Atmospheric Oxidation of C(12)-C(14) n-Alkylcyclohexanes Initiated by Cl Atoms and OH Radicals: Mechanistic Insights, Kinetics, and the Inhibiting Effect of Water.

Long-chain alkanes are key intermediate volatility organic compounds (IVOCs) in the atmosphere and recognized as significant precursors to the formation of secondary organic aerosol (SOA). C-C -alkylcyclohexanes are representative IVOCs that undergo oxidation by both OH radicals and Cl atoms. In this work, the initial hydrogen abstraction and subsequent multigeneration oxidation mechanisms of C-C -alkylcyclohexanes initiated by Cl atoms and OH radicals were systematically investigated using quantum chemical methods. The results show that both Cl atoms and OH radicals preferentially abstract a hydrogen atom from the tertiary carbon of the cyclohexane ring, proceeding via low-energy barriers. The formed alkyl radicals undergo barrierless additions to O subsequently and reactions with NO, forming alkoxy radicals (CHO, CHO, and CHO) that are consistent with experimentally observed products. The calculated rate constants for the reactions with Cl atoms and OH radicals at 298 K are in the ranges of 1.59-2.17 × 10 and 1.55-3.75 × 10 cm molecule s, respectively, showing a slight increasing trend with carbon-chain length. Notably, the presence of a single water molecule was found to significantly increase the energy barriers for H-abstraction, indicating that water vapor exerts a negative catalytic effect on these atmospheric oxidation reactions. This finding suggests that under high-humidity conditions, the gas-phase oxidation of such IVOCs may be slower than currently predicted, with potential implications for SOA formation estimates. By elucidating the mechanistic details and kinetic parameters for this important class of IVOCs, this work provides a theoretical foundation for improving chemical mechanisms in atmospheric models.