<b>Mechanistic studies on rocket fuels via quantum chemical calculations, identification of hydrocarbons in Jet A via GC×GC/TOF mass spectrometry, and detection of isomers via laser-induced acoustic desorption/molecular rotational resonance spectroscopy</b>

<p dir="ltr">Hypergolic rocket propellants are bipropellants that spontaneously ignite upon contact without an ignition source. The commonly used fuels that are hypergolic with the oxidizer dinitrogen tetroxide, such as monomethylhydrazine, are very toxic. Therefore, safer alternative fuels are of interest. However, the reactions and chemical properties that relate to hypergolic behavior, and specifically to ignition delay, are unknown. Based on the product ions detected in drop-on-drop tests performed for low-toxicity hypergolic fuels and dinitrogen tetroxide in front of a linear quadrupole ion trap mass spectrometer, mechanisms were proposed for the formation of the detected product ions. Quantum chemical calculations with the SMD solvation model were used to investigate the proposed reactions. Reactions of the nitronium cation with three low-toxicity hypergolic fuels, 1,3,5-cycloheptatriene, triallylamine, and 2,3-dihydrofuran, were found to be highly exergonic, with free energy changes as favorable as 100 kcal/mol. The reactions involved addition followed by dissociation and a hydrogen-atom transfer to produce resonance stabilized final products. This reactivity may rationalize the hypergolic behavior of these fuels. While the overall free energy changes calculated for different fuels for the addition reactions did not correlate with the previously measured ignition delays, the free energy changes upon the formation of certain intermediates, the radical cation of the fuel molecules and the nitrogen dioxide radical, were found to correlate with the ignition delays for these fuels. If this finding proves to be general, this correlation might be used as a predictor for hypergolic behavior and short ignition delays.</p><p dir="ltr">Aviation fuels are complex mixtures containing mostly hydrocarbons. The composition and amounts of specific hydrocarbon compounds and their different classes influence many physical and chemical properties of aviation fuels in different ways. However, the determination of the chemical composition of aviation fuels is challenging. Therefore, development of better methods for the identification of compounds in jet fuels was attempted. Electron ionization mass spectrometry with library matching is commonly used to identify unknown compounds; however, this method frequently results in the misidentification of hydrocarbons due to the extensive fragmentation of their molecular radical cations. An alternative ionization method is chemical ionization, which can be used to softly ionize analytes via gas-phase ion-molecule reactions with minimal fragmentation. Previously, a student in the Kenttämaa group developed a method based on two-dimensional gas chromatography coupled with positive-ion mode methane chemical ionization high-resolution mass spectrometry to establish the patterns of ionization, fragmentation, and adduct formation for various hydrocarbon classes by using model compounds. These results and the same experimental method were used in this thesis research to identify over thirty hydrocarbons in Jet A. These results demonstrate that chemical ionization mass spectrometry can be used to identify compounds found in complex hydrocarbon mixtures, such as Jet A, when electron ionization mass spectrometry fails.</p><p dir="ltr">Molecular rotational resonance spectroscopy (MRR) is an analytical technique that can be used for the identification of small polar gaseous molecules based on their angular momentum and the energies of the transitions between quantized rotational states based on microwave emission. The energies of the transitions are directly related to the geometries of the gaseous molecules and their moments of inertia. Detection has been limited to volatile and thermally stable molecules because the compounds must be in the gas phase. Laser ablation has been coupled with MRR to desorb nonvolatile and thermally labile compounds. However, laser ablation can cause fragmentation of the desorbed compounds and often requires a matrix, which can lead to the formation of gas-phase analyte-matrix clusters upon ablation. Laser-induced acoustic desorption (LIAD) is an alternative desorption method that can be used to desorb neutral nonvolatile compounds without fragmentation or the use of heat. LIAD involves coating one side of a thin titanium foil with a sample and translating a laser across the backside of the foil to desorb the intact neutral compounds in the sample into the gas phase. Therefore, LIAD was coupled to MRR (LIAD/MRR) to facilitate the analysis of nonvolatile and thermally labile compounds. LIAD/MRR was demonstrated to allow the detection and differentiation of constitutional isomers in neat samples and in mixtures. Additionally, a linear correlation was observed between the area of the characteristic transition frequency peak and the analyte molar flow rate into the instrument, suggesting that this method may be well-suited for quantitative applications.</p>