Unimolecular Kinetics of Alkoxy Radicals: A Computational Approach to Understanding Branching Ratios

Alkoxy and hydroxyalkoxy radicals are key intermediates in the reactions of volatile organic compounds (VOCs) in the troposphere. The end products of these reactions contribute to secondary organic aerosol formation and ground-level ozone (O3). To understand the various unimolecular decomposition and isomerization reactions of these intermediates, we implement a composite form of ΔE, in which the correlation effects are captured using the LNO–CCSD(T) approach. Rate constants are computed using canonical transition state theory (TST) with correction factors for entropic, quantum, and pressure effects. The pressure dependence is evaluated using the chemically significant eigenvalue scheme for solving the master equation. Entropic effects are treated using the hindered rotor approximation to account for the anharmonicity caused by large-amplitude motion, along with the small-curvature tunneling correction to incorporate quantum effects. Because multidimensional tunneling approaches are computationally intensive, machine learning models are incorporated to reduce computation time and cost. The new theoretical framework agrees well with experimental results for both alkoxy and β-hydroxyalkoxy systems within a factor of 2. The proposed workflow can be extended to larger systems at a modest computational cost.