Heterogeneous reactions have the potential to significantly impact urban ozone formation and total reactivity of the atmosphere. This dissertation used comparisons between photochemical modeling predictions and field measurements to examine heterogeneous N₂O₅ chemistry in Southeast Texas. Heterogeneous reactions of N₂O₅ can lead to two different products: nitric acid (HNO₃) and nitryl chloride (ClNO₂). The formation of HNO₃ results in a loss of reactive nitrogen from the atmosphere. In contrast ClNO₂ photolysis forms Cl radicals and NO₂, both of which promote ozone formation in the troposphere. Preliminary modeling identified key uncertainties and the need to perform more refined modeling which included updated PM emissions estimates, an updated gas-phase N₂O₅ hydrolysis reaction rate constant, updated reactive uptake coefficients, and the inclusion of ClNO₂ as a product of heterogeneous N₂O₅ uptake. Refined modeling which incorporated all of these improvements was carried out and was the first comprehensive modeling of this chemistry performed for an urban air pollution episode. Comparisons of aerosol surface area concentrations, N₂O₅ concentrations, HNO₃ concentrations, and ClNO₂ concentrations with ambient data showed that model predictions were reasonable. The exceptions to this were 1) over-predictions of aerosol surface area concentration peaks at altitudes above 1500 meters and 2) over-prediction of N₂O₅ concentrations in the Houston Ship Channel. Further analysis is needed to identify the reasons for these over-predictions. Other key findings from this modeling include the model prediction of inland chlorine concentrations high enough to form ClNO₂ and the prediction that a large portion of atmospheric chlorine is cycled through ClNO₂, therefore making the inclusion of ClNO₂ into photochemical models essential for properly simulating chlorine chemistry. In addition, modeling suggested that the chemistry leads to significant increases of NO[subscript x] at night, but decreases in daytime NO[subscript x] concentrations and that the overall effect was to decrease ozone concentrations. Further investigation into the effect of ClNO₂ as a chlorine source showed that likely ozone increases in the Houston area caused by the presence of this compound are on the order of several ppb. Further analyses showed that vertical dispersion and local atmospheric composition moderated the effect of nitryl chloride on ozone mixing ratios. / text

The reduction of dinitrogen (N2) to ammonia (NH3) will continue to play a vital role in society as the population of the world grows and maintains its dependence on artificial fertilizers. This energy-intensive transformation is achieved industrially by the Haber-Bosch process and naturally via nitrogenase enzymes. Recent synthetic systems attempt to produce NH3 artificially but with lower energy costs than Haber-Bosch by modeling their designs after nitrogenase. This dissertation describes the progress made in one iron-phosphine system, the water-soluble Fe(DMeOPrPE)2N2, capable of producing NH3 at room temperature and pressure. Chapter I describes the history of the coordination chemistry of N2 to a variety of metals, with a focus on iron complexes. In addition to exploring the range of coordination geometries and supporting ligands of such complexes, the application of N2 coordination complexes towards NH3 formation is analyzed. Chapter II discusses the various methods for quantifying yields of ammonia. Along with a historical perspective on the popular indophenol method, the challenges and best conditions for measuring NH3 in the Fe-DMeOPrPE system are defined. Chapter III explores a series of trans-hydrido intermediates along a potential protonation pathway of Fe(DMeOPrPE)2N2. The complete series of reduced dinitrogen ligands (N2, N2H2, N2H4, and NH3) on the Fe(DMeOPrPE)2H+ scaffold is described. Chapter IV highlights the discovery and characterization of a unique bridged Fe(I) dimer, observed during the protonation of Fe(DMeOPrPE)2N2 as a dark purple intermediate. Chapter V describes the electrochemistry of certain intermediates in the Fe-DMeOPrPE system. This insight should open new avenues for future investigations. By altering the electronics of the system, more NH3 may eventually be produced. Chapter VI provides a summary of this work. This dissertation includes previously published and unpublished co-authored material. / 10000-01-01

Reductive functionalization of methyl ligands by 3d metal catalysts and two possible side reactions has been studied. Selective oxidation of methane, which is the primary component of natural gas, to methanol (a more easily transportable liquid) using organometallic catalysis, has become more important due to the abundance of domestic natural gas. In this regard, reductive functionalization (RF) of methyl ligands in [M(diimine)2(CH3)(Cl)] (M: VII (d3) through CuII (d9)) complexes, has been studied computationally using density functional techniques. A SN2 mechanism for the nucleophilic attack of hydroxide on the metal-methyl bond, resulting in the formation of methanol, was studied. Similar highly exergonic pathways with very low energy SN2 barriers were observed for the proposed RF mechanism for all complexes studied. To modulate RF pathways closer to thermoneutral for catalytic purposes, a future challenge, paradoxically, requires finding a way to strengthen the metal-methyl bond. Furthermore, DFT calculations suggest that for 3d metals, ligand properties will be of greater importance than metal identity in isolating suitable catalysts for alkane hydroxylation in which reductive functionalization is used to form the C—O bond. Two possible competitive reactions for RF of metal-methyl complexes were studied to understand the factors that lower the selectivity of C—O bond forming reactions. One of them was deprotonation of the methyl group, which leads to formation of a methylene complex and water. The other side reaction was metal-methyl bond dissociation, which was assessed by calculating the bond dissociation free energies of M3d—CH3 bonds. Deprotonation was found to be competitive kinetically for most of the 1st row transition metal-methyl complexes (except for CrII, MnII and CuII), but less favorable thermodynamically as compared to reductive functionalization for all of the studied 1st row transition metal complexes. Metal-carbon bond dissociation was found to be less favorable than the RF reactions for most 3d transition metal complexes studied. The first dinitrogen dicopper (I) complex has been characterized using computational and experimental methods. Low temperature reaction of the tris(pyrazolyl)borate copper(II) hydroxide {iPr2TpCu}2(µ-OH)2 with triphenylsilane under a dinitrogen atmosphere gives the µ -N2 complex, {iPr2TpCu}2(µ -N2). X-ray crystallography reveals an only slightly activated N2 ligand (N-N: 1.111(6) Å) that bridges between two iPr2TpCuI fragments. While DFT studies of mono- and dinuclear copper dinitrogen complexes suggest a weak µ-backbonding between the d10 CuI centers and the N2 ligand, they reveal a degree of cooperativity in the dinuclear Cu-N2-Cu interaction.

A major driving force for development of new catalyst systems is the need for more efficient synthesis of chemical compounds essential to modern life. Catalysts having superior performance offer significant environmental and economic advantages, but their discovery is not trivial. Well-defined, homogeneous catalysts can offer unparalleled understanding of ligand effects, which proves invaluable in directing redesign strategies. This thesis work focuses on the design of ruthenium complexes for applications in dinitrogen activation and olefin metathesis. The complexes developed create new directions in small-molecule activation and asymmetric catalysis by late-metal complexes. Also examined are the dual challenges, ubiquitous in catalysis, of adequate interrogation of catalyst structure and performance. Insight into both is essential to enable correlation of ligand properties with catalyst activity and/or selectivity. Improved methods for accelerated assessment of catalyst performance are described, which expand high-throughput catalyst screening to encompass parallel acquisition of kinetic data. A final aspect focuses on direct examination of metal complexes, both as isolated species, and under catalytic conditions. Applications of charge-transfer MALDI mass spectrometry to structural elucidation in organometallic chemistry is described, and the technique is employed to gain insight into catalyst decomposition pathways under operating conditions.

A major driving force for development of new catalyst systems is the need for more efficient synthesis of chemical compounds essential to modern life. Catalysts having superior performance offer significant environmental and economic advantages, but their discovery is not trivial. Well-defined, homogeneous catalysts can offer unparalleled understanding of ligand effects, which proves invaluable in directing redesign strategies. This thesis work focuses on the design of ruthenium complexes for applications in dinitrogen activation and olefin metathesis. The complexes developed create new directions in small-molecule activation and asymmetric catalysis by late-metal complexes. Also examined are the dual challenges, ubiquitous in catalysis, of adequate interrogation of catalyst structure and performance. Insight into both is essential to enable correlation of ligand properties with catalyst activity and/or selectivity. Improved methods for accelerated assessment of catalyst performance are described, which expand high-throughput catalyst screening to encompass parallel acquisition of kinetic data. A final aspect focuses on direct examination of metal complexes, both as isolated species, and under catalytic conditions. Applications of charge-transfer MALDI mass spectrometry to structural elucidation in organometallic chemistry is described, and the technique is employed to gain insight into catalyst decomposition pathways under operating conditions.

The novel chelating ligand, Me[NOON]H2 (N,N'-((ethane-1,2-diylbis(oxy))-bis(ethane-2,1-diyl))- bis(2,6-dimethylaniline)), have been synthesized successfully and characterized by NMR. The lithium complexes of the aryl diamide ligand have also been synthesized by n-BuLi react with neutral ligand, Me[NOON]H2. And the lithium complexes is a ether adduct according to the 1H NMR. The lithium complex, Me[NOON]Li2(OEt2) react with Ti(OiPr)4 and TiCl4(THF)2 to form the NOON titanium alkoxide and dichloride complexes respectively, and they have been characterized by 1H NMR and X-ray diffraction. These NOON titanium complexes are expected to be a catalyst for the ring opening polymerization of lactide or caprolactone in the future. The tantalum complexes of diphenolate phosphine ligands have been synthesized and characterized successfully by NMR, X-ray diffraction, and elemental analysis. The tantalum complexes, [tBuOPO]2TaX (X=Me, Et, H) is produced by the reaction of [tBuOPO]2TaCl with Grignard reagent (MeMgBr and EtMgCl) and superhydride(LiHBEt3). These tantalum complexes will be applied in dinitrogen activation in the future work.

A major driving force for development of new catalyst systems is the need for more efficient synthesis of chemical compounds essential to modern life. Catalysts having superior performance offer significant environmental and economic advantages, but their discovery is not trivial. Well-defined, homogeneous catalysts can offer unparalleled understanding of ligand effects, which proves invaluable in directing redesign strategies. This thesis work focuses on the design of ruthenium complexes for applications in dinitrogen activation and olefin metathesis. The complexes developed create new directions in small-molecule activation and asymmetric catalysis by late-metal complexes. Also examined are the dual challenges, ubiquitous in catalysis, of adequate interrogation of catalyst structure and performance. Insight into both is essential to enable correlation of ligand properties with catalyst activity and/or selectivity. Improved methods for accelerated assessment of catalyst performance are described, which expand high-throughput catalyst screening to encompass parallel acquisition of kinetic data. A final aspect focuses on direct examination of metal complexes, both as isolated species, and under catalytic conditions. Applications of charge-transfer MALDI mass spectrometry to structural elucidation in organometallic chemistry is described, and the technique is employed to gain insight into catalyst decomposition pathways under operating conditions.

The linkage between denitrification activity, N gas emissions, and the size of the denitrifier community in soils of an upland pasture was investigated. Special emphasis was placed on soil pH as a regulating factor, the spatial distribution of denitrification, and the degree of cattle impact. The thesis has been based on field and laboratory measurements using both conventional and modern methods of soil ecology.

A major driving force for development of new catalyst systems is the need for more efficient synthesis of chemical compounds essential to modern life. Catalysts having superior performance offer significant environmental and economic advantages, but their discovery is not trivial. Well-defined, homogeneous catalysts can offer unparalleled understanding of ligand effects, which proves invaluable in directing redesign strategies. This thesis work focuses on the design of ruthenium complexes for applications in dinitrogen activation and olefin metathesis. The complexes developed create new directions in small-molecule activation and asymmetric catalysis by late-metal complexes. Also examined are the dual challenges, ubiquitous in catalysis, of adequate interrogation of catalyst structure and performance. Insight into both is essential to enable correlation of ligand properties with catalyst activity and/or selectivity. Improved methods for accelerated assessment of catalyst performance are described, which expand high-throughput catalyst screening to encompass parallel acquisition of kinetic data. A final aspect focuses on direct examination of metal complexes, both as isolated species, and under catalytic conditions. Applications of charge-transfer MALDI mass spectrometry to structural elucidation in organometallic chemistry is described, and the technique is employed to gain insight into catalyst decomposition pathways under operating conditions.

Les sédiments sont des sites importants d’élimination d’azote (N) puisqu’ils possèdent des gradients d’oxydoréduction leur conférant les conditions idéales pour les réactions microbiennes de transformation de N. L’eutrophisation des régions côtières peut altérer ces gradients, par des changements dans la concentration d’oxygène (O2) de l’eau interstitielle, et modifier l’importance relative des processus transformant le N. Afin de mieux comprendre comment l’O2 pourrait influencer les transformations de N, nous avons mesuré les flux diffusifs de diazote (N2), nitrate (NO3-), oxygène et ammonium (NH4+) dans les sédiments de l’Estuaire Maritime du St-Laurent (EMSL), et nous avons estimé les taux de dénitrification. L’importance du couple nitrification-dénitrification en fonction d’un gradient de concentrations d’O2 dans la zone d’hypoxie de l’EMSL fut aussi évaluée. La concentration des gaz dissous fut mesurée en utilisant une nouvelle approche développée dans cette étude. Les flux diffusifs de N2, O2, NO3- et NH4+ variaient de 5.5 à 8.8, de -37.1 à -84.8, de -4.0 à -5.8 et de 0.6 à 0.8 μmol N m-2 h-1 respectivement. Les concentrations de N2 et NO3- dans l’eau porale et les flux de NO3- et de N2 des sédiments, suggèrent que la diffusion de NO3- provenant de l’eau à la surface des sédiments ne peut pas expliquer par elle-même la production de N2 observée. En utilisant une approche stoichiométrique, les taux de nitrification potentielle estimés comptent pour 0.01 à 52% du flux total de NO3 nécessaire pour aboutir aux flux de N2 observés et diminuent avec l’augmentation de l’hypoxie. / Sediments display strong redox gradients and represent important sites of various microbially mediated nitrogen (N) transformation pathways resulting in the loss of fixed N. Increased eutrophication of coastal systems will likely impact the overall function of these sediments via changes in the oxygen (O2) concentration in the overlying water, thus influencing the redox gradient and the relative importance of different N processes. To have a better understanding of how O2 could influence N transformations, we measured dinitrogen (N2), nitrate (NO3-), oxygen and ammonium (NH4+) diffusive fluxes in the sediments of the Lower St. Lawrence Estuary (LSLE) and estimated denitrification rates. The importance of the nitrification-denitrification couple as a function of a gradient of hypoxic O2 concentrations was also evaluated. Dissolved gas concentrations were measured using a new approach developed in this study. N2, O2, NO3- and NH4+ diffusive fluxes varied respectively from 5.5 to 8.8, from -37.1 to -84.8, from -4 to -5.8 and from 0.6 to 0.8 μmol N m-2 h-1 among the five stations sampled. N2 and NO3- concentrations in sediment pore water and measured fluxes of NO3- in and N2 out of the sediments suggested that NO3- diffusion from the overlying water alone could not support N2 production. Using a stoichiometric approach, estimated potential nitrification rates varied among sites, from 0.01 to 52% of the total NO3 supply rate. The relative importance of nitrification decreased with increasing hypoxia.