<p> Small nitrogen or silicon containing molecules and their ionized counterparts have attracted a great deal of attention from both experimentalists and theoreticians. This is because such species may play an important role in interstellar chemistry. Small nitrogen containing heterocyclic molecules and ions are of biological importance and are of increasing interest to chemists and biochemists. Therefore, studying the chemistry of such species in the gas phase as solitary ions and neutrals is of considerable importance.</p> <p> The ions studied in this thesis were generated in the rarefied gas phase of the mass spectrometer by (dissociative) electron ionization of selected precursor molecules. The characterization of their structure and reactivity was realized using a variety of tandem mass spectrometry based techniques. These include metastable ion spectra to probe the dissociation chemistry of the low energy ions and (multiple) collision experiments to establish the structure (atom connectivity) of the stable ions. The technique of neutralization-reionization mass spectrometry (NRMS) was used to probe the structure and stability of the neutral counterparts of the ions. The results of theoretical calculations involving the CBS-QB3 model chemistry formed an essential component in the interpretation of the experimental findings.</p> <p> The above approach was used to study various isomers of the CH2N2•+ family of ions, in particular the elusive carbodiimide ion HN=C=NH•+. Unlike other isomers including ionized cyanamide H2N-C≡N•+ and nitrilimine HC=N=NH•+, the carbodiimide ion could only be identified by a strategy that involved the use of the technique of collision-induced dissociative ionization (CIDI) mass spectrometry. Guided by the results of the theoretical calculations, the carbodiimide ion, the global minimum on the CH2N2•+ potential energy surface, could also be generated by the molecule-assisted isomerization of its 1,3-H shift isomer H2N-C≡N•+. This occurs through an ion-molecule interaction with a single molecule of H2O under conditions of chemical ionization : a process termed proton-transport catalysis.</p> <p> The amino-substituted carbodiimide ion, H2N-N=C=NH•+, and nine more isomers have all been calculated to be minima on the potential energy surface of the CH3N3•+ family of ions. Four of these, viz. the aminocarbodiimide ion, the aminonitrilimine ion, H2N-C=N=NH•+, the cyclic C-aminoisodiazirine ion, and the ionized biradical, H2N-N=C(H)=N•+, were generated and characterized by tandem mass spectrometry as stable species in the gas phase. Neutralization-reionization experiments show, in agreement with the CBS-QB3 computational results, that aminocarbodiimide and aminonitrilimine are stable species in the gas phase.</p> <p> Next, the α-distonic ions of a number of heterocyclic nitrogen-containing compounds including pyrazole, pyridazine and aminopyrazine, were characterized and differentiated by tandem mass spectrometry. The stabilities of the neutral counterparts of the ylid ions and their conventional isomers were probed by neutralization-reionization experiments. From these experiments, it follows that the ylid ions have stable neutral counterparts, as predicted by the theoretical calculations.</p> <p> The final component of this work deals with the chemistry of a silicon containing species of potential interest in interstellar chemistry. The reactions of protonated silicic acid, Si(OH)3+, and the mechanisms of these reactions were studied using tandem mass spectrometric experiments and CBS-QB3 model chemistry. The low energy (metastable) ions of Si(OH)3+ dissociate by loss of H2O and SiO2 to form O=SiOH+ and H3O+ ions. Neutralization-reionization experiments show that Si(OH)3• is a stable species in the rarefied gas phase.</p> / Thesis / Master of Science (MSc)

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No description available.

No description available.

<p> The organic ions studied in this thesis were generated in the rarefied gas phase of the mass spectrometer by electron ionization of selected precursor molecules. The characterization of their structure and reactivity was probed by using a variety of tandem mass spectrometry techniques. These include metastable ion spectra to probe the dissociation chemistry of the low energy ions and collision experiments to establish the atom connectivity of the ions. The technique of neutralization-reionization mass spectrometry (NRMS) was used to probe the structure and stability of the neutral counterparts of the ions. Computational results involving the CBS-QB3 model chemistry formed an integral component in the interpretation of the experimental findings.</p> <p> The above approach was used to study proton-transport catalysis in the formaldehyde elimination from low energy 1,3-dihydroxyacetone radical cations. Solitary ketene-water ions, CH2=C(=O)OH2·+, do not readily isomerize into its more stable isomer, CH2=C(OH)2·+. A mechanistic analysis using the CBS-QB3 model chemistry shows that metastable 1,3-dihydroxyacetone radical cations will rearrange into hydrogen-bridged radical cations [CH2C(=O)O(H)-H•••OCH2]·+, where the CH2=O will catalyze the transformation of CH2=C(=O)OH2·+ into CH2=C(OH)2·+.</p> <p> Metastable pyruvic acid radical cations, CH3C(=O)COOH·+, have been shown to undergo decarboxylation to yield m/z 44 ions, C2H4O·+, in competition with the formation of CH3C=O+ + COOH· by direct bond cleavage. Collision induced dissociation experiments agree with an earlier report that oxycarbene ions CH3COH·+ are formed but they also suggest the more stable isomer CH3C(H)=O·+ may be co-generated. Using the CBS-QB3 model chemistry, a mechanism is proposed to rationalize these results.</p> <p> Next, the isomeric ions CH3O-P=S·+ and CH3S-P=O·+ were characterized and differentiated by tandem mass spectrometry. Metastable CH3O-P=S·+ and CH3S-P=O·+ ions both spontaneously lose water to yield m/lz 74 cyclic product ion [-S-CH=]P·+. Using the CBS-QB3 model chemistry a mechanism is proposed for the water loss from CH3O-P=S·+ and CH3S-P=O·+. Our calculations also show that these two isomers communicate via a common intermediate, the distonic ion CH2S-P-OH·+, prior to the loss of water.</p> <p> The final component of this work details the computational study addressing the long standing question on the mechanism for the water elimination from metastable ethyl acetate radical cations. The CBS-QB3 results show that low energy ethyl acetate ions isomerize into ionized 4-hydroxy-2-butanone prior to the loss of water.</p> / Thesis / Master of Science (MSc)

El polietileno es un polímero versátil de bajo costo usado en numerosas aplicaciones, desde partes de automóviles hasta envases de comida, juguetes, films, pañales y bolsas de residuos, cuya demanda mundial es de aproximadamente 76 millones de toneladas métricas al año. Junto con el polipropileno, representa casi dos tercios de los termoplásticos “commodities” usados en el mundo, estimados en 130 millones de toneladas métricas en 2011. Una de las tecnologías más utilizadas en el mundo para la fabricación comercial de polietileno es la de fase gas. La inversión de capital y los requerimientos energéticos para operar los reactores de fase gas son relativamente bajos comparados con otros procesos existentes para la producción de polietileno. Esta ventaja, sumada a la gran flexibilidad en cuanto a las variaciones de índices de fluidez y densidades de los productos que se pueden producir en estos reactores, hace de este proceso un tema de estudio sumamente interesante. En esta tesis se desarrolló un modelo integrado del reactor de lecho fluidizado y sus equipos auxiliares, para modelar la operación de una planta que produce polietileno de baja densidad lineal, utilizando etileno como monómero y 1‐buteno como comonómero, en presencia de un catalizador tipo Ziegler‐Natta. Para ello se realizó inicialmente una búsqueda bibliográfica donde se analizaron los diferentes modelos para polimerización de etileno en fase gas ya desarrollados por otros autores. Para la representación de la operación del reactor de lecho fluidizado, se implementaron dos modelos: un modelo simplificado de mezclado perfecto, y un modelo más complejo de dos fases, donde la fase gas tiene burbujas de tamaño constante. Luego se ampliaron ambos modelos incorporando los equipos que conforman el reciclo de la planta, es decir, el compresor, el intercambiador, y el mezclador de la corriente fresca con la corriente de reciclo. Finalmente, se realizó un ajuste inicial de los parámetros cinéticos de cada uno de los modelos y también de algunas condiciones operativas (caudal de catalizador y velocidad superficial del gas) con el objetivo de minimizar la diferencia entre los valores calculados y experimentales de temperatura del reactor y peso molecular promedio en peso y en número del polímero. Los valores de pesos moleculares promedio en peso y en número se obtuvieron experimentalmente a partir de muestras de polietileno de dos productos comerciales que se producen en una planta de tecnología fase gas ubicada en Bahía Blanca, utilizando cromatografía por exclusión de tamaños. Ambos modelos del reactor autónomo, con sus respectivos set de parámetros estimados, fueron capaces de predecir apropiadamente los valores típicos de temperatura del reactor, conversión y los pesos moleculares promedio en número y en peso, para las condiciones de proceso industriales estudiadas en esta tesis. Asimismo, ambos modelos integrados fueron capaces de reproducir adecuadamente los valores de las variables de los estados estacionarios para los cuales se ajustaron los parámetros. En lo que respecta a las perturbaciones en ciertas variables, la respuesta de los modelos integrados fue disímil. El comportamiento del modelo de mezclado perfecto resultó inestable ante perturbaciones en ciertas variables del proceso, por causas que fue imposible precisar en detalle. Por el contrario, el modelo de dos fases de burbuja constante fue capaz de reproducir apropiadamente el comportamiento del proceso industrial ante perturbaciones acotadas de variables clave como la temperatura y caudal de agua de enfriamiento, velocidad superficial del gas, flujo másico del catalizador, y concentraciones de monómero, comonómero e hidrógeno. En consecuencia, el modelo integrado de dos fases con tamaño de burbuja constante, se utilizó para realizar diversos análisis de sensibilidad para determinar el impacto que tienen diferentes variables operativas y de diseño sobre la estabilidad térmica del reactor, su capacidad productiva y las características del producto obtenido. Esto permitió profundizar la comprensión del comportamiento de los reactores de lecho fluidizado, lo que sin duda redunda en una mejor preparación para la toma de decisiones en planta. Finalmente, se puede mencionar que entre los usos potenciales de este modelo matemático se incluyen la simulación y testeo de esquemas de control de calidad en línea, la predicción de los efectos de políticas de transición de grado en las distribuciones de peso molecular y composición; y el ser útil como herramienta de optimización del proceso, por ejemplo definidas las características moleculares que debe tener el polietileno para cierto tipo de aplicaciones finales, encontrar las condiciones operativas que permitan obtenerlo y además, maximicen la productividad.

<p dir="ltr">While electrospray ionization facilitates the mass determination of smaller analytes, ESI of macromolecular native protein complexes is complicated by narrow charge state distributions and overlapping charge states, hindering mass analysis. This problem is further exacerbated with heterogeneous protein mixtures that yield ions of similar <i>m/z</i> values. Charge-reduction of a selected precursor population via ion/ion reaction provides further <i>m/z</i> separation and utilizes an extended mass range, allowing for mass determination. All experiments were performed on a TripleTOF 5600 quadrupole TOF mass spectrometer (SCIEX), modified for ion/ion reactions. Alternatively pulsed nano-electrospray ionization allowed for sequential injection of reagent and analyte ions. Selected cations were reacted with different anions, and charge-reduced product ions were mass analyzed on a TOF with modifications and tuning for an extended <i>m/z</i> range of 200,000+. Charge reduction via proton transfer reaction (PTR) involves a perfluorinated anion reacting with a multiply charged cation and results in the loss of one proton removal at a time. Through multiple iterations of PTR, the overlapping charge states of protein ions (from an unstained protein standard mixture consisting of 12 recombinant proteins of masses10kDa-200kDa) within the <i>m/z</i> 6500-8500 and <i>m/z</i> 4000-5000 ranges, generated under native conditions by ESI, transforms to a product spectrum with single-digit charge states, thereby deconvoluting the precursor “blob”.</p>

No / Crystal structure prediction simulations are reported on 5-hydroxymethyl-2-oxazolidinone and 4-hydroxymethyl-2-oxazolidinone to establish the feasibility of predicting the spontaneous resolution of racemates of small organic molecules. It is assumed that spontaneous resolution occurs when the enantiomorph is more stable than the racemic solid. The starting point is a gas phase conformational search to locate all low-energy conformations. These conformations are used to predict the possible crystal structures of 5- and 4-hydroxymethyl-2-oxazolidinone. In both cases, the racemic crystal structure is predicted to have the lowest energy. The energy differences between the lowest-energy racemic solids and the lowest-energy enantiomorphs are 0.2 kcal mol-1 for 5-hydroxymethyl-2-oxazolidinone and 0.9 kcal mol-1 for 4-hydroxymethyl-2-oxazolidinone. In the case of 4-hydroxymethyl-2-oxazolidinone, where the racemic crystal is known to be more stable and the experimental crystal structures of both the racemate and the enantiomorph are available, the simulation results match the observed data. For 5-hydroxymethyl-2-oxazolidinone, where only enantiopure crystals are observed experimentally, the known experimental structure is found 1.6 kcal mol-1 above the lowest-energy predicted structure. This work shows that it is possible to predict whether the racemate of a small chiral molecule can be resolved spontaneously, although further advances in the accuracy of lattice energy calculations are required.

In this thesis, I have investigated gas phase infrared spectroscopy of environmentally as well as astrophysical important large organic molecules such as naphthalene, methy-lated naphthalene, fluorine, methyalted fluorine etc. which are commonly known as polycyclic aromatic hydrocarbons (PAHs). Depending upon the molecular weight these organic molecules can exist both in gaseous as well as in the particulate state at room temperature hence they are the major environmental pollutants. They are also responsible for the unidentified infrared emission bands in the interstellar medium. Chapter 1 provides a brief introduction to my thesis work. A detailed literature survey on the importance, abundance of the PAHs in the environment as well as various spectroscopic techniques useful for identifying the PAHs has been done. Since the objective of my thesis work is to assign the observed fundamental infrared bands of large organic molecules with the help of high level quantum mechanical calculations, a brief introduction to the various high level quantum mechanical techniques that I have used in assigning the bands have been described in this chapter. In Chapter 2 I have presented the experimental and the theoretical methodologies in details. The chapter begins with a detailed description of the experimental procedure used for recording the infrared spectrum of these molecules followed by the theoretical methodologies used for the assignment of the observed infrared bands as well as for identifying the Fermi resonances. In Chapters 3 and 4, of this thesis I have recorded infrared spectrum of 1-and 2-methylnaphthalene (1-and 2-MN), fluorine (FL), 1-methylfluorene (1-MFL) and 1,8-dimethylfluorene (1,8-DMF) in the gas phase. The observed bands were assigned with the help of scaled harmonic frequency, scaled quantum mechanical harmonic force field (SQMFF) and enharmonic frequency calculations. The first two methods are based on the harmonic approximation, whereas the enharmonic frequency calculation is based on the standard second order perturbation theory. All these calculations gave me a partial fit to the fundamental bands in both aromatic and aliphatic C-H stretching as well as in the non C-H stretching region. At the end of both the chapters an error analysis in fitting the spectrum from all the three different calculations have been presented. Evidently the non linear least square fitting method employed in SQMFF calculation gives much better agreement between the experiment and theory than the other two methods. It has been observed in the experimental spectrum of methylated naphthalene that the band structure near the C-H stretch around 3000 cm−1 is very complicated and many bands and shoulders remain unassigned by the methods described in Chapters 3 and 4. Fermi resonance is one of the potential reason for the complicated band structure in this region. In Chapter 5, I have taken naphthalene and have investigated the Fermi resonance around the C-H stretching region using an effective vibrational hamiltonian (EVH) approach. In this method I have constructed an EVH consisting of 8 C-H stretches and 8 H-C-C in-plane bend overtones and 28 H-C-C in-plane bend combination modes as the basis. Both type 1 (stretch overtone) and type 2 (stretch combination) Fermi resonances were investigated. Calculated frequencies belonging to B1u and B2u irreducible representation were compared with the observed bands. Many bands and shoulders have been assigned as the overtone and combination modes of low frequency H-C-C bend motion obtained from the EVH approach. How-ever some bands remain unassigned in this method. This is perhaps due to the neglect of the carbon framework motion in the construction of the EVH. To improve upon the results obtained from the EVH formalism I included the carbon frame degrees of freedom and have carried out a full variation treatment in curvilinear coordinates. I have considered the 8 C-H stretches and 8 H-C-C in-plane bends of naphthalene as local mode oscillators and 17 coordinates belonging to the carbon framework motion as curvilinear normal mode oscillators. A quartic hamiltonian in a mixed local mode -normal mode basis was constructed including up to three body terms in both kinetic and potential energy part. The hamiltonian was subsequently recast into the ladder operator form and diagonal zed in a symmetry adapted basis with polyad constraints. Frequencies so obtained were compared to the experiment All these findings have been presented in Chapter 6 of this thesis. The concluding remark of the thesis and the future direction is presented in Chapter 7

This research is divided into three major parts. In part I, the critical supersaturations required for the homogeneous nucleation of 2,2,2-trifluorothanol (TFE) vapor have been measured over a temperature range (266-296 K) using an upward thermal diffusion cloud chamber (DCC). The measured supersaturations are in agreement with the predictions of both the classical and the scaled theory of nucleation. Moreover, the condensation of supersaturated TFE vapor on laser-vaporized magnesium nanoparticles has been studied under different experimental conditions, such as the supersaturation, the pressure and the electric field. In part II, the laser vaporization controlled condensation (LVCC) technique was used to prepare Au-Ag alloy nanoparticles in the vapor phase using designed targets of compressed Au and Ag micron-sized powder mixtures of selected composition. The results showed that the optical properties of these nanoparticles could be tuned depending on the alloy composition and the laser wavelength. Different intermetallic nanoparticles (FeAl and NiAl) from the vapor phase has also been prepared, using the same approach.In this work, the fraction of the charged particles generated during the laser vaporization process was used to prepare a new class of nanoparticle assemblies in the LVCC chamber under the influence of an electric field. The results showed that the electric field required to induce the formation of these nanoassemblies is material and field dependent. By coupling the LVCC chamber with the differential mobility analyzer, size-selected nanoparticles have been prepared in the vapor phase. The prepared nanoparticles were characterized by different techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-visible spectroscopy. In part III, new methods were developed to prepare nanoparticle-polymer composites from the vapor phase. In the first method, the LVCC method was used to prepare a carbonaceous cross-linked resin, with different nanoparticles (Ni, Pt and FeAl) embedded inside. In the second method, free radical-thermally initiated polymerization was used to polymerize a monomer vapor of styrene on the surfaces of activated Ni nanoparticles.