Stepping Stones to Soot and Stardust: A Computational and Mass Spectrometric Investigation of Gas-Phase Molecular Weight Growth via HACA and Other Reactions

Molecular weight growth chemistry is where small gas-phase molecules combine over time to form large aggregates. It describes the formation of soot in combustion reactions, and tholins in extraterrestrial environments such as Titan’s atmosphere or cold interstellar nebulae. Modelling systems in which molecular weight growth chemistry is prominent requires detailed understanding of the underlying reactions to characterise the pathways towards larger particles. This thesis investigates a selection of molecular weight growth reactions, including both neutral and charged radicals. Computational studies and statistical models are paired with ion-trap mass spectrometry to assess rates of reaction and product branching ratios.The first study describes a detailed kinetic model of the Bittner-Howard pathway of the phenyl Hydrogen Abstraction, C2H2 Addition (HACA) reaction, including both trans and cis geometries. The model is simulated at temperatures ranging from 550 to 1800 K and pressures from 5.33Å~102 to 107 Pa. The inclusion of trans geometries significantly alters product branching ratio at every simulated temperature and pressure, which is most pronounced at high pressure and high temperature. Alternative C10H8 isomers to naphthalene are predicted, including benzofulvene (maximum abundance of 8% at 1200 K and 5.33Å~102 Pa) and E-2-ethynyl-1-phenylethylene (maximum abundance of 26% at 1800 K with little pressure dependence). Pathways involving trans geometries are shown to be also relevant in several other neutral HACA reactions, indicating that they should be considered in future chemical models of combustion-relevant reactions to better predict soot formation.Molecular weight growth is studied using distonic radical ions, where the charge and radical sites are separated. The second study investigates the HACA chemistry of the nine dehydroanilinium, dehydropyridinium and dehydrobenzonitrylium distonic radical cations, with 2-, 3- and 4-substitution. These reactions are explored using room-temperature ion-trap mass spectrometry as well as quantum chemical calculations. Each second-order reaction rate is reported and can be categorised into three groups. The slowest reactions involve the 3- and 4-dehydroanilinium radical cations. Benzonitrylium radical cations react with acetylene at a moderate rate. The 2-dehydroanilinium and all dehydropyridinium radical cations react most quickly with acetylene. All reactions are rapid notwithstanding this grouping, with the slowest (4-dehydroanilinium) reacting with acetylene at (1.895ű0.004)Å~1012 cm3 s-1. A simple and computationally inexpensive empirical model for the estimation of reaction rate is proposed, exponentially correlating reaction rate to the energy of the submerged inner transition state to adduct formation (relative to the energy of the separated reactants). This model provides a means to rapidly estimate the rate of reaction of related molecular weight growth chemistry using minimal computational resources.The final investigation covers non-HACA reactions between the three aforementioned distonic dehydroanilinium radical cations with a propene co-reactant. These reactions are investigated using room-temperature ion-trap mass spectrometry, quantum chemical calculations, and statistical RRKM-ME analysis at 0 K. All three reactions are rapid, with the reaction rate increasing in the order para → meta → ortho. 4- and 3- dehydroanilinium exhibit similar chemistry when reacted with propene, usually eliminating either a methyl radical or ethylene, with a small proportion (4%ű1%) of H atom loss. The 2-dehydroanilinium + propene reaction has markedly different chemistry, and primarily eliminates an ethyl radical (71%ű1% branching ratio). The product of the ethyl loss channel is tentatively assigned as the aminotropylium ion, a seven-membered ring. Seven-membered ring formation in a simple molecular weight growth reaction at low temperature may indicate the possibility of such chemistry occuring in low-temperature environments such as the atmosphere of Titan.These results provide new data to be included in models of soot and tholin formation and further insight into patterns of reactivity to suggest and successfully implement novel studies of molecular weight growth chemistry using computation and mass spectrometry.