19 research outputs found

    TREATING CONTAMINATED WATER: PREDICTING REACTIVITIES OF AQUEOUS ORGANIC CONTAMINANTS WITH HYDRATED ELECTRONS, AND DEVELOPMENT OF A BIOGARDEN TO MANAGE GREYWATER DISCHARGE IN MONTEVERDE, COSTA RICA

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    Advanced reduction processes (ARPs) that generate highly reactive solvated electrons (e-aq) are a promising method for the destruction of conventional and emerging aqueous organic contaminants. While there is a large database of contaminant reactivity with e-aq in the literature, there is little information on the detailed elementary mechanisms for reduction of multifunctional group compounds and the impact of those functional groups on reactivity. As it is difficult to determine specific mechanisms through experiments and time consuming to measure reactivity, the development of computational approaches to elucidate mechanisms and predict reactivity is becoming increasingly important. In chapter 2, I use density functional theory to calculate the aqueous-phase one electron reduction potential (E∘red,aq) of 251 diverse conventional organic compounds. I then use E∘red,aq to investigate the occurrence of three possible reduction mechanisms (association, concerted cleavage, stepwise cleavage) at all reactive sites and develop linear free energy relationships (LFERs) between E∘red,aq and the experimentally measured rate constant. Using the LFERs, I predict the reactivity of the Environmental Protection Agency per- and polyfluoroalkyl substance (PFAS) priority subset (EPA-75). In chapter 3, I develop group contribution methods (GCMes) for each reduction mechanism for 262 aliphatic and aromatic organic contaminants. The GCMes effectively predict compound reactivity through chemical structure fragmentation. The LFER and GCMe tools can be used to screen thousands of organic contaminants for degradability by homogeneous and heterogeneous ARPs, along with prioritizing them for further study. Although Costa Rica has established water discharge laws, a lack of enforcement and resources has resulted in greater than 95% of all greywater discharged to the environment untreated, leading to eutrophication and an increased risk to public health. The Monteverde Zone, located at the top of the Guacimal watershed, is a key player in this issue, as greywater discharged in this zone flows downstream, impacting a large number of communities. While centralized water treatment is currently not feasible in this developing country, biogardens are an affordable, decentralized, nature-based solution to Costa Rica’s greywater problem that utilize filtration techniques, plant uptake, and microbial degradation to effectively treat greywater. In chapter 4, I design and construct a horizontal sub-surface flow biogarden system for the Centro Educacion Creativa K-12 school, located in the Monteverde Zone, that effectively treats the campus’ greywater to compliance levels. In addition to engineering a natural and effective treatment system, I use the biogarden as a local pilot project to educate the Monteverde community on the greywater issue and biogarden development

    Vibrational mode-specific dynamics of the F- + CH3CH2Cl multi-channel reaction

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    We investigate the mode-specific dynamics of the ground-state, C-Cl stretching (v(10)), CH2 wagging (v(7)), sym-CH2 stretching (v(1)), and sym-CH3 stretching (v(3)) excited F- + CH3CH2Cl(v(k) = 0, 1) [k = 10, 7, 1, 3] -> Cl- + CH3CH2F (S(N)2), HF + CH3CHCl-, FHMIDLINE HORIZONTAL ELLIPSISCl- + C2H4, and Cl- + HF + C2H4 (E2) reactions using a full-dimensional high-level analytical global potential energy surface and the quasi-classical trajectory method. Excitation of the C-Cl stretching, CH2 stretching, and CH2/CH3 stretching modes enhances the S(N)2, proton abstraction, and FHMIDLINE HORIZONTAL ELLIPSISCl- and E2 channels, respectively. Anti-E2 dominates over syn-E2 (kinetic anti-E2 preference) and the thermodynamically-favored S(N)2 (wider reactive anti-E2 attack angle range). The direct (a) S(N)2, (b) proton abstraction, (c) FHMIDLINE HORIZONTAL ELLIPSISCl- + C2H4, (d) syn-E2, and (e) anti-E2 channels proceed with (a) back-side/backward, (b) isotropic/forward, (c) side-on/forward, (d) front-side/forward, and (e) back-side/forward attack/scattering, respectively. The HF products are vibrationally cold, especially for proton abstraction, and their rotational excitation increases for proton abstraction, anti-E2, and syn-E2, in order. Product internal-energy and mode-specific vibrational distributions show that CH3CH2F is internally hot with significant C-F stretching and CH2 wagging excitations, whereas C2H4 is colder. One-dimensional Gaussian binning technique is proved to solve the normal mode analysis failure caused by methyl internal rotation

    How solvation influences the S(N)2 versus E2 competition

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    We have quantum chemically investigated how solvation influences the competition between the S(N)2 and E2 pathways of the model F- + C2H5Cl reaction. The system is solvated in a stepwise manner by going from the gas phase, then via microsolvation of one to three explicit solvent molecules, then last to bulk solvation using relativistic density functional theory at (COSMO)-ZORA-OLYP/QZ4P. We explain how and why the mechanistic pathway of the system shifts from E2 in the gas phase to S(N)2 upon strong solvation of the Lewis base (i.e., nucleophile/protophile). The E2 pathway is preferred under weak solvation of the system by dichloromethane, whereas a switch in reactivity from E2 to S(N)2 is observed under strong solvation by water. Our activation strain and Kohn-Sham molecular orbital analyses reveal that solvation of the Lewis base has a significant impact on the strength of the Lewis base. We show how strong solvation furnishes a weaker Lewis base that is unable to overcome the high characteristic distortivity associated with the E2 pathway, and thus the S(N)2 pathway becomes viable.Bio-organic Synthesi

    Catalytic de-halogenation of halogen-containing solid wastes by nanostructures of transition metals and their oxide configurations

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    This thesis presents a series of scientific investigations examining the potential dual role for nanostructure of several 3d transition metals and their oxides in the catalytic de-halogenation of halogen laden materials as well as their mediating formation of toxic halogenated aromatics through accurate density functional theory (DFT) calculations. These investigations have been instrumental to (i) understand, on a precise atomic scale, mechanisms operating in fixation of halogens on transitional metal and their oxides, (ii) design of a large-scale catalytic upgrading unit that operates to extract the valuable metals loads from electric arc furnace dust (EAFD) and (iii) provide important fingerprints for environmental burdens associated with thermal recycling of e-waste and subsequent generation of notorious dioxins compounds and phenoxy-type environmental persistent free radicals (EPFRs). Herein, we have carefully benchmarked the accuracy of systematically obtained results versus experimental values pertinent to investigated systems, namely, lattice parameters, density of states and surface relaxations. The initial part of the dissertation focuses on the reaction mechanisms of major products from thermal decomposition of polyvinyl chloride (PVC) and brominated flame retardants (BFRs) with nanostructures (clusters and surfaces) of hematite (α-Fe2O3), zincite (ZnO) and magnetite (Fe3O4). The detailed kinetic analysis indicates that dissociative adsorption of hydrogen halides molecules, the major halogen fragments from thermal degradation of halogen laden materials, over those metal oxide structures affords oxyhalides structures via modest activation barriers. Transformation of oxyhalides into metal halides occurs through two subsequent steps, further dissociative adsorption of hydrogen halides over the same structures followed by the release of H2O molecule. In the course of the interaction of halogenated alkanes and alkenes with the selected metal oxide structures, the opening channel in the dissociative addition route requires lower activation barriers in reference to the direct HCl/Br elimination pathways. However, sizable activation barriers are encountered in the subsequent β C-H bond elimination step. The obtained accessible reaction barriers for reactions of halogenated alkanes and alkenes with the title metal oxides demonstrate that the latter serve as active catalysts in producing clean olefins streams from halogenated alkanes. Finally, we examined the dissociative adsorption of a phenolic molecules on Cu and Fe surfaces and their partially oxidized configurations to elucidate the specific underpinning mechanism of the title reaction and the kinetics feasibility to germane to generate phenolate-type EFPRs. Our simulated results show that dissociative adsorption of the phenol molecule is kinetically and thermodynamically preferred over the partially oxidized configuration in reference to neat surfaces. Computed charge transfer and density of state (DOS) indicates accumulation of spin density on the phenolic’s O following fission of the O-H bond. Obtained results shall be instrumental in efforts that aim to recycle the non-metallic fraction in e-waste

    Espectroscopia eletrônica de vácuo em soluções aquosas

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    Tese (doutorado)—Universidade de Brasília, Instituto de Física, 2014.Nesta tese, descrevemos as propriedades a nível molecular da superfície (PNMS) de diferentes soluções aquosas, estudadas por espectroscopia de fotoelétrons de raios X(XPS), usando a técnica de micro-jato. Estes são os primeiros resultados obtidos pelo grupo de Espectroscopia Eletrônica da Universidade de Brasília (LEE-UnB), nesta área, em colaboração com outros pesquisadores. No corpo da tese encontramos quatro diferentes estudos: 1) Deslocamentos químicos no espectro de XPS para avaliar as formas iônicas em soluções aquosas com diferentes pH. 2) Utilizando-se da sensibilidade superficial da espectroscopia de XPS, estudamos a distribuição de solutos, próximos a interface da solução. 3) Observamos a razão não estequiométrica de uma solução para uma ampla faixa de energia de fótons. 4) Estudamos o comportamento de um composto muito usado como tampão em química e biologia, em função da sua concentração.Estudamos, pela primeira vez, as PNMS dos dois únicos aminoácidos essenciais que contêm enxofre (Cisteína e Metionina). Obtivemos os deslocamentos químicos da energia de ligação e alargamento nas bordas do C1s, N1s e S2p na Cisteína(HSCH2CH(NH2)COOH)aq e Metionina (CH3SCH2CH2CH(NH2)COOH)aq aquosas em função da mudança do pH da solução. Mostramos que a Cisteína apresenta um padrão complexo de protonação mesmo se considerarmos apenas o corpo (bulk) da solução. A partir da análise do espectro de XPS da Cisteína aquosa observamos que o pH de uma solução aquosa contendo biomoléculas de interesse não varia entre o corpo da solução e a superfície, apesar de estudos anteriores em água pura apontarem para um pH mais ácido presente na superfície molecular do líquido. Apesar desta constatação isotrópica de pH,observamos uma mudança drástica na concentração de duas espécies protonadas entre obulk e a superfície no pH 9,5. Explicamos o fenômeno com base em um modelo simples.Comparamos as bordas do S2p e do N1s e encontramos que as formas neutras dos grupos(SH e NH2) são mais propensas a superfície da solução do que as formas carregadas (SH+e NH3+). Estes resultados são importantes e podem ter um grande impacto na compreensão das questões ambientais, como a química de superfície em aerossóis atmosféricos, química da membrana celular e aprisionamento de gases nocivos por soluções aquosas.Realizamos estudos das intensidades do sinal do S2p da solução aquosa de Tiossulfato de Sódio (Na2S2O3) em função da energia dos fótons. Observamos que o espectro obtido se assemelha a um espectro de EXAFS, com oscilações que diminuem de amplitude na medida em que se aumenta a energia dos fótons. Concluímos que as oscilações observadas ocorrem devido à interferência dos fotoelétrons ejetados que podem ou não colidir com os átomos vizinhos, o que produz uma modulação na seção de choque de fotoionização, a qual precisa ser considerada na análise quantitativa dos espectros deXPS.Analisamos o comportamento da borda P2p do íon dihidrogeno fosfato (H2PO4-) em função da concentração das soluções aquosas de NaH2PO4 e observamos que a intensidade dos picos cresce linearmente na faixa de concentrações estudada. _______________________________________________________________________________________ ABSTRACTThis thesis describes the properties at the molecular level of surfaces (PMLS) for different aqueous solutions. These properties are probed by X-ray Photoelectron Spectroscopy (XPS), using the micro-jet technique. These are the first results, for aqueous solutions, obtained by the group of Electron Spectroscopy at the University of Brasilia(LEE/UnB), in collaboration with other researches. We performed in this thesis four differentstudies. First, we explored the chemical shifts in the XPS spectra to probe the chemical states in aqueous solutions with different pHs. Second, we explored the surface sensitivity ofthe XPS spectroscopy to study the distribution of solutes near the solution surface. Third, we observed the nonstoichiometric intensities in a core photoelectron spectroscopy of a solution for a wide range of photon energies. Fourth, we studied a compound very used as buffer inchemistry and biology, to verify its behavior in function of solute concentration.We studied, for the first time, the PMLS of two essential amino acids containingsulfur: Cysteine and Methionine. The XPS chemical shifts and line broadening of the C1s,N1s and S2p levels were obtained for the aqueous Cysteine (HSCH2CH(NH2)COOH)aq andaqueous Methionine (CH3SCH2CH2CH(NH2)COOH)aq as a function of the solution pH.Cysteine has a complex pattern of protonation even if only the bulk solution is considered.From the analysis of the XPS spectrum of the aqueous Cysteine, we reached to theconclusion that the pH of an aqueous solution containing biomolecules does not vary frombulk and surface regions. This was surprising since previous studies suggested that purewater is more acid at molecular surface. Despite the fact that the pH is isotropic in thesolution, we observed a dramatic change in concentration of two protonated speciesbetween bulk and surface. We formulated a simple model to explain this change. By comparing the S2p and N1s levels we found that neutral fragment forms (SH and NH2) showenhanced surface propensities than the charged forms (S+ and NH3+). The results areimportant and can have a great impact on the understanding of environmental issues, suchas, surface chemistry in atmospheric aerosols, cell membrane chemistry and trapping ofharmful gases by aqueous solutions.We performed studies on the intensities of the S2p level of Sodium Thiosulfate(Na2S2O3) solution as a function of the photon energy. The spectrum obtained resemblesEXAFS oscillations with decreasing amplitude as the photon energy increases. The observedoscillations are due to the interference of the ejected photoelectrons scattered, or not, byneighboring atoms. This interference produces a modulation in the photoionization crosssection that should be considered in the quantitative analysis of the XPS spectra of a vastnumber of aqueous solutions.We studied the behavior of the P2p level of dihydrogenphosphate ion (H2PO43- ) as a function of the aqueous solutions concentration (NaH2PO4). We observed that the peakintensity increases linearly in the concentration range studied

    Applications of electronic structure theory to problems in strong-field chemistry, inorganic chemistry, and nanomaterial systems

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    This dissertation covers research performed on applications of electronic structure theory to various fields of chemistry and is divided into eight chapters. Chapters 2 through 4 describe a series of related works which explore applications of excited state electronic structure methods to problems in strong field chemistry. Chapters 5, 6, and 7 discuss the application of electronic structure theory methods to solving problems in inorganic chemistry. Finally, Chapter 8 looks at an application of electronic structure theory to nanomaterials. Chapter 2 covers the modeling of electron dynamics of butadiene interacting with a short, intense laser pulse in the absence of ionization This chapter lays down the ground work for the following two chapters by examining the effects of basis set size and number of excited states included in the TD-CI simulation on the amount of population transferred from the ground state into the excited states by the interaction with an short, intense, non-resonant laser pulse. This chapter focuses mostly on TD-CI simulations using excited state energies and transition dipole matrices found by wavefunction based methods: TD-HF, TD-CIS, and TD-CIS(D). Chapter 3 expands on the work established in Chapter 2 by examining the excited state populations of butadiene using excitation energies and transition dipoles calculated by time-dependent density functional theory. Several DFT functionals are tested including GGA, meta-GGA, hybrid and long-range corrected functionals. The degree to which excited state energies and transition dipoles contribute to the final populations of the excited states is also examined. Chapter 4 wraps up the series by including ionization using a heuristic ionization model. This chapter examines the strong-field ionization of a series of linear polyenes of increasing length: ethylene, butadiene, hexatriene, and octatetraene. Also tested is the ionization dependence on parameters of the ionization model, basis set size, and number of states included in the simulation. Chapters 5-7 discuss collaborative works with members of the inorganic division of chemistry at Wayne State University. Chapter 5 describes a study on a chiral pentadenate ligand synthesized by the Kodanko group and the geometrical preference for a single isomer out of five possible isomers. Electronic structure theory indicates that the favored geometry is due to the chiral ligand, which prefers to be in a single conformation in metal complexes due to steric interactions. Chapter 6 covers a paddlewheel dinculear Cu(II) complex synthesized by the Winter group. This complex has the shortest Cu--Cu separation reported to date and electronic structure theory is used to explore the cause of this small separation. A simple model is proposed where the metal separation is governed by twisting of the ligand due to interligand π orbital interactions. Chapter 7 describes work done in collaboration with the Verani group, exploring the redox properties of some five-coordinate Fe(III) complexes. Chapter 8 sets out to develop an inexpensive model that can be used to optimize guest systems inside single walled nanotubes. The model takes advantage of the highly polarizable nature of nanotubes. The model is calibrated using a simple hydrogen bonded system and comparisons are made to test the reliability of the model

    Ab initio Studies of Aluminium Halides

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    The catalytic fluorination of hydrocarbons facilitates the large-scale production of chlorofluorocarbons for a wide range of applications including aerosol propellants, refrigerants and solvents. Lewis acid catalysts, such as Swarts catalysts based on antimony pentafluoride, are commonly used. Recently, a sol-gel based synthesis method has been developed which yields very high surface area aluminium fluoride (HS-AlF3) that has a Lewis acidity comparable to that of the Swarts catalysts. This makes HS-AlF3 a promising candidate for use in several Lewis acid catalysed reactions. Despite the importance of the surface in the catalytic process little is known about the detailed atomic scale structure of AlF3 surfaces. Surface thermodynamics calculations, based on hybrid-exchange density functional theory, are employed to predict the composition and structure of AlF3 surfaces. The surfaces of AlF3 expose under coordinated Al ions that are potential Lewis acid sites. Under standard atmospheric conditions the AlF3 surfaces are shown to adsorb water above the under coordinated Al ions. Theoretical characterisation of the under coordinated Al ions shows that the most reactive type of site is not exposed on crystalline α-AlF3 samples, however, it is predicted to occur in small quantities on β crystallites. It is speculated that such sites occur in higher quantities on the high surface area materials. This result may explain the different reactivity of α-, β- and HS-AlF3. Our detailed understanding of AlF3 surfaces allows us to propose a reaction centre and mechanism for the dismutation of CCl2F2 on β-AlF3. Aluminium chloride is extensively used as a catalyst in Friedal-Crafts reactions. It is therefore, commonly assumed that pure crystalline AlCl3 is strongly Lewis acidic. Ab initio surface thermodynamics calculations are used to study the surfaces of crystalline AlCl3 and show that it is chemically inert

    Reactividad atmosférica de alcoholes, tioles y y fluoroalcoholes con cloro atómico

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    En la troposfera, los átomos de cloro pueden tener un papel importante en la oxidación de compuestos orgánicos volátiles (covs) y en la producción de ozono. Actualmente, las reacciones de átomos de cloro son consideradas de gran importancia en la capa limítrofe marina y en regiones costeras, done la concentración de átomos de CI puede llegar a ser importante. Los átomos de cloro son muy reactivos con una amplia variedad de compuestos orgánicos e inorgánicos, de manera que concentraciones relativamente pequeñas pueden competir con los principales oxidantes troposféricos, los radicales hidroxilos (oh) y nitrano (n03) y la molécula de ozono, en la oxidación troposférica de estos compuestos. Estos átomos de cloro en regiones costeras contaminadas pueden iniciar la oxidación de covs de manera anloga a como lo hace el radical oh, que es el principal oxidante troposférico de compuestos orgánicos. De ese modo, es muy importante la obtención de medidas precisas dse las constantes de velocidad de las reacciones con ci, para determinar el papel de estas reacciones en química atmosférica a escala global y regional. En este trabajo se ha realizado un estudio cinético en función de la presión y la temperatura, mediante la técnica absoluta de fotolisis laser pulsada y fluorescencia resonante (flp-fr), de las reacciones en fase gaseosa de cloro atómico con una serie de alcoholes, tioles y fluoroalcoholes de interés en química atmosférica. También se ha realizado un estudio cinético de las reacciones de cloro atómico con la serie de alcoholes mencionada mediante la técnica de fotolisis continua en una cámara de smog y detección mediante espectroscopia infrarroja con transformada de fourier (ftir). Asi mismo, se ha empleado la espectroscopia ftir para la determinación de los productos de esas reacciones. El estudio de productos de reacción también se ha realizado mediante reprivatización con 2,4 dinitrofenilhidrazina (2,4-dnph) y análisis por cromatografía liquida de alta resolución (hplc). Los diferentes estudios experimentales se han completado con un estudio teórico a nivel qcisd(t)/6 311g**//mp2(full)/6-311g** de los distintos caminos de reacción posibles para cada una de las reacciones estudiadas, con el fin de determinar los más favorecidos energéticamente. Gracias al conjunto de los distintos estudios cinéticos, teóricos y de productos de reacción llevados a cabo, ha sido posible proponer diversos mecanismos para las reacciones estudiadas. Finalmente, para los diferentes compuestos estudiados se han calculado los tiempos e permanencia troposféricos relativos a su reacción con cloro atómico y se han comparado con los correspondientes tiempos relativos a la reacción con otros oxidantes troposféricos
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