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A quantum chemical study is conducted to analyze and characterize the O-O bond cleavage in several dinuclear peorox complexes of iron porphyrins. The analysis reveals that the cleavage occurs either at an increased temperature or on the addition of a strongly coordinating base.

Density functional theory using the B3LYP hybrid functional is employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H2O2 is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(super III)(TPA)-OOH is explored, and is revealed that a high-valent iron-oxo intermediate is formed, where an Fe(super v) oxidation state is attained.

This article applies density functional theory to biologically important large metal enzyme complexes in the elucidation of mechanism, structure, and modeling. Metalloenzymes comprising methane monooxygenase, photosystem II complex, nitrogenase, the copper dimer active site in tyrosinase, and hemocyanin are described.

The potential energy surface of the endothermic reaction Rh+ + CH4 -- RhCh2+ H2 was calculated using high-accuracy quantum mechanical methods. The results agreed with results from ion beam mass spectrometry. This indicates that there is no barrier in excess of the endothermicity of the H2 elimination reaction. All structures on the potential surface were optimized at the density functional theory level, using the empirically parametrized hybrid functional B3LYP. Relative energies were calculated both at the B3LYP level and at the parametrized configuration interaction level.

Research was conducted to examine the charge-separation process in both the bacterial reaction center in Rhodobacter sphaeroides and in photosystem II. Calculations were performed in three steps. Computational results for the bacterial reaction center agree with experimental results on charge separation energetics. Charge separation to the P680+Pheo- state is shown to be less strongly driven in photosystem II.

The mechanism of nickel-iron hydrogenases are examined through the use of density functional theory (DFT) quantum chemical methods. When a NiFe(II,III) oxidation state is assigned to the resting cluster, the DFT quantum chemical approach was able to reproduce the structure and reactivity pattern on the hydrogenases. Under the proposed mechanism of H2 oxidation by the enzyme, H2 initially attaches to Fe in a molecular hydrogen complex before experiencing heterolytic splitting. This process is influenced by the spin states of the cluster.

Quantum chemical calculations were used to test a model of water oxidation in photosystem II, involving direct abstraction of hydrogen atoms from water molecules ligated to manganese ions in an oxygen-evolving complex by oxidized tyrosine TyrZ. The empirically parametrized hybrid density function method was used. The first O-H bond strength of water was lowered by coordination to a manganese center. The second hydrogen abstraction energy was found to be similar

The stability constants for certain rhodium(I)-alkene complexes correlated with 103Rh-NMR shifts due to similar metal-alkene bonding on different compounds, and the relatively long rhodium-alkene bonds for the methyl-substituted ethylenes. The d-d excitation energy contribution to the paramagnetic term in the Ramsey equation determined the charges in Rh-NMR shifts when the alkenes on RH(beta-diketonato)(alkene)2 complexes bind the same way.

The oxidative addition interaction is particularly and greatly affected by the occurrence of the ammonia lone pair. The insertion products show varying structures of atoms in the left and the right due to the lone pair. Planar product complexes are seen in the atoms on the left while the right atoms reveal nonplanar and pyramidal MNH2 subunits. The atoms on the left also show higher exothermicities and less barrier heights for ammonia. The reactions between the metal electrons and the lone pair are used to represent the structural and energetic influences.

The second row, left most metals in the first and second-row transition metals of two structures of M2N2 alone can form the type of M2N2 complex where the N-N distance corresponds to a single bond. The N2 molecule is perpendicularly coordinated in this type of molecule, as has been observed in the case of zirconium. Ab initio quantum chemical computations performed on two varying structures of M2N2 for first and second-row transition metal systems involving titanium, yttrium and zirconium indicated that N2 bridges the metal atoms in both, either in a perpendicular side-on coordination mode or in a parallel end-on coordination mode, giving a linear M-N-N-M.

A study was done on the effects of covalent ligands on the oxidative addition reaction between second-row transition-metal atoms and methane. Reaction energies and barrier heights were determined and the reaction energies for the oxidative addition of the hydrogen molecule were compared to these results. The entire sequence of second-row transition metals from yttrium to palladium was also studied.

Quantum chemical model studies were performed for the transition metal activation of C-C bonds in ethane, cyclopropane and cyclobutane. The reactions between ethane and cyclobutane and the entire second row of transition metal atoms were studied. The cyclopropane reaction has been studied for rhodium and palladium. It was found that palladium has the lowest barrier for breaking the C-C bonds, and the C-C bond in cyclopropane is easiest to break.

Quantum mechanical calculations, are performed for the insertion of the transition metals Cr, Mo, Fe, Co, Ni, Cu and Ag into the C-O bond of ethene oxide. The planar structure of the metallaoxetane is used and geometrical parameters for the nickeloxetane are optimized. Results indicate that metallaoxetanes are logical intermediates in the epoxidation reactions of olefins and in the deoxygenation reactions of epoxides. The deoxygenation reaction is preferred by Mo, Co, Fe and Ni, while epoxidation is preferred by Cu and Ag.

The isotopic composition of nitrous oxide (N2O) provides useful information for evaluating N2O sources and budgets. Due to the co‐occurrence of multiple N2O transformation pathways, it is, however, challenging to use isotopic information to quantify the contribution of distinct processes across variable spatiotemporal scales. Here, we present an overview of recent progress in N2O isotopic studies and provide suggestions for future research, mainly focusing on: analytical techniques; production and consumption processes; and interpretation and modelling approaches. Comparing isotope‐ratio mass spectrometry (IRMS) with laser absorption spectroscopy (LAS), we conclude that IRMS is a precise technique for laboratory analysis of N2O isotopes, while LAS is more suitable for in situ/inline studies and offers advantages for site‐specific analyses. When reviewing the link between the N2O isotopic composition and underlying mechanisms/processes, we find that, at the molecular scale, the specific enzymes and mechanisms involved determine isotopic fractionation effects. In contrast, at plot‐to‐global scales, mixing of N2O derived from different processes and their isotopic variability must be considered. We also find that dual isotope plots are effective for semi‐quantitative attribution of co‐occurring N2O production and reduction processes. More recently, process‐based N2O isotopic models have been developed for natural abundance and 15N‐tracing studies, and have been shown to be effective, particularly for data with adequate temporal resolution. Despite the significant progress made over the last decade, there is still great need and potential for future work, including development of analytical techniques, reference materials and inter‐laboratory comparisons, further exploration of N2O formation and destruction mechanisms, more observations across scales, and design and validation of interpretation and modelling approaches. Synthesizing all these efforts, we are confident that the N2O isotope community will continue to advance our understanding of N2O transformation processes in all spheres of the Earth, and in turn to gain improved constraints on regional and global budgets. [ABSTRACT FROM AUTHOR]

The formation of methyl-Ni([F.sub.430]) species in methyl-coenzyme M reductase (MCR) is examined by using hybrid density functional method and an active-site model built on the basis of X-ray crystal structure. The calculations have shown that the formation of C[H.sub.3]-Ni(F.sub.430]) in MCR is dependent on the acidity of the substrate leaving group and a C[H.sub.3]-Ni([F.sub.430]) species is observed with methyl halides as substrates.

The hybrid density functional theory functional B3LYP is used to study the binding of O2 to CuTPA and the effects of solvents as well as sustituents on the TPA ligand are investigated. Calculated thermodynamic and kinetic parameters are compared to experimental data, and in general there is a high agreement between theoretical and experimental results.

The analysis of the nature of the highly oxidized species and their need for O-O bond formation in Mn-complexes is investigated. A density functional theory (DFT) is used for this purpose.

Proton translocation in cytochrome oxidase is studied by using hybrid density functional theory and pK(sub a) values and redox potentials for the active site of cytochrome oxidase is calculated. A proton coming from the inside of the membrane thermodynamically prefer to go to the propionate for further pumping rather than go to the binuclear center for consumption in the oxygen chemistry.

The examination of the effects of a variety of ortho-substituents on the acidity of phenol is presented. The substitutions are made at the ortho-position to model modified tyrosine residues found in enzymes and a comparison of the substituent effects on the O--H bond strengths and the acidities indicate no simple correlation between the two.

Hybrid density functional theory is used where full geometry optimizations are done with a medium basis and final energies are evaluated using much larger basis. Models consisting of 40-50 atoms containing the active site metal complex, including the first-shell amino acid ligands, are normally employed.

The de novo protein design is employed to examine the structural basis for the creation and maintenance of a tryptophanyl radical. It is concluded that the radical protein maquette provides a unique platform to explore the structural basis for the extraordinary radical chemistry of a critically important class of enzymes.