Sulfur containing radical chemistry: CH3SO2 → CH3 + SO2

Sulfur-containing radicals in the troposphere play a dynamic role in the oxidation of dimethyl sulfide to sulfur dioxide. The formation of SO₂ and its subsequent conversion into the sulfate anion (SO₄ ^2-) is of significant environmental concern due to its role in acid rain and its effect on global climate. These velocity map imaging and crossed laser-molecular beam scattering experiments investigate the dissociation dynamics of methylsulfonyl radicals generated from the photodissociation of CH₃SO₂Cl. The data evidences three primary photodissociation channels of the precursor: S-Cl fission to produce Cl atoms and ground electronic state CH₃SO₂ radicals, S-Cl fission to produce Cl atoms and electronically excited CH₃SO₂ radicals and S-CH₃ fission to produce methyl radicals. Some of the vibrationally excited CH₃SO₂ radicals undergo subsequent dissociation to CH₃ + SO₂. The velocities of the SO₂ products show that the vibrationally excited ground state CH₃SO₂ radicals dissociate via a loose transition state with a small exit barrier beyond the endoergicity, so a near-statistical recoil kinetic energy distribution fits the distribution of velocities imparted to the SO₂ products. The electronically-excited CH₃SO₂ radicals also dissociate to CH₃ + SO₂, but with a larger release to relative kinetic energy. Interestingly, though photoionization of the stable CH₃SO₂ radicals results in parent CH₃SO₂ + ions, 200 eV electron bombardment detection of the stable radicals gives signal only at the CH₃+ 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 barrier of 14.6 kcal/mol for the dissociation of CH₃SO₂ to CH₃ + SO₂ calculated at the CCSD(T) level of theory.