Ab Initio Kinetics of Pyrene + OH Reaction: A Revisited Study

Abstract

Pyrene is a compound belonging to the highly toxic class of polycyclic aromatic hydrocarbons and has carcinogenic properties; thus, the kinetics of its OH-initiated oxidation is theoretically investigated in a wide range of conditions (T = 200 -2000 K & P = 7.6 -76,000 Torr). The T&P-dependent kinetic behaviors were studied within the stochastic RRKM-based Master Equation framework with the potential energy profile constructed at the ROCBS-QB3//M06-2X/aug-cc-pVTZ level. The computed total rate constants total k are in good agreement with the laboratory values, which helps resolve the controversy between the prior calculation and the measured study.The model reveals the detailed mechanistic insights: (i) the OH-addition channels from the Cα and Cγ atoms of pyrene (to form the adducts, 5-hydroxy-4,5-dihydropyren-4-yl (I1) and 1-hydroxy-1H-pyren-10a-yl (I2)) predominate under atmospheric conditions, while the direct H-abstraction pathways become dominating at T > 500 K; (ii) the U-shaped T-dependent behaviors and the slightly positive P-dependence at low T (e.g., T ≤ 500 K & P = 760 Torr) of total k are observed due to the T-dependent mechanism shift. Additionally, pyrene should not be considered a persistent organic pollutant (POP) due to its short calculated atmospheric lifetime (~4.1 hours toward OH), which is significantly shorter than that resulting from degradation by the abundant oxidants (i.e., Cl, NO 3 , and O 3 ) in the atmosphere. Moreover, pyren-4-ol (& pyrene-4,5-dione) and pyren-1-ol (& pyrene-1,2-dione) are suggested as the primary products of I1 and I2, respectively, by the further oxidation of O 2 /NO. It is also revealed that pyrene is highly toxic to aquatic organisms in both short and long-term exposures, with its main degraded products, I1 and I2, showing lower toxicity but still posing a significant threat to marine organisms and the ecosystem.