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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. In the first year of our funding we investigated 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 dissociation 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.

In the second year of funding we investigated an isomer of the CH₃SO₂ radical, CH₃OSO (the O atom is bonded to the C of the methyl). Our collaborator F. Blase (Haverford) developed a synthesis for the photolytic precursor to this radical isomer. The data evidences a barrier to dissociate that is more than 10 kcal/mol higher than the endoergicity. This is in agreement with high-level CCSD(T) calculations of the bond fission transition state. Prior experimental studies of these isomers had concluded that the isomerizaiton barrier was low enough between these isomers that the CH₃OSO radical would isomerize to CH₃SO₂ en route to dissociating. Our theoretical and experimental results suggest the barrier for isomerization is higher than the barrier for CH₃OSO to directly dissociate to CH₃ + SO₂, even though the latter is well above the asymptotic endoergcity.

The PRF funding supported the stipend of the lead author on the CH₃OSO work, Bridget Alligood, who will file her Ph.D. thesis this academic year. The PRF-supported work on the imaging apparatus, complemented by NSF-supported work on our scattering apparatus, represents two of her thesis chapters. Several other graduate students contributed to the work, as well as two talented undergraduates, Emily Jane Glassman (BA from Chicago, now doing her Ph.D. at Berkeley) and Ethan Alquire (BA from Haverford, now doing his Ph.D. at Penn.).