Despite previous extensive study of the widely-employed ruthenium-catalysed olefin metathesis reaction, the finer mechanistic details have not been elucidated. An area that is noticeably lacking is spectroscopic exploration of the relevant complexes. In this work, organometallic ruthenium complexes of importance to olefin metathesis have been investigated using Ru K-edge X-ray absorption spectroscopy. The lowest energy feature in the Ru K-edge spectrum has been unambiguously assigned as due to Ru 4d←1s transitions. These electric-dipole-forbidden transitions are extremely sensitive to geometry. For centrosymmetric complexes, the pre-edge feature has very low intensity because it is limited by the weak electric quadrupole mechanism. By contrast, non-centrosymmetric complexes exhibit a substantial increase in pre-edge intensity because Ru 5p-4d mixing introduces electric-dipole-allowed character to the Ru 4d←1s transitions. The energy of the edge feature in the Ru K-edge spectrum corresponds to ionisation of 1s electrons and is a good indicator of the charge on the metal centre. Unexpectedly, we found that the first-generation (L = PCy₃) Grubbs precatalyst (1) has a higher 1s ionisation energy than the second-generation (L = H₂IMes) complex (2). This effect provides a compelling rationale for the unexplained differences in phosphine dissociation kinetics for complexes 1 and 2: the phosphine dissociation rate of 2 is slower than 1 because the metal centre is more electron-deficient in 2. Density functional theory calculations confirm the charge differences and offer some insight into the nature of bonding in these complexes, particularly with regard to the N-heterocyclic carbene and trialkylphosphine ligands. On the basis of these results, we propose that, for this system, the NHC ligand is a weaker σ-charge donor than the phosphine ligand, and that the NHC accepts significant π-electron density from the metal; both interactions function to reduce the electron density on the ruthenium centre. An ultimate goal is to investigate reactive species in the olefin metathesis mechanism; accordingly, we have made considerable progress toward collecting XAS data for a metallacyclobutane species, and we are pursuing methods to trap the four-coordinate intermediate in the metathesis cycle.

Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (p. [83]-91).

Thesis (Ph.D.) OGI School of Science & Engineering at OHSU, October 2007. / Includes bibliographical references (leaves 132-158).

Despite previous extensive study of the widely-employed ruthenium-catalysed olefin metathesis reaction, the finer mechanistic details have not been elucidated. An area that is noticeably lacking is spectroscopic exploration of the relevant complexes. In this work, organometallic ruthenium complexes of importance to olefin metathesis have been investigated using Ru K-edge X-ray absorption spectroscopy. The lowest energy feature in the Ru K-edge spectrum has been unambiguously assigned as due to Ru 4d←1s transitions. These electric-dipole-forbidden transitions are extremely sensitive to geometry. For centrosymmetric complexes, the pre-edge feature has very low intensity because it is limited by the weak electric quadrupole mechanism. By contrast, non-centrosymmetric complexes exhibit a substantial increase in pre-edge intensity because Ru 5p-4d mixing introduces electric-dipole-allowed character to the Ru 4d←1s transitions. The energy of the edge feature in the Ru K-edge spectrum corresponds to ionisation of 1s electrons and is a good indicator of the charge on the metal centre. Unexpectedly, we found that the first-generation (L = PCy₃) Grubbs precatalyst (1) has a higher 1s ionisation energy than the second-generation (L = H₂IMes) complex (2). This effect provides a compelling rationale for the unexplained differences in phosphine dissociation kinetics for complexes 1 and 2: the phosphine dissociation rate of 2 is slower than 1 because the metal centre is more electron-deficient in 2. Density functional theory calculations confirm the charge differences and offer some insight into the nature of bonding in these complexes, particularly with regard to the N-heterocyclic carbene and trialkylphosphine ligands. On the basis of these results, we propose that, for this system, the NHC ligand is a weaker σ-charge donor than the phosphine ligand, and that the NHC accepts significant π-electron density from the metal; both interactions function to reduce the electron density on the ruthenium centre. An ultimate goal is to investigate reactive species in the olefin metathesis mechanism; accordingly, we have made considerable progress toward collecting XAS data for a metallacyclobutane species, and we are pursuing methods to trap the four-coordinate intermediate in the metathesis cycle. / Science, Faculty of / Chemistry, Department of / Graduate

The alkylation of pyrimidyl aldehyde by diisopropylzinc has received immense attention over the last decade. This is mainly because the reaction which was discovered by Soai and coworkers is capable of achieving a homochiral product from an essentially achiral precursor. The strong amplification of the enantiomeric excess occurs because of a transition state complex which is responsible for autocatalysis. Clarifying the structural nature of the organometallic species involved in the reaction is vital for understanding the mechanism of the chiral amplification process. Known mechanistic details are patchy and based on studies that address molecular level details by NMR, computational chemistry, calorimetric and kinetic studies. The studies reported in this thesis for the first time directly addressed the nature of the intermediate by structural analysis with X-ray Absorption Fine Structure (XAFS) Spectroscopy at the Zn K-edge. These measurements provide bond distances, local coordination numbers and the geometry of ligands in the local environment around the Zn centres. First, the molecular level origin of the solvent dependencies in the asymmetric amplification by the Soai process is elucidated. A rationale for the behaviour of dialkylzinc compounds in polar and non-polar solvents is reported. Structural causes for often observed chirality depletion in polar solvents are elucidated. Further studies then examined the nature of the products formed by the reaction of various chiral ligands and dialkylzinc compounds. Different chiral ligands are examined and the complex structure was determined.Finally, first results of in situ studies of the reaction progress in continuous flow channel cells are reported.

A new spectrometric technique for measuring diffusion coefficients of Ordovician Queenston Formation shale from the Michigan Basin of southwest Ontario, Canada is presented; in this case pore diffusion coefficients (Dp) were determined for the conservative (iodide, I^-) and reactive (cesium, Cs^+) tracers in porous media. Furthermore, diffusion-reaction parameters such as Cation exchange capacity (CEC) were obtained for the reactive tracer, Cs^+ by the reactive-transport modeling. The principle of X-ray absorption spectrometry (XAS) is based on the attenuation of high atomic number of diffusive species (e.g. Cs^+and I^-) in porous media, allowing for the quantification of the resulting spatial changes. The technique employs a 1.1 mm collimated X-ray beam to resolve the tracer presence in a slice; small region of a rotating sample for a fixed time. The x-ray beam is incident on a sample of porous rock while the transmitted beam is detected on the opposite side of the sample, allowing acquisition of a transmitted X-ray energy spectrum vs intensity (counts). Additional analyses such as mineralogical studies by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) were conducted to improve the understanding of solute transport. The comparison of the data obtained by the XAS method indicating a good agreement with established radiography and other conventional diffusion methods provide an alternate approach for quantifying the diffusion coefficient of porous media.

<p> This thesis examines available x-ray absorption spectroscopy (XAS) data for NiO, NaNi02 , and LiNi02 . The XAS examined is the Ni L-edge, 3d^n2p^6 →t 3d^(n+1)2p^5 . The experimental spectra are compared to spectra calculated using a configuration interaction approach. This approach reproduces the spectra accurately. The NaNi02 spectrum is shown to be sensitive to the Jahn-Teller distortion, while the LiNi02 spectrum is reproduced by a hybridized d^7-d^8 state that explains the lack of Jahn-Teller distortion in LiNi02 </p> / Thesis / Master of Science (MSc)

El objetivo de esta tesis es estudiar la agrupación y el crecimiento de especies metálicas confinadas o soportadas en materiales porosos mediante espectroscopia de absorción de rayos X in situ. Para lograrlo, las especies de paladio y plata se han introducido en materiales porosos (¿-alúmina, carbón activo y zeolitas) mediante impregnación vía húmeda y métodos de intercambio iónico, respectivamente. Luego, el agrupamiento de estas especies metálicas se ha controlado mediante tratamientos de activación en diferentes atmósferas (inerte, oxidativa y reductiva) y seguido por XAS de manera detallada. El objetivo principal del trabajo actual es demostrar que tanto XANES como EXAFS pueden proporcionar información valiosa y, en cierto punto, innovadora durante el control de especies metálicas (en términos de tipo y tamaño de las especies). Aprovechando los procedimientos de análisis inusuales, como el análisis de los cumulantes, el ajuste de la parte imaginaria de la transformada de Fourier y otros, es posible obtener información refinada sobre los sistemas investigados. En la sección de introducción, se proporciona una compilación de estudios en los que se ha utilizado XAS como técnica importante para caracterizar especies metálicas en materiales porosos. Conscientes de que las personas pueden usar dicha introducción como base para estudios más complejos en el futuro, la discusión se ha dirigido tentativamente hacia este objetivo. El capítulo 4 se centra en el estudio de la influencia de los precursores de paladio y la naturaleza del soporte en las nanopartículas resultantes. El proceso de activación completo, es decir, la transformación precursor --> nanopartícula, ha sido seguido por XAS in situ. El análisis estuvo compuesto por el punto de partida (material impregnado), calcinación en flujo de O2 y reducción posterior con H2. La consecuencia del uso de diferentes precursores metálicos y soportes se ha discutido en términos del número de coordinación promedio obtenido a partir del análisis de datos de EXAFS, que fue respaldado por técnicas de caracterización de laboratorio. El capítulo 5 está dedicado al estudio de la agrupación de plata durante y después de los tratamientos de activación utilizando zeolitas de poro pequeño intercambiadas con plata como precursores y nanocontenedores. Se ha estudiado la influencia de la estructura y la composición química de los materiales basados en plata sobre las especies metálicas formadas en diferentes condiciones de agrupamiento y redispersión del metal (calcinación usando atmósferas distintas, reducción en H2, redispersión en O2) utilizando métodos de caracterización in situ o ex situ. Después, se discuten las consecuencias catalíticas de las zeolitas que contienen Ag en la reacción de SCO-NH3. En esta sección, la combinación de XAS in situ con varias técnicas de laboratorio ha demostrado ser fundamental para un completo entendimiento del trabajo. Finalmente, una lista de proyectos desarrollados en paralelo a esta tesis se proporciona al final de este documento. / The aim of this thesis is to study the clustering and growth of metallic species either confined or supported in porous materials by in situ X-ray absorption spectroscopy. To accomplish this task, palladium and silver species were introduced into porous materials (¿-alumina, activated carbon and zeolites) by wetness impregnation and ion-exchange methods, respectively. Then, the clustering of these metallic species was controlled by activation treatments in different atmospheres (inert, oxidative and reductive) and followed by XAS in a comprehensive way. The principal goal of current work is to demonstrate that both XANES and EXAFS can provide valuable and, at certain point, innovative information during tuning of metallic species (in terms of type and size). Taking advantage of unusual analysis procedures, such as cumulant approach, fitting of imaginary part of Fourier transform and others, it is possible to obtain refined information about the investigated systems. In the introduction section, a compilation of studies in which XAS was used as important technique to characterize metallic species in porous materials is provided. Conscious that people can use such introduction as a basis for more complex studies in the future, the discussion has been tentatively directed toward this goal. The chapter 4 is focused on the study of the influence of palladium precursors and the nature of support on the resultant nanoparticles. The whole activation process, i.e. the transformation precursor --> nanoparticle, was followed in situ by XAS. The analysis pathway was composed by the starting point (as-impregnated), calcination in O2 flow and posterior reduction with H2. The consequence of using distinct metal precursors and supports were discussed in terms of average coordination number obtained from EXAFS data analysis, which was co-supported by laboratory characterization techniques. The chapter 5 is dedicated to the study of silver clustering during and after activation treatments using Ag-containing small-pore zeolites as precursors and nanocontainers. The influence of framework structure and chemical composition of Ag-based materials on formed Ag species at different clustering and metal redispersion conditions (calcination using distinct atmospheres, reduction in H2, redispersion in O2) were studied using either in situ or ex situ characterization methods. After, the catalytic consequences of tuned Ag-containing zeolites in SCO-NH3 are discussed. In this section, the combination of in situ XAS with several laboratory techniques proved to be pivotal to have a full picture of the investigated system. Finally, a list of projects developed in parallel to this thesis is provided at the end of this document. / L'objectiu d'aquesta tesi és estudiar l'agrupació i el creixement d'espècies metàl·liques confinades o suportades en materials porosos mitjançant espectroscòpia d'absorció de raigs X in situ. Per a això, les espècies de pal·ladi i plata s'han introduït en materials porosos (¿-alúmina, carbó activat i zeolites) per mitjà de la impregnació via humida i mètodes d'intercanvi iònic, respectivament. Una vegada preparats els materials, l'agrupament de les espècies metàl·liques s'ha controlat fent ús de tractaments d'activació en diferents atmosferes (inert, oxidant i reductora) s'ha estudiat exhaustivament per XAS. L'objectiu principal del treball és demostrar que tant el XANES com l'EXAFS proporcionen informació rellevant i, en certa manera, innovadora per al control d'espècies metàl·liques (en termes de tipus i grandària d'aquestes espècies). Fent ús de procediments de tractament de dades no molt habituals com l'anàlisi de cumulants, l'ajust de la part imaginària de la transformada de Fourier i altres, és possible obtenir informació detallada sobre els sistemes estudiats. En l'apartat de la introducció, es proporciona una recopilació d'estudis en els quals s'ha utilitzat XAS com a tècnica principal per a caracteritzar les anomenades espècies metàl·liques en materials porosos. Aquesta introducció ha estat redactada per a que puga servir com a punt de partida per a futurs estudis que requereixen la utilització de XAS per a la caracterització de les espècies metàl·liques presents en els catalitzadors. El capítol 4 es centra en l'estudi de la influència dels precursors de pal·ladi i la naturalesa del suport front a les nanopartícules resultants. El procés d'activació, és a dir, la transformació precursor --> nanopartícula, ha sigut estudiat per XAS in situ. L'anàlisi per XAS va comprendre els següents passos: punt de partida (material impregnat), calcinació en flux d'O2 i reducció posterior amb H2. La utilització de diferents precursors i suports metàl·lics ha permès dur a terme una discussió, referent al nombre de coordinació mitjà obtingut a partir de l'anàlisi de dades de la zona EXAFS, que ha estat recolzat per altres tècniques de caracterització. El capítol 5 s'ha dedicat a l'estudi de l'agrupació de plata intercanviada en els catalitzadors durant i després dels tractaments d'activació. S'han utilitzat zeolites de porus xicotet, com la CHA i RHO, intercanviades amb plata. L'estudi de la influència de l'estructura zeolítica i la composició química dels materials enfront dels diferents tractaments d'activació (calcinació utilitzant diferents atmosferes, reducció en presència d'H2, re-dispersió en atmosfera d'O2) es va realitzar fent ús de mètodes de caracterització in situ o ex situ. A continuació, es discuteix la influència d'aquestes espècies metàl·liques formades, utilitzant els diferents mètodes d'activació, per a la reacció d'SCO-NH3. En aquest sentit, s'ha demostrat que la combinació de XAS in situ amb diverses tècniques habituals de laboratori és fonamental per al desenvolupament d'aquest treball. Finalment, es presenta una llista de projectes, en els quals també s'ha treballat paral·lelament, on s'ha utilitzat XAS com a tècnica de caracterització. / Wittee Lopes, C. (2018). Characterization of metallic species on porous materials by in situ XAS [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/107953

Given the wide range of properties and applications of intermetallic compounds, it is important to achieve a detailed understanding of their structure and bonding. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) were used to study the electronic structure of several types of pnictides (compounds containing Pn = P, As). ZrAs2, forming a PbCl2-type structure, has been established to be a genuine binary phase that is strictly stoichiometric. At 900 °C, it supports extensive solubility of Ge to form the ternary extension Zr(GexAs1−x)As (0 ≤ x ≤ 0.4). XPS analysis and band structure calculations confirmed that the Ge and As atoms are anionic in character and that the substitution of Ge for As is driven by a depopulation of anion–anion antibonding states. ZrCuSiPn and REMAsO are important representatives of ZrCuSiAs-type materials. The small magnitudes of the binding energy shifts in the XPS spectra of ZrCuSiPn suggest significant covalent character in the Zr–Si, Zr–Pn, and Cu–Pn bonds, consistent with a three-dimensional structure. On progressing from ZrCuSiP to ZrCuSiAs, the charge transfer from metal to Pn atoms becomes less pronounced, as indicated by changes in the intensity of the Cu K-edge and Zr K, L-edge XANES spectra. Binding energy shifts and satellite features of the XPS spectra of REMAsO indicated that bonding in the [REO] layer is ionic, whereas bonding in the [MAs] layer is strongly covalent. Altering the electronic structure of one layer (by M or RE substitution) does not affect the electronic structure of the other layer, consistent with a two-dimensional structure in REMAsO. Metal-rich phosphides M2P (forming Cr2P-, Fe2P-, and Co2P-type structures) and M3P (forming Ni3P-type structures) were examined by XPS and XANES. The P 2p3/2 binding and P K-edge absorption energies decrease with greater ionic character of the M−P bonding and indicate the presence of anionic phosphorus. Interatomic effects play a more important role in affecting the energy shifts in these metal-rich phosphides than in the monophosphides, becoming more pronounced with higher metal concentration. Surprisingly, intraatomic effects dominate in mixed-metal phosphides (Ni1-xMx)2P despite evidence of metal-to-metal charge transfer from the Ni XANES spectra and Ni 2p XPS satellite features.

Summary Serpentinites from the northern Vermont were examined for the distribution and abundance of As. XRD and electron microprobe showed the samples are composed of antigorite, chromite, magnetite, and carbonate minerals (magnesite, dolomite, calcite). The concentration in As when the samples were dissolved in H3PO4 was 10% of the concentration in As when the samples were dissolved in concentrated HF/HNO3, suggesting that As is mainly incorporated in the structure of antigorite. X-ray absorption near-edge structure spectra showed that the As is As(III) in the samples. Extended X-ray absorption fine structure spectra suggested that the As has a tetrahedral coordination and is located in the Si-site in serpentine.