Dissociation dynamics of the methylsulfonyl radical and its photolytic precursor CH3 SO2 Cl

The dissociation dynamics of methylsulfonyl radicals generated from the photodissociation of CH₃ SO₂ Cl at 193 nm is investigated by measuring product velocities in a crossed laser-molecular beam scattering apparatus. The data evidence three primary photodissociation channels of the precursor: S-Cl fission to produce Cl atoms and ground electronic state CH ₃ SO₂ radicals, S-Cl fission to produce Cl atoms and electronically excited CH₃ SO₂ radicals, and S- CH ₃ fission. Some of the vibrationally excited CH₃ SO ₂ radicals undergo subsequent dissociation to CH₃ SO ₂, as do all of the electronically excited radicals. The velocities of the SO₂ products show that the vibrationally excited ground state CH₃SO₂ radicals dissociate via a loose transition state having a small exit barrier beyond the endoergicity. Hence, a statistical recoil kinetic energy distribution should and does fit the distribution of velocities imparted to these SO₂ products. The electronically excited CH ₃ SO₂ radicals also dissociate to CH₃ + SO ₂, but with a larger average release to relative kinetic energy. Interestingly, when using 200 eV electron bombardment detection, the ground electronic state CH₃ SO₂ radicals having too little internal energy to dissociate are not observed at the parent CH₃ SO₂⁺₂ ion, but only at the CH⁺ ₃ daughter ion. They are distinguished by virtue of the velocity imparted in the original photolytic step; the detected velocities of the stable radicals are consistent with the calculated barrier of 14.6 kcal/mol for the dissociation of CH₃ SO₂ to CH₃ + SO ₂. We present CCSD(T) calculations of the adiabatic excitation energy to the lowest excited state of CH₃ SO₂ radicals, the 1² A ″ state, as well as the vertical energy from the equilibrium geometry of that excited state to the 2² A ″ state, to aid in the experimental assignment. © 2009 American Institute of Physics.