Thermochemical properties of the hydroxy-formyl radical, HO– [rad]CO, and the formyloxy radical, HC(O)O [rad], and their role in the reaction [rad]OH+CO→H [rad]+CO 2. Computational G3MP2B3 and CCSD(T)-CBS studies

Abstract

The relative enthalpies, Δ H o(0) and Δ H o(298.15), of stationary points (seven minimum and five transition structures) on the CHO 2 · 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, Δ H well o ( 298.15 ) = − 110.9 kJ mol − 1 , as well as the reaction enthalpy of the overall reaction, OH+CO→H +CO 2, Δ r H o(298.15)=−102.5 kJ mol –1, are in close agreement with the most recent experimental values, Δ H well o ( 298.15 ) = − 108.4 kJ mol − 1 and Δ r H o(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 C p o ( T ) , entropy S o( T), thermal energy content H o( T) – H o(0), and enthalpy of formation Δ f H o( T) are tabulated in the range of 50 – 3000 K. The reaction OH+CO→H +CO 2 via rotation of the initially formed trans-HOCO to cis-HOCO proceeds over a quite small barrier (TS 3, Δ H o(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 (TS 4, Δ H o(298.15)=36.0 kJ mol –1). The first electronically excited state ( 2B 2) of HC(O)O is found to lie just 7.0 kJ mol –1 above the 2A 1 ground state (experimental T 0=2.6 kJ mol –1). For HC(O)O ( 2A 1) the CCSD(T)-CBS (W1U) method gives Δ f H o(298.15)=−127.1 kJ mol –1 (experimental Δ f H o(298.15)=−129.7±12.6 kJ mol –1).