PHYS 569

The translational energy distribution of the nascent CH₃SO₂ radicals is precisely determined by detecting the momentum-matched Cl atoms from the C-Cl bond fission of methylsulfonyl chloride. The spin-orbit state selected Cl(²P_3/2) and Cl(²P_1/2) atomic fragments are detected via [2+1] resonance enhanced multi-photon ionization (REMPI) at 235 nm. Using energy conservation, the total recoil translational energy distribution allows us to determine the internal energy distribution of the nascent CH₃SO₂ radicals.

The measurement shows a bimodal recoil energy distribution, indicating that the CH₃SO₂ radicals are produced in both the ground electronic and low-lying excited electronic states. The measured internal energy distribution for the component assigned to electronically-excited radicals is consistent with electronic structure calculations at the EOM-CCSD/cc-PVTZ level of theory of the adiabatic excitation energy to the the ²A' and ²A" excited states of CH₃SO₂. Our results suggest that prior attempts to measure the dissociation rate of methylsulfonyl radicals by Baronavski and co-workers were inadvertently probing the decay of electronically excited radicals. The vertical IE of CH₃SO₂ (9.72eV) is even higher than two 260 nm photons used in the prior experiment. Our work also includes calculations of RRKM unimolecular dissociation rates using the transition state structures and frequencies calculated at the CCSD(T)/6-311G(2df,p) and the G3//B3LYP/6-311++G(3df,2p) levels of theory. These are compared to previous predictions of the transition states for dissociation to CH₃ + SO₂ and the isomerization to CH₃OSO. Experiments to experimentally deterimine the energetic barrier for dissociation to CH₃ + SO₂ are in progress.