Thermochemical properties of the hydroxy-formyl radical, HO-CO, and the formyloxy radical, HC(O)O, and their role in the reaction OH+CO→H+CO2. Computational G3MP2B3 and CCSD(T)-CBS studies

The relative enthalpies, ΔHo(0) and ΔHo(298.15), of stationary points (seven minimum and five transition structures) on the CHO2· potential energy surface were calculated with the aid of the G3MP2B3 as well as the CCSD(T)-CBS (W1U) procedures from which we earlier found mean absolute deviations of 3.9 and 2.3 kJ mol-1, respectively, between experimental and calculated enthalpies of formation of a set of 32 free radicals. For CCSD(T)-CBS (W1U) both the well depth of trans-HOCO, ΔHwello(298.15)=−110.9kJmol−1, as well as the reaction enthalpy of the overall reaction, OH+CO→H+CO2, ΔrHo(298.15)=−102.5 kJ mol-1, are in close agreement with the most recent experimental values, ΔHwello(298.15)=−108.4kJmol−1 and ΔrHo(298.15)=−102.3 kJ mol-1, respectively. In addition, for radicals trans-HOCO, cis-HOCO, and formyloxy HC(O)O, the thermodynamic functions heat capacity Cpo(T), entropy So(T), thermal energy content Ho(T) - Ho(0), and enthalpy of formation ΔfHo(T) are tabulated in the range of 50 - 3000 K. The reaction OH+CO→H+CO2 via rotation of the initially formed trans-HOCO to cis-HOCO proceeds over a quite small barrier (TS3, ΔHo(298.15)=1.1 kJ mol-1 with respect to the asymptote OH+CO), considerably lower than that for rearrangement to the formyloxy radical HC(O)O (TS4, ΔHo(298.15)=36.0 kJ mol-1). The first electronically excited state (2B2) of HC(O)O is found to lie just 7.0 kJ mol-1 above the 2A1 ground state (experimental T0=2.6 kJ mol-1). For HC(O)O (2A1) the CCSD(T)-CBS (W1U) method gives ΔfHo(298.15)=−127.1 kJ mol-1 (experimental ΔfHo(298.15)=−129.7±12.6 kJ mol-1).