Emily Jane

for Beckman Foundation Conference Investigation of the Unimolecular Dissociation of CH3SO2 As coal-burning technologies expand, introducing sulfur-containing molecules into the atmosphere, it is increasingly imperative to understand the reactions of the CH3SO2 radical. While the radicals and their dissociation products, CH3 + SO2, play a significant role in the atmospheric oxidation cascade of sulfur, the relative energy barriers for the dissociation of CH3SO2 have never been definitively determined experimentally. In this study, we have investigated the generation of this important radical, CH3SO2, from an appropriate photolytic precursor, methylsulfonyl chloride, CH3SO2Cl. Studies of the unimolecular dissociation rates of CH3SO2 have been reported by Baronavski and coworkers. Our measurements of the velocity distribution of the Cl atom co-fragment have allowed us to determine the nascent internal energy distribution of the CH3SO2 radicals produced at 193 nm. This data allows us to reinterpret the previously studied ultrafast unimolecular dissociation rate of the radicals. Experiments with CH3SO2Cl show that the photolytically-produced radicals have a bimodal energy distribution. Further studies of the radical co-fragment, CH3SO2, at a range of internal energies promise a direct determination of the energy barriers for dissociation of each radical by determining the internal energy distribution of the radicals which are not able to dissociate. In this project, we have also sought to extend our understanding of these reactions from first-principle quantum mechanics. Electronic structure calculations at the CCSD(T) and G3/B3LYP levels of theory indicate that in order to get an accurate prediction of the energetic barriers and rates of unimolecular dissociation of sulfur-containing systems, it is necessary to use a larger basis set than had been previously used by Zhu and Bozzelli. i J. C. Owrutsky, H. H. Nelson, and A. P. Baronavski. J. Phys. Chem. 105, 1440 (2001). ii L. Zhu, J. W. Bozzelli. J. Mol. Structure. 728, 147 (2005). Methylsulfonyl Radical Unimolecular Dissociation Studied by Deep-UV Ultrafast Photoionization Spectroscopy† J. C. Owrutsky,* H. H. Nelson, and A. P. Baronavski* Chemistry DiVision, Code 6111, NaVal Research Laboratory, Washington, D.C. 20375-5342 ReceiVed: May 31, 2000; In Final Form: September 12, 2000 The secondary dissociation dynamics of the methylsulfonyl radical following the photodissociation near 193 nm of methylsulfonyl chloride and methylsulfonyl ethanol has been studied using femtosecond mass-resolved photoionization spectroscopy. The primary dissociation is instrument-limited (<200 fs) for both precursors. The methylsulfonyl unimolecular dissociation is measured to have a lifetime of 0.34 ps with complete secondary dissociation following methylsulfonyl chloride photolysis. For the case of methylsulfonyl ethanol, the measured lifetime is 1.1 ps and ∼40% of the radical remains undissociated for the 18 ps duration of the experiment. We have successfully modeled the experimental results assuming an impulsive dissociation process to yield the internal energy distributions in the primary photoproducts and RRKM calculations based on structures and energetics of the methylsulfonyl radical obtained from the literature.