Browsing by Author "Yee, Gordon T."

Now showing 1 - 20 of 29
  • Thumbnail Image
    Ab initio Calculations of Optical Rotation
    Tam, Mary Christina (Virginia Tech, 2006-04-18)
    Coupled cluster (CC) and density functional theory (DFT) are highly regarded as robust quantum chemical methods for accurately predicting a wide variety of properties, such as molecular structures, thermochemical data, vibrational spectra, etc., but there has been little focus on the theoretical prediction of optical rotation. This property, also referred to as circular birefringence, is inherent to all chiral molecules and occurs because such samples exhibit different refractive indices for left- and right- circularly polarized light. This thesis focuses on the theoretical prediction of this chiroptic property using CC and DFT quantum chemical models. Several small chiral systems have been studied, including (S)-methyloxirane, (R)-epichlorohydrin, (R)-methylthiirane, and the conformationally flexible molecules, (R)-3-chloro-1-butene and (R)-2-chlorobutane. All predicted results have been compared to recently published gas-phase cavity ringdown polarimetry data. When applicable, well-converged Gibbs free energy differences among confomers were determined using complete-basis-set extrapolations of CC energies in order to obtain Boltzmann-averaged specific rotations. The overall results indicate that the theoretical rotation is highly dependent on the choice of optimized geometry and basis set (diffuse functions are shown to be extremely important), and that there is a large difference between the CC and DFT predicted values, with DFT usually predicting magnitudes that are larger than those of coupled cluster theory.
  • Thumbnail Image
    Analytical Methods Development for High-Throughput Photochemisty With Led Arrays
    Brown, Jared R. (Virginia Tech, 2007-05-03)
    This thesis describes the design, construction, and evaluation of a series of LED array photolysis systems for high throughput photochemistry. Three generations of array systems of increasing sophistication are evaluated using calorimetric measurements and potassium tris(oxalato)ferrate(III) chemical actinometry. The results are analyzed using descriptive statistics and analysis of variance (ANOVA). The LEDs in the third generation array were shown to be statistically equivalent, with respect to light output, according to physical and chemical actinometry experiments. The third generation LED array was compared with a traditional 1000 W Xe arc lamp source in terms of cost, light intensity, and light stability. Two constant current drivers were evaluated with respect to LED array performance. The optimized third generation LED array was evaluated as the photolysis source for photochemical hydrogen production experiments using the supramolecular catalyst [{(bpy)2Ru(dpp)}2RhCl2](PF6)5.
  • Thumbnail Image
    Correlation of Structure and Magnetic Properties in Charge-Transfer Salt Molecular Magnets Composed of Decamethylmetallocene Electron Donors and Organic Electron Acceptors
    Tyree, William Stuart (Virginia Tech, 2005-08-02)
    Di-n-propyl dicyanofumarate (DnPrDCF) and di-isopropyl dicyanofumarate (DiPrDCF) have been used as one-electron acceptors in the synthesis of charge-transfer salt magnets with decamethylmetallocenes, MCp*2 (M = Mn, Cr). Salts of each acceptor with each metallocene have been characterized and the structures of the chromium analogues have been solved. The two acceptors are structurally similar to dimethyl dicyanofumarate (DMeDCF) and diethyl dicyanofumarate (DEtDCF), which have been previously studied and found to form charge-transfer salt magnets with the aforementioned decamethylmetallocenes. A typical structural motif is present in these types of charge-transfer salts which allows for the comparison of magnetic properties based on the length or size of the alkyl group of the dialkyl dicyanofumarate. Some trends were established based on the magnetic properties of the homologous series including ordering temperature/bulkiness of the alkyl group and intrastack distances/theta values. Correlation of magnetic and structural properties may give some insight into "through-space" magnetic coupling, of which little is understood.
  • Thumbnail Image
    Development of an Ionically-Assembled On-Column Enzyme Reactor for Capillary Electrophoresis
    Hooper, Stephanie Elaine (Virginia Tech, 2007-06-26)
    This work describes the integration of a separation capillary for capillary electrophoresis (CE) with an on-column enzyme reactor for selective determination of the enzyme substrate. The enzyme reaction occurs during a capillary separation, allowing selective determination of the substrate in complex samples without the need for pre- or post- separation chemical modification of the analyte. The overall goal of this work is to develop a system in which sample introduction, separation of the analyte/substrate from other biological species, enzymatic conversion of the analyte/substrate into a detectable product, and sensitive detection are all included within a single analysis scheme. Immobilization of the enzyme is achieved by electrostatic assembly of poly(diallydimethylammonium chloride) (PDDA) followed by adsorption of a mixture of the negatively charged enzyme glucose oxidase (GOx) and anionic poly(styrenesulfonate) (PSS). The reaction of glucose with the immobilized glucose oxidase produces H2O2 which migrates the length of the capillary under the influence of electroosmotic flow and is detected amperometrically at the capillary outlet. The optimal response, kinetics, and stability for the enzyme reactor are determined through characterization of several parameters including the concentration ratio of PSS:GOx, applied separation voltage, and the inner diameter of the separation capillary. Various analyte mixtures containing the substrate and other biological species were evaluated to illustrate selective separation and determination of the substrate from other biomolecules. Optimization of this electrostatically assembled capillary enzyme reactor lead to application of these parameters to similar enzymes such as glutamate oxidase. Future application to similar enzymes like L-amino acid oxidase and possible microfluidic systems is a long-term goal of the system.
  • Thumbnail Image
    Electrochemical oxidation of aliphatic carboxylates: Kinetics, thermodynamics, and evidence for a shift from a concerted to a stepwise mechanism in the presence of water
    Abdel Latif, Marwa K. (Virginia Tech, 2016-09-22)
    The mechanism and the oxidation potential of the dissociative single electron transfer for tetra-n-butylammonium acetate has been investigated via conventional (cyclic voltammetry) and convolution voltammetry. The oxidation potential for tetra-n-butylammonium acetate was determined to be 0.60 ± 0.10 (vs. Ag/ (0.1 M) AgNO₃) in anhydrous acetonitrile. The results also indicated the mechanism of oxidation was concerted dissociative electron transfer (cDET), rather than stepwise as was previously reported. To further investigate the mechanism, a series of aliphatic and aromatic tetra-n butylammonium carboxylates were synthesized and investigated via convolution and conventional methods under anhydrous conditions (propionate, pivalate, phenyl acetate, and benzoate). The reported results showed high reproducibility and consistency with a concerted dissociative electron transfer for aliphatic carboxylates with a systematic shift in the oxidation potentials (0.60 ± 0.09 V for acetate, 0.47 ± 0.05 V for propionate, and 0.40 ± 0.05 V for pivalate) within the series which is expected trend based on radical stabilization energies of the alkyl groups on the aliphatic carboxylates. Hydrogen bonding was investigated as a possible source for the discrepancy between our results and the reported mechanism of the dissociative electron transfer. Because of the extreme hygroscopic nature of carboxylate salts, it was hypothesized that the presence of small amounts of water might alter the reaction mechanism. Deionized water and deuterium oxide additions to anhydrous acetonitrile were performed to test this hypothesis. The mechanism was noted to shift towards a stepwise mechanism as water was added. In addition, the derived oxidation potentials became more positive with increasing concentrations of water. Several explanations are presented with regards to water effects on the shift in the electron transfer mechanism. Indirect electrolysis (homogeneous redox catalysis) was also employed as an alternative and independent approach to quantify the oxidation potentials of carboxylates. A series of substituted ferrocenes were investigated as mediators for the oxidation of tetra-n-butylammonium acetate. Preliminary data showed redox catalysis was feasible for these systems. Further analyses of the electrochemical results suggested a follow-up chemical step (addition to mediator) that competes with the redox catalysis mechanism. As predicted from theoretical working curves, a plateau region in the ip/ipd plots (where no meaningful kinetic information could be obtained) was observed. Products mixture analyses verified the consumption of the mediator upon electrolysis, but no further information with regards to the nature of the mechanism was deduced. In a related study the effects of hydrogen bonding and ions on the reactivity of neutral free radicals were examined by laser flash photolysis. The rate of the β-scission of the cumyloxyl radical is influenced by cations (Li⁺ > Mg²⁺ ≈ Na⁺ > nBu₄N⁺) due to stabilizing ion-dipole interactions in the transition state of the developing carbonyl group. Experimental findings are in a good agreement with theoretical work suggesting metal ion complexation can cause radical clocks to run fast with a more significant effect if there is an increase in dipole moment going from the reactant to the transition state.
  • Thumbnail Image
    Enabling Synthesis Toward the Production of Biocompatible Magnetic Nanoparticles With Tailored Surface Properties
    Thompson, Michael Shane (Virginia Tech, 2007-07-10)
    Amphiphilic tri- and penta-block copolymers containing a polyurethane central block with pendant carboxylic acid groups flanked by hydroxyl functional polyether tails were synthesized. Our intention was to investigate the activities of these copolymers as dispersants for magnetite nanoparticles in biological media. A benzyl alkoxide initiator was utilized to prepare poly(ethylene oxide) (BzO-PEO-OH), poly(propylene oxide) (BzO-PPO-OH) and poly(ethylene oxide-b-propylene oxide) (poly(BzO-EO-b-PO-OH)) oligomeric tail blocks with varying lengths of PEO and PPO. The oligomers had a hydroxyl group at the terminal chain end and a benzyl-protected hydroxyl group at the initiated end. The polyether oligomers were incorporated into a block copolymer with a short polyurethane segment having approximately three carboxylic acid groups per chain. The block co-polyurethane was then hydrogenated to remove the benzyl group and yield primary hydroxyl functionality at the chain ends. End group analysis by 1H NMR showed the targeted ratio of PEO to PPO demonstrating control over block copolymer composition. Number average molecular weights determined by both 1H NMR and GPC were in agreement and close to targeted values demonstrating control over molecular weight. Titrations of the pentablock copolymers showed that the targeted value of approximately three carboxylic acid groups per chain was achieved. Heterobifunctional poly(ethylene oxide) (PEO) and poly(ethylene oxide-b-propylene oxide) (PEO-b-PPO) copolymers were synthesized utilizing heterobifunctional initiators to yield polymers having a hydroxyl group at one chain end and additional moieties at the other chain end. For PEO homopolymers, these moieties include maleimide, vinylsilane, and carboxylic acid functional groups. Heterobifunctional PEO oligomers with a maliemide end group were synthesized utilizing a double metal cyanide coordination catalyst to avoid side reactions that occur with a basic catalyst. PEO oligomers with vinylsilane end groups were synthesized via alkoxide-initiated living ring-opening polymerization, and this produced polymers with narrow molecular weight distributions. Heterobifunctional PEO-b-PPO block copolymers were synthesized in two steps where the double metal cyanide catalyst was used to polymerize propylene oxide (PO) initiated by 3-hydroxypropyltrivinylsilane. The PPO was then utilized as a macroinitiator to polymerize ethylene oxide (EO) with base catalysis. Heterobifunctional PEO and PEO-b-PPO block copolymers possessing carboxylic acid functional groups on one end were synthesized by reacting the vinyl groups with mercaptoacetic acid via an ene-thiol addition.
  • Thumbnail Image
    Heterogeneous Redox Chemistries in Layered Oxide Materials for Lithium-Ion Batteries
    Xu, Zhengrui (Virginia Tech, 2022-01-05)
    The invention of the lithium-ion battery has revolutionized the passenger transportation field in recent years, and it has emerged as one of the state-of-the-art solutions to address greenhouse gases emission and air pollution issues. Layered oxide lithium-ion battery cathode materials have become commercially successful in the past few decades due to their high energy density, high power density, long cycle life, and low cost. Yet, with the increasing demand for battery performance, it is crucial to understand the material fading mechanisms further to improve layered oxide materials' performance. A heterogeneous redox reaction is a dominant fading mechanism, which limits the utilization percentage of a battery materials' redox capability and leads to adverse effects such as detrimental interfacial reactions, lattice oxygen release, and chemomechanical breakdown. Crystallographic defects, such as dislocations and grain boundaries, are rich in battery materials. These crystallographic defects change the local lithium-ion diffusivity and have a dramatic effect on the redox reactions. To date, there is still a knowledge gap on how various crystallographic defects affect electrochemistry at the microscopic scale. Herein, we adopted synchrotron-based diffraction, imaging, and spectroscopic techniques to systematically study the correlation between crystallographic defects and redox chemistries in the nanodomain. Our studies shed light on design principles of next-generation battery materials. In Chapter 1, we first provide a comprehensive background introduction on the battery chemistry at various length scales. We then introduce the heterogeneous redox reactions in layered oxide cathode materials, including a discussion on the impacts of heterogeneous redox reactions. Finally, we present the different categories of crystallographic defects in layered oxide materials and how these crystallographic defects affect electrochemical performance. In Chapter 2, we use LiCoO2, a representative layered oxide cathode material, as the material platform to quantify the categories and densities of various crystallographic defects. We then focus on geometrically necessary dislocations as they represent a major class of crystallographic defects in LiCoO2. Combining synchrotron-based X-ray fluorescence mapping, micro-diffraction, and spectroscopic techniques, we reveal that geometrically necessary dislocations can facilitate the charging reactions in LiCoO2 grains. Our study illustrates that the heterogeneous redox chemistries can be potentially mitigated by precisely controlling the defects. In Chapter 3, we systematically investigated how grain boundaries affect redox reactions. We reveal that grain boundaries can guide redox reactions in LiNixMnyCo1-x-yO2 (NMC) materials. Specifically, NMC materials with radially aligned grains have a more uniform charge distribution, less stress mismatch, and better cycling performance. NMC materials with randomly orientated grains have a more heterogeneous redox reaction. These heterogeneous redox reactions are related to the lattice strain mismatch and worse cycling performance. Our study emphasizes the importance of tuning grain orientations to achieve improved performance. Chapter 4 systematically investigated how the grain boundaries and crystallographic orientations affect the thermal stability of layered oxide cathode materials. Combining diffraction, spectroscopic, and imaging techniques, we reveal that a cathode materials' microstructure plays a significant role in determining the lattice oxygen release behavior and, therefore, determines cathode materials' thermal stability. Our study provides a fundamental understanding of how the grain boundaries and crystallographic orientations can be tuned to develop better cathode materials for the next-generation Li-ion batteries. Chapter 5 summarizes the contributions of our work and provides our perspective on future research directions.
  • Thumbnail Image
    Investigating the parameters of metal-organic framework crystal growth control for reverse osmosis membrane nanofillers and direct air capture of CO2
    Bonnett, Brittany Lauren (Virginia Tech, 2022-06-02)
    Inorganic nano- and micromaterials (NMMs) exhibit unique properties including high surface areas, tunable optical and electronic properties, low densities, thermal and chemical robustness, and catalytic capabilities, among others. One of the more novel subclasses of NMMs, metal-organic frameworks (MOFs), are crystalline porous coordination polymers consisting of metal nodes connected by organic linkers to form one-, two-, or three-dimensional frameworks. While the mechanism of MOF formation is complex, tuning the metal:ligand ratios, reaction temperature and vessel pressure, ligand concentration, modulator concentration, and H+ activity impacts particle size, morphology, dispersity, and isotropy of these materials. MOFs also exhibit post-synthetic modification capabilities, which, along with their tunable synthetic nature, make them promising candidates for composite materials such as functionalized nanofillers for reverse osmosis (RO) desalination. The work described herein investigates synthetic parameters of a zirconium-based porphyrinic MOF, PCN-222, to selectively control its crystal size, aspect ratio, and dispersity. Size-constrained PCN-222 was post-synthetically modified with fatty acids and zwitterions to be used as RO thin-film composite (TFC) membranes with improved membrane flux, salt rejection, and anti-fouling properties. The synthetic parameters of MOFs were also considered for the commercial scale-up of CO2 direct air capture (DAC) solid sorbents, including UiO-66, MIL-101-Cr, and Mg-MOF-74, to preserve CO2 uptake capacities between lab and industrial scales.
  • Thumbnail Image
    Investigation of the Magnetic Properties of Non-Thiolated Au Nano-Structures Grown by Laser Ablation
    Zhao, Chenlin (Virginia Tech, 2014-09-09)
    Although it is known that gold (Au) is diamagnetic in bulk form, it has been reported that Au displays magnetic properties when reduced to the nano-scale. Researchers found magnetism in Au nanoparticles (NPs) in a size range from 2 to 10 nanometers. Moreover, the Au nanoparticles are usually coated by thiol-containing organic caps, which are believed to be responsible for the magnetism. However, others suggest that organic capping is not necessary to observe magnetism in Au NPs, and magnetism may be an intrinsic property for nano-structured gold. For this investigation, we used pulsed laser deposition to prepare nano-structured gold of different sizes and concentrations to investigate the magnetic properties. Our experiment results confirmed that for the samples in which Au is in the metallic state as nanoparticles with ~5 nm diameter, as well as inthe alloy form, bonded with indium, the samples show ferromagnetism when embedded in an Al2O3 matrix without any thiol-containing organic capping. Our results suggest that ferromagnetism is an intrinsic property of Au nano-structures, which means that it is not necessary to incorporate Au-S bonds with organic coatings in order to observe this phenomenon. We believe due to the significant broken symmetry at the surface of the nanoparticles, holes are generated in d bands of the surface Au atoms. These holes are most possibly responsible for ferromagnetism in Au nanoparticles. The realization of magnetism in Au coupled with the lack of clear understanding of its origin makes the investigation of magnetism of diamagnetic metals ripe for further inquiry.
  • Thumbnail Image
    Local Correlation: Implementation and Application to Molecular Response Properties
    Russ, Nicholas Joel (Virginia Tech, 2006-04-17)
    One of the most promising methods for surmounting the high-degree polynomial scaling wall associated with electron correlating wave function methods is the local correlation technique of Pulay and Saebø. They have proposed using a set of localized occupied and virtual orbitals free of the canonical constraint commonly employed in quantum chemistry, resulting in a method that scales linearly (in the asymptotic limit) with molecular size. Pulay and Saeb$oslash; first applied their methods to configuration interaction and later to M$oslash;ller-Plesset perturbation theory. Werner et al. have have extended the local correlation scheme of Pulay and Saeb$oslash; to coupled-cluster theory. One of the pitfalls of the local correlation methods developed by Pulay and Saeb$oslash; is the dependence of domain selection on the molecular geometry. In other words, as the geometry changes the domain structure of the local correlation calculation can change also, leading to discontinuities in the potential energy surface. We have examined the size of these discontinuities for the homolytic bond cleavage of fluoromethane and the heterolytic bond dissociation of singlet ketene and propadienone. Properties such as polarizabilities and optical rotation are realized through linear response theory, where the Hamiltonian is subject to an external perturbation and the wave function is allowed to respond to the applied perturbation. Within the context of local correlation it is necessary to understand how the domain structure alters in response to an applied perturbation. We have proposed using solutions to the CPHF equations (coupled-perturbed Hartree-Fock) in order to predict the correlation response to an applied perturbation. We have applied this technique to the calculation of polarizabilities, with very favorable results, and also to optical rotation, with mixed results.
  • Thumbnail Image
    Metal Oxide Nanoparticles: Optical Properties and Interaction with Chemical Warfare Agent Simulants
    Gordon, Wesley Odell (Virginia Tech, 2006-11-06)
    Materials with length scales in the nanometer regime demonstrate properties that are remarkably different from analogous bulk matter. As a result, researchers are striving to catalog the changes in properties that occur with decreasing size, and more importantly, understand the reason behind novel nanomaterial properties. By learning the true nature of nanomaterials, scientists and engineers can design better materials for a variety of applications. Inert gas-phase condensation synthesis of metal oxide nanoparticles was used to develop materials to explore the optical and chemical properties of metal oxide nanoparticles. One potential application for nanomaterials is use in optical applications. The possibility of interparticle energy transfer was investigated for lanthanide-doped yttrium oxide nanoparticles using laser spectroscopy. Experimental evidence collected with this study indicates that interparticle, lanthanide-mediated energy transfer may have been observed. In addition, lanthanide-doped gadolinium oxide nanoparticles were synthesized and investigated with optical spectroscopy to identify the best potential candidates for bioanalytical applications of this material. The influence of particle annealing and dopant concentration were also studied. Nanoparticle film structure was investigated with scanning electron microscopy. Two different film structures composed of oxide nanoparticles were found to grow under different synthesis conditions. The film structure was found to be determined by the degree of particle aggregation in the gas phase during synthesis. Aggregation of the particles was found to be controlled by a combination of gas pressure and properties. Chemical properties of metal oxide nanoparticles also are very important. Reflection-absorption Infrared Spectroscopy and vacuum surface analytical techniques were used to explore the chemistry of the chemical warfare agent dimethyl methylphosphonate (DMMP) on yttrium oxide as well as other metal oxide nanoparticles. DMMP was found to dissociate at room temperature on several types of metal oxide nanoparticles. Hydroxyl groups were found to be critical for the adsorption of DMMP onto the particles. Finally, the reactivity of the nanoparticles was found to increase with decreasing particle size. This was attributed to a relative increase in the number of high-energy surface defects for the smaller particles.
  • Thumbnail Image
    Microwave-assisted Synthesis of Modified Cyclopentadienyl Iridium and Rhodium Chloro-bridged Dimers
    Brown, Loren (Virginia Tech, 2016-06-16)
    The present work describes the design and synthesis of a series of dimers [(η5 - ring)MCl]2(μ2 -Cl)2, (where (η5 -ring)MCl = (η5 -Me4C5R)Rh(III)Cl or (η5 -Me4C5R)Ir(III)Cl). Iridium and rhodium dimeric complexes were synthesized via a microwave reaction and directly compared through single-crystal X-ray crystallography. Finally, the dimeric complexes were evaluated as potential oxidation catalysts. The modified HCp*R (R = isopropyl, n-butyl, isobutyl, sec-butyl, n-pentyl, n-hexyl, nheptyl, n-octyl, phenyl, benzyl, phenethyl, cyclohexyl, and cyclopentyl) type ligands were synthesized by reaction of 2,3,4,5-tetramethylcyclopent-2-en-1-one with the respective Grignard reagent (RMgX), followed by elimination of water under acidic conditions to produce the tetramethyl(alkyl or aryl)cyclopentadienes in moderate to excellent yields (39 - 98%). Reaction of the HCp*R ligands with [M(COD)](μ2 -Cl)2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) gave the dimeric complexes [Cp*RMCl]2(μ2 -Cl)2 in yields ranging from 16 - 96%. The dimers were characterized by nuclear magnetic resonance (NMR)spectroscopy, single-crystal X-ray diffraction (XRD) (supplemented by powder XRD), high-resolution mass spectrometry (HRMS), and elemental analysis. Complexes studied by XRD were analyzed to understand the bond lengths and bond angles throughout each complex. The dimeric complexes synthesized, will facilitate a complete study on how the R group influences catalytic activity.
  • Thumbnail Image
    Multimetallic Supramolecular Complexes: Synthesis, Characterization, Photophysical Studies and Applications in Solar Energy Utilization and Photodynamic Therapy
    Miao, Ran (Virginia Tech, 2007-12-04)
    This thesis describes the study of a series of multimetallic supramolecules containing varied metals and ligands, synthesized by a building block method and characterized by mass spectrometry, electronic absorption spectroscopy, and electrochemistry. Incorporating different functional units into complex systems allowed these multimetallic supramolecules to perform various light activated tasks including DNA cleavage and hydrogen generation from water. The complex [({(bpy)₂Os(dpp)}₂Ru)₂(dpq)](PF₆)12 and [{(bpy)₂M(dpp)}₂Ru(BL)PtCl₂](PF₆)₆ were synthesized (M = OsII or RuII; BL = dpp or dpq; bpy = 2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, dpq = 2,3-bis(2-pyridyl)quinoxaline). The building blocks displayed varied electrochemical properties upon complexation. The bridging ligands dpp and dpq display their reduction potentials shifted to less negative values when they changed from monochelating to bischelating. The electronic absorption spectra of the multimetallic systems displayed transitions of each contributing chromophore, with overlapping metal to ligand charge transfer (MLCT) transitions in visible region of spectrum. Spectroelectrochemistry revealed the nature of MLCTs and helped to identify fingerprint features of complex supramolecules. Photophysical measurements include emission spectroscopy with quantum yield measurements and emission lifetime measurements. Photophysical data provided detailed information to aid in developing an understanding of excited state properties of these complexes. Supported by the electrochemical data and spectroelectrochemistry, the hexametallic complex was suggested to have a HOMO localizing in the peripheral Os and a LUMO localizing in the central dpq, separating by a Ru energy barrier. This research systematically investigated photophysical properties of some building blocks and the mixed-metal, mixed-ligand supramolecules constructed by a variety of building blocks coupling light absorbing subunits to a reactive Pt metal center. Preliminary studies suggested [{(bpy)₂Ru(dpp)}₂Ru(dpq)PtCl₂](PF₆)₆ was a photocatalyst for H2 production from water in the presence of a sacrificial electron donor. The complex [{(bpy)₂Ru(dpp)}₂Ru(dpq)PtCl₂](PF₆)₆ had been studied for its catalytic ability in generating hydrogen and was found to have 34 product turnovers after 3 h photolysis. Photolysis and gel electrophoresis revealed that the tetrametallic complexes were able to bind to and then photocleave DNA through an oxygen mediated mechanism. The independence of ionic strength variation when [{(bpy)₂Ru(dpp)}₂Ru(dpp)PtCl₂](PF₆)₆ interacted with DNA, suggested the covalent interaction nature of the complex. These results suggest future work on understanding the excited state properties of supramolecular complexes is suggested. The designs of future photocatalysts for hydrogen production from water and anticancer photodynamic therapy drugs are also proposed.
  • Thumbnail Image
    A New Family of High Tc Molecule-Based Magnetic Networks: V[x-ClnPTCE]2·yCH2Cl2 (PTCE = Phenyltricyanoethylene)
    Tatum, David S.; Zadrozny, Joseph M.; Yee, Gordon T. (MDPI, 2019-08-01)
    Using the structural and electronic tunability of molecules to control magnetism is a central challenge of inorganic chemistry. Herein, a ten-member family of the high-ordering temperature (Tc) molecule-based magnetic coordination networks of the form V[x-ClnPTCE]2·yCH2Cl2 (PTCE = phenyltricyanoethylene, y < 0.5) were synthesized and characterized, where x is (are) the position(s) and n is the number of chlorine substitutions on the phenyl ring. These chlorophenyltricyanoethelenes are tunable analogs of the more commonly investigated tetracyanoethylene (TCNE). Varying the number and position of chlorine substitution around the phenyl ring engendered a family of network solids with significantly different magnetic ordering temperatures ranging from 146 to 285 K. The Tcs of these ferrimagnets were rationalized with the aid of cyclic voltammetry and Density Functional Theory (DFT) calculations.
  • Thumbnail Image
    Phase Behavior of Poly(Caprolactone) Based Polymer Blends As Langmuir Films at the Air/Water Interface
    Li, Bingbing (Virginia Tech, 2007-01-31)
    Poly (caprolactone) (PCL) has been widely studied as a model system for investigating polymer crystallization. In this thesis, PCL crystallization along with other phase transitions in PCL-based polymer blends are studied as Langmuir films at the air/water (A/W) interface. In order to understand the phase behavior of PCL-based blends, surface pressure induced crystallization of PCL in single-component Langmuir monolayers was first studied by Brewster angle microscopy (BAM). PCL crystals observed during film compression exhibit butterfly-shapes. During expansion of the crystallized film, polymer chains detach from the crystals and diffuse back into the monolayer as the crystals "melt". Electron diffraction on Langmuir-Schaefer films suggests that the lamellar crystals are oriented with the chain axes perpendicular to the substrate surface, while atomic force microscopy (AFM) reveals a crystal thickness of ~ 7.6 nm. In addition, the competition between lower segmental mobility and a greater degree of undercooling with increasing molar mass produces a maximum average growth rate at intermediate molar mass. PCL was blended with poly(t-butyl acrylate) (PtBA) to study the influence of PtBA on the morphologies of PCL crystals grown in monolayers. For PCL-rich blends, BAM studies reveal dendritic morphologies of PCL crystals. The thicknesses of the PCL dendrites are ~ 7-8 nm. BAM studies during isobaric area relaxation experiments at different surface pressure reveal morphological transitions from highly branched dendrites, to six-arm dendrites, four-arm dendrites, seaweedlike crystals, and distorted rectangular crystals. In contrast, PCL crystallization is suppressed in PtBA-rich blend films. For immiscible blends of PCL and polystyrene (PS) with intermediate molar masses as Langmuir films, the surface concentration of PCL is the only factor influencing surface pressure below the collapse transition. For PS-rich blends, both BAM and AFM studies reveal that PS nanoparticle aggregates formed at very low surface pressure form networks during film compression. For PCL-rich blends, small PS aggregates serve as heterogeneous nucleation centers for the growth of PCL crystals. During film expansion, BAM images show a gradual change in the surface morphology from highly continuous networklike structures (PS-rich blends) to broken ringlike structures (intermediate composition) to small discontinuous aggregates (PCL-rich blends).
  • Thumbnail Image
    Pin1 Catalytic and WW Domain Ligands
    Chen, Xingguo Ronald (Virginia Tech, 2011-04-27)
    Pin1 is a peptidyl prolyl isomerase (PPIase) enzyme with two domains, the catalytic domain and the WW domain. Both domains specifically bind pSer/pThr–Pro motifs. Pin1 plays an important role in regulating the cell cycle, and it is involved in many diseases, such as cancer, HIV-1, Alzheimer's disease, asthma, hepatitis B, and rheumatoid arthritis. Pin1 is a very promising target for new drug development. Three stereoisomers: (R,S)-, (S,R)- and (S,S)-Ac–pSer–Ψ[(Z)CH=C]–Pip–2-(2-naphthyl)ethylamine were synthesized as inhibitors binding to the Pin1 catalytic domain. The (R,S)- and (S,R)-isomers were synthesized via a 13-step route, with overall yields of 2.0% and 1.4%, respectively. The newly formed stereogenic center in the piperidyl ring was introduced by a Luche reduction, followed by a stereoselective [2,3]-Still-Wittig rearrangement. The configuration of the stereocenter was determined by NOESY of a bicyclic derivative. The (Z)- to (E)-alkene ratio in the rearrangement was (5.5:1). The (S,S)-isomer was obtained as the epimerized by-product resulting from the (S,R)-isomer in the Na/NH3 deprotection step. The IC50 values for Pin1 inhibition were: 52, 85, and 141 μM, respectively. We concluded that in this Z-alkene isostere, the R-configuration would be preferred at both stereogenic centers, as mimics of L-Ser and L-Pip, to improve the affinity. Combinatorial chemistry is a powerful method to discover biologically active compounds, and solid-phase synthesis is most commonly used to synthesize combinatorial libraries. To identify ligands for the Pin1 WW domain, a library, R1CO–pSer–Pro–NHR2, was designed. A new solid-phase phosphorylating reagent (SPPR) containing a phosphoramidite function was synthesized in one step from commercially available Wang resin. The SPPR was applied in the preparation of a designed library through parallel synthesis. The library contained 357 members (17 Ã 21), and was screened by an enzyme-linked enzyme binding assay (ELEBA). The best hits were resynthesized, and the competitive dissociation constants, Kd-rel, were measured by ELEBA, with a Kd-rel value of 130 μM for the best ligand. The absolute dissociation constants will be measured by our collaborator, Prof. Jefferey Peng, University of Notre Dame, using NMR methods. Besides the identification of the Pin1 WW domain ligands, I created a practical method for solid-phase synthesis of phosphopeptides.
  • Thumbnail Image
    Probing Organometallic Reactions With 19F NMR
    Hawrelak, Eric James (Virginia Tech, 2002-11-18)
    This dissertation explores fundamental aspects of the reaction of group 4 metallocenes with methylaluminoxane (MAO) that lead to active Ziegler-Natta olefin polymerization catalysts. A novel experimental approach is described, in which a unique spectroscopic probe (a fluorinated substituent) is attached to the metallocene ancillary ligands and the metallocene/MAO mixtures are analyzed using 19F NMR spectroscopy. Group 4 metallocene dimethides bearing pentafluorophenyl (C6F5) substituents were synthesized and treated with MAO in benzene-d6. 19F NMR spectroscopic analysis demonstrated reversible methide transfer to form "cation-like" methylmetallocenium methylaluminates. A series of quantitative titration studies showed that fewer than 10% of the aluminum centers in MAO actually participate in the methide transfer process. A systematic study of metallocene substituent effects suggested that MAO contains active centers of extremely high but varying Lewis acidity. Activation of group 4 metallocene dichlorides using MAO was also analyzed using 19F NMR. Initial Cl/CH3 exchange was followed by Cl transfer to aluminum, whereas "normal" subsequent transfer of CH3 from Al to the methylmetallocenium cation was apparently inhibited by the abstracted chloride. Additional studies showed that the 19F NMR probe is sensitive to the interactions of Zr-Cl bonds with simple alkylaluminum species such as Me3Al, Me2AlCl, MeAlCl2, and Et3Al. However, the method was arguably less useful than 1H NMR spectroscopy in following the metathesis of Zr-Cl and Al-R (R = Me, Et) bonds. New methods of preparing methylhalometallocenes were investigated. The reactions of eleven metallocene dimethyls with triphenylmethyl chloride were highly selective (> 95%) with the five most electron-deficient metallocenes studied. Two other examples showed good selectivity on an NMR scale but could not be isolated from the 1,1,1-triphenylethane byproduct. Reactions of dimethylmetallocenes with benzyl bromide were also selective for formation of the corresponding methylbromo-metallocenes, however the reactions were too slow to be of practical value. The observation of long initation periods and the analysis of organic byproduct distributions suggested that these halogenation reactions may proceed by a radical chain mechanism rather than simple sigma bond metathesis. To demonstrate "proof of concept" in the use of 19F NMR to analyze the reactions of paramagnetic metallocenes, the coordination of CO and CN- to C6F5-substituted chromocenes were analyzed. Whereas CO coordinates readily to chromocene, cyanide coordinates effectively to 1,1'-bis(pentafluorophenyl)chromocene. This observation is interpreted in terms of the electron-withdrawing effect of the C6F5 substituent, which should strengthen bonding to sigma-donor ligands (CN-) and weaken bonding to pi-acceptors (CO).
  • Thumbnail Image
    Probing the Redox and Photophysical Properties of Ru(II)-Pt(II) Supramolecular Complexes as Efficient Photodynamic Therapy Agents
    Higgins, Samantha Lake Hopkins (Virginia Tech, 2012-02-16)
    Mixed-metal Ru(II)-Pt(II) supramolecular complexes having the [(Ph₂phen)₂Ru(BL)PtCl₂]₂+ (Ph₂phen = 4,7-diphenyl-1,10-phenanthroline, and BL (bridging ligand) = dpp = 2,3-bis(2-pyridyl)pyrazine, or dpq = 2,3-bis(2-pyridyl)quinoxaline) structural motif were synthesized and their redox, photophysical, and photochemical properties studied. Subsequently the application of the Ru(II)-Pt(II) bimetallic complexes in light activated DNA modification and cytotoxicity were evaluated. The supramolecular design entails covalently coupling an efficient Ru(II) chromophore for photodynamic therapy (PDT) activity through a polyazine bridging ligand (dpp or dpq) to a cis-PtCl₂ bioactive site for covalent binding to biological substrates. The bioactive site is comparable to the first generation Pt-based chemotherapy agent cisplatin, cis-[PtCl₂(NH₃)₂]. The Ph₂phen ligand is known in [Ru(Ph₂phen)₃]²+ to provide enhanced excited state lifetime and increase quantum efficiency for singlet oxygen generation in comparison to the phen analog (Φ₁₀₂ = 0.97, Ph₂phen and Φ₁₀₂ = 0.54, phen). The redox and photophysical properties were analyzed at each synthetic step providing systematic evaluation of the complex properties. The [(Ph₂phen₂2Ru(BL)PtCl₂](PF₆)₂ complexes display reversible RuII/III oxidations at +1.61 (dpp) and +1.63 (dpq) V vs. Ag/AgCl with an irreversible PtII/IV oxidation occurring prior at +1.51 V vs. Ag/AgCl. Four reversible ligand reductions occur at -0.45 (dpp0/-), -1.15 (dpp-/2-), -1.33 (Ph₂phen0/-), and -1.52 (Ph₂phen0/-) V vs. Ag/AgCl. For the [(Ph₂phen)₂Ru(dpq)PtCl₂](PF₆)₂ complex, the first two reductions shift to more positive potentials at -0.19 and -0.95 V vs. Ag/AgCl, while the TL reductions remain generally unperturbed. The electronic absorption spectroscopy for the [(Ph₂phen)₂Ru(dpq)PtCl₂](PF₆)₂, BL = dpp or dpq, complexes is dominated in the UV region by Ph₂phen (274 nm) and BL-based (310-320 nm) π⟶ π* transitions and in the visible region by metal-to-ligand charge transfer (MLCT) transitions at 424 nm (Ru(dπ)→ Ph₂phen(π*) 1CT) and 517 nm (Ru(dπ)→ dpp(π*) 1CT) or 600 nm (Ru(dπ)→ dpq(π*) 1CT). Steady-state and time-resolved emission spectroscopy shows that upon attaching Pt to the Ru monometallic precursor the λmaxem shifts from 664 nm for [(Ph₂phen)2Ru(dpp)](PF₆)₂ to 740 nm for [(Ph₂phen)₂Ru(dpp)PtCl₂](PF₆)₂ and the excited state lifetime is reduced from 820 ns to 44 ns in accordance with the energy gap law. The τ = 44 ns for the Ru(dπ)→ dpp(π*) 3CT excited state was somewhat unexpected upon TL variation given the lack of formal involvement of Ph₂phen in the emissive state. This likely results from the Ph₂phen contribution to the formally Ru(dπ) donor orbital. Although not typically done, given the complexity of the study the Φ₁₀₂ was quantified for the [(Ph₂phen)₂Ru(BL)PtC₂]Cl₂ (BL = dpp, Φ₁₀₂ = 0.07 or dpq, Φ₁₀₂ = 0.03) complexes supporting 1O2 generation via energy transfer from the 3MLCT excited state. The thermal and photochemical interactions of the [(Ph₂phen₂2Ru(BL)PtCl₂]Cl₂ (BL = dpp or dpq) supramolecular complexes were studied in the presence of DNA and U87MG cancer cells. Thermal binding at the cis-PtCl₂ BAS in the Ru(II)-Pt(II) architecture was compared to cisplatin displaying similar reduced migration through the gel attributed to covalent binding to DNA. DNA photocleavage studies provided evidence of efficient strand cleavage when excited at 455 nm likely enhanced by producing 1O2 locally at the DNA target. DNA photobinding by the [(Ph₂phen)₂Ru(dpp)PtCl₂]Cl₂ complex was observed utilizing low energy light where typical Pt(II) agents do not absorb. This is the first example of MLCT excitation of a Ru(II)-Pt(II) complex to induce a photobinding event. MLCT excitation enhances electron density on the dpp making the Pt(II) a weaker Lewis acid and promoting halide loss. In addition, this system is photoactivated with low energy red light in the therapeutic window. These studies validate the supramolecular design and show that coupling a Ru(II) chromophore for PDT activity and a cis- PtCl₂ binding moiety for covalent DNA targeting affords a complex applicable in photochemotherapies. Analysis of cytotoxicity in the dark for [(Ph₂phen)₂Ru(dpp)PtCl₂]Cl₂ and cisplatin afforded LC50 values of 100 μM, which are confirmed by previous reports for cisplatin and the currently used chemotherapy, TMZ in U87MG cells. Photolysis of the [(Ph₂phen)₂Ru(dpp)PtCl₂]Cl₂ resulted in substantial reduction in the observed LC50 values to approximately 5 μM. The enhanced cytotoxicity via excitation into the formally Ru(dπ)→ BL(π*) CT excited state of [(Ph₂phen)2Ru(dpp)PtCl2]Cl2 indicates that the bimetallic complex undergoes an efficient light activated mechanism of action. The Ru(II)-Pt(II) complex displays substantially lower LC50 values through PDT action than currently used clinical treatments with LC50 values of 100 μM. The [(Ph₂phen)₂Ru(BL)PtCl₂]₂+ (BL = dpp or dpq) mixed-metal supramolecules utilizing the Ph₂phen TL have displayed surprising results. The direct coupling of the cis-PtCl₂ moiety to the (Ph₂phen)₂Ru(BL) chromophore display dramatically enhanced photophysical properties, relative to the bpy and phen systems with a longer excited state lifetime and improved light activated interactions with DNA, which was not previously observed for directly coupled Ru(II)- Pt(II) systems. The Ph₂phen TL positively influence the bioactivity compared to the typical deactivation observed in the bpy and phen systems. Probing the [(Ph₂phen)₂Ru(BL)PtCl₂]₂+ (BL = dpp or dpq) biological interactions confirms the importance of coupling an efficient light absorbing and 1O2 generating PDT-type unit with a cis-PtCl2 DNA binding unit for applications in covalent DNA photomodification, DNA photocleavage, and photocytotoxicity. It is proposed that excitation using visible light into the formally Ru(dπ)→ BL(π*) CT excited state leads to enhanced electron density on the BL and weakened Lewis acidity at the Pt(II) center, which facilitates halide loss for efficient biological substrate modification. Upon coordination of the Ru(II)-Pt(II) complexes at the biological substrate, 1O2 is localized providing effective targeting of the highly reactive oxygen species. The visible light induced activity of the [(Ph₂phen)₂Ru(BL)PtCl₂]₂+ (BL = dpp or dpq) supramolecules suggests a new mode of action in relation to cisplatin, which was further supported by the enhanced photocytotoxicity observed in the presence of U87MG cells. The results indicate that the Ru(II)-Pt(II) supramolecular structural motif hold great promise as a future photochemotherapy agent.
  • Thumbnail Image
    Rapid Synthesis, Characterization, and Catalytic Function of Rhodium(III) and Iridium(III) Chloro-bridged Dimers
    Brown, Loren (Virginia Tech, 2019-06-03)
    Rh(III) and Ir(III) dimeric complexes with tunable cyclopentadienyl (Cp) rings have proven versatile for both catalysis and as synthetic precursors. An efficient microwave method to synthesize Rh(III) and Ir(III) dimeric complexes [(η5-ring)MCl]2(μ2-Cl)2, (where (η5-ring)MCl = (η5-Me4C5R)Rh(III)Cl or (η5-Me4C5R)Ir(III)Cl) was developed. A modular design for the substituted cyclopentadienes HC5Me4R was based on Grignard reactions of 2,3,4,5-tetramethylcyclopent-2-en-1-one (R = alkyl, 12 examples; R = aryl, 3 examples) or by SNAr reactions of potassium tetramethylcyclopentadienide with perfluoroarenes (R = perfluoroaryl, 3 examples). Reaction of the Me4CpHR ligands with [M(COD)](μ2-Cl)2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) produced the dimeric complexes [Cp*RMCl]2(μ2-Cl)2 in moderate to excellent yield. The resulting dimers were characterized by nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction (XRD), high-resolution mass spectrometry (HRMS), elemental analysis, and examined as catalysts for oxidative lactonization of 1,4- and 1,5-diols. Oxidative lactonization of 1,4-butanediol to afford γ-butyrolactone proceeded selectively and efficiently using [(η5-Me4C5R)IrCl]2(μ2-Cl)2 as the catalyst. Several R substituents were tested to assess electronic substituent effects. The most active complex contained an electron donating group, R = CHMe2 and successfully catalyzed the formation of diols to lactones across a range of 1,4- and 1,5-diols, generally in high yield. Computational analysis of the rate-determining b-hydrogen elimination reactions provided an atomistic account of observed trends in reaction yield and selectivity as a function of substrate structure, while accounting neatly for the observed selective formation of lactones (vs. succinaldehyde) in the transfer dehydrogenation of 1,4-butyrolactone.
  • Thumbnail Image
    Rare Earth Extraction from Clayey Waste Materials by Alkali Pretreatment
    Liu, Wei (Virginia Tech, 2023-04-12)
    The increasing demand for rare earth elements (REEs) and the depletion of conventional rare earth deposits have enabled secondary REE resources to be promising feedstocks for REEs. Studies have been conducted in developing technologies that can physically preconcentrate and/or chemically extract REEs from low-REE-grade clayey waste materials (e.g., coal-based clays). However, the low REE grades and poor leachability of REE-bearing species still make the recovery of REEs from coal-based clays challenging. The primary objective of this study is to develop leaching technologies that can extract REEs from clayey waste materials under mild conditions (<100 oC). In the first part of this work, a novel leaching process consisting of NaOH pretreatment followed by ammonium sulfate leaching has been proposed to recover REEs from monazite, which served as a proxy for the rare earth phosphates identified in coal-based clays. In this process, monazite can be decomposed at 80 oC. The following ammonium sulfate leaching was conducted under less aggressive conditions (i.e., pH 4 and room temperature) to recover REEs. After releasing RE3+ ions from RE(OH)3(s) by acid, the role of ammonium sulfate in the leaching process may be explained by an ion exchange mechanism. Sulfate ions also benefit the leaching process by complexing with RE3+ ions. The influences of temperature and particle size on the leaching kinetics of REEs from the NaOH-treated monazite by ammonium sulfate were also investigated based on the shrinking core model. It was found that the leaching process is controlled by a chemical reaction with an activation energy of 61.28 kJ/mol. Besides ammonium sulfate, ammonium formate is a promising lixiviant for NaOH-treated monazite. However, other carboxylate ligands tested were inefficient at room temperature, mainly due to the slow dissolution kinetics of RE(OH)3(s) resulting from the passivation of the binuclear surface complexes. Subsequently, the feasibility of decomposing rare earth phosphates by NaOH in the presence of ethylenediaminetetraacetic acid (EDTA) was explored by constructing the stability diagrams for La-, Nd-, and Y-PO4-H2O systems, respectively. The simulation results were validated using three coal-based clay samples. The leaching results of both HCl and ammonium sulfate indicated that the pretreatment conducted by combining EDTA with dilute NaOH solutions (5-10%) could significantly enhance the REE leachability of the clay samples, with the light REEs (LREEs) being preferentially extracted compared to heavy REEs (HREEs). Under optimal conditions, the co-extraction of Al and Si can be significantly reduced. Besides liberated phosphate mineral particles, X-Ray photoelectron spectroscopy (XPS) analyses conducted on the synthetic ion adsorption clay samples revealed that phosphate could also passivate the REEs adsorbing on the surface of clay minerals in the form of “clay-RE-PO4”. This finding may partially explain the poor ion exchangeability of REEs in coal-based clays. After subjecting to the proposed NaOH pretreatment technique, the passivated REEs on the surface of clay can be effectively removed. Lastly, the possibility of preconcentrating REEs from a kaolinite flotation reject material was explored by froth flotation and the hydrophobic-hydrophilic separation (HHS). A final concentrate assaying 10,765 ppm of REEs and 71% of recovery was obtained by the HHS process, which was superior to flotation in dealing with ultrafine particles. The microscopic characterization of the concentrate revealed that rare earth phosphates were the major REE-bearing species. The leaching results showed that the proposed NaOH pretreatment followed by ammonium sulfate leaching was also an effective method to recover REEs from the upgraded clayey waste material under mild conditions (<100 oC).