An integrated protocol to study hydrogen abstraction reactions by atomic hydrogen in flexible molecules: application to butanol isomers

This work presents a protocol designed to study hydrogen abstraction reactions by atomic hydrogen in molecules with multiple conformations.

Theoretical and Experimental Study of 3-Pentanol Autoignition: Ab Initio Calculation, Shock Tube Experiments, and Kinetic Modeling

3-Pentanol is a potential alternative fuel or a green fuel additive for modern engines. The H-abstraction reactions from 3-pentanol by H, CH₃, HO₂, and OH radicals are significant in the 3-pentanol oxidation process. However, corresponding rate constants are forced to rely on either analogy from sec-butanol or estimation from alkanes due to a lack of direct experimental and theoretical study. In this work, stationary points on the potential energy surfaces (PESs) were calculated with the high-level DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ method, which is further used to benchmark against the CBS-QB3 method. Then, the high-pressure limit rate constants for target reactions, over a broad range of temperature (400-2000 K), were calculated with the phase-space theory and conventional transition state theory. A comparison was made between the calculated rate constants and the values available in Carbonnier et al. [ Proc. Combust. Inst. 2019, 37(1), 477-484]. The rate constants for the above H-abstraction reactions in the Carbonnier model were updated with the calculated results, followed by a modification based on the computed results of 3-pentanol + HO₂ to obtain the revised model. Validation against the shock tube (ST) and the jet-stirred reactor (JSR) measurements from the literature proved the revised model an optimal one. Furthermore, using an ST, ignition delay times (IDTs) for the 3-pentanol/air mixtures were measured spanning a temperature range of 920-1450 K, pressures of 6, 10, and 20 bar, and equivalence ratios of 0.5, 1.0, and 1.5. Generally, IDTs decrease with increasing temperature and reflected shock pressure. Improved predictions to present experimental data were obtained by using the revised model as compared with the Carbonnier model. Finally, sensitivity analysis was conducted using the revised model to gain an in-depth comprehension of the 3-pentanol autoignition.

Computational Studies on the Thermodynamic and Kinetic Parameters of Oxidation of 2-Methoxyethanol Biofuel via H-Atom Abstraction by Methyl Radical

In this work, a theoretical investigation of thermochemistry and kinetics of the oxidation of bifunctional 2-Methoxyethanol (2ME) biofuel using methyl radical was introduced. Potential-energy surface for various channels for the oxidation of 2ME was studied at density function theory (M06-2X) and ab initio CBS-QB3 levels of theory. H-atom abstraction reactions, which are essential processes occurring in the initial stages of the combustion or oxidation of organic compounds, from different sites of 2ME were examined. A similar study was conducted for the isoelectronic n-butanol to highlight the consequences of replacing the ϒ CH₂ group by an oxygen atom on the thermodynamic and kinetic parameters of the oxidation processes. Rate coefficients were calculated from the transition state theory. Our calculations show that energy barriers for n-butanol oxidation increase in the order of α ‹ O ‹ ϒ ‹ β ‹ ξ, which are consistent with previous data. However, for 2ME the energy barriers increase in the order α ‹ β ‹ ξ ‹ O. At elevated temperatures, a slightly high total abstraction rate is observed for the bifunctional 2ME (4 abstraction positions) over n-butanol (5 abstraction positions).