Stationary points on the energy hypersurface of the reaction O3 + H•→ [•O3H]* ⇆ O2 + •OH and thermodynamic functions of •O3H at G3MP2B3, CCSD(T)-CBS (W1U) and MR-ACPF-CBS levels of theory

Abstract

The relative enthalpies, ΔHo (0) and ΔHo (298.15), of stationary points (four minimum and three transition structures) on the •O3H 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 (MAD) of 3.9 kJ mol−1 and 2.3 kJ mol−1, respectively, between experimental and calculated standard enthalpies of the formation of a set of 32 free radicals. For CCSD(T)-CBS (W1U) the well depth from O3 + H• to trans-•O3H, ΔHowell(298.15) = −339.1 kJ mol−1, as well as the reaction enthalpy of the overall reaction O3 + H•→O2 + •OH, ΔrHo(298.15) = −333.7 kJ mol−1, and the barrier of bond dissociation of trans-•O3H → O2 + •OH, ΔHo(298.15) = 22.3 kJ mol−1, affirm the stable short-lived intermediate •O3H. In addition, for radicals cis-•O3H and trans-•O3H, the thermodynamic functions heat capacity Cop(T), entropy So (T), and thermal energy content Ho(T) − Ho(0) are tabulated in the range of 100 − 3000 K. The much debated calculated standard enthalpy of the formation of the trans-•O3H resulted to be ΔfHo(298.15) = 31.1 kJ mol −1 and 32.9 kJ mol −1, at the G3MP2B3 and CCSD(T)-CBS (W1U) levels of theory, respectively. In addition, MR-ACPF-CBS calculations were applied to consider possible multiconfiguration effects and yield ΔfHo(298.15) = 21.2 kJ mol −1. The discrepancy between calculated values and the experimental value of −4.2 ± 21 kJ mol−1 is still unresolved.