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

Abstract

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 cm³ 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.