH)-C(O)H, +CH2−O−C(O)H, and +CH2O−C−OH have been calculated as 147, 131, 157, 144, 144, 140, and 177 kcal/mol, respectively.