Ab Initio Chemical Kinetics for Oxidation of CH₃OH by N₂O₄: Elucidation of the Mechanism for Major Product Formation and Its Relevancy to Tropospheric Chemistry

Next to CH₄, CH₃OH is the most abundant C₁ organics in the troposphere. The redox reaction of CH₃OH with N₂O₄ had been shown experimentally to produce CH₃ONO, instead of CH₃ONO₂. 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 N₂O₄ to ONONO₂ through a very loose transition state within the N₂O₄-CH₃OH collision complex with a 14.3 kcal/mol barrier, followed by the rapid attack of ONONO₂ at CH₃OH producing CH₃ONO and HNO₃. The predicted mechanism for the redox reaction compares closely with the hydrolysis of N₂O₄. The computed rate constant, k₁ = 1.43 × 10^-8 T^1.96 exp (−9092/T) (200-2000 K) cm³molecule^-1s^-1, for the formation of CH₃ONO and HNO₃ agrees reasonably with available low-temperature kinetic data and is found to be similar to that of the isoelectronic N₂O₄ + CH₃NH₂ reaction. We have also estimated the kinetics for the termolecular reaction, 2 NO₂ + CH₃OH, and compared it with the direct bimolecular process; the latter was found to be 4.4 × 10⁵ times faster under the troposphere condition. On the basis of the known pollution levels of NO₂, N₂O₄, and CH₃OH, both processes were estimated to be of negligible importance to tropospheric chemistry, however.