Temperature dependence of the reaction of OH with SO

✍ Scribed by Mark A. Blitz; Kenneth W. McKee; Michael J. Pilling

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 181 KB
• Volume: 28
• Category: Article
• ISSN: 1540-7489
• DOI: 10.1016/s0082-0784(00)80664-5

✦ Synopsis

The rate coefficient for OH ‫ם‬ 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] k [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 (226 nm excitation) produced in the reaction SO ‫ם‬ NO 2 → SO 2 ‫ם‬ 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 ‫ס‬ (8.28 ‫ע‬ 0.37) ‫ן‬ 10 ‫11מ‬ (T/295) ‫11.0ע53.1מ‬ cm 3 s ‫1מ‬ . 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 ‫ם‬ SO 2 . A simple canonical model suggested that TS1 lies about 20 kJ mol ‫1מ‬ below the input channel energy. The canonical model did not, however, reproduce the strong high-temperature curvature observed experimentally in the Arrhenius plot.