Role of water on the H-abstraction from methanol by ClO.

The influence of a single water molecule on the reaction mechanism and kinetics of hydrogen abstraction from methanol (CH3 OH) by the ClO radical has been investigated using ab initio calculations. The reaction proceeds through two channels: abstraction of the hydroxyl H-atom and methyl H-atom of CH3 OH by ClO, leading to the formation of CH3 O+HOCl (+H2 O) and CH2 OH+HOCl (+H2 O), respectively. In both cases, pre- and post-reactive complexes were located at the entrance and exit channel on the potential energy surfaces. Results indicate that the formation of CH2 OH+HOCl (+H2 O) is predominant over the formation of CH3 O+HOCl (+H2 O), with ambient rate constants of 3.07×10-19 and 3.01×10-23 cm3 /(molecule·sec), respectively, for the reaction without water. Over the temperature range 216.7-298.2K, the presence of water is seen to effectively lower the rate constants for the most favorable pathways by 4-6 orders of magnitude in both cases. It is therefore concluded that water plays an inhibitive role on the CH3 OH+ClO reaction under tropospheric conditions.