Quantum chemical and dynamical characterization of the reaction OH + SO2 .dblharw. HOSO2 over an extended range of temperature and pressure.
The recombination reaction of the OH radical with SO2 has been investigated using current theor. methods. Quantum chem. calcns. were performed to locate the stationary points on the potential energy surface (PES) for the recombination/dissocn. process, including structures and vibrational frequencies calcd. at the B3LYP/aug-cc-pVTZ(+1) and QCISD/6-311G(d,p) level of theory. Furthermore, the energetics are characterized by the application of G3X-theory which provides all sensitive data such as the crit. energy barrier and the enthalpy of reaction. The results obtained suggest a compact transition state with regards to the dissocn. of HOSO2. Combining the calcd. enthalpy of formation for the HOSO2 radical (DHf(298.15 K) = -368.8 kJ mol-1) and the well known enthalpy of formation for OH and SO2 this allows the calcn. of the temp. dependent equil. const. Keq(T). In addn., a very shallow barrier (.apprx.0.5 kJ mol-1) is predicted for the recombination process at T = 0 K. The novel data have been used as input to statistical kinetic calcns. Focusing on the reverse process (e.g. the HOSO2 dissocn.), RRKM-theory in conjunction with a subsequent soln. of the master equation has been utilized to derive complete fall-off curves for the thermal decompn. of HOSO2 as well as for the OH + SO2 recombination reaction, both in a weak collision scenario ([M] = N2,He). Furthermore, the temp. dependence of the recombination reaction has been analyzed in the temp. range between 150 and 1500 K and Arrhenius expressions have been derived from the \"numerically exact\" fall-off data in different pressure regimes.
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