Theoretical, experimental and modelling studies of the reactions of hydrogen atoms with C2 – C5 alkenes and cyclisation reactions of hydroperoxyl-alkyl radicals forming cyclic ethers and hydroxyl radicals

This thesis presents a hierarchical study of the reactions of Ḣ atom addition to and abstraction from both linear and branched C2 – C5 alkenes. The subsequent C–C and C–H β-scission reactions and H-atom transfer reactions are also considered. As mentioned throughout this thesis, alkyl radicals are prominent in combustion chemistry as they are formed by hydrogen abstraction from a stable molecule or from radical attack on hydrocarbons. The addition of Ḣ atoms to the carbon-carbon double bond (C=C) plays a significant role in controlling experimental high-temperature ignition delay times, flame speeds, and species profiles measured as a function of temperature and/or time in reactors including jet-stirred (JSR) and flow reactors. Therefore, accurate determinations of the thermochemistry and kinetics of their unimolecular isomerisation and decomposition reactions and related addition reactions to alkenes are important in simulating the combustion chemistry of virtually all hydrocarbon fuels. Despite their importance, alkenes have not been as extensively studied as alkanes, especially the larger alkenes such as pentene. By having a consistent set of rate constants for C2 – C5 alkenes + Ḣ using the same level of theory, the calculation results help constrain available models and the development of recommended rules for rate constants associated with certain reaction types. This will provide a tool in developing mechanisms describing the pyrolysis and oxidation of larger alkenes for which calculations do not exist in the literature. Thermochemical values for species on the Ċ2H5, Ċ3H7, Ċ4H9 and Ċ5H11 potential energy surfaces (PESs) are 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 are calculated using Rice-Ramsperger-Kassel-Marcus (RRKM) theory coupled with a one-dimensional (1-D) master equation (ME). Geometries are optimised using the density functional theory (DFT) ωB97XD method coupled with the aug-cc-pVTZ basis set. Harmonic frequency analysis is simultaneously carried out at the same level of theory to verify the nature of each stationary point. Low-frequency torsional modes are treated via relaxed PES scans in 10-degree increments with the ωB97XD / 6-311++G(d,p) method, with the potential energies as a function of dihedral angle used as input for a one-dimensional (1-D) hindered rotor approximation as implemented in the Master Equation System Solver (MESS). To compute barrier heights, single point energies for minima and transition states are calculated with coupled cluster theory, specifically (CCSD(T)), and Møller-Plesset perturbation theory (MP2), with cc-pVXZ basis sets, where X = D, T and Q levels of theory. As a validation of the theoretical results calculated in this thesis, the results are implemented into kinetic models (AramcoMech3.0, NUIGMech1.0 and NUIGMech1.2) and simulations are compared to new hydrogen atomic resonance absorption spectrometry (Ḣ-ARAS) experimental measurements taken as part of a collaboration with Dr. Sebastian Peukert at Duisburg-Essen University. The Ḣ-ARAS experiments measured for 1- and 2-pentene + Ḣ provide the first measurements of the global rates of reaction of Ḣ atoms with 1- and 2-pentene, with the theoretical results predicting the experiments well. Satisfactory agreement is also observed for the theoretical results compared to the single-pulse shock tube pyrolysis experiments of linear and branched 1-alkenes recorded at NUIG, both of which serve as direct and in-direct validation targets for the current calculations. Additionally, as part of this thesis, rate constants for the low-temperature reaction class: cyclisation of hydroperoxyl-alkyl (QOOH) radicals to form cyclic ethers and hydroxyl radicals (QOOH ↔ cyclic ether + ȮH) are calculated, involving species ranging in size from C2H5Ȯ2 to C5H11Ȯ2. These rate constants are determined using density functional theory (DFT) and ab initio approaches. Geometry optimisations are conducted using the M06-2X method, coupled with the 6-311++G(d,p) basis set. Single point energies are calculated using coupled cluster (CCSD), specifically CCSD(T) and second-order Møller-Plesset perturbation theory (MP2) methods, with relatively large basis sets (cc-pVXZ, where X = D,T,Q). Standard statistical thermodynamics and canonical transition state theory are employed to derive the kinetic data of interest. The use of these new rate coefficients in the NUIG pentane oxidation model produces favourable agreement with C5 cyclic ether concentration measurements in JSRs at Nancy and Orléans. These had previously been over-predicted by the model utilising literature rate constant values