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

TL;DR

This study explains how methanol reacts with dinitrogen tetroxide to form specific products and assesses its impact on atmospheric chemistry.

Contribution

The paper provides a detailed ab initio mechanism for the oxidation of methanol by N2O4, revealing the formation of CH3ONO and HNO3.

Findings

The reaction proceeds via isomerization of N2O4 to ONONO2 with a 14.3 kcal/mol barrier.

The rate constant for CH3ONO and HNO3 formation agrees with low-temperature kinetic data.

Both the bimolecular and termolecular reactions are negligible in tropospheric chemistry.

Abstract

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 T1.96 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.

Figures