Theoretical Perspectives on the Gas-Phase Oxidation Mechanism and Kinetics of Carbazole Initiated by OH Radical in the Atmosphere

Abstract

Carbazole is one of the typical heterocyclic aromatic compounds (NSO-HETs) observed in polluted urban atmosphere, which has become a serious environmental concern. The most important atmospheric loss process of carbazole is the reaction with OH radical. The present work investigated the mechanism of OH-initiated atmospheric oxidation degradation of carbazole by using density functional theory (DFT) calculations at the M06-2X/6-311++G(3df,2p)//M06-2X/6-311+G(d,p) level. The rate constants were determined by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The lifetime of carbazole determined by OH was compared with other typical NSO-HETs. The theoretical results show that the degradation of carbazole initiated by OH radical includes four types of reactions: OH additions to “bend” C atoms, OH additions to “benzene ring” C atoms, H abstractions from C-H bonds and the H abstraction from N-H bond. The OH addition to C1 atom and the H abstraction from N-H bond are energetically favorable. The main oxidation products are hydroxycarbazole, dialdehyde, carbazolequinone, carbazole-ol, hydroxy-carbazole-one and hydroperoxyl-carbazole-one. The calculated overall rate constant of carbazole oxidation by OH radical is 6.52 × 10−12 cm3 molecule−1 s−1 and the atmospheric lifetime is 37.70 h under the condition of 298 K and 1 atm. The rate constant of carbazole determined by OH radical is similar with that of dibenzothiophene oxidation but lower than those of pyrrole, indole, dibenzofuran and fluorene. This work provides a theoretical investigation of the oxygenated mechanism of NSO-HETs in the atmosphere and should help to clarify their potential health risk for determining the reaction pathways and environmental influence of carbazole.