Theoretical, Experimental, and Modeling Study of the Reaction of Hydrogen Atoms with 1- and 2-Pentene

Abstract

Alkyl radicals are prominent in combustion chemistry as they are formed by hydrocarbon decomposition or from a radical attack on hydrocarbons. Accurate determinations of the thermochemistry and kinetics of their unimolecular isomerization and decomposition reactions and related addition reactions of alkenes are therefore important in simulating the combustion chemistry of virtually all hydrocarbon fuels. In this work, a comprehensive potential energy surface (PES) for H ̇-atom addition to and abstraction from 1-and 2-pentene, and the subsequent C−C and C−H β-scission reactions, and H-atom transfer reactions has been considered. Thermochemical values for the species on the C ̇5H 11 PES were calculated as a function of temperature (298−2000 K), with enthalpies of formation determined using a network of isodesmic reactions. High-pressure limiting and pressure-dependent rate constants were calculated using the Rice-Ramsperger-Kassel-Marcus theory coupled with a one-dimensional master equation. As a validation of our theoretical results, hydrogen atomic resonance absorption spectrometry experiments were performed on the H ̇-atom addition and abstraction reactions of 1-and 2-pentene. By incorporating our calculations into a detailed chemical kinetic model (AramcoMech 3.0), excellent agreement with these experiments is observed. The theoretical results are further validated via a comprehensive series of simulations of literature data. Our a priori model is found to reproduce important absolute species concentrations and product ratios reported therein.

Hierarchical Study of the Reactions of Hydrogen Atoms with Alkenes: A Theoretical Study of the Reactions of Hydrogen Atoms with C2–C4 Alkenes

“…The current study uses the most recent ATcT values for the molecular and radical chaperones. 52 , 53 Similar to previous work, 4 , 5 ATcT, ANL0, and ANL1 formation enthalpies do not exist for the species Ċ 4 H 7 -11, Ċ 4 H 7 -12, Ċ 4 H 7 -13, Ċ 4 H 7 -14, Ċ 4 H 7 -22, iĊ 4 H 7 , and iĊ 4 H 7 -i1. Quantum chemical composite methods (CBS–QB3, CBS–APNO, G3, and G4) 55 − 57 were therefore used to calculate their formation enthalpies at 0 K via isodesmic reactions suitable for each species, using ATcT values as chaperones.…”

“…The rate constants for the reaction Ċ 5 H 11 -1 ↔ C 2 H 4 + nĊ 3 H 7 by Awan, 61 Comandini, 62 and Jitariu et al 63 are in good agreement with our previously calculated rate constant. 4 At 500 K, the values from Awan 61 and Comandini 62 are a factor of ∼4.5 times faster than our calculated rate constant for this reaction at 500 K, with the rate constants converging at high temperatures. The difference of 7.76 kJ mol –1 in the energy barrier accounts for the observed difference.…”

“…The rate constant for the reaction aĊ 5 H 11 ↔ C 3 H 6 + Ċ 2 H 5 calculated in our most recent study 5 is a factor of ∼3 times faster at 500 K than our calculated rate constant for Ċ 5 H 11 -2 ↔ C 3 H 6 + Ċ 2 H 5 . 4 An energy barrier difference of 1.9 kJ mol –1 accounts for a factor of 1.6 of this difference. For the decomposition of an alkyl radical forming an olefin and an ethyl radical, an average of the rate constants calculated in our current and previous studies 4 , 5 as well as those by Awan, Comandini, and Jitariu , 61 − 63 is taken, with a factor of 2 of the recommended represented as orange dotted lines.…”

“…Larger differences are observed between the values calculated in this work and by Awan and Comandini at temperatures below 500 K; therefore, the recommended rate constant for Ċ 5 H 11 -1 ↔ C 2 H 4 + n Ċ 3 H 7 is taken as an average of the rate calculated in the current work and by Jitariu et al 63 The rate constant by Jitariu et al is in excellent agreement with our calculated rate constant for the same reaction. 4 Our calculated rate constant for dĊ 5 H 11 ↔ C 2 H 4 + iĊ 3 H 7 is also plotted in this graph, which is taken as the recommended rate constant for alkyl radical decomposition forming an olefin and an iĊ 3 H 7 radical ( Table 11 ).…”

“…In the current work, the reactions of Ḣ atoms with C 2 –C 4 alkenes are studied, while the reactions of Ḣ atom addition to C 5 alkenes were studied previously. 4 , 5 There have been a number of theoretical and experimental studies of Ḣ atoms with C 2 –C 4 alkenes. 17 − 16 This study aims to complement these by providing a comprehensive hierarchical set of rate constants for Ḣ atom addition and abstraction potential energy surfaces (PESs), including their chemically activated pathways for C 2 –C 5 alkenes, determined at the same levels of theory.…”