Determining the CH3 SO2 + CH3 + SO 2 barrier from methylsulfonyl chloride photodissociation at 193 nm using velocity map imaging

These imaging experiments study the formation of the methylsulfonyl radical, CH3_SO2, from the photodissociation of CH₃ SO ₂ Cl at 193 nm and determine the energetic barrier for the radical's subsequent dissociation to CH₃ + SO₂. We first state-selectively detect the angular and recoil velocity distributions of the Cl(²P_3/2) and Cl (²P _1/2) atoms to further refine the distribution of internal energy partitioned to the momentum-matched CH₃ SO₂ radicals. The internal energy distribution of the radicals is bimodal, indicating that CH₃ SO ₂ is formed in both the ground state and low-lying excited electronic states. All electronically excited CH₃ SO₂ radicals dissociate, while those formed in the ground electronic state have an internal energy distribution which spans the dissociation barrier to CH₃ + SO₂. We detect the recoil velocities of the energetically stable methylsulfonyl radicals with 118 nm photoionization. Comparison of the total recoil translational energy distribution for all radicals to the distribution obtained from the detection of stable radicals yields an onset for dissociation at a translational energy of 70±2 kcal/mol. This onset allows us to derive a CH₃ SO₂ → CH₃ + SO₂ barrier height of 14±2 kcal/mol; this determination relies on the S-Cl bond dissociation energy, taken here as the CCSD(T) predicted energy of 65.6 kcal/mol. With 118 nm photoionization, we also detect the velocity distribution of the CH₃ radicals produced in this experiment. Using the velocity distributions of the SO₂ products from the dissociation of CH ₃ SO₂ to CH₃ + SO₂ presented in the following paper, we show that our fastest detected methyl radicals are not from these radical dissociation channels, but rather from a primary S- CH₃ bond photofission channel in CH₃ SO₂ Cl. We also present critical points on the ground state potential energy surface of CH₃ SO₂ at the CCSD (T) /aug-cc-pV (Q+d) Z//CCSD (T) /6-^311++G (2d_f,p) level. We include harmonic zero-point vibrational corrections as well as core-valence and scalar-relativistic corrections. The CCSD(T) predicted barrier of 14.6 kcal/mol for CH₃ SO₂ → CH₃ + SO₂ agrees well with our experimental measurement. These results allow us to predict the unimolecular dissociation kinetics of CH₃ SO₂ radicals and critique the analysis of prior time-resolved photoionization studies on this system. © 2009 American Institute of Physics.