UNT Theses and Dissertations - 10 Matching Results

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Design Considerations and Implementation of Portable Mass Spectrometers for Environmental Applications

Description: Portable mass spectrometers provide a unique opportunity to obtain in situ measurements. This minimizes need for sample collection or in laboratory analysis. Membrane Inlet Mass Spectrometry (MIMS) utilizing a semi permeable membrane for selective rapid introduction for analysis. Polydimethylsiloxane membranes have been proven to be robust in selecting for aromatic chemistries. Advances in front end design have allowed for increased sensitivity, rapid sample analysis, and on line measurements. Applications of the membrane inlet technique have been applied to environmental detection of clandestine drug chemistries and pollutants. Emplacement of a mass spectrometer unit in a vehicle has allowed for large areas to be mapped, obtaining a rapid snapshot of the various concentrations and types of environmental pollutants present. Further refinements and miniaturization have allowed for a backpackable system for analysis in remote harsh environments. Inclusion of atmospheric dispersion modeling has yielded an analytical method of approximating upwind source locations, which has law enforcement, military, and environmental applications. The atmospheric dispersion theories have further been applied to an earth based separation, whereby chemical properties are used to approximate atmospheric mobility, and chemistries are further identified has a portable mass spectrometer is traversed closer to a point source.
Date: May 2017
Creator: Mach, Phillip Michael
Partner: UNT Libraries

Design, Synthesis and Optoelectronic Properties of Monovalent Coinage Metal-Based Functional Materials toward Potential Lighting, Display and Energy-Harvesting Devices

Description: Groundbreaking progress in molecule-based optoelectronic devices for lighting, display and energy-harvesting technologies demands highly efficient and easily processable functional materials with tunable properties governed by their molecular/supramolecular structure variations. To date, functional coordination compounds whose function is governed by non-covalent weak forces (e.g., metallophilic, dπ-acid/dπ-base stacking, halogen/halogen and/or d/π interactions) remain limited. This is unlike the situation for metal-free organic semiconductors, as most metal complexes incorporated in optoelectronic devices have their function determined by the properties of the monomeric molecular unit (e.g., Ir(III)-phenylpyridine complexes in organic light-emitting diodes (OLEDs) and Ru(II)-polypyridyl complexes in dye-sensitized solar cells (DSSCs)). This dissertation represents comprehensive results of both experimental and theoretical studies, descriptions of synthetic methods and possible application allied to monovalent coinage metal-based functional materials. The main emphasis is given to the design and synthesis of functional materials with preset material properties such as light-emitting materials, light-harvesting materials and conducting materials. In terms of advances in fundamental scientific phenomena, the major highlight of the work in this dissertation is the discovery of closed-shell polar-covalent metal-metal bonds manifested by ligand-unassisted d10-d10 covalent bonds between Cu(I) and Au(I) coinage metals in the ground electronic state (~2.87 Å; ~45 kcal/mol). Moreover, this dissertation also reports pairwise intermolecular aurophilic interactions of 3.066 Å for an Au(I) complex, representing the shortest ever reported pairwise intermolecular aurophilic distances among all coinage metal(I) cyclic trimetallic complexes to date; crystals of this complex also exhibit gigantic luminescence thermochromism of 10,200 cm-1 (violet to red). From applications prospective, the work herein presents monovalent coinage metal-based functional optoelectronic materials such as heterobimetallic complexes with near-unity photoluminescence quantum yield, metallic or semiconducting integrated donor-acceptor stacks and a new class of Au(III)-based black absorbers with cooperative intermolecular iodophilic (I…I) interactions that sensitize the harvesting of all UV, all visible, and a broad spectrum of near-IR ...
Date: August 2017
Creator: Ghimire, Mukunda Mani
Partner: UNT Libraries

Diphosphine Ligand Activation Studies with Organotransition-Metal Compounds

Description: Thermolysis of CoRu(CO)7(m -PPh2) (1) in refluxing 1,2-dichloroethane in the presence of the diphosphine ligands 2,3-bis(diphenylphosphino)maleic anhydride (bma) and 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) furnishes the new mixed-metal complexes CoRu(CO)4(μ -P-P)(μ -PPh2) [where P-P = bma (3); bpcd (6)], along with trace amounts of the known complex CoRu(CO)6(PPh3)(μ -PPh2) (4). The requisite pentacarbonyl intermediates CoRu(CO)5(μ -P-P)(μ -PPh2) [where P-P = bma (2); bpcd (5)] have been prepared by separate routes and studied for their conversion to CoRu(CO)4(μ -P-P)(μ -PPh2). The complexes 2/3 and 5/6 have been isolated and fully characterized in solution by IR and NMR spectroscopy. The kinetics for the conversion of 2→3 and of 5→6 were measured by IR spectroscopy in chlorobenzene solvent. On the basis of the first-order rate constants, CO inhibition, and the activation parameters, a mechanism involving dissociative CO loss as the rate-limiting step is proposed. The solid-state structure of CoRu(CO)4(μ -bma)(μ -PPh2) (3) reveals that the two PPh2 groups are bound to the ruthenium center while the maleic anhydride π bond is coordinated to the cobalt atom. Thermolysis of the cluster Ru3(CO)12 with the bis(phosphine)hydrazine ligand (MeO)2PN(Me)N(Me)P(OMe)2 (dmpdmh) in toluene at 75°C furnishes the known clusters Ru4(CO)12[μ -N(Me)N(Me)] (9) and Ru3(CO)11[P(OMe)3] (10), in addition to the new cluster Ru3(CO)10(dmpdmh) (8) and the phosphite-tethered cluster Ru3(CO)9[μ -P(OMe)3] (11). The simple substitution product Ru3(CO)10(dmpdmh), a logical intermediate to clusters 9-11, was synthesized by treating Ru3(CO)12 and dmpdmh with Me3NO in CH2Cl2 at room temperature, and independent thermolysis reactions using cluster 8 were shown to yield clusters 9-11. The tetrahedrane cluster FeCo2(CO)9(μ3-S) reacts with the redox-active ligand 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) to give the disubstituted cluster FeCo2(CO)7(bpcd)(μ3-S) as the sole product. This diphosphine-substituted cluster contains a cobalt-bound, chelating bpcd ligand. The solid-state structure has been unequivocally established by X-ray diffraction analysis. Cyclic voltammetric studies on FeCo2(CO)7(bpcd)(μ3-S) reveal the presence of two quasireversible ...
Date: December 2000
Creator: Wang, Jiancheng
Partner: UNT Libraries

Interfacial Studies of Bimetallic Corrosion in Copper/Ruthenium Systems and Silicon Surface Modification with Organic and Organometallic Chemistry

Description: To form Cu interconnects, dual-damascene techniques like chemical mechanical planarization (CMP) and post-CMP became inevitable for removing the "overburden" Cu and for planarizing the wafer surface. During the CMP processing, Cu interconnects and barrier metal layers experience different electrochemical interactions depending on the slurry composition, pH, and ohmic contact with adjacent metal layers that would set corrosion process. Ruthenium as a replacement of existing diffusion barrier layer will require extensive investigation to eliminate or control the corrosion process during CMP and post CMP. Bimetallic corrosion process was investigated in the ammonium citrate (a complexing agent of Cu in CMP solutions) using micro test patterns and potentiodynamic measurements. The enhanced bimetallic corrosion of copper observed is due to noble behavior of the ruthenium metal. Cu formed Cu(II)-amine and Cu(II)-citrate complexes in alkaline and acidic solutions and a corrosion mechanism has been proposed. The currently used metallization process (PVD, CVD and ALD) require ultra-high vacuum and are expensive. A novel method of Si surface metallization process is discussed that can be achieved at room temperature and does not require ultra-high vacuum. Ruthenation of Si surface through strong Si-Ru covalent bond formation is demonstrated using different ruthenium carbonyl compounds. RBS analysis accounted for monolayer to sub-monolayer coverage of Si surface. Interaction of other metal carbonyl (like Fe, Re, and Rh) is also discussed. The silicon (111) surface modifications with vinyl terminated organic compounds were investigated to form self-assembled monolayers (SAMs) and there after these surfaces were further functionalized. Acrylonitrile and vinylbenzophenone were employed for these studies. Ketone group of vinylbenzophenone anchored to Si surface demonstrated reactivity with reducing and oxidizing agents.
Date: August 2006
Creator: Nalla, Praveen Reddy
Partner: UNT Libraries

Investigations of Thermochemistry and the Kinetics of H Atom Radical Reactions

Description: The thermochemistry of several species, and the kinetics of various H atom radical reactions relevant to atmospheric and combustion chemistry were investigated using ab initio theoretical techniques and the flash photolysis / resonance fluorescence technique. Using ab initio quantum mechanical calculations up to the G3 level of theory, the C-H bond strengths of several alkanes were calculated. The bond strengths were calculated using two working reactions. From the results, it is apparent that the bond strengths decrease as methyl groups are added to the central carbon. The results are in good agreement with recent experimental halogenation kinetic studies. Hydrogen bond strengths with sulfur and oxygen were studied via CCSD(T) theory, together with extrapolation to the complete basis set limit. The results for the bond dissociation energies (ground state at 0 K, units: kJ mol-1) are: S-H = 349.9, S-D = 354.7, HS-H = 376.2, DS-D = 383.4, and HO-H = 492.6. These data compare well with experimental literature. The rate constants for the isotopic reactions of H + H2S, D + H2S, H + D2S, and D + D2S are studied at the QCISD(T)/6-311+G(3df,2p) level of theory. The contributions of the exchange reaction versus abstraction are examined through transition state theory. The energy of NS was computed via CCSD(T) theory, together with extrapolation to the complete basis set limit. The results were employed with three working reactions to find ΔfH0(NS) = 277.3 ± 2 kJ mol-1 and ΔfH298(NS) = 278.0 ± 2 kJ mol-1. This thermochemistry is consistent with, but much more precise than, earlier literature values. A kinetic study of the reaction of H + CH2CCl2 was conducted over the temperature range of 298 - 680 K. The reaction was found to be pressure dependent and results of the rate constants and their interpretation via unimolecular rate theory are ...
Date: December 2002
Creator: Peebles, Lynda Renee
Partner: UNT Libraries

Layered Double Hydroxides: Morphology, Interlayer Anion, and the Origins of Life

Description: The preparation of layered double hydroxides via co-precipitation of a divalent/trivalent metal solution against a base results in 1 mm LDH particles with a disorganized metal lattice. Research was performed to address these morphological issues using techniques such as Ostwald ripening and precipitation via aluminate. Another interesting issue in layered double hydroxide materials is the uptake and orientation of anions into the interlayer. Questions about iron cyanide interlayer anions have been posed. Fourier transform infared spectroscopy and powder x-ray diffraction have been used to investigate these topics. It was found that factors such as orientation, anion charge, and anion structure depended on the divalent/trivalent metal ratio of the hydroxide layer and reactivity time. The cyanide self-addition reaction is an important reaction of classical prebiotic chemistry. This reaction has been shown to give rise to amino acids, purines and pyrimidines. At cyanide concentrations similar to that expected on the early earth, hydrolysis to formamide rather than self-addition occurs. One theory to alleviate this side reaction is the use of minerals or clays that are thought to concentrate and catalyze prebiotics of interest. Layered double hydroxides have been studied as a catalyst for this reaction.
Date: December 2002
Creator: Halcom-Yarberry, Faith Marie
Partner: UNT Libraries

Mechanisms of Methoxide Ion Substitution and Acid- Catalyzed Z/E Isomerization of N-Methoxyimines

Description: The second order rate constants for nucleophilic substitution by methoxide of (Z)- and (E)-O-methylbenzohydroximoyl fluorides [C6H4C(F)=NOCH3] with various substituents on the phenyl ring [p-OCH3 (1h, 2h), p-CH3 (1g, 2g), p-Cl (1f, 2f), p-H (1e, 2e), (3,5)-bis-CF3 (1i, 2i)] in 90:10 DMSO:MeOH have been measured. A Hammett plot of these rate constants vs σ values gave positive ρ values of 2.95 (Z isomer) and 3.29 (E isomer). Comparison of these rates with methoxide substitution rates for Omethylbenzohydroximoyl bromide [C6H4C(Br)=NOCH3] and Omethylbenzohydroximoyl chloride [C6H4C(Cl)=NOCH3] reveal an element effect for the Z isomers of Br:Cl:F(1e) = 2.21:1.00:79.7 and for the E isomers of Cl:F(2e) = 1.00:18.3. With the p-OCH3-imidoyl halides the following element effects are found: Br:Cl:F(1h) = 2.78:1.00:73.1 for the Z isomer and Br:Cl:F(2h) = 1.97:1.00:12.1 for the E isomer. Measurement of activation parameters revealed ∆S≠ = -17 eu for 1e and ∆S≠ = -9.9 eu for 2e. Ab initio calculations (HF/6-31+G*, MP2/6-31+G*//HF/6-31+G*, B3LYP/6- 31+G*//HF/6-31+G*, HF-SCIPCM/6-31+G*//HF/6-31+G*) were performed to define the reaction surface. These calculations demonstrate a relatively large barrier for nucleophilic attack in relation to halogen loss and support the experimental findings that this reaction proceeds by an addition-elimination mechanism (AN# + DN). The imidoyl fluorides have been used to synthesize highly functionalized O-methyloximes by reaction with enolate anions derived from malononitrile, ethyl cyanoacetate, and diethyl malonate. Acid-catalyzed isomerization of compounds containing the O-methyloxime moiety have been investigated with ab initio calculations (HF/6-31+G*, MP2/6- 31+G*//HF/6-31+G*, B3LYP/6-31+G*//HF/6-31+G*). Barriers for rotation around the C-N bond following protonation have been calculated. The calculated barriers are discussed in relation to an isomerization mechanism of protonation-rotation versus a nucleophilic catalysis.
Date: December 2001
Creator: Dolliver, Debra D.
Partner: UNT Libraries

Microwave-Assisted Synthesis and Photophysical Properties of Poly-Imine Ambipolar Ligands and Their Rhenium(I) Carbonyl Complexes

Description: The phenomenon luminescence rigidochromism has been reported since the 1970s in tricarbonyldiimine complexes with a general formula [R(CO)3LX] using conventional unipolar diimine ligands such as 2,2;-bipyridine or 1,10-phenanthroline as L, and halogens or simple solvents as X. As a major part of this dissertation, microwave-assisted synthesis, purification, characterization and detailed photoluminescence studies of the complex fac-[ReCl(CO)3L], 1, where L = 4-[4,6-bis(3,5-dimethyl-1H-pyrazol-1-yl]-N,N-diethylbenzenamine are reported. The employment of microwaves in the preparation of 1 decreased the reaction time from 48 to 2 hours compared to the conventional reflux method. Stoichiometry variations allows for selective preparation of either a mononuclear, 1, or binuclear, fac-[Re2Cl2(CO)6], 2, complex. The photophysical properties of 1 were analyzed finding that it possesses significant luminescence rigidochromism. The steady state photoluminescence emission spectra of 1 in solution shift from 550 nm in frozen media to 610 nm when the matrix becomes fluid. Moreover, a very sensitive emission spectral analysis of 0.1 K temperatures steps shows a smooth transition through the glass transition temperature of the solvent host. Furthermore, synthetic modifications to L have attained a family of ambipolar compounds that have tunable photophysical, thermophysical and other material properties that render them promising candidates for potential applications in organic electronics and/or sensors - either as is or for their future complexes with various transition metals and lanthanides.
Date: August 2017
Creator: Salazar Garza, Gustavo Adolfo
Partner: UNT Libraries

Rational Design of Metal-organic Electronic Devices: a Computational Perspective

Description: Organic and organometallic electronic materials continue to attract considerable attention among researchers due to their cost effectiveness, high flexibility, low temperature processing conditions and the continuous emergence of new semiconducting materials with tailored electronic properties. In addition, organic semiconductors can be used in a variety of important technological devices such as solar cells, field-effect transistors (FETs), flash memory, radio frequency identification (RFID) tags, light emitting diodes (LEDs), etc. However, organic materials have thus far not achieved the reliability and carrier mobility obtainable with inorganic silicon-based devices. Hence, there is a need for finding alternative electronic materials other than organic semiconductors to overcome the problems of inferior stability and performance. In this dissertation, I research the development of new transition metal based electronic materials which due to the presence of metal-metal, metal-?, and ?-? interactions may give rise to superior electronic and chemical properties versus their organic counterparts. Specifically, I performed computational modeling studies on platinum based charge transfer complexes and d10 cyclo-[M(?-L)]3 trimers (M = Ag, Au and L = monoanionic bidentate bridging (C/N~C/N) ligand). The research done is aimed to guide experimental chemists to make rational choices of metals, ligands, substituents in synthesizing novel organometallic electronic materials. Furthermore, the calculations presented here propose novel ways to tune the geometric, electronic, spectroscopic, and conduction properties in semiconducting materials. In addition to novel material development, electronic device performance can be improved by making a judicious choice of device components. I have studied the interfaces of a p-type metal-organic semiconductor viz cyclo-[Au(µ-Pz)]3 trimer with metal electrodes at atomic and surface levels. This work was aimed to guide the device engineers to choose the appropriate metal electrodes considering the chemical interactions at the interface. Additionally, the calculations performed on the interfaces provided valuable insight into binding energies, charge redistribution, change in the energy ...
Date: December 2012
Creator: Chilukuri, Bhaskar
Partner: UNT Libraries

Substituent Effects: A Computational Study on Stabilities of Cumulenes and Low Barrier Hydrogen Bonds

Description: The effect of substituents on the stabilities of cumulenes-ketenes, allenes, diazomethanes and isocyanates and related systems-alkynes, nitriles and nitrile oxides is studied using the density functional theory (B3LYP, SVWN and BP86) and ab initio (HF, MP2) calculations at the 6-31G* basis set level. Using isodesmic reactions, correlation between stabilization energies of cumulenes and substituent group electronegativities (c BE) is established and the results from DFT and MP2 methods are compared with the earlier HF calculations. Calculations revealed that the density functional methods can be used to study the effect of substituents on the stabilities of cumulenes. It is observed that the cumulenes are stabilized by electropositive substituent groups from s -electron donation and p -electron withdrawal and are destabilized by electronegative substituent groups from n-p donation. The calculated geometries of the cumulenes are compared with the available experimental data.High level ab initio and density functional theory calculations have been used to study the energetics of low-barrier hydrogen bond (LBHB) systems. Using substituted formic acid-formate anion complexes as model LBHB systems, hydrogen bond strength is correlated to the pKa mismatch between the hydrogen bond donor and the hydrogen bond acceptor. LBHB model systems are characterized by the 1H-NMR chemical shift calculations. A linear correlation between the calculated hydrogen bond strength and the predicted 1H-NMR chemical shift was established. It is concluded that the pKa matching within the enzyme active site of the two species involved in the LBHB is important to maximizing catalytic stabilization.
Date: August 2000
Creator: Kumar, Ganesh Angusamy
Partner: UNT Libraries