Ab Initio Chemical Kinetics for Oxidation of CH 3 OH by N 2 O 4 : Elucidation of the Mechanism for Major Product Formation and Its Relevancy to Tropospheric Chemistry.

Next to CH 4 , CH 3 OH is the most abundant C 1 organics in the troposphere. The redox reaction of CH 3 OH with N 2 O 4 had been shown experimentally to produce CH 3 ONO, instead of CH 3 ONO 2 . The mechanism for the reaction remains unknown to date. We have investigated the reaction by ab initio MO calculations at the UCCSD(T)/6-311+G(3df,2p)//UB3LYP/6-311+G(3df,2p) level. The result indicates that the reaction takes place primarily by the isomerization of N 2 O 4 to ONONO 2 through a very loose transition state within the N 2 O 4 -CH 3 OH collision complex with a 14.3 kcal/mol barrier, followed by the rapid attack of ONONO 2 at CH 3 OH producing CH 3 ONO and HNO 3 . The predicted mechanism for the redox reaction compares closely with the hydrolysis of N 2 O 4 . The computed rate constant, k 1 = 1.43 × 10 -8 T 1.96 exp (-9092/T) (200-2000 K) cm 3 molecule -1 s -1 , for the formation of CH 3 ONO and HNO 3 agrees reasonably with available low-temperature kinetic data and is found to be similar to that of the isoelectronic N 2 O 4 + CH 3 NH 2 reaction. We have also estimated the kinetics for the termolecular reaction, 2 NO 2 + CH 3 OH, and compared it with the direct bimolecular process; the latter was found to be 4.4 × 10 5 times faster under the troposphere condition. On the basis of the known pollution levels of NO 2 , N 2 O 4 , and CH 3 OH, both processes were estimated to be of negligible importance to tropospheric chemistry, however.