Molecular conformational effects on the overall rate constant and the branching ratios of the n-heptane + OH reaction: an ab initio and variational transition state theory study

The oxidation of fuels by hydroxyl radical (OH) is a crucial initiation step in both combustion and atmospheric processes. In the present work, we explore the site-specific hydrogen atom abstraction reactions by OH from n-heptane through high level ab initio and sophisticated kinetic calculations. Rate constants for each of the four distinct abstraction sites were computed, for the first time, over a wide temperature range (200-3000 K) and using multistructural torsional variational transition state theory (MS-T-VTST) with small curvature tunneling (SCT) corrections. Optimized geometries, vibrational frequencies and minimum energy paths (MEPs) were determined using the M06-2X/aug-cc-pVTZ level of theory, with single-point energy refinements performed at the CCSD(T)/aug-cc-pVTZ level of theory to refine the energy of the optimized stationary points whose energy is up to 1 kcal mol⁻¹ of their respective global minimum conformer. Our findings reveal that the overall rate constants obtained from our site-specific calculations align closely with experimental data, with a maximum deviation factor (k^exp/k^theory) of 1.5 at 1000 K. More importantly, we provide branching ratios that are based on first-principles calculations, identifying the secondary site neighboring the primary site as the dominant abstraction channel—data not previously reported in the literature, thereby adding value to our kinetic study. Moreover, we demonstrate that, as long as multi-structural torsional anhamonicity effects are implemented, appropriate density functional methods with large enough basis sets can yield rate constants that are comparable to those from much more computationally demanding levels of theory that are based on wave function methods. The demonstrated protocol, in which we have also tested the effect of different energy cuttoffs for the computationally intensive single-point energy refinements, effectively handles large and conformationally complex chemical systems, offering a reliable framework for future investigations in this field involving similar chemical systems whose kinetics is barely known or simply based on estimations.