Measurement of the Intramolecular Hydrogen-Shift Rate Coefficient for the CH 3 SCH 2 OO Radical between 314 and 433 K.

The intramolecular hydrogen-shift rate coefficient of the CH 3 SCH 2 O 2 (methylthiomethylperoxy, MSP) radical, a product formed in the oxidation of dimethyl sulfide (DMS), was measured using a pulsed laser photolysis flow tube reactor coupled to a high-resolution time-of-flight chemical ionization mass spectrometer that measured the formation of the DMS degradation end product HOOCH 2 SCHO (hydroperoxymethyl thioformate). Measurements performed over the temperature range of 314-433 K yielded a hydrogen-shift rate coefficient of k 1 ( T ) = (2.39 ± 0.7) × 10 9 exp(-(7278 ± 99)/ T ) s -1 Arrhenius expression and a value extrapolated to 298 K of 0.06 s -1 . The potential energy surface and the rate coefficient have also been theoretically investigated using density functional theory at the M06-2X/aug-cc-pVTZ level combined with approximate CCSD(T)/CBS energies yielding k 1 (273-433 K) = 2.4 × 10 11 × exp(-8782/ T ) s -1 and k 1 (298 K) = 0.037 s -1 in fair agreement with the experimental results. Present results are compared with the previously reported values of k 1 (293-298 K).