TEMPERATURE DEPENDENCE OF THE REACTION OF OH WITH SO

摘要

The rate coefficient for OH + SO was measured over the temperature range 295-703 K, using laser flash photolysis coupled with laser-induced fluorescence (LIF), under pseudo-first-order conditions, with [SO] [OH]. SO was generated by the photolysis of both SO_2 and SOC1_2 at 193 nm; the majority of the experiments were performed with a SOC1_2 precursor. The absolute SO concentration was determined by monitoring the NO via LIF (226 nm excitation) produced in the reaction SO + NO_2→ SO_2 + NO. This NO signal was compared with a known NO concentration. OH was monitored by LIF (282 nm excitation) as a function of time and [SO] was varied, typically, over the range 0.5 to 6 mTorr. The rate coefficient decreased with T, falling by a factor of 5.5 over the experimental range. A fit to a negative temperature exponent gives κ = (8.28 +- 0.37) X 10~(-11) (T/295)~(1.35+- 0.11) cm~3 s~(-1). The mechanism was discussed by reference to published ab initio potential energy surface data, and it was suggested that the temperature dependence derives from a competition between isomerization of the adduct, HOSO, over a low-energy, tight transition state (TS1) and its decomposition over a higher-energy, loose transition state to regenerate the reactants. The isomerization leads to formation of HSO_2, which rapidly decomposes to form the products, H + SO_2. A simple canonical model suggested that TS1 lies about 20 kJ mol~(-1) below the input channel energy. The canonical model did not, however, reproduce the strong high-temperature curvature observed experimentally in the Arrhenius plot.