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 (CHOH) 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 CHOH by ClO, leading to the formation of CHO+HOCl (+HO) and CHOH+HOCl (+HO), 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 CHOH+HOCl (+HO) is predominant over the formation of CHO+HOCl (+HO), with ambient rate constants of 3.07×10 and 3.01×10cm/(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 CHOH+ClO reaction under tropospheric conditions.