Metabolically active microbial communities are present in a wide range of subsurface environments. Techniques like enumeration of microbial cells, activity measurements with radiotracer assays and the analysis of porewater constituents are currently being used to explore the subsurface biosphere, alongside with molecular biological analyses. However, many of these techniques reach their detection limits due to low microbial activity and abundance. Direct measurements of microbial turnover not just face issues of insufficient sensitivity, they only provide information about a single specific process but in sediments many different process can occur simultaneously. Therefore, the development of a new technique to measure total microbial activity would be a major improvement. A new tritium-based hydrogenase-enzyme assay appeared to be a promising tool to quantify total living biomass, even in low activity subsurface environments. In this PhD project total microbial biomass and microbial activity was quantified in different subsurface sediments using established techniques (cell enumeration and pore water geochemistry) as well as a new tritium-based hydrogenase enzyme assay. By using a large database of our own cell enumeration data from equatorial Pacific and north Pacific sediments and published data it was shown that the global geographic distribution of subseafloor sedimentary microbes varies between sites by 5 to 6 orders of magnitude and correlates with the sedimentation rate and distance from land. Based on these correlations, global subseafloor biomass was estimated to be 4.1 petagram-C and ~0.6 % of Earth's total living biomass, which is significantly lower than previous estimates. Despite the massive reduction in biomass the subseafloor biosphere is still an important player in global biogeochemical cycles. To understand the relationship between microbial activity, abundance and organic matter flux into the sediment an expedition to the equatorial Pacific upwelling area and the north Pacific Gyre was carried out. Oxygen respiration rates in subseafloor sediments from the north Pacific Gyre, which are deposited at sedimentation rates of 1 mm per 1000 years, showed that microbial communities could survive for millions of years without fresh supply of organic carbon. Contrary to the north Pacific Gyre oxygen was completely depleted within the upper few millimeters to centimeters in sediments of the equatorial upwelling region due to a higher supply of organic matter and higher metabolic activity. So occurrence and variability of electron acceptors over depth and sites make the subsurface a complex environment for the quantification of total microbial activity. Recent studies showed that electron acceptor processes, which were previously thought to thermodynamically exclude each other can occur simultaneously. So in many cases a simple measure of the total microbial activity would be a better and more robust solution than assays for several specific processes, for example sulfate reduction rates or methanogenesis. Enzyme or molecular assays provide a more general approach as they target key metabolic compounds. Since hydrogenase enzymes are ubiquitous in microbes, the recently developed tritium-based hydrogenase radiotracer assay is applied to quantify hydrogenase enzyme activity as a parameter of total living cell activity. Hydrogenase enzyme activity was measured in sediments from different locations (Lake Van, Barents Sea, Equatorial Pacific and Gulf of Mexico). In sediment samples that contained nitrate, we found the lowest cell specific enzyme activity around 10^(-5) nmol H_(2) cell^(-1) d^(-1). With decreasing energy yield of the electron acceptor used, cell-specific hydrogenase activity increased and maximum values of up to 1 nmol H_(2) cell^(-1) d^(-1) were found in samples with methane concentrations of >10 ppm. Although hydrogenase activity cannot be converted directly into a turnover rate of a specific process, cell-specific activity factors can be used to identify specific metabolism and to quantify the metabolically active microbial population. In another study on sediments from the Nankai Trough microbial abundance and hydrogenase activity data show that both the habitat and the activity of subseafloor sedimentary microbial communities have been impacted by seismic activities. An increase in hydrogenase activity near the fault zone revealed that the microbial community was supplied with hydrogen as an energy source and that the microbes were specialized to hydrogen metabolism. / Mikrobielle Gesellschaften und ihre aktiven Stoffwechselprozesse treten in einer Vielzahl von Sedimenten unterschiedlichster Herkunft auf. In der Erforschung dieser tiefen Biosphäre werden derzeit Techniken wie Zellzählungen, Aktivitätsmessungen mit Radiotracer-Versuchen und Analysen der Porenwasserzusammensetzung angewendet, darüber hinaus auch molekularbiologische Analysen. Viele dieser Methoden stoßen an ihre Nachweisgrenze, wenn Sedimente mit geringer Zelldichte und mikrobieller Aktivität untersucht werden. Bei der Untersuchung von Stoffwechselprozessen mit herkömmlichen Techniken kommt dazu, dass von mehreren Prozessen, die zeitgleich ablaufen können, jeweils nur einer erfasst wird. Deswegen wäre die Entwicklung einer neuartigen Messtechnik für die gesamte mikrobielle Aktivität ein wesentlicher Fortschritt für die Erforschung der tiefen Biosphäre. Ein vielversprechender Ansatz, um die gesamte lebende Biomasse auch in Proben mit geringer Aktivität zu bestimmen, ist eine Hydrogenase-Enzym-Versuchsanordnung mit Tritium als quantifizierbarer Messgröße. In dieser Doktorarbeit wurde die gesamte mikrobielle Biomasse und Aktivität von unterschiedlichen Sedimentproben einerseits mit herkömmlichen Methoden (Zellzählungen, Analyse der Porenwasserzusammensetzung) als auch mit einer neu entwickelten Hydrogenase-Enzym-Versuchsanordnung quantifiziert. Mit einer großen Anzahl eigener Zellzählungsdaten von Sedimenten aus dem Äquatorialpazifik und dem Nordpazifik und ergänzenden publizierten Daten konnte gezeigt werden, dass Zellzahlen sich in ihrer globalen geographischen Verteilung je nach Bohrlokation um 5 bis 6 Größenordnungen unterscheiden. Dabei bestehen Korrelationen zur Sedimentationsrate und zur Entfernung zum Land, mit deren Hilfe sich die Gesamtbiomasse in Tiefseesedimenten zu 4,1 Petagramm-C abschätzen lässt. Das entspricht ~0,6 % der Gesamtbiomasse der Erde und ist damit erheblich weniger als in früheren Schätzungen angegeben. Trotz der Korrektur auf diesen Wert spielt die Biomasse der tiefen Biosphäre weiterhin eine erhebliche Rolle in biogeochemischen Kreisläufen. Um die Zusammenhänge zwischen Aktivität der Mikroben, der Häufigkeit ihres Auftretens und Zustrom von organischem Material zu verstehen, wurde eine Expedition ins Auftriebsgebiet des Äquatorialpazifiks und zum nordpazifischen Wirbel durchgeführt. Daten der Sauerstoffaufnahme in Sedimenten des nordpazifischen Wirbels, die mit Sedimentationsraten von 1 mm pro 1000 Jahren abgelagert werden, zeigen, dass mikrobielle Gesellschaften über Millionen von Jahren ohne Zufuhr von frischem organischen Kohlenstoff überleben konnten. Im Gegensatz zum nordpazifischen Wirbel wird in Sedimenten des äquatorialpazifischen Auftriebsgebiets Sauerstoff bei höherer mikrobieller Aktivität und Verfügbarkeit organischer Verbindungen oberflächennah in den ersten Milli- bis Zentimetern komplett umgesetzt. Auftreten und Variabilität von Elektronenakzeptoren nach Tiefe und Bohrlokation machen die tiefe Biosphäre zu einer komplexen Umgebung für die Quantifizierung der gesamten mikrobiellen Aktivität. Aktuelle Studien zeigen das verschiedene Elektronenakzeptorprozesse gleichzeitig ablaufen können, obwohl man bisher davon ausgegangen war, dass diese sich thermodynamisch ausschließen. In vielen Fällen wäre also eine einfache Methode zur Messung der gesamten mikrobiellen Aktivität eine bessere und verlässlichere Lösung aktueller Analyseaufgaben als Messungen mehrerer Einzelprozesse wie beispielsweise Sulfatreduktion und Methanogenese. Enzym-oder Molekular-Versuchsanordnungen sind ein prozessumfassender Ansatz, weil hier Schlüsselkomponenten der Stoffwechselprozesse untersucht werden. Das Hydrogenase-Enzym ist eine solche Schlüsselkomponente und in Mikroben allgegenwärtig. Deshalb kann die Quantifizierung seiner Aktivität mit der neu entwickelten Hydrogenase-Enzym-Versuchsanordnung als Parameter für die gesamte mikrobielle Aktivität der lebenden Zellen verwendet werden. Hydrogenase-Aktivitäten wurden in Sedimenten unterschiedlicher Lokationen (Vansee, Barentssee, Äquatorialpazifik, und Golf von Mexico) gemessen. In Sedimentproben, die Nitrat enthielten, haben wir mit ca. 10^(-5) nmol H_(2) cell^(-1) d^(-1) die geringste zellspezifische Hydrogenase-Aktivität gefunden. Mit geringerem Energiegewinn des genutzten Elektronenakzeptors steigt die zellspezifische Hydrogenase-Aktivität. Maximalwerte von bis zu 1 nmol H_(2) cell^(-1) d^(-1) wurden in Sedimentproben mit >10 ppm Methankonzentration gefunden. Auch wenn die Hydrogenase-Aktivität nicht direkt in die Umsatzrate eines spezifischen Prozesses konvertierbar ist, können zellspezifische Aktivitätsfaktoren verwendet werden, um die metabolisch aktive Mikrobenpopulation zu quantifizieren. In einer weiteren Studie mit Sedimenten des Nankai-Grabens zeigen Daten der Zelldichte und der Hydrogenase-Aktivität einen Einfluss von seismischen Ereignissen auf Lebensraum und Aktivität der mikrobiellen Gesellschaften. Ein Anstieg der Hydrogenase-Aktivität nahe der Verwerfungszone machte deutlich, dass die mikrobiellen Gesellschaften mit Wasserstoff als Energiequelle versorgt wurden und dass die Mikroben auf einen Wasserstoff-Stoffwechsel spezialisiert waren.

Rhodobacter sphaeroides O.U.001 is a purple non-sulphur bacterium producing hydrogen under photoheterotrophic, nitrogen limited conditions. Hydrogen is produced by Mo-nitrogenase but substantial amount of H2 is reoxidized by a membrane bound uptake hydrogenase. In this study, hydrogen production and the expression of structural nitrogenase genes were investigated by varying molybdenum and iron ion concentrations. These two elements are found in the structure of Mo-nitrogenase and they are important for functioning of the enzyme. The results showed that hydrogen production and nifD gene expression increased upon increase in molybdenum concentration. Increasing iron concentration had also positive effect on hydrogen production and nifK gene expression. To improve the hydrogen producing capacity of R. sphaeroides O.U.001, hupSL genes encoding uptake hydrogenase were disrupted in two different methods. In the first method, hup genes were disrupted by gentamicin resistance gene insertion. In the second method, part of the hup gene was deleted without using antibiotic resistance gene. The wild type and the hup- mutant cells showed similar growth patterns but substantially more hydrogen was produced by the mutant cells. The genes coding for hox1 hydrogenase of Thiocapsa roseopersicina was aimed to be expressed in R. sphaeroides O.U.001 to produce H2 under nitrogenase repressed and mixotrophic conditions. The hox1 hydrogenase genes of T. roseopersicina were cloned and transferred to R. sphaeroides. Although the cloning was successful, the expression of hydrogenase was not achieved by using either the native promoter of hox1 hydrogenase or the crtD promoter of T. roseopersicina.

All the eighteen Frankia strains isolated from ten different actinorhizal host plants showed uptake hydrogenase activity. The activity of this enzyme is further increased by addition of nickel. Nickel also enhanced the degree of hydrogenase transfer into the membranes of Frankia, indicating the role of this metal in the processing of this enzyme. The uptake hydrogenase of Frankia is most probably a Ni-Fe hydrogenase. Genome characterization revealed the presence of two hydrogenase genes (syntons) in Frankia, which are distinctively separated in all the three available Frankia genomes. Both hydrogenase syntons are also commonly found in other Frankia strains. The structural, regulatory and accessory genes of both hydrogenase synton #1 and #2 are arranged closely together, but in a clearly contrasting organization. Hydrogenase synton #1 and #2 of Frankia are phylogenetically divergent and that hydrogenase synton #1 is probably ancestral among the actinobacteria. Hydrogenase synton #1 (or synton #2) of Frankia sp. CcI3 and F. alni ACN14a are similar in gene arrangement, content and orientation, while the syntons are both reduced and rearranged in Frankia sp. EANpec. The hydrogenases of Frankia sp. CcI3 and F. alni ACN14a are phylogenetically grouped together but never with the Frankia sp. EAN1pec, which is more closely related to the non-Frankia bacteria than Frankia itself. The tree topology is indicative of a probable gene transfer to or from Frankia that occurred before the emergence of Frankia. All of the available evidence points to hydrogenase gene duplication having occurred long before development of the three Frankia lineages. The uptake hydrogenase synton #1 of Frankia is more expressed under free-living conditions whereas hydrogenases synton #2 is mainly involved in symbiotic interactions. The uptake hydrogenase of Frankia can also be manipulated to play a larger role in increasing the efficiency of nitrogen fixation in the root nodules of the host plants, there by minimizing the need for environmentally unfriendly and costly fertilizers. The hydrogen-evolving hydrogenase activity was recorded in only four Frankia strains: F. alni UGL011101, UGL140102, Frankia sp. CcI3 and R43. After addition of 15mM Nicl2, activity was also detected in F. alni UGL011103, Frankia sp. UGL020602, UGL020603 and 013105. Nickel also increased the activity of hydrogen-evolving hydrogenases in Frankia, indicating that Frankia may have different types of hydrogen-evolving hydrogenases, or that the hydrogen-evolving hydrogenases may at least be regulated differently in different Frankia strains. The fact that Frankia can produce hydrogen is reported only recently. The knowledge of the molecular biology of Frankia hydrogenase is, therefore, of a paramount importance to optimize the system in favor of hydrogen production. Frankia is an attractive candidate in search for an organism efficient in biological hydrogen production since it can produce a considerable amount of hydrogen.

A multitude of microorganisms possess the ability to metabolise molecular hydrogen (H2). The major enzyme family involved in hydrogen metabolism are Hydrogenases. These enzymes catalyse the reversible conversion of molecular hydrogen to protons and electrons (H2 ↔ 2H+ + 2e-). These enzymes have the potential to be utilised for biotechnological applications such as hydrogen fuel cells, but they also represent promising drug targets for inhibition of bacterial energy metabolism both within the gastrointestinal tract and after infection. Therefore, further understanding and discoveries made in the hydrogenase field warrants progression into applied medical and biotechnological research areas. Hydrogenases are also interesting due to their phylogeny and physiology in a large number of microbial species. These enzymes are categorised by their active site architecture. One well studied, ancient group is termed the [NiFe]-hydrogenases, which all harbour a complex NiFe(CN-)2CO active site in the 'large' catalytic subunit and usually have three iron-sulfur clusters within a 'small' electron transferring partner subunit. [NiFe]-hydrogenases have undergone massive diversification, with four major phylogenetic subgroups arising. The major part of this Thesis concerns work on a Group 4 [NiFe]-hydrogenase that functions in partnership with a formate dehydrogenase as a formate hydrogenlyase (FHL). This FHL complex generates H2 and CO2 from the disproportionation of formate (CHOO- + H+ ↔ H2 + CO2). In this Thesis, genetic and biochemical characterisation of Pectobacterium atrosepticum SCRI1043, a potato pathogen, led to the identification of a novel FHL complex. The [NiFe]-hydrogenase in this organism is similar to that of Escherichia coli Hydrogenase-4, with an extended membrane domain similar to that of respiratory Complex I. Importantly, the P. atrosepticum formate dehydrogenase is selenium-free, while previously characterised FHL complexes have selenocysteine-containing formate dehydrogenases. Using genetic and biochemical approaches it was shown that the [NiFe]-hydrogenase and a formate dehydrogenase were vital for H2 production by P. atrosepticum. Using plant infection assays it was also shown that the gene encoding the formate dehydrogenase was important for full infective ability of P. atrosepticum in potato plants and tubers. The latter part of this Thesis focuses on developing genetic tools to study this novel FHL from P. atrosepticum as well as Hydrogenase-1 and -2 from E. coli.

A variety of unicellular green algae have the ability to photo-produce molecular hydrogen (H2). Using sunlight to power the production of H2 from water is attractive due to the abundant supply of both resources and the potential for the technology to address global warming and energy supply concerns. Increasing levels of H2 production from those currently achievable with algal systems is a necessity for the technology to become economically feasible. Green unicellular algae are rare amongst organisms in that some have an ability to switch to an H2-producing metabolism when environmental conditions become anaerobic. The process of H2 production is greatly accentuated in the light due to the role of the photosynthetic apparatus directing electron flow to hydrogenase enzymes located in the chloroplast. Difficulties in maintaining continuous systems of H2 production largely result from the O2 sensitivity of hydrogenase enzymes. As O2 is generally produced through photosynthesis, the process of H2 production has always been short-lived. Recently, a process of inducing H2 production for several days was accomplished by depriving the growth medium of sulphur (Melis et al., 2000). Lacking sulphur, photosystem II activity diminishes to a point where any O2 evolved is consumed by respiration; this leads to the culture becoming anaerobic and to the onset of H2 production. The method of sulphur depletion has proven to be very useful for studies of H2 production due to enhanced rates over longer time periods being possible. This work was performed to search for new H2-producing Australian algal species and to shed light upon the molecular and biochemical interactions occurring when algal species move from aerobic photosynthetic growth to an anaerobic H2-producing status. An assay to test new species for an H2-producing ability was developed and implemented; leading to the isolation of new H2-producing species from Australian waters. The assay involved purging algal cultures in the dark with N2, sealing them in bioreactors and then exposing them to light. Metabolic profiling performed during this assay revealed cells to rapidly enter a fermentative metabolism upon the onset of anoxia. Acetate, formate and ethanol were key metabolites produced alongside H2 during this period. Metabolomics was used as a tool to understand the biochemical interactions occurring during 120 h of sulphur depleted H2 production. Extraction protocols were developed that allowed the detection and identification of over 100 metabolites using gas chromatography coupled to mass spectrometry, nuclear magnetic resonance spectroscopy and thin layer chromatography. Shifts in primary energy metabolism when cells switch from O2 production to H2 production were revealed. Indications are that both starch and triacylglyceride accumulate during the first 24 h of sulphur depletion prior to anoxia. Following the onset of anoxia, fermentative metabolism begins, H2 is produced and amino acids generally increase. A build-up of toxic fermentative end products and a lack of sulphur are believed to cause the termination of H2 production, rather than a lack of energy reserves. Key achievements of this work have been: • The establishment of an assay that can be used for future bio-prospecting work aimed at finding H2-producing algal species. • The isolation of new H2-producing green algal species from Australian waters. • The establishment of protocols for the extraction of metabolites from small volumes (1 ml) of Chlamydomonas reinhardtii cultures for analysis on a variety of analytical platforms. • The mapping of changes in metabolism of C. reinhardtii during the switch from an aerobic environment to an anaerobic H2-producing environment. • A range of recommendations for future research that may lead to higher H2 production.

La synthèse de modèles du site actif de l'hydrogénase à fer, responsable de la catalyse réversible de la réduction des protons en dihydrogène, est un enjeu crucial dans l'optique d'une meilleure compréhension du fonctionnement du système enzymatique et du développement de nouveaux types de catalyseurs à base de clusters fer-soufre. Durant cette thèse ont été abordés les aspects synthétiques (ligands soufrés et complexes fer-carbonyle) aussi bien que les études spectroscopiques et électrochimiques.

No description available.

Molecular hydrogen has been proposed as an energy store to help meet the world's ever increasing demand for clean energy because the oxidation product (produced by either combustion or in a fuel cell) results in the formation of water. To realize this goal, energy efficient catalysts comprised of earth abundant elements must be used. The work in this dissertation describes investigations of diiron dichalcogen catalysts used for proton reduction. These complexes are inspired by the active site of the [FeFe]-hydrogenase enzyme. Catalysts were extensively studied with cyclic voltammetry in conjunction with photoelectron spectroscopy and density functional theory calculations in order to determine the effects that bridging ligands and 2Fe2E (E = S or Se) core substitutions have on the electronic structure and catalytic ability of these complexes. The complex µ-(pyrazine-2,3-dithiolato)diironhexacarbonyl (pyrazine-cat) was prepared and found to catalyze proton reduction at a -0.49 V overpotential, which represents a 16% decrease over the previously studied complex µ-(benzene-1,2-dithiolato)diironhexacarbonyl (benz-cat). Electrochemical investigations in conjunction with DFT calculations indicated the possibility of two mechanisms for proton reduction, both of the ECEC type. The first mechanism is Fe-based and analogous to the mechanism reported for benz-cat. The second is a nitrogen-based mechanism which occurs at more negative potentials than the Fe-based mechanism. Overall, pyrazine-cat maintained the ability to mediate successive redox states similar to benz-cat and the electron withdrawing nature of the pyrazine caused the initial reduction to occur at a lower potential than benz-cat. Ultimately this results in the decreased overpotential for catalytic proton reduction by pyrazine-cat. Investigations of the electronic structure and catalytic ability of complexes of the type (µ-ECH₂XCH₂E-µ)Fe₂(CO)₆ where E = S or Se and X = CH₂, S or Se were also carried out. All complexes were found to catalyze H₂ production from acetic acid in acetonitrile. DFT calculations indicate that when X = S or Se the HOMO changes character from predominatly metal based (X = CH2) to containing significant chalcogen lone pair character. The presence of the chalcogen lone pair character helps to facilitate a rotated structure in either the oxidized or reduced forms of these complexes. Through computations it was found that oxidation of the X = S or Se complexes results in a CO ligand rotating into a semi-bridging position, which opens a vacant site on one of the Fe-centers. The bridgehead bends toward this vacant site donating electron density greatly stabilizing the cation and more interestingly forming a structure which strongly resembles the active site of the [FeFe]-hydorgenase. Complexes which contain a chalcogen in the bridgehead undergo potential inversion, leading to a two-electron initial reduction. This is in part due to electron-electron repulsion between chalcogen lone pair electrons and the reduced Fe-centers, which leads to the formation of a rotated dianion.Complexes with the general structure (µ-E (CH₂)nE-µ)Fe₂ (CO)₆ where E = S or Se and n = 3, 4, or 5 were investigated using cyclic voltammetry, photoelectron spectroscopy, and DFT calculations. Substitution of Se in the 2Fe2E core for S resulted in a lengthening of the FeFe bond. As the linker length increased from n =3 to 5, one of the apical CO's is pushed down due to a steric interaction creating a more obtuse Fe-Fe-C angle. Larger effects of the linker length were seen in the oxidation and reduction chemistry. CV and UPS show that linker length has little effect on the oxidation potential or onset ionization energy. Computations predict that the oxidized structure is rotated, and as the linker length increases there is an agostic interaction which forms between a methylene proton and the vacant site on the rotated Fe-center. Reduction potentials for these complexes are found to decrease with increasing linker length, which was attributed to the steric interaction between the alkane linker and the apical CO helping to facilitate rotation of the anion. Interestingly catalytic potentials were found to depend almost entirely on chalcogen character in the 2Fe2E core, with S-containing catalysts having a lower catalytic potential than Se-containing catalysts. The long known complex [η⁵-CpFe(CO)SMe]₂ was investigated as both a proton reduction and H₂ oxidation catalyst. Reduction of [η⁵-CpFe(CO)SMe]₂ revealed that the complex undergoes a two electron irreversible reduction and the reduced species precipitates onto the glassy carbon electrode surface. The new species on the electrode surface facilitates proton reduction at a -0.3 V overpotential, which is significantly lower (0.9 V) than the most similar complex Fp₂. Unlike previous catalysts of this type, [η⁵-CpFe(CO)SMe]₂ catalytic current does not decrease as overpotential decreases. [η⁵-CpFe(CO)SMe]₂ was also shown to undergo two one-electron oxidations, and in the presence of H₂ and the dication, appears to oxidize H₂. The ability of [η⁵-CpFe(CO)SMe]₂ to both oxidize H₂ and reduce protons to H₂ addresses a known deficiency for catalysts mimicking the function of the active site of the [FeFe]-hydrogenase.

The transfer of electrons to and from a molecule is one of the more fundamental and important chemical processes. One such important example is the reduction-oxidation (redox) cycles in catalysts and enzymes. In the hydrogenase enzymes, adding and removing electrons is one of the key processes for generating H₂ from water molecules. Finding a direct free energy relation between the vertical ionization energies (IE(V)) measured spectroscopically by gas-phase photoelectron spectroscopy and the oxidation potentials (E(1/2)) measured thermodynamically in solution by cyclic voltammetry (CV) for molecules is an important aspect for developing effective catalysts. In this study, a series of organometallic compounds such as metallocenes were used for investigating the free energy relationships and catalysts inspired by the active sites of [FeFe]-hydrogenases enzymes were evaluated for their ability to produce H₂ from electrocatalytic reduction of weak acids. The first part of the dissertation explores metallocenes of the form (η⁵-C₅H₅)₂M (M= Fe, Ru, Os, Co, Ni) as the model for developing the free energy relation between gas phase ionization energies (IE(V)) and solution oxidation potentials (E(1/2)). It was found that computing the electronic properties of Cp₂Fe, Cp₂Ru, and Cp₂Os using VWN-Stoll and OPBE density functional theory (DFT) functional was successful with root mean square deviation (RMSD) of 0.02 eV between the experimental and calculated ionization energies. However, calculated ionization energies of Cp₂Co and Cp₂Ni were less successful with RMSD of 0.3 eV between the experimental and calculated ionization energies. Introduction of the B3LYP or M06 hybrid DFT functionals yielded much improved results (0.1 eV) over the previous combinations of DFT functional for Cp2Co and Cp2Ni. The energy relation between the two experimental measurements was established and further computational studies revealed that the solvation energy was the largest energy contribution between IE(V) and E(1/2) in the five studied metallocenes. The RMSD of the calculated oxidation potentials, after adjusting for the error in gas-phase ionization energies, was 0.09 V. The second part of the dissertation explores a series of catalysts inspired by the active sites of [FeFe]-hydrogenase enzymes; μ-(2,3-pyrazinedithiolato)diironhexacarbonyl (PzDT-cat), Fe₂(μ-X₂C₅H₈O)(CO)₆ (where X = S, Se, Te), and Fe₂(μ-1,3-SC₃H₆X)(CO)₆ (where X = Se and Te) for their ability to produce H₂ from weak acids utilizing the computational techniques and knowledge gained from the metallocene study. Even though the overall electronic perturbation from μ-(1,2-benzenedithiolato)diironhexacarbonyl (BDT-cat) to μ-(2,3-pyridinedithiolato)diironhexacarbonyl (PyDT-cat) to PzDT-cat is found to be small, the reduction potential of PzDT-cat was found to be 0.15 V less negative than that of BDT-cat resulting in less energy required for initiating electrocatalytic H₂ production over the BDT-cat and PyDT-cat. Lower reorganization energy has been achieved by substitutions of larger chalcogens at the Fe₂S₂ core. However, the electrocatalytic production of H₂ from acetic acid in acetonitrile was found to be diminished upon going from analogous S to Se to Te species. This is ascribed to the increase in the Fe–Fe bond distance with a corresponding increase in the size of the chalcogen atoms from S to Se to Te, disfavoring the formation of a carbonyl-bridged structure in the anion which is thought to be critical to the mechanism of H₂ production.

Techn. University, Diss., 2001--Berlin.