G2 Theory and Experiment in Concert: Enthalpy of Formation of CH3O-C=O+ 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-C=O⁺] = 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-C=O⁺], 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 H2C-O-C(H)OH⁺, HOCH2-C=O⁺, ⁺CH2-H⋯O=C=O, ⁺CH(OH)-C(=O)H, ⁺CH2-O-C(=O)H, CH2-O-C(H)-O⁺, and ⁺CH2O-C̈-OH have been calculated as 147, 131, 157, 144, 144, 140, and 177 kcal/mol, respectively.