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 be en used to deduce the enthalpy of formation of the methoxycarbonyl ion as D elta H-f(298)[CH3O-C=O+] = 130 +/- 2 kcal/mol. From the G2 computed ionizat ion energy (IEa = 7.32 eV) and Delta H-f(298) (-40 kcal/mol) of the parent radical CH3O-C-.=O, we arrive at 129 kcal/mol for its ionic counterpart. Co mbining these theoretical findings with a reevaluation of existing experime ntal data (appearance energy measurements) yields 129 +/- 2 kcal/mol as our recommended value for Delta H-f(298)[CH3O-C=O+], a large upward revision o f the current literature value of 120 kcal/mol. By use of the new value as the anchor point, G2 derived Delta H-f(298) 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-<(C)double over dot>-OR have been calculated as 147, 131, 157, 144. 144, 140, and 177 kcal/mol, respectively.