The gas-phase reaction of methane sulfonic acid with the hydroxyl radical without and with water vapor.

The gas phase reaction between methane sulfonic acid (CH3SO3H; MSA) and the hydroxyl radical (HO), without and with a water molecule, was investigated with DFT-B3LYP and CCSD(T)-F12 methods. For the bare reaction we have found two reaction mechanisms, involving proton coupled electron transfer and hydrogen atom transfer processes that produce CH3SO3 and H2O. We also found a third reaction mechanism involving the double proton transfer process, where the products and reactants are identical. The computed rate constant for the oxidation process is 8.3 × 10(-15) cm(3) s(-1) molecule(-1). CH3SO3H forms two very stable complexes with water with computed binding energies of about 10 kcal mol(-1). The presence of a single water molecule makes the reaction between CH3SO3H and HO much more complex, introducing a new reaction that consists in the interchange of H2O between HO and CH3SO3H. Our kinetic calculations show that 99.5% of the reaction involves this interchange of the water molecule and, consequently, water vapor does not play any role in the oxidation reaction of methane sulfonic acid by the hydroxyl radical.

On the possible catalysis by single water molecules of gas-phase hydrogen abstraction reactions by OH radicals.

The gas phase hydrogen abstraction reaction between OH and CY(2)XH, where X = H, F, OH, or NH(2) and Y = H, CH(3) or F, in the absence and presence of a single water molecule is investigated using both density function theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. We find that a single water molecule could have a catalytic effect at low temperatures possible in laboratory experiments, but does not seem to catalyze these reactions at 298 K, and will not play a role under relevant atmospheric conditions.

A computational study of the oxidation of SO2 to SO3 by gas-phase organic oxidants.

We have studied the oxidation of SO(2) to SO(3) by four peroxyradicals and two carbonyl oxides (Criegee intermediates) using both density functional theory, B3LYP, and explicitly correlated coupled cluster theory, CCSD(T)-F12. All the studied peroxyradicals react very slowly with SO(2) due to energy barriers (activation energies) of around 10 kcal/mol or more. We find that water molecules are not able to catalyze these reactions. The reaction of stabilized Criegee intermediates with SO(2) is predicted to be fast, as the transition states for these oxidation reactions are below the free reactants in energy. The atmospheric relevance of these reactions depends on the lifetimes of the Criegee intermediates, which, at present, is highly uncertain.