Reaction kinetics of 1,4-cyclohexadienes with OH radicals: an experimental and theoretical study.

This work presents the OH-initiated oxidation kinetics of 1,4-cyclochexadiene (1,4-CHD). The temperature dependence of the reaction was investigated by utilizing a laser flash photolysis flow reactor and laser-induced fluorescence (LPFR/LIF) technique over the temperature range of 295-438 K and a pressure of ∼50 torr. The kinetics of the reaction was followed by measuring the LIF signal of OH radicals near 308 nm. The reaction of OH radicals with 1,4-CHD exhibited a clear negative temperature dependence. To discern the role of various channels, ab initio and RRKM-based ME calculations (RRKM-ME) were performed over temperatures of 200-2000 K and pressures of 0.76-7600 torr. The computed energy profile revealed that the reaction proceeds via the formation of a pre-reaction van der Waals complex at the entrance channel. The complex was found to be more stable than that usually seen in other alkenes + OH reactions. Both the addition channel and the abstraction reaction of allylic hydrogen were found to have negative energy barriers. Interestingly, the abstraction reaction exhibited a negative temperature dependence at low temperatures and contributed significantly (∼37%) to the total rate coefficients even under atmospheric conditions. At T ≥ 900 K, the reaction was found to proceed exclusively (>95%) via the abstraction channel. Due to the competing channels, the reaction of OH radicals with 1,4-CHD displays complicated kinetic behaviours, reflecting the salient features of the energy profile. The role of competing channels was fully characterized by our kinetic model. The calculated rate coefficients showed excellent agreement with the available experimental data.

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 cm3/molecule/s) as kabstraction(P1) = 3.41 × 101 × T-4.56 × exp (-255.2 K/T) for 200-2000 K and kaddition(I2) = 3.68 × 109 × T-7.39 × 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 ktotal(T) are also observed. The disagreement in ktotal(T) between the previous calculations and experimental studies is also resolved, and atmospheric aniline is found to be primarily removed by OH radicals (τOH ∼ 1.1 h) in the daytime. Also, via TD-DFT simulations, it is recommended to include P1 and I2 in any atmospheric photolysis-related model.