HO₂-Driven Atmospheric Aniline Oxidation: Kinetic, Mechanism, and Quantum Chemical Insights

This study explores the mechanistic and kinetic details of aniline (AN) oxidation by hydroperoxyl radicals (HO₂) using DFT (M06-2X/jun-cc-pVTZ) and high-level ab initio methods (CCSD(T), BD(T), CBS-QB3). Eight prereactive complexes were identified, with TD-DFT confirming their photolysis under atmospheric conditions. The most stable complex (CR2) exhibits the complexation energy of -11.06 to 4.29 kcal mol^-1 across 300-3000 K. Among 12 reaction pathways (four H-abstractions, four additions, and four addition-eliminations), NH₂ H-abstraction (P1) and ortho-addition (P6) are the most favorable. Rate constants increase with temperature and pressure, indicating thermally and pressure-activated kinetics. At 300 K, activation energies for H-abstraction (R1-R4) range from 5.47 to 29.31 kcal mol^-1, while addition reactions (R5-R8) require 9.81-14.29 kcal mol^-1, significantly lower than addition-elimination routes (R9-R12). Branching ratio analysis reveals dominant H-abstraction from the NH₂ group (R1, 67-99% at <500 K) and ortho-addition (R6, up to 24%), with minor contributions from ipso/para-additions (R5/R8, 8-16% at 500-1000 K) and secondary H-abstractions (R3/R4, 20-33% at >2500 K). These findings highlight NH₂ H-abstraction and ortho-addition as the primary low-temperature pathways for AN atmospheric degradation.