An Experimental Study of the Kinetics of OH/OD(v = 1,2,3) + SO 2 : The Limiting High-Pressure Rate Coefficients as a Function of Temperature.

The kinetics of the reaction OH/OD(v = 1,2,3) + SO2 were studied using a photolysis/laser-induced fluorescence technique. The rate coefficients OH/OD(v = 1,2,3) + SO2 , k1 , over the temperature range of 295-810 K were used to determine the limiting high-pressure limit k1 ∞ . This method is usually applicable if the reaction samples the potential well of the adduct HOSO2 and if intramolecular vibrational relaxation is fast. In the present case, however, the rate coefficients showed an additional fast removal contribution as evidenced by the increase in k1 with vibrational level; this behavior together with its temperature dependence is consistent with the existence of a weakly bound complex on the potential energy surface prior to adduct formation. The data were analyzed using a composite mechanism that incoporates energy-transfer mechanisms via both the adduct and the complex, and yielded a value of k1 ∞ (295 K) equal to (7.2 ± 3.3) × 10-13 cm3 molecule-1 s-1 (errors at 1σ), a factor of between 2 and 3 smaller than the current recommended IUPAC and JPL values of (2.0-1.0 +2.0 ) and (1.6 ± 0.4) × 10-12 cm3 molecule-1 s-1 at 298 K, respectively, although the error bars do overlap. k1 ∞ was observed to only depend weakly on temperature. Further evidence for a smaller k1 ∞ is presented in the companion paper.