An Experimental Study of the Kinetics of OH/OD(v=1,2,3) + SO₂: the Limiting High Pressure Rate Coefficients as a Function of Temperature

The kinetics of the reaction OH/OD(ν=1,2,3) + SO₂ have been studied using a photolysis / laser induced fluorescence technique. The rate coefficients OH/OD(v=1,2,3) + SO₂, k₁, over the temperature range 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 behaviour 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 analysed 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 two to three smaller than the current recommended IUPAC and JPL values of (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.