Ozonolysis of isoprene, the most abundant alkene, produces three distinct Criegee intermediates (CIs): CH2OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO2 by CIs is a potential source of H2SO4, an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti-MACRO with SO2 has been found, kSO2 = (1.5 ± 0.4) × 10−10 cm3 s−1 (±1σ, σ is the standard deviation of the data) at 298 K (150 − 500 Torr), which is ca. 4 times the value for syn-MVKO. However, the reaction of anti-MACRO with water vapor has been observed to be quite slow with an effective rate coefficient of (9 ± 5) × 10−17 cm3 s−1 (±1σ) at 298 K (300 to 500 Torr), which is smaller than current literature values by 1 or 2 orders of magnitude. Our results indicate that anti-MACRO has an atmospheric lifetime (best estimate ca. 18 ms at 298 K and RH = 70%) much longer than previously thought (ca. 0.3 or 3 ms), resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti-MACRO on the oxidation of atmospheric SO2 would be substantial, even more than that of syn-MVKO.

To gain an understanding of the substitution effect on the unimolecular reaction rate coefficients for Criegee intermediates (CIs), we performed ab initio calculations for CHOO, CHCHOO, (CH)COO, CHCHCHOO, CHCHCHOO and CHCCHOO. The energies of the CIs, products and transition states were calculated with QCISD(T)/CBS//B3LYP/6-311+G(2d,2p), while the rate coefficients were calculated with anharmonic vibrational correction by using second order vibrational perturbation theory. It was found that for single bonded substitutions, the hydrogen transfer reaction dominates for the syn-conformers, while the OO bending reaction dominates for the anti-conformers. However once a double bond or a triple bond is added, the OO bending reaction dominates for both syn and anti-conformers. The rate coefficients for OO bending reaction show a significant increase when adding a methyl group or ethyl group. On the other hand, the addition of unsaturated vinyl and acetylene groups usually results in a slower thermal decomposition compared to the substitution with saturated carbon groups. Interestingly, for syn_Syn-CHCHCHOO, a special five member ring closure reaction forming dioxole was calculated to have an extremely fast rate coefficient of 9312 s at room temperature.

We report a full-dimensional spin-orbit-corrected analytical potential energy surface (PES) for the HBr + C2H5 → Br + C2H6 reaction and a quasi-classical dynamics study on the new PES. For...

syn-CH3CHOO reacts quickly with 2CH3OH molecules; this reaction shows a strong negative temperature dependence.

The role of water in gas-phase reactions has gained considerable interest. Here we report a direct kinetic measurement of the reaction of syn-CH 3 CHOO (a Criegee intermediate or carbonyl oxide) with methanol at various relative humidity (RH = 0−80%) under near-ambient conditions (298 K, 250−755 Torr). The data indicate that a single water molecule expedites the reaction by up to a factor of three. The rate coefficient of the corresponding reaction, syn-CH 3 CHOO + CH 3 OH + H 2 O → products, has been determined to be (1.95 ± 0.11) × 10 −32 cm 6 s −1 at 298 K, with no observable pressure dependence for 250−755 Torr. Quantum chemistry calculation shows that the dominating pathway involves a hydrogen-bonded ring structure, in which methanol is donating a hydrogen atom to water, water is donating a hydrogen atom to the terminal oxygen atom of the Criegee intermediate, and, on the product side, H 2 O is reformed and acts as a catalyst.