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An Investigation of the Reaction of CH3S with CO

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journal contribution
posted on 17.06.2004, 00:00 by L. C. Koch, Paul Marshall, A. R. Ravishankara
The reaction of CH3S with CO was studied by using laser-induced fluorescence (LIF) to detect CH3S radicals that were produced through pulsed laser photolysis of CH3SSCH3. There was no increased loss of CH3S in the presence of CO. We place an upper limit of 1.4 × 10-16 cm3 molecule-1 s-1 on the rate coefficient, k1, for the reaction CH3S + CO → products (1), based on the precision in our measurements. This upper limit applies between 208 and 295 K. The addition of O2 did not enhance the rate coefficient for reaction 1. The reaction between CH3S and CO is therefore not expected to be a significant source of OCS or loss process for CH3S in the atmosphere. We also conclude that the reaction of CH3SOO with CO is not sufficiently rapid to be a significant loss process for CH3S in the atmosphere. The rate coefficients for the removal of electronically excited CH3S (CH3S(A)) at 295 K by CO, N2, and O2 were found to be (8.0 ± 0.2) × 10-11, (7.0 ± 1.6) × 10-12, and (4.8 ± 0.8) × 10-12 cm3 molecule-1 s-1 (±2σ precision), respectively. Ab initio calculations indicate that formation of a weakly bound complex between ground-state CH3S and CO is thermodynamically unfavorable, and that there are significant barriers to CH3S·CO adduct formation and its subsequent dissociation to CH3 + OCS, consistent with our experimental observations.