The electronic structure of carbonyl metal clusters : I. An INDO investigation of mono-, di- and trinuclear nickel complexes

A series of mono-, di- and trinuclear nickel carbonyl complexes has been investigated by means of the MO-LCAO method. The total molecular energies, one-electron energies and electron distributions have been computed and their variations along the series are discussed. The wavefunction of the complex has been considered in terms of contributions of the individual MO's of the free CO molecule in order to compute the variations of the σ-donor and π-accepting ability of the carbonyl ligand.     

Structural properties and reactivity of bimetallic silver-gold clusters

Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic Au_nAg_m ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au₈Ag₁₂ cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O₂ and CO molecules. As a starting point we consider reactivity towards CO and O₂ molecules on the example of AgAu^- dimer. The results show that the catalytic cycle can be fullfilled.     

Structural insights into the active-ready form of [FeFe]-hydrogenase and mechanistic details of its inhibition by carbon monoxide

[FeFe]-hydrogenases harbor a {2Fe3S} assembly bearing two CO and two CN- groups, a mu-CO ligand, and a vacant coordination site trans to the mu-CO group. Recent theoretical results obtained studying the isolated {2Fe3S} subsite indicated that one of the CN- ligands can easily move from the crystallographic position to the coordination site trans to the mu-CO group; such an isomerization would have a major impact on substrates and inhibitors binding regiochemistry and, consequently, on the catalytic mechanism. To shed light on this crucial issue, we have carried out hybrid QM/MM and free energy perturbation calculations on the whole enzyme, which demonstrate that the protein environment plays a crucial role and maintains the CN- group fixed in the position observed in the crystal structure; these results strongly support the hypothesis that the vacant coordination site trans to the mu-CO group has a crucial functional relevance both in the context of CO-mediated inhibition of the enzyme and in dihydrogen oxidation/evolution catalysis.     

Masses of S and P wave mesons and pseudoscalar decay constants using a confinement scheme

In the framework of relativistic harmonic confinement model for quarks and antiquarks, the masses of S- and P-wave mesons and pseudoscalar decay constants from light flavour to heavy flavour sectors are computed. The residual two-body Coulomb interaction and the spin-dependent interaction of the confined one gluon exchange effects (COGEP) such as spin-spin and spin-orbit interactions are perturbatively incorporated with the confinement energy to get the respective vector-pseudoscalar meson mass differences. Here we employ the same parametrization and model parameters as used in a recent study of low-lying hadron masses and leptonic decay widths. The results are being compared with the values obtained from other theoretical models and the experimental values.     

Fast generation of broken‐symmetry states in a large system including multiple iron–sulfur assemblies: Investigation of QM/MM energies,clusters charges,and spin populations

A density functional theory study is presented regarding the energetics and the Mulliken population analyses of a quantum mechanical/molecular mechanical (QM/MM) system including multiple iron–sulfur clusters in the QM region. The [FeFe]‐hydrogenase from Desulfovibrio desulfuricans was studied, and both the active site (an Fe₆S₆ assembly generally referred to as the H‐cluster) and an ancillary Fe₄S₄ site were treated at the BP86‐RI/TZVP level. The antiferromagnetic coupling that characterizes both sites was modeled using the broken‐symmetry (BS) approach. For such a QM system, 36 different BS couplings can be defined, depending on the localization of spin excess on the various spin centers. All the BS states were obtained by means of an effective and simple method for spin localization, that is here described and compared with more sophisticated approaches already available in literature. The variation of the QM/MM energy among the various geometry‐optimized protein models was found to be less than 25 kJ mol^–1. This energy variation almost doubles if no geometry optimization is performed. A detailed analysis of the additive nature of these variations in QM/MM energy is reported. The Mulliken charges show very small variations among the 36 BS states, whereas the Mulliken spin populations were found to be somewhat more variable. The relevance of such variations is discussed in light of the available Mössbauer and Electron Paramagnetic Resonance (EPR) spectroscopic data for the enzyme. Finally, the influence of the basis set on the spin populations, charges, and structural parameters of the models was investigated, by means of QM/MM computations on the same system at the BP86‐RI/SVP level. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Isocyanide in Biochemistry? A Theoretical Investigation of the Electronic Effects and Energetics of Cyanide Ligand Protonation in [FeFe]‐Hydrogenases

The presence of Fe‐bound cyanide ligands in the active site of the proton‐reducing enzymes [FeFe]‐hydrogenases has led to the hypothesis that such Brønsted–Lowry bases could be protonated during the catalytic cycle, thus implying that hydrogen isocyanide (HNC) might have a relevant role in such crucial microbial metabolic paths. We present a hybrid quantum mechanical/molecular mechanical (QM/MM) study of the energetics of CN^? protonation in the enzyme, and of the effects that cyanide protonation can have on [FeFe]‐hydrogenase active sites. A detailed analysis of the electronic properties of the models and of the energy profile associated with H₂ evolution clearly shows that such protonation is dysfunctional for the catalytic process. However, the inclusion of the protein matrix surrounding the active site in our QM/MM models allowed us to demonstrate that the amino acid environment was finely selected through evolution, specifically to lower the Brønsted–Lowry basicity of the cyanide ligands. In fact, the conserved hydrogen‐bonding network formed by these ligands and the neighboring amino acid residues is able to impede CN^? protonation, as shown by the fact that the isocyanide forms of [FeFe]‐hydrogenases do not correspond to stationary points on the enzyme QM/MM potential‐energy surface.     

Flux line lattice symmetries in the borocarbide superconductor LuNi2B2C

We compare the results of small angle neutron scattering on the flux line lattice (FLL) obtained in the borocarbide superconductor LuNi₂B₂C with the applied field along the c- and a-axes. For H‖c the temperature dependence of the FLL structural phase transition from square to hexagonal symmetry was investigated. Above 10 K the transition onset field. H ₂(T), rises sharply, bending away from H _c2(T) in contradiction to theoretical predictions of the two merging. For H‖a a first order FLL reorientation transition is observed at H _tr=3–3.5 kOe. Below H _tr the FLL nearest neighbor direction is parallel to the b-axis, and above H _tr to the c-axis. This transition cannot be explained using nonlocal corrections to the London model.     

Influence of the [2Fe]H Subcluster Environment on the Properties of Key Intermediates in the Catalytic Cycle of [FeFe] Hydrogenases: Hints for the Rational Design of Synthetic Catalysts

Nature's recipe : A theoretical study analyzes how the environment of the [FeFe] hydrogenase's catalytic cofactor affects its chemical properties, particularly the relative stability of complexes with bridging and terminal hydride ligands (see picture; Fe teal, S yellow, C green, N blue, O red, H gray). The results help to elucidate key rules for the design of bioinspired synthetic catalysts for H₂ production.