AmericanChemicalSociety.com

Sulfur-containing radicals in the troposphere play a dynamic role in the oxidation of dimethyl sulfide (DMS) to sulfur dioxide (SO₂). While the major anthropogenic source of SO₂ is the combustion of sulfur-containing fossil fuels, the major natural sources of SO₂ in the troposphere is from the atmospheric oxidation of DMS, from ocean phytoplankton. This work investigates the unimolecular dissociation of CH₃SO₂, a key radical intermediate in the DMS oxidation mechanism originally proposed by A. R. Ravishankara. We use state-of-the-art molecular beam scattering and imaging techniques to generate the radical under collision free conditions and probe it's dissociation to CH₃ + SO₂. We analyze the experimental results in conjunction with high level ab initio electronic structure calculations of our collaborator, K. -C. Lau. The experiments directly determine the energetic barrier for dissociation of CH₃SO₂ to CH₃ + SO₂ to be 14 +/- 2 kcal/mol, in good agreement with the theoretical result of 14.6 kcal/mol. (Prior theoretical predictions of the barrier had ranged from 9 to 17 kcal/mol.) Our dynamics studies show that the unimolecular dissociaition of CH₃SO₂. occurs via a loose transition state with negligible barrier beyond the endoergicity. Our experiments also reveal a low-lying excited state of the CH₃SO₂ radical; these dissociate to CH₃ + SO₂, but with more energy imparted to relative kinetic energy between the products. Our work indicated that the excited state radicals had compromised prior attempts by Baronavski and co-workers to measure the unimolecular dissociation of CH₃SO₂ using ultrafast photoionization methods. We use statistical transition state theories to estimate the unimolecular dissociation rate constants for CH₃SO₂ as a function of temperature for inclusion in atmospheric models.