Temperature dependence of the reaction of OH with SO

Abstract

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 SOCl 2 at 193 nm; the majority of the experiments were performed with a SOCl 2 precursor. The absolute SO concentration was determined by monitoring the NO via LIF (226nm 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 k =(8.28±0.37)×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 (TSI) 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.