New Mechanistic Insights into Atmospheric Oxidation of Aniline Initiated by OH Radicals

This study theoretically reports the comprehensive kinetic mechanism of the aniline + OH reaction in the range of 200-2000 K and 0.76-7600 Torr. The temperature- and pressure-dependent behaviors, including time-resolved species profiles and rate coefficients, were studied within the stochastic RRKM-based master equation framework with the reaction energy profile, together with molecular properties of the species involved, characterized at the M06-2X/aug-cc-pVTZ level. Hindered internal rotation and Eckart tunneling treatments were included. The H-abstraction from the -NH2 moiety (to form C6H5NH (P1)) is found to prevail over the OH-addition on the C atom at the ortho site of aniline (to form 6-hydroxy-1-methylcyclohexa-2,4-dien-1-yl (I2)) with the atmospheric rate expressions (in cm(3)/molecule/s) as k(abstraction)(P1) = 3.41 X 10(1) x T(-4.56 )x exp (-255.2 K/T) for 200-2000 K and k(addition )(12) = 3.68 X 10(9) X T-7.39 x exp (-1163.9 K/T) for 200-800 K. The U-shaped temperature-dependent characteristics and weakly positive pressure dependence at low temperatures (e.g., T <= 800 K and P = 760 Torr) of k(total)(T) are also observed. The disagreement in k(total)(T) between the previous calculations and experimental studies is also resolved, and atmospheric aniline is found to be primarily removed by OH radicals (tau(OH) similar to 1.1 h) in the daytime. Also, via TD-DFT simulations, it is recommended to include P1 and 12 in any atmospheric photolysis-related model.

Automated summary

The comprehensive kinetic mechanism of the aniline + OH reaction has been theoretically reported, with the abstraction of hydrogen from the NH2 moiety prevailing over OH addition on aniline. The temperature-dependent behavior shows a U-shaped pattern, with weakly positive pressure dependence at low temperatures, and it is recommended to include P1 and 12 in atmospheric photolysis-related models.