Chemistry & Biochemistry Theses and Dissertations
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Item Experimental and Theoretical Characterization of Effective Interactions Near 132Sn(1987) Stone, Craig A.; Walters, William B.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)Experimental investigations have been undertaken to study the multiplet structure in six nuclei near 132Sn: 132,130Sb, 131,129,127Sb, and 132Te. Experiments were performed using ion beams of mass-separated fission products produced by the TRISTAN mass separator at Brookhaven National Laboratory. Extensive four-detector gamma-gamma coincidences, gamma-multiscaling and conversion-electron data have been collected. Ultralarge shell model calculations were performed using the VLADIMIR shell model code on the Cray/CDC 7600 supercomputer system at Lawrence Livermore National Laboratory. These calculations were designed to look at the performance of the Kallio-Kolltveit and Siemen's g-matiix potentials on the 1-3 quasiparticle nuclides in the gddsh model space. Results show that realistic potentials work well on nuclei near 132Sn but show problems with 129,130Sn and 131Sb which can not be accounted for by core-polarization corrections. Problems are shown to be due to the use of a potential derived with the Scott-Moszkowski separation metl1od. The separation distance was demonstrated to have a weak dependence on the principal quantum number but a strong dependence on the orbital angular momentum. This suggests the Kallio-Kolltveit potential is underestimating the strength of the h11/2 interactions in 129,130Sn and 131Sb.Item Spacial and Temporal Variations in 36CI Deposition in the Northern United States(1994) Hainsworth, Laura J.; Mignerey, Alice C.; Chemistry and Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Chlorine-36, a cosmogenic radioisotope, has been developed for use as a tracer in hydrological systems. The deposition of atmospheric 36Cl, although of primary importance to hydrological applications, has not been well studied. To begin to address this problem, 36Cl has been measured in monthly, wet-only, precipitation samples collected from February, 1991, to January, 1993, at the Elms Environmental Education Center in St. Mary's County, Maryland. In addition, bulk deposition samples were collected over a 1 y period at seven sites across the Northern United States and analyzed for 36CI. The mean, wet-only 36Cl/Cl ratio for the 2 y sampling period is 68±19 (x10 -15), and the mean 36 CI concentration is 1.2±0. 1 (x10 6) atoms/L. The 36Cl wet deposition flux data reveal a distinct seasonal deposition pattern, with peaks occurring in March and April. This pattern is attributed to stratospheric/ tropospheric exchange. The mean 36Cl wet deposition flux is 38.2±5 atoms/m2s. Comparison between wet-only and bulk deposition samples indicates that the difference accounts for approximately 25% of the total 36Cl deposition flux at the Elms site. A new model, using 90Sr to predict the 36CI deposition pattern, is developed to predict 36Cl/Cl ratios across the United States. Chlorine-36/Cl ratios in bulk deposition samples collected across the northern United States agree well with the model predictions. A mean global 36Cl production rate of approximately 28 to 38 atoms/m2s is indicated by these samples. A comparison between 36Cl concentrations in the Aquia and Magothy aquifers is southern Maryland and bulk deposition samples collected at the Elms, MD, site indicated that modern precipitation can account for the 36Cl content in the youngest water in these aquifers. Surface water samples from the Susquehanna River basin reveal 36 Cl and stable chloride concentrations an order of magnitude higher than in bulk deposition samples collected at State College, PA. The source of excess 36Cl in the Susquehanna is not known. Possible explanations include 'bomb-pulse' 36Cl and in-situ 36CI production in surface rocks.Item Investigation of the Tetrahymena Pyriformis 2-Aminoethylphosphonic Acid Biosynthetic Pathway and the P-C Bond Forming Enzyme Phosphoenolpyruvate Mutase(1991) McQueney, Michael Scott; Dunaway-Mariano, Debra; Chemistry and Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The biosynthetic pathway leading to 2-aminoethylphosphonate in Tetrahymena pyriformis was determined. A cell-free homogenate of T. pyriformis converted phosphoenolpyruvate to AEP in 37% yield, phosphonopyruvate to AEP in a 11 % yield and phophonoacetaldehyde to AEP in an 83% yield. The Tetrahymena pyriformis enzyme, PEP mutase was purified. The PEP Mutase catalyzes the rearrangement of phosphoenolpyruvate to phosphonopyruvate and the equilibrium constant is >500:1 in favor of phosphoenolpyruvate. To distinguish between an intra-and intermolecular reaction pathway for this process an equimolar mixture of [P= ^18O, C(2)- ^18 O]thiophosphonopyruvate and (all - ^16Q)thiophosphonopyruvate was reacted with the PEP mutase and the resulting products were analyzed by ^31P-NMR. The absence of the cross over product [C(2)-18O]thiophosphonoenolpyruvate in the product mixture was interpreted as evidence for an intramolecular reaction pathway. To distinguish between a concerted and stepwise intramolecular reaction pathway the pure enantiomers of the chiral substrate [P= ^18Q]thiophosphonopyruvate were prepared and the stereochemical course of their conversion to chiral [P= ^18O]thiophosphoenolpyruvate was determined. Based on the observed conversion of (Sp) - [P= ^18O]thiophosphonopyruvate to (Sp)-[P= ^18O]thiophosphoenolpyruvate and (Rp) - [P = ^18O]thiophosphonopyruvate to (Rp)[P= ^18O]thiophosphoenolpyruvate it was concluded that the PEP phosphomutase reaction proceeds with retention of the phosphorus configuration and therefore by a stepwise mechanism. The similar reactivity of the oxo and thio substituted phosphonopyruvate substrates (i.e., nearly equal Vmax) was interpreted to suggest that addition to the phosphorus atom is not rate limiting among the reaction steps. Lastly, single turnover experiments failed to trap a pyruvate in the PEP mutase reaction.Item The Role of Hydrogen Cyanide in Chemical Evolution(1989) Navarro-González, Rafael; Ponnamperuma, Cyril; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Two major research areas are investigated: The electrosynthesis of hydrogen cyanide; and the role of cyanocomplexes in the free - radical oligomerization of hydrogen cyanide. The electric discharge production of hydrogen cyanide from a simulated primitive atmosphere composed of methane, nitrogen and water vapor was investigated. The radiation chemical yield (G) of formation of HCN was determined to be 0.26. A free radical mechanism was proposed to account for the observed chemical changes. Computer simulations of the reaction mechanism could effectively model the early stages of electrolysis of the gas mixture, and permitted the estimation of the rate of electrosynthesis of hydrogen cyanide under various atmospheric conditions . The possible role of cyanocomplexes of transition elements on the free- radical oligomerization of hydrogen cyanide was investigated. Aqueous, oxygenfree, dilute solutions of hydrogen cyanide and hexacyanoferrate(II) or (III) were submitted to various doses of gamma irradiation. The presence of either cyanocomplex led to a significant decrease in the rate of decomposition of hydrogen cyanide. The major products were ammonia and carbon dioxide . Computer simulations of these systems permitted the elucidation of the reaction mechanism and the derivation of rates of reactions of free- radicals with the cyanocomplexes. The results obtained provide an insight into the possible role of cyanocomplexes of transition elements in chemical evolution.Item CHROMIUM OXIDATION AND REDUCTION BY HYDROGEN PEROXIDE IN DIVERSE SOILS AND SIMPLE AQUEOUS SYSTEMS(1999) Rock, Melanie Louise; Helz, George R.; James, Bruce R.; Chemistry; Environmental Science & Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Hydrogen peroxide is being tested for in situ remediation of buried contaminants - either as a direct chemical oxidant in Fenton-type reactions or as a source of oxidizing equivalents in bioremediation. How it affects a common co-contaminant, Cr, is explored here in four chemically diverse high-Cr soils. Soils contaminated with high levels of soluble Cr(VI) from ore processing and soils containing high levels of recently reduced Cr(III) from electroplating waste showed marked increases in chromate after single applications of J-25 mM peroxide. Cr(VI) in the leachates exceeded the drinking water standard (2μM) by 1-3 orders of magnitude. Soluble Cr(III), in the form of dissolved organic complexes, contributed to the likelihood of Cr(III) oxidation. Anaerobic soil conditions at a tannery site prevented oxidation of Cr(III). Naturally occurring Cr in serpentine soil also resisted oxidation. Ambient soluble Cr(VI) in a contaminated aquifer disappeared from peroxide leachates below pH 5, then reappeared as peroxide levels declined. In solutions prepared under environmentally relevant conditions, aged 280 μM Cr(III) treated with 100 μM H2O2 showed increases in Cr(VI) over weeks with maximum oxidation rates achieved in solutions prepared with 2:1 and 4:1 OH^-:Cr. Although Cr(III) speciation differs in fresh and aged aqueous systems, a similar mechanism involving the pre-equilibrium step: Cr(OH)/ + OH- .,. Cr(OH)/ may account for Cr(III) oxidation in both systems. Under alkaline conditions, H2O2 enhanced the oxidative dissolution of Crn(OH)3n^0. The formation of peroxochromium compounds in the presence of H2O2 and Cr(VI) may account for the disappearance and reappearance of Cr(VI) in H2O2 treated soils; as does the possible formation and subsequent reoxidation of Crm\(OH)3n-2^2+ oligomers. Mobilization of hazardous Cr(VI) must be considered in plans to use H2O2 for remediation of chemically complex wastes. Once Cr(III) is oxidized to Cr(VI) by H2O2 it may persist long after applied H2O2 treatments have disappeared. Further, hexavalent Cr will behave as a catalyst toward H20 2 in soils, enhancing its oxidative capacity while helping to dissipate high levels of applied H2O2.Item Transition Metal Polypnictides from Zintl Ions(1995) Charles, Scott; Eichhorn, Bryan W.; Chemistry & Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The chemistry of the soluble E7 3- ions (E = P, As, Sb) with various transition metal complexes has been investigated. Alloys of K3E7 react with (arene)M(CO)3 (M = Cr, Mo, W) complexes in the presence of three equivalents of 2,2,2-crypt in ethylenediamine to give [K(2,2,2-crypt)]3[E7M(CO)3] complexes. Nine [E7M(CO)3] 3- compounds (E = P, As, Sb; M = Cr, Mo, W) have been prepared. The compounds have norbomadiene-like E7 fragments, with a formal negative charge associated with the unique two-coordinate pnictogen furthest from the transition metal. The bonding is described as E7 π-type interactions with the metal center. 31p NMR studies show that these compounds undergo an intramolecular wagging process in solution. The [E7M(C0)3] 3- ions are modestly basic and highly nucleophilic. The complexes react with weak acids, tetraalkylammonium salts and (arene)M'(CO)3 complexes to form the [K(2,2,2-crypt)]2[HE7M(CO)3], [K(2,2,2- crypt)]2[RE7M(CO)3] and [K(2,2,2-crypt)]3[(L2)(CO)3M'E7M(CO)3] compounds, respectively. Each of the structures contains a norbornadiene-like E7 fragment bound η4 to the M(CO)3 fragment and η1 to the appended moiety [H+, R+, M(CO)3(en)] that is attached to the pnictogen atom furthest from the M(CO)3 center. The [K(2,2,2-crypt)]3 [η4-E7M(CO)3] complexes also react with carbon monoxide at the M(CO)3 center and reversible binds CO to form [K(2,2,2-crypt)]3[η4-E7M(CO)4] complexes. These complexes are more easily protonated and undergo faster alkylations of the E7 cages than the parent [K(2,2,2-crypt)]3[η4-E7M(CO)3] compounds. Each of these structures contains an E7 fragment bound η2 to the M(C0)4 center. Products were characterized by various physical and spectroscopic techniques including UV-vis, IR, 1H, 13C, and 31P NMR spectroscopies, mass spectrometry, elemental analyses, and single crystal X-ray diffraction studies.Item Release of Aluminum, Arsenic, Cadmium, Chromium, Copper, Iron, Lead, and Zinc in a Coal Leachate, and their Removal from Solution Undergoing Neutralization(1992) Tatum, Thomas Lee; Helz, George R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Whole coal contains significant amounts of iron pyrite which is oxidized ultimately to ferric acid sulfate. As a result, trace elements are released from the coal and other minerals in potentially hazardous concentrations. The purpose of this research was to: 1) study the release and mobility of selected trace elements during the weathering of coal; 2) seek to understand factors controlling solubility of trace elements in a synthetic, acidic leachate undergoing gradual neutralization; and 3) develop a chemical thermodynamic computer model to predict the effects of dilution and neutralization of leachate on trace element mobility and speciation. Samples collected periodically from a slurry of whole ground coal in water were filtered and analyzed for dissolved sulfate (by ion chromatrography), iron (by flame atomic absorption spectrophotometry), and Al, Zn, Cd, Cu, Cr, Pb, As, and Se (by graphite furnace AAS). Iron, copper, and probably arsenic tracked the production of sulfate, while aluminum, zinc, chromium, and cadmium concentrations were stable or rose slightly. A synthetic leachate of ferric sulfate and sulfuric acid was doped with trace levels of Al, Zn, Cu, Cd, Cr, Pb, As, and Se. Slow injection of sodium bicarbonate solution neutralized the stirred system, though hydrolysis of iron buffered the pH near 2.5. Computer modeling of the sample analyses indicated that sulfate complexes dominated the speciation of iron and the trace elements. The other findings were used to develop a thermodynamic equilibrium model based on the aqueous geochemistry computer model PHREEQE. Iron and sulfate removal were best modeled by the precipitation of Fe16O16 (OH) 12 (SO4 ) 2. Aluminum solubility was modeled by precipitation of jurbanite below pH 4, of bayerite and basaluminite for pH 4 - 5, and of gibbsite at pH above 5. Chromium, copper, and lead removal was modeled by solid solution formation with the ferric oxyhydroxysulfate precipitates. Program convergence failures above pH 5 precluded the modeling of zinc and cadmium, but it is hypothesized that their ions are adsorbed onto suspended particles of hydrous ferric oxyhydroxides. The model was tested with our laboratory data, and field data from a creek system contaminated with acid sulfate mine drainage.