Ab Initio/Transition-State Theory Study of the Reactions of Ċ5H9 Species of Relevance to 1,3-Pentadiene, Part I: Potential Energy Surfaces, Thermochemistry, and High-Pressure Limiting Rate Constants

Yanjin Sun; Chong-Wen Zhou; Kieran P Somers; Henry J Curran

doi:10.1021/acs.jpca.9b06628

Ab Initio/Transition-State Theory Study of the Reactions of Ċ₅H₉ Species of Relevance to 1,3-Pentadiene, Part I: Potential Energy Surfaces, Thermochemistry, and High-Pressure Limiting Rate Constants

J Phys Chem A. 2019 Oct 24;123(42):9019-9052. doi: 10.1021/acs.jpca.9b06628. Epub 2019 Oct 15.

Authors

Yanjin Sun¹, Chong-Wen Zhou^{1

2}, Kieran P Somers¹, Henry J Curran¹

Affiliations

¹ Combustion Chemistry Centre, School of Chemistry, Martin Ryan Institute MaREI , National University of Ireland , Galway H91 TK33 , Ireland.
² School of Energy and Power Engineering , Beihang University , Beijing 100191 , P. R. China.

PMID: 31566374
DOI: 10.1021/acs.jpca.9b06628

Abstract

In this study, the reactions of Ċ₅H₉ radicals are theoretically investigated, with a particular emphasis on hydrogen atom addition reactions to 1,3-pentadiene (C₅H₈) to form Ċ₅H₉ radicals, although the subsequent isomerization and decomposition reactions of the Ċ₅H₉ radicals are also of direct relevance to the radicals formed from the pyrolysis and oxidation of species including pentene and cyclopentane. Moreover, H-atom abstraction reactions by hydrogen atoms from 1,3-pentadiene are also investigated. The geometries and frequencies of 63 potential energy surface (PES) minima and 88 transition states are optimized at the ωB97XD/aug-cc-pVTZ level of theory. Spin-unrestricted open-shell single-point energies for all the species are calculated at the CCSD(T)/aug-cc-pVTZ level of theory with basis set corrections from MP2/aug-cc-pVXZ (where X = T and Q). A one-dimensional hindered rotor treatment is employed for torsional modes, with the M06-2X/6-311++G(D,P) method used to compute the potential energy as a function of the dihedral angle. The high-pressure limiting rate constants and the thermochemical properties for C₅ species are calculated using the Master Equation System Solver (MESS) with conventional transition-state theory and comparisons made with existing available literature data. A hydrogen atom can add to the terminal carbon atom of 1,3-pentadiene to form the 2,4-Ċ₅H₉ radical and/or the internal carbon atoms to form 2,5-Ċ₅H₉, 1,4-Ċ₅H₉, and 1,3-Ċ₅H₉ radicals. Among the four entrance channels for Ḣ atom addition reactions, the formation of 2,4-Ċ₅H₉ and 1,3-Ċ₅H₉ radicals is more exothermic in comparison to the other Ċ₅H₉ isomers (2,5-Ċ₅H₉, 1,4-Ċ₅H₉) because of the resonantly stabilized allylic structure. Consequently, the formation of the former is generally dominant in terms of barrier heights. Ḣ atom addition reactions to 1,3-pentadiene are compared to available C₃-C₅ alkenes and dienes, with external addition calculated to be kinetically favored over internal addition. However, the correlation between heats of formation and energy barriers for Ḣ atom addition to 1,2-dienes is different from that for 1,3- and 1,4-dienes. Hydrogen atom addition and abstraction rate constants are also compared for 1,3-pentadiene, with addition found to be dominant. The subsequent unimolecular reactions on the Ċ₅H₉ PES are found to be highly complex with reactions taking place on a multiple-well multiple-channel PES. For clarity, the reaction mechanism and kinetics of each Ċ₅H₉ radical are discussed individually in terms of the computed enthalpy of the reaction and activation, the transition-state structure/reaction class, and also in terms of the combustion species for which the reactions are of potential importance. The reactions on the Ċ₅H₉ PES are divided into three reaction classes (H-shift isomerization, cycloaddition, and β-scission reactions), and the reactivity-structure-based estimation rules for energy barriers are derived for these three reaction classes and compared to literature results for alkyl radicals.