Ab Initio/Transition-State Theory Study of the Reactions of Ċ₅H₉Species of Relevance to 1,3-Pentadiene, Part I: Potential Energy Surfaces, Thermochemistry, and High-Pressure Limiting Rate Constants

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 al...

Experimental and DFT Calculated IR Spectra of Guests in Zeolites: Acyclic Olefins and Host–Guest Interactions

“…As discussed above in Methods, we model the pore space of deAl-BEA by using DFT to compute properties of molecule I in all-silica MOR, which contains channels very similar in size to those of BEA. Regarding guest species I , it has been reported that molecules of this nature exist in either s-cis or s-trans conformations, , prompting us to begin by investigating the equilibrium between the two conformers using computed Gibbs and Helmholtz energies, and the impact of these conformers on the IR spectra.…”

“…Dienes such as molecule I have a dihedral angle along the single bond in between the two C=C bonds, resulting in two stable conformations denoted as s-trans and s-cis, where the ‘s’ indicates torsion around the s ingle bond. Previous reports on substituted dienes suggest that the s-trans conformer can be lower in energy by as much as 3 kcal/mol, thus exhibiting higher population at thermal equilibrium. , We investigated the thermodynamic stabilities of conformers of I by computing free energy profiles for the s-cis and s-trans conformers in vacuum and over all-silica MOR (Table S1 in Supporting Information). Interestingly, we found that a gauche s-cis conformation of I has lower free energies than its s-trans counterpart at 20 °C.…”

An ab Initio/Transition State Theory Study of the Reactions of Ċ5H9 Species of Relevance to 1,3-Pentadiene, Part II: Pressure Dependent Rate Constants and Implications for Combustion Modeling

“…The aim of this work is to develop comprehensive kinetics for the title reaction for use in a detailed chemical kinetic model describing C 5 species oxidation. Ḣ atom addition reactions to the pentadiene isomers and cyclopentene have been extensively studied theoretically and literature papers describing the Ċ 5 H 9 PES have been documented in part I of this study . Moreover, the thermochemistry of the C 5 species used in this study has been comprehensively validated against literature data in part I.…”

“…As highlighted in part I of this study, olefins and dienes are important intermediates generated via heavy petroleum cracking. The reactions of Ċ 5 H 9 radicals have an important effect on the performance of a C 5 combustion mechanism and may also influence the combustion chemistry of larger dienes and alkenes.…”

“…H ̇atom addition reactions to the pentadiene isomers and cyclopentene have been extensively studied theoretically and literature papers describing the C ̇5H 9 PES have been documented in part I of this study. 1 1.…”