Advanced kinetic calculations with multi-path variational transition state theory for reactions between dimethylamine and nitrogen dioxide in atmospheric and combustion temperature ranges

Abstract

Rate constants of the reactions between dimethyl amine (DMA) and NO₂ are accurately determined using the advanced multi-path canonical variational theory with a small-curvature tunneling correction. To select a suitable method for the direct kinetic calculations, various combinations with nine DFT methods and seven basis sets are evaluated, and the M08-HX/ma-TZVP method is identified as the best one for the current reaction system, with a mean unsigned deviation of 1.1 kcal mol⁻¹ in comparison with the benchmark CCSD(T)-F12/jun-cc-pVTZ. A total of 13 elementary reactions are found, but only the H-abstraction reactions are kinetically favorable and included in the kinetic calculations. The recrossing and tunneling effects are different among the various H-abstraction reaction channels as well as different reaction paths. The recrossing effects are comparably more significant for reactions at the N-site, and tunneling coefficients of the reaction channels producing a trans-HONO are the largest. The higher-energy reaction paths have much higher tunneling coefficients, which should be considered in the rate constant calculation, especially at low temperatures. Branching ratio analysis demonstrates that the most important products are CH₃NCH₃ + cis-HONO at 200–2000 K. Additionally, by comparing our calculations with the available literature data, the importance of the investigated reactions is discussed at combustion and atmospheric temperatures.