Ab initio kinetics and thermochemistry of hydrogen atom abstraction from solketal by hydroxyl radicals.

Abstract

As oxygenated biofuels gain prominence in sustainable energy strategies, understanding their reaction kinetics with hydroxyl (˙OH) radicals is essential for predicting combustion behavior. In this work, all six hydrogen-atom abstraction (HAA) pathways in the solketal (2,2-dimethyl-1,3-dioxolane-4-methanol) + ˙OH reaction were characterized using high-level ab initio calculations and variational transition state theory (VTST). Molecular geometries and vibrational frequencies were obtained at the M06-2X/6-311++G(d,p) level, with single-point energies refined at the QCISD(T)/CBS level. Low-frequency torsional modes were treated using one-dimensional hindered rotor scans. All abstraction channels proceed through low or partially submerged transition states (-3.1 to 3.2 kcal mol^-1), necessitating a variational treatment. VTST calculations with asymmetric Eckart tunneling corrections were performed over 300-2000 K. At 298 K, the α (hydroxymethyl C-H) and r₁ (ring methine C-H) channels dominate the reactivity, yielding a total rate coefficient of k_total (298 K) ≈ 1.4 × 10¹³ cm³ mol^-1 s^-1. With increasing temperature, the ring-centered channels (r₁ and r₂) become dominant, while O-H abstraction becomes increasingly competitive, reaching ∼24% of the total flux at 2000 K. A systematic sensitivity analysis shows that the β₁, β₂, and O-H channels are strongly influenced by the treatment of low-frequency vibrational modes, leading to variations in the predicted rate coefficients of up to several orders of magnitude. In contrast, the α, r₁, and r₂ channels are comparatively robust. Temperature-dependent thermochemical properties (Δ_fH°, S°, ) were computed and fitted to NASA polynomials, and modified Arrhenius parameters were derived for all channels. The resulting dataset provides a consistent foundation for incorporation into detailed combustion models and highlights the importance of entropic and variational effects in hydrogen abstraction from multifunctional oxygenates.