Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009. / Title from title screen (site viewed June 26, 2009). PDF text: xxvii, 209 p. : ill. (some col.) ; 17 Mb. UMI publication number: AAT 3350440. Includes bibliographical references. Also available in microfilm and microfiche formats.

This thesis focuses on the theoretical investigation of the electronic structure and magnetic interactions present in 3d and 4d/5d transition metal compounds. We use many-body quantum chemistry methods that provide a theoretical frame for the rigorous construction and systematic improvement of correlated N-electron wave-functions. In Chapter 3 we compute d-d transitions fully ab initio and assign excitation peaks of experimental spectra measured in spin-Peierls TiPO4 compound. In this material we find that the d1 ground state is composed of an admixture of dz2 and dxz orbital character, which is related to the large positive ionic charge at P sites in the xz plane (defining the shortest Ti-P links) and of Ti nearest-neighbors along the z axis. In addition, the magnitude of the nearest-neighbors Heisenberg magnetic coupling calculated by quantum chemistry methods compares well with resonant inelastic X-ray scattering (RIXS) experimental data. We further demonstrate that the intersite exchange is very sensitive to the Ti-Ti interatomic distance, which is relevant in the context of spin-Peierls physics in TiPO4. In Chapter 4 we have studied the magnetic anisotropy of Fe ions within the Li3N lattice. The calculated magnetic anisotropy splitting of 26.3 meV for Fe2+ d6 ions in D6h symmetry compares favorably to values measured or computed by similar theoretical methods for Fe1+ d7 species with linear coordination. This substantial spin-reversal energy barrier of the Fe2+ ion is associated with a a^1_{1g}e^3_{2g}e^2_{1g} ground-state electron configuration. Our study therefore puts into the spotlight the linearly coordinated Fe2+ d6 ion as candidate for viable single molecule magnet behavior. In Chapter 5 we address the effect of electron-lattice interactions on the magnetic properties of 4d and 5d TM ions with a formally degenerate t^1_{2g} electron configuration in the double-perovskite materials Ba2YMoO6, Ba2LiOsO6 and Ba2NaOsO6. Our analysis indicates that the sizable magnetic moments and g-factors found experimentally are due to both strong TM d -- ligand p hybridization and dynamic Jahn-Teller effects. Our results also point out that cation charge imbalance in the double-perovskite structure allows a fine tuning of the gap between the t2g and eg levels. The mechanism has not been explored so far experimentally but seems to hold much potential in the context of orbital engineering in transition metal compounds. In Chapter 6 we report a study of magnetic exchange interactions in the S=3/2 orthorhombic perovskite NaOsO3. We mapped the ab initio quantum chemistry results onto model Hamiltonians including both isotropic Heisenberg interactions and anisotropic Dzyaloshinskii-Moriya exchange. We found antiferromagnetic nearest-neighbors Heisenberg exchange interactions of J_ac = 24.4 meV and J_b = 20.9 meV, twice larger than the J extracted from the magnon excitation spectra. The quantum chemistry results motivate further experimental measurements or theoretical analysis to clarify the magnitude of the nearest-neighbors Heisenberg couplings. In Chapter 7 we provide valuable insights on the effective magnetic interactions in 5d and 4d oxides with face-sharing oxygen octahedra, BaIrO3 and BaRhO3, for different bond-angles and bond-lengths. The large antiferromagnetic Heisenberg interactions computed here emphasize the subtle interplay among strong spin-orbit interactions, direct intersite orbital overlap and orbital bonding, and couplings to the lattice degrees of freedom in face-sharing compounds. In Chapter 8 we apply a computational scheme for computing intensities as measured in X-ray absorption and RIXS experiments. We take into account the readjustment of the charge distribution in the vicinity of an excited electron for the modeling of RIXS. For L3-edge spectra of Cu2+ 3d9 ions in KCuF3, we discuss the way to consider orbital ordering effects (alternately occupied d_x2-z2 and d_y2-z2 orbitals). For L3-edge spectra of Ni2+ 3d8 ions in La2NiO4, the computed spectra reproduce trends found experimentally for the incoming-photon incident-angle and polarization dependence.

The main focus of the present work was the Ge-rich part of the binary Ba – Ge system, in which by inspecting the behavior of the clathrate-I Ba8Ge43 under pressure, several new phases were found. The new phases in this system have the following compositions: BaGe3 (with two modifications), BaGe5, BaGe5.5 and BaGe6, therefore they are quite close in composition range: 75% - ~85% at. Ge. Concerning the conditions required for the synthesis of each phase, several combinations of temperature and pressure were employed in order to find a stability range. It was possible to establish such a formation range for all phases. In some cases two phases were found for a given conditions and in many other cases three or more phases were found to coexist. Thus, the stability range of pressure and temperature for single phase formation turned out to be very narrow. By inspecting of some structural features, for instance the interatomic distances, it is found that the average of the Ge – Ge distances change in line with the composition, i.e. the shorter contacts belong to BaGe6 while the longer distances are present in BaGe3 (both modification). An opposite trend is observed for the calculated density of each phase (neglecting the tI32 form of BaGe3): the lower density is found for BaGe3 and the denser compound is found to be BaGe6. Of course this is not coincidence, since due to the Ge content, BaGe6 has the largest molar mass. Similarly, by examining the density as a function of the interatomic distance. In such case, the denser compound is characterized by shorter Ge – Ge contacts, while the less dense phase holds the longest Ge – Ge contacts. This is in agreement with the building motifs within each crystal structure: columns in BaGe3 (open framework) passing through layers in BaGe5, ending in a three-dimensional network (closed framework) in BaGe6.

Luminescent lanthanide containing coordination polymers and metal-organic frameworks hold great potential in many applications due to their distinctive spectroscopic properties. While the ability to design coordination polymers for specific functions is often mentioned as a major benefit bestowed upon these compounds, the lack of a meaningful understanding of the crystal engineering and luminescence in lanthanide coordination polymers remains a significant challenge toward functional design. Currently, the study of luminescence attributed to these compounds is based on the antenna effect as derived from molecular systems, where organic antennae are used to facilitate lanthanide-centered luminescence. This molecular based approach does not take into account the unique features of extended network solids, particularly the formation of band structure. By comparing molecular and band-based approaches, it was determined that the band structure of the organic sensitizing linker needs to be considered when evaluating the luminescence of lanthanide coordination polymers. This new model, as well as work on the crystal engineering and sensor applications of these materials will be presented. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

[Truncated abstract] This thesis describes various synthetic approaches toward the synthesis of highly conjugated complexes incorporating multiple transition metal centres. Particular attention is given to the synthesis of mononuclear complexes that allow for the facile assembly of discrete oligo- and poly-nuclear complexes in a controlled, stepwise fashion. Conjugated multi-metallic materials are of interest on account of their unique photophysical and electronic properties, with a particular emphasis on elucidating the nature of intramolecular communication between multiple metal centres. Chapter 1 provides a survey of these topics and current research efforts in the field. Chapter 2 describes the synthesis of Group-VIII allenylidene complexes incorporating a terminal bipyridyl moiety that provides a site for further coordination. The new compound 9-hydroxy-9-ethynyl-4,5-diazafluorene was synthesised, and reaction of this proligand with a coordinatively unsaturated metal fragment yields the allenylidene complexes [MCl(PnP)2=C=C=(4,5-diazafluoren-9-yl)]PF6 (M = Ru, PnP = dppm, dppe, dmpe; M = Os, PnP = dppm) and [CpRu(dppe)=C=C=(4,5-diazafluoren- 9-yl)]PF6. The dmpe-ligated complex is particularly susceptible to decomposition, though it was possible to obtain partial spectroscopic characterisation in addition to a single-crystal X-ray structural determination. The remaining allenylidene complexes are stable compounds readily characterised by standard spectroscopic and electrochemical means, with the bis(diphosphine) complexes characterised by single crystal X-ray structural determinations. ... Reactions of the proligand with [RuCl(PnP)2]+ (PnP = dppm, dppe) led to the isolation of a product spectroscopically consistent with the formation of the target cationic allenylidene complexes, though the complexes were not readily purified and the identity of the accompanying anion was not elucidated. The new compound 4-hydroxy-4- ethynyl-cyclopentadithiophene was also prepared, though the compound was found to be highly unstable and susceptible to rapid decomposition. The derived allenylidene complexes [RuCl(PnP)2=C=C=(4-cyclopentadithiophene)]PF6 (PnP = dppm, dppe) were isolated in a pure form and the complexes stable toward spontaneous decomposition. The thienyl-derived allenylidene complexes were characterised by spectroscopic and electrochemical techniques, with a single-crystal X-ray structural determination undertaken for [RuCl(dppm)2=C=C=(4-cyclopentaditiophene)]PF6. Electrochemical properties are significantly different between the complexes, and also show significant variation between electrodes and solvents. The terminal thienyl substituents are electroactive and show one or two oxidation processes consistent with oligomerisation of the thienyl moiety in dichloromethane solvent, and in acetonitrile solvent cyclic voltammograms are consistent with the deposition of an electroactive film on the electrode surface. The electro-polymerisation of the thienylallenylidene complexes offers a promising new route toward multi-metallic allenylidene complexes.

The main focus of the present work was the Ge-rich part of the binary Ba – Ge system, in which by inspecting the behavior of the clathrate-I Ba8Ge43 under pressure, several new phases were found. The new phases in this system have the following compositions: BaGe3 (with two modifications), BaGe5, BaGe5.5 and BaGe6, therefore they are quite close in composition range: 75% - ~85% at. Ge. Concerning the conditions required for the synthesis of each phase, several combinations of temperature and pressure were employed in order to find a stability range. It was possible to establish such a formation range for all phases. In some cases two phases were found for a given conditions and in many other cases three or more phases were found to coexist. Thus, the stability range of pressure and temperature for single phase formation turned out to be very narrow. By inspecting of some structural features, for instance the interatomic distances, it is found that the average of the Ge – Ge distances change in line with the composition, i.e. the shorter contacts belong to BaGe6 while the longer distances are present in BaGe3 (both modification). An opposite trend is observed for the calculated density of each phase (neglecting the tI32 form of BaGe3): the lower density is found for BaGe3 and the denser compound is found to be BaGe6. Of course this is not coincidence, since due to the Ge content, BaGe6 has the largest molar mass. Similarly, by examining the density as a function of the interatomic distance. In such case, the denser compound is characterized by shorter Ge – Ge contacts, while the less dense phase holds the longest Ge – Ge contacts. This is in agreement with the building motifs within each crystal structure: columns in BaGe3 (open framework) passing through layers in BaGe5, ending in a three-dimensional network (closed framework) in BaGe6.

Freie Leitungselektronen sind bisher in oxidischen Perowskiten mit Elektronenspinresonanz (ESR) selten beobachtet worden. Meist sind solche Elektronen, insbesondere bei tiefen Temperaturen, an Gitterstörungen gebunden. In akzeptorarmem BaTiO3 ist es gelungen, mit ESR Elektronen zu identifizieren, die auch bei tiefen Temperaturen nicht an Defekte gekoppelt sind. Sie entstehen beim Einbau von Nb5+Ti in das Gitter und lokalisieren sich auf Titan-Platz als Ti3+ Jahn-Teller-Polaronen selbst. Die beiden Gebiete Jahn-Teller-Effekt und Polaronen, die sich bisher weitgehend getrennt voneinander entwickelt haben, werden hier in einer experimentellen Untersuchung vereinigt. Beim Jahn-Teller-Effekt wird eine elektronische Bahn-Entartung durch eine spontane Erniedrigung der Punktsymmetrie aufgehoben. Die Polaronenbildung erfolgt unter Bruch der Translationssymmetrie. Dabei wird die Energie des Systems erniedrigt, indem ein Zustand unter die quasi-entarteten Bandzustande abgesenkt wird. Beim System Ti3+ treten beide Effekte gleichzeitig auf: Die nach Lokalisierung eines freien Elektrons an Titan aus dem Leitungsband auftretende Bahn-Entartung des resultierenden T2-Grundzustandes wird durch einen Jahn-Teller-Effekt aufgehoben. Es bildet sich ein sogenanntes Jahn-Teller-Polaron. Als Vorbereitung der experimentellen Untersuchungen an Jahn-Teller- Polaronen dienen ESR-Messungen an verschiedenen reinen Jahn-Teller-Systemen in BaTiO3 (Mo5+, Ni+, Rh2+). Im Gegensatz zu einem Polaron ist das durch eine Störstelle im Kristall hervorgerufene Potential schon vorhanden, bevor dort ein Ladungsträger eingefangen wird. Beim Polaron bewirkt die Anwesenheit des Ladungsträgers selbst die Bildung einer Potentialmulde. Man spricht daher von Selbsteingrabung. Die untersuchten Jahn-Teller-Zentren reagieren auf äußeren uniaxialen Druck mit einer Reorientierung: Die spontan erfolgten Jahn-Teller-Verzerrungen werden entlang der Druckachse ausgerichtet. Auch beim Jahn-Teller-Polaron ist diese Reorientierung zu beobachten. Die Jahn-Teller-Kopplung ist jedoch schwächer als die des isoelektronischen Zentrums Mo5+ (4d1 ). Aufgrund der Äquivalenz aller Titan-Plätze im Kristall ist das ungepaarte Elektron an Ti3+ stärker auf seine Nachbarn delokalisiert als im Fall des Mo5+. Es wird gezeigt, daß eine delokalisierte Wellenfunktion schwächer an das Gitter ankoppelt als eine lokalisierte und daher eine schwächere Jahn-Teller-Kopplung aufweist. Es treten kleine und intermediäre Jahn-Teller-Polaronen gleichzeitig auf. BaTiO3 besitzt die Tendenz, breite Polaronenbänder zu bilden. Polaronen in diesem Material zeigen daher die Tendenz zur Delokalisierung, d. h. sie sind über mehr als einen Gitterplatz ausgedehnt und werden als intermediär bezeichnet. Eine Lokalisierung auf einen Gitterplatz, also ein kleines Polaron, bildet sich nur bei Anwesenheit von lokalen Potentialfluktuationen aus (Anderson-Lokalisierung). Unter uniaxialem Druck erfolgt eine Verringerung dieser Fluktuationen durch die Reorientierung, und ein Übergang vom kleinen zum intermediären Polaron wird beobachtet.

A new approach to the discovery of high absorbing semiconductors for solar cells was taken by working under a set of design principles and taking a systemic methodology. Three transition metal chalcogenides at varying states of development were evaluated within this framework. Iron pyrite (FeS���) is well known to demonstrate excellent absorption, but the coexistence with metallic iron sulfides was found to disrupt its semiconducting properties. Manganese diselenide (MnSe���), a material heavily researched for its magnetic properties, is proposed as a high absorbing alternative to iron pyrite that lacks destructive impurity phases. For the first time, a MnSe��� thin film was synthesized and the optical properties were characterized. Finally, CuTaS���, a known but never characterized material, is also proposed as a high absorbing semiconductor based on the design principles and experimental results. / Graduation date: 2013

Metallohydrolases with dinuclear-zinc active sites perform many important biological hydrolytic reactions on a variety of substrates. In this regard, metallo-β-lactamases (mβ1, class B) represent a unique subset of zine hydrolases that hydrolyze the β-lactam ring in several antibiotics. The antibiotic resistance that results from this hydrolysis is becoming an increased threat for the clinical community. These metalloenzymes can hydrolyze a wide range of β-lactam substrates, such as cephamycins and imipenem that are generally resistant t the serine-containing β-lactamases. Therefore, the clinical application of the entire range of antibiotics is severely compromised in bacteria that produce mβls. Due to the lack of information on the mechanism of mβls, to-date, no clinically known inhibitors is there for mβls. In this present study, we synthesized several mono and dizinc complexes as models for the mβls and investigated the differences in their hydrolytic properties. This study supports the assumption that the second zinc in the dinuclear enzymes does not directly involve in the catalysis, but may orient the substrates for hydrolysis and the basic amino acid residues such as Asp and His may activate the zinc-bound water molecules, fulfilling the role of the second zinc in the mononuclear enzymes. The effect of various side chains on the hydrolysis of some commonly used cephalosporin antibiotics by mβl from B.cereus is described. It is shown that the cephalosporins having heterocyclic thiol side chains are more resistance to mβl-mediated hydrolysis than the antibiotics that do not have such side chains. This is partly due to the inhibition of enzyme activity by the thiol moieties eliminated during the hydrolysis. It is also observed that the heterocyclic side chains in pure form inhibit the lactamase activity of mβl as well as its synthetic mimics. The mode of binding of these heterocyclic side chains to the zinc has been analyzed from the crystal structure of the tetranuclear zinc complexes. The theoretical studies suggest that the eliminated heterocyclic thiols undergo a rapid tautomerism to produce the corresponding thiones. These thiones are found to irreversibly inhibit the LPO-catalyzed iodination reaction. The reaction of various thiones with I2 leads to the formation of thione-iodine complexes similar to that of the most commonly used antithyroid drug methimazole(MMI). These observations suggest that some of the latest generation of antibiotics may show negative effects on thyroid gland upon hydrolysis. Synthetic organophosphorus compounds have been used extensively as pesticides and petroleum additives. These compounds are very toxic to mammals and their widespread use in agriculture leads to serious environmental problems. Therfore, degradation of organophosphorus trimesters and remediation of associated contaminated sites are of worldwide concern. In this regards, the bacterial phsophotriesterase (PTE) enzyme plays an important role in degrading a wide range of organophosphorus esters and the active side of PTE has been shown to be very similar to that of mβl. This identification prompted us to check the hydrolysis of phosphotriesters by the mβl and its mimics. It has been observed that the dinuclear zine(II) complexes that do not allow a strong binding of phosphodiestes would be a better PTE mimics.

In dieser Dissertation wird die Darstellung tetrelreicher Seltenerdmetall–Verbindungen in den Systemen SE:Tt (SE = La, Nd, Sm, Gd, Tb, Ho, Lu; Tt = Si, Ge) und die Charakterisierung ihrer Eigenschaften beschrieben. Diese Verbindungen, welche mittels der Hochdruck–Hochtemperatur–Methode dargestellt wurden, zeigen neuartige Verknüpfungsmuster in der Tetrel–Partialstruktur. Neben der Charakterisierung der Verbindungen hinsichtlich der thermischen Stabilität und der physikalischen Eigenschaften bei Normaldruck wurde bei den Germanium–reichen Seltenerdmetall–Verbindungen eine Untersuchung der Veränderungen der Kristallstruktur bei Variation des Drucks oder der Temperatur vorgenommen. Die dargestellten Seltenerdmetall–Trisilicide SESi3 (SE = Gd, Ho, Lu) kristallisieren tetragonal isotyp zu YbSi3. LuSi3 zeigt Supraleitung mit Tc = 7.0 K. In den Systemen Gd–Si und SE–Ge (SE = La, Nd, Sm, Gd, Tb) wurden die Verbindungen GdSi5 und SEGe5 (SE = La, Nd, Sm, Gd, Tb) synthetisiert. Sie kristallisieren orthorhombisch isotyp zu LaGe5. Durch in–situ Röntgenbeugungsexperimente bei erhöhten Temperaturen kann die Existenz metastabiler Germanium–ärmerer Verbindungen SE2Ge9 (SE = Nd, Sm) nachgewiesen werden. Es handelt sich um Defektvarianten der Verbindungen SEGe5 (SE = Nd, Sm). Die strukturelle Verwandtschaft zum Aristotyp SEGe5 wird anhand einer Gruppe–Untergruppe–Beziehung aufgezeigt. Eine alternative Synthesemethode zur Darstellung tetrelreicher Verbindungen mit Gerüststrukturen bei extremen Reaktionsbedingungen stellt das Spark–Plasma–Sinterverfahren (SPS) dar. Die Darstellung der Clathratphase Ba8Ga16±xGe30∓x (x = 0, 1) erfolgte mittels SPS aus den Precursoren BaGa2±x (x = 0, 0.125) und Germanium. Die Untersuchungen der thermoelektrischen Eigenschaften zeigen, dass durch Variation der nominellen Zusammensetzung sowohl n–leitende als auch p–leitende Eigenschaften erhalten werden können. / In this thesis the synthesis of tetrel–rich rare–earth metal compounds in the systems RE:Tt (RE = La, Nd, Sm, Gd, Tb, Ho, Lu; Tt = Si, Ge) and the characterization of their properties is described. These compounds, synthesized by means of high–pressure high–temperature method, show new structural motifs in the tetrel partial structure. The compounds were characterized with respect to their thermal stability and their physical properties at ambient pressure. In addition, the changes in the crystal structure of the germanium-rich rare–earth metal compounds by variation of pressure or temperature were investigated. The synthesized rare–earth trisilicides SESi3 (SE = Gd, Ho, Lu) crystallize tetragonal, isotypic to YbSi3. LuSi3 is a superconductor with Tc = 7.0 K. In the systems Gd–Si and SE–Ge (SE = La, Nd, Sm, Gd, Tb) the compounds GdSi5 and SEGe5 (SE = La, Nd, Sm, Gd, Tb) were synthesized. They crystallize orthorhombic isotypic to LaGe5. Using in-situ high–temperature X-ray experiments the metastable germanium-poorer compounds SE2Ge9 (SE = Nd, Sm) could be observed. These compounds are defect variants of the pentagermanides SEGe5 (SE = Nd, Sm). The structural relationship to the aristotype SEGe5 is given via a group-subgroup relation. An alternative synthesis route for the preparation of tetrel–rich compounds with framework structures at extreme reaction conditions is the spark plasma sintering method (SPS). The clathrate phase Ba8Ga16±xGe30∓x (x = 0, 1) was synthesized from the precursors BaGa2±x (x = 0, 0.125) and germanium by means of SPS. The investigation of the thermoelectric properties shows, that through variation of the nominal composition both n-type and p-type conduction properties can be obtained.