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An Investigation of Linked Physical And Biogeochemical Processes In Heterogeneous Soils In The Vadose ZoneHansen, David Joseph 2011 August 1900 (has links)
Chemical dynamics in the vadose zone are poorly understood due to the transient nature of chemical and hydrologic conditions, but are nonetheless critical to understanding contaminant fate and transport. This work explored the effects of soil structure (i.e. layers, lenses) on linked geochemical, hydrological, and microbiological processes under changing hydrologic conditions (e.g. rainfall, introduction of groundwater, and fluctuating water table heights). A homogenized medium-grained sand, homogenized organic-rich loam and a sand-over-loam layered column were constructed for the first series of experiments. The second series of experiments employed two soil columns with lenses that were packed identically with sterilized and untreated sediments. Each consisted of two lenses of organic-rich loam in a medium-grained sand matrix. Lenses were located at different vertical depths and were horizontally offset. In-situ collocated probes collected soil hydrologic and chemical data.
In the layered column, enhanced biogeochemical cycling was observed over the texturally homogeneous soil columns. Enumerations of Fe(III) and SO42- reducing microorganisms also show 1-2 orders of magnitude greater community numbers in the layered column. The greatest concentrations of aqueous FeS clusters (FeSaq) were observed in close proximity to the soil interface. To our knowledge, this was the first documentation of FeSaq in partially saturated sediments. Mineral and soil aggregate composite layers were also most abundant near the soil layer interface; the presence of which, likely contributed to an order of magnitude decrease of hydraulic conductivity.
In the live lens column, Fe-oxide bands formed at the fringes of the lenses that retarded water flow rates by an order of magnitude compared to the sterilized column. Microbial activity also produced insoluble gases and that led to the creation of a separate gas phase that reduced hydraulic conductivity. This limited the interaction between groundwater with soil-pore waters that led to the formation of geochemically distinct water masses in relatively close proximity to one another. No such changes were observed in the sterilized column.
When compared to homogenous columns, the presence of soil heterogeneities altered biogeochemical and hydrologic processes considerably which highlights the need to consider soil heterogeneity in contaminant fate and transport models. These findings suggest that quantifying coupled hydrologic-biogeochemical processes occurring at small scale soil interfaces is critical to accurately describing and predicting chemical changes at the larger system scale.
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Estudis teòrics de clusters, superfícies i cristalls d'òxids metàl.lics. Propietats estructurals, electròniques i catalítiquesCalatayud Antonino, Monica 27 June 2001 (has links)
La present tesi doctoral tracta la modelitzacio de diferents estats de la materia: clusters o agregats, superficies i cristalls d'oxids metal.lics. Els metodes disponibles per abordar cada sistema han estat breument descrits, per justificar la seua utilitzacio en diverses aplicacions a materials. Primer, s'han proposat geometries per als agregats de V2O5 cations i neutres en fase gas, i s'han caracteritzat els estats electronics, estructura, espectre de vibracio i enllac quimic. Segon, s'han simulat superficies d'oxids metal.lics i processos d'adsorcio de molecules o atoms metal.lics: CH3OH i O2 sobre SnO2, Cu sobre ZnO. La interaccio adsorbat-superficie te lloc per un mecanisme acid/base, i s'ha caracteritzat la geometria i energia de cada supersistema (mode d'adsorcio, paper dels defectes de superficie). Finalment, s'ha estudiat el cristall de TiO2 en fase anatasa des de multiples punts de vista: descripcio de l'estructura en poliedres, estructura electronica (bandes i densitat d'estats), termodinamic (equacio d'estat) i d'enllac quimic ("Atoms In Molecules"). Les superficies mes estables han estat construides i els efectes de relaxacio avaluats.
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Quantum Cascade Lasers for Mid-Infrared Chemical SensingCharlton, Christy 23 November 2005 (has links)
The mid-infrared (MIR) spectral range (2-20 m) is particularly useful for chemical sensing due to the excitation of fundamental rotational and vibrational modes. In the fingerprint region (10-20 m), most organic analytes have unique absorption patterns; absorption measurements in this region provide molecule-specific information with high sensitivity.
Quantum cascade lasers (QCLs) present an ideal light source for (MIR) chemical sensing due to their narrow linewidth, high spectral density, compact size, and ease of fabrication of nearly any MIR wavelength. As the emission wavelength is dependent on layer size within the heterostructure rather than material composition, various wavelengths in the MIR can be achieved through bandstructure engineering.
High sensitivity measurements have been achieved in both gas and liquid phase by developing integrated sensing systems. The laser emission frequency is selected to match a strong absorption feature for the analyte of interest where no other interfering bands are located. A waveguide is then developed to fit the application and wavelength used.
Gas sensing applications incorporate silica hollow waveguides (HWG) and an OmniGuide fiber (or photonic bandgap HWG). Analyte gas is injected into the hollow core allowing the HWG or OmniGuide to serve simultaneously as a waveguide and miniaturized gas cell. Sensitivities of parts per billion are achieved with a response time of 8 s and a sample volume of approximately 1 mL.
Liquid sensing is achieved via evanescent wave measurements with planar waveguides of silver halide (AgX) and gallium arsenide (GaAs). GaAs waveguides developed in this work have a thickness on the order of the wavelength of light achieving single-mode waveguides, providing a significant improvement in evanescent field strength over conventional multimode fibers. Liquid samples of L volume at the waveguide surfaces are detected.
QCLs have begun to be utilized as a light source in the MIR regime over the last decade. The next step in this field is the development of compact and highly integrated device platforms which take full advantage of this technology. The sensing demonstrations in this work advance the field towards finding key applications in medical, biological, environmental, and atmospheric measurements.
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Electric deflection measurements of sodium clusters in a molecular beamLiang, Anthony 10 November 2009 (has links)
Rotationally averaged polarizabilities and intrinsic electric dipole moments of sodium clusters are measured and reported. The experimental method is a molecular beam deflection. Our precision is the highest (<5%) and the range of the cluster sizes is the broadest to date (Na₁₀ ∼ Na₃₀₀). Compared to the earlier measurements, our data covers all sizes with no gaps up to the largest cluster. The fine structure in the polarizability curve is previously unobserved. We have carefully ruled out several possible explanations. And we find an earlier existing theory could explain the facts but will lead to magic numbers which were not seen in some previous experiments. A detailed theory is needed to understand the behaviors we see.
Intrinsic electric dipole moments (EDM) of sodium clusters are probed to answer the intriguing question: Do metal clusters develop electric dipole moments like molecules? Some theories have predicted the existence of EDM in ground state sodium clusters and gave their magnitudes. We put upper bounds on the EDM of sodium clusters and find that they are orders of magnitude smaller than the predictions. This provokes an interesting question: how can one define metallicity in metal clusters?
Our measurements are performed at cryogenic temperature 20 Kelvin. At this temperature the clusters are believed to be in their vibronic ground states.
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Laser flash photolysis studies of some gas phase reactions of atmospheric interestZhao, Zhijun 20 August 2009 (has links)
Radical reactions play central roles in regulating regional air quality and global climate. Some potentially important gas phase radical reactions are being investigated in this research project, including Cl reactions with acetone, butanone, 3-pentanone, pyridine, and dimethyl selenide (DMSe), HO2 complex formation and dissociation with formic and acetic acids, and reactive and non-reactive quenching of O(1D) by the potent greenhouse gases SO2F2, NF3, and SF5CF3. The involved radicals are generated by laser flash photolysis (LFP). Temporal profiles of either the radical reactant or a product are monitored in "real time" using atomic resonance fluorescence spectroscopy (RF), time-resolved UV-visible absorption spectroscopy (TRUVVAS), or tunable diode laser absorption spectroscopy (TDLAS), allowing kinetic and mechanistic information of these reactions to be obtained. These studies provide new knowledge of the investigated radical reactions and facilitate a better understanding of their significance in atmospheric chemistry.
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Studies of transport in some oxides by gas phase analysisDong, Qian January 2004 (has links)
No description available.
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Development and Application of ESI-MS Based Techniques to Study Non-Covalent Protein-Ligand Complexes in Solution and the Gas PhaseDeng, Lu Unknown Date
No description available.
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Spectroscopy and Kinetics of Weakly Bound Gas-Phase Adducts of Atmospheric InterestDookwah-Roberts, Venus Maria Christina 21 May 2008 (has links)
A number of weakly bound adducts play important roles in atmospheric chemistry, such as DMS OH and CS2 OH. The work comprising this dissertation involves kinetic and spectroscopic studies of adducts formed between halogen atoms and the important atmospheric trace gases CS2, CH3SCH3 (DMS), CH3I, and C2H5I. The results reported in these studies are useful for developing an understanding of the reactivity of these species and for testing the ability of electronic structure theory and reaction rate theory to predict or rationalize any observed trends. Oxidative pathways of both alkyl halides and sulfur compounds, especially DMS, are of atmospheric interest based on the roles of these species in affecting the oxidizing capacity of the troposphere and in the formation of new particles which impact the Earth s radiation budget and climate variability.
The experimental approach employed laser flash photolysis (LFP) coupled with time resolved UV-visible absorption spectroscopy (TRUVVAS) to investigate the spectroscopy and kinetics of the gas phase adducts: SCS Cl, CH3I Cl, C2H5I Cl, (CH3)2S Br, and (CH3)2S I.
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A utilização da química computacional em processos químicos relacionados à ionização por electrospray / The use of computational chemistry in the studies of chemical processes involved in electrospray ionizationRicardo Vessecchi Lourenço 09 June 2009 (has links)
Nas últimas décadas, o desenvolvimento das técnicas de ionização à pressão atmosférica impulsionou a espectrometria de massas, na caracterização e elucidação estrutural de compostos de grande massa molecular. O surgimento dessas técnicas foi o responsável pela amplitude nas aplicações e estudos de espectrometria de massas, sendo a ionização por electrospray a mais versátil dentre essas fontes de ionização. O caráter eletrolítico da fonte de ionização por electrospray permite-se obter íons provenientes de três processos químicos: i) ácido-base; ii) redox e iii) complexação. A extensão com que cada um desses processos ocorrerá dependerá de fatores relacionados à operação da fonte de ionização e grandezas termoquímicas do analito. O notável progresso em técnicas experimentais, processamento de dados e integração entre as mais diversas áreas de aplicação da química, tem estimulado e beneficiado a aplicação da química teórica em estudos de reações em fase gasosa. A aplicação da química computacional fornece uma compreensão quantitativa das variações estruturais e energéticas dos possíveis íons formados durante a ionização da amostra, permitindo também a compreensão das possíveis vias de dissociação. É neste sentido, que o sinergismo entre a aplicação de conceitos derivados da química quântica pode auxiliar nas análises de espectrometria de massas. O objetivo desta tese foi o de se aplicar os modelos fundamentados na mecânica quântica para obtenção de grandezas termoquímicas relacionadas aos fenômenos que ocorrem durante as análises de espectrometria de massas por electrospray. Inicialmente, a comparação entre métodos ab initio, modelos compostos e aqueles embasados na teoria do funcional de densidade foram empregados nos cálculos das grandezas termoquímicas para a -butirolactona e 2-pirrolidinona, com a finalidade de se obter parâmetros termoquímicos de alta qualidade. Os modelos compostos G2, G2(MP2), CBS-Q, CBS-QB3 e os métodos B3LYP, B3P86, B98, PW91PW91 e MP2 foram testados. Os valores obtidos para a entalpia de formação, afinidade protônica e basicidade em fase gasosa para essas duas moléculas foram comparados aos dados experimentais disponíveis na literatura. Os melhores resultados para os valores de entalpia de formação foram obtidos ao se empregar o modelo B3LYP/6-31+G(d,p). A afinidade protônica e basicidade em fase gasosa foram mais bem descritas por B3LYP e G2(MP2). Posteriormente, foram estudadas a 1,4-benzoquinona, 1,4-naftoquinona, bem como seus derivados (2-acetilamina-1,4-naftoquinona; 2-propionilamina-1,4-naftoquinona; 2-butirilamina-1,4-naftoquinona, 2-benzoilamina-1,4-naftoquinona, 2-succinilamina-1,4-naftoquinona e lapachol). A escolha do modelo teórico empregado foi realizada por se comparar as geometrias, afinidade protônica, basicidade em fase gasosa, energia de ionização e afinidade eletrônica obtidos para a 1,4-benzoquinona com aqueles disponíveis na literatura. O modelo B3LYP/6-31+G(d,p) foi o mais exato com relação as grandezas termoquímicas supracitadas, assim, este modelo foi empregado no cálculo dessas mesmas grandezas para a 1,4-naftoquinona e seus derivados. A influência do substituinte na estrutura eletrônica dessas moléculas protonadas, desprotonadas, oxidadas, reduzidas e cationizadas foi estudada utilizando-se análises energéticas, geométricas, eletrônicas e topológicas. O desenvolvimento desses estudos compreendeu as análises das grandezas termoquímicas e análises da densidade eletrônica pelos métodos NBO (Natural Bond Orbital), NSA (Natural Steric Analysis), NRT (Natural Resonance Theory) e AIM (Atoms in molecules). A ionização por electrospray e a dissociação induzida por colisão foram realizadas para os derivados da 1,4-naftoquinona, sendo analisados suas moléculas protonadas, desprotonadas, reduzidas, oxidadas e cationizadas com Na+. As vias mecanísticas de dissociação foram embasadas nas análises da superfície de energia obtidas pelo cálculo das energias de Gibbs e entalpias. / In recent decades, the development of atmospheric ionization techniques improved mass spectrometry, principally for characterization and structural elucidation of high-molecular weight compounds. The development of spray ionization was responsible for the spread of applications and studies of mass spectrometry, where the electrospray ionization is the most versatile among the ionization sources. The electrolytic character of electrospray source allows obtaining ions by three different chemical processes: i) acid-base; ii) redox and, iii) metal complexation. These processes will occur through several factors which can be related to the ionization source and thermochemical parameters of analyte. The notable progress of experimental analyses, computational data and the integration between several areas of chemical application have stimulated the use of theoretical chemistry at gas-phase studies. Computational chemistry can furnish a quantitative understanding of the structure and energy of possible ions during the ionization process. For this reason, the synergism between the concepts from quantum chemistry and gas-phase chemistry can help mass spectrometry analysis. The main purpose of this thesis was the application of several quantum mechanical models to obtain thermochemical parameters which can be related to mass spectrometry phenomena. Firstly, the comparison between ab initio, composite model and DFT methods were employed to obtain the thermochemical parameters to -butyrolactone e 2-pyrrolidinona, in order to obtain high performance of thermochemical parameters. The composite G2, G2(MP2), CBS-Q and, the B3LYP, B3P86, B98, PW91PW91 and MP2 methods were tested. The calculated values were compared to experimental values reported in the literature. The best results for enthalpies of formation were obtained when B3LYP/6-31+G(d,p) model was employed. The proton affinity and gas-phase basicity were better described by using of B3LYP and G2(MP2). Secondly, the studies with quinones were performed, where the 1,4-benzoquinone, 1,4-naphthoquinone and its derivatives (2-acylamino-1,4-naphthoquinone; 2-propyonilamino-1,4-naphthoquinone; 2-butyrilamino-1,4-naphthoquinone; 2-benzoylamino-1,4-naphthoquinone; 2-succynilamino-1,4-naphtoquinone and, lapachol) were studied. A search for a theoretical model was made to compare the geometries, proton affinity, gas-phase basicity, ionization energy and electron affinity to 1,4-benzoquinone with those reported in the literature. The most accurate results were obtained by using of B3LYP/6-31+G(d,p). Thus, this model was applied in all studies with 1,4-naphthoquinone derivatives. The influence of substituent groups on electronic structure of protonated, deprotonated, reduced, oxidized and cationized molecules were studied by energetic, geometrics, electronics and topological analyses. The development of these studies and the determination of the thermochemical parameters and wave function analysis was achieved by means of Natural Bond Orbitals, Natural Steric Analysis, Natural Resonance Theory and Atoms in Molecules The electrospray ionization and gas-phase collision-induced dissociation were made for the 1,4-naphtoquinone derivatives by analyzing their protonated, deprotonated species, the radicalar and sodiated ones. The main fragmentation pathways were elucidated on the basis of the energy surface by using Gibbs energies and enthalpies.
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A computational study on indium nitride ALD precursors and surface chemical mechanismRönnby, Karl January 2018 (has links)
Indium nitride has many applications as a semiconductor. High quality films of indium nitride can be grown using Chemical Vapour Deposition (CVD) and Atomic Layer Deposition (ALD), but the availability of precursors and knowledge of the underlaying chemical reactions is limited. In this study the gas phase decomposition of a new indium precursor, N,N-dimethyl-N',N''-diisopropylguanidinate, has been investigated by quantum chemical methods for use in both CVD and ALD of indium nitride. The computations showed significant decomposition at around 250°C, 3 mbar indicating that the precursor is unstable at ALD conditions. A computational study of the surface chemical mechanism of the adsorption of trimethylindium and ammonia on indium nitride was also performed as a method development for other precursor surface mechanism studies. The results show, in accordance with experimental data, that the low reactivity of ammonia is a limiting factor in thermal ALD growth of indium nitride with trimethylindium and ammonia.
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