Theoretical Study on the Atmospheric and Combustion Chemistry of 4-Methylcyclohexanol Initiated by •OH Radicals: Mechanism, Kinetics, and Environmental Risks.

4-Methylcyclohexanol (4MCHexOH) is widely utilized as a biofuel additive and chemical feedstock, yet its environmental fate and potential risks remain poorly understood. In this study, density functional theory combined with multistructural variational transition state theory was applied to investigate the reaction mechanism and kinetic properties of 4MCHexOH with •OH radicals. A total of 12 hydrogen abstraction pathways (R1-R12) were identified, among which R2 (-CH< adjacent to -CH3) exhibited the lowest energy barrier. The calculated rate coefficient at 298 K was 1.84 × 10-11 cm3 molecule-1 s-1, in good agreement with experimental measurements. The tunneling effect significantly influences the rate coefficient within 200-900 K, while the multistructural effect becomes notable between 900 and 2500 K. The atmospheric lifetime of 4MCHexOH was determined to range from 0.92 to 16.96 h under tropospheric temperatures (200-278 K), whereas it drops to approximately 14 s under combustion conditions at 2500 K. The risk assessment results show that coupling reactions involving HO2• or NO produce toxic and persistent substances, while hydrogen migration pathways and functional group elimination reactions result in less toxic and more environmentally benign products. This study advances the understanding of biofuel optimization and environmental risk management associated with multicarbon cyclic saturated alcohols.