Cluster Models of FeMoco with Sulfide and Carbyne Ligands: Effect of Interstitial Atom in Nitrogenase Active Site

<p>Nitrogen-fixing organisms perform dinitrogen reduction to ammonia at an iron-M (M = Mo, Fe, or V) cofactor (FeMco) of nitrogenase. FeMoco displays eight metal centers bridged by sulfides and a carbide having the MoFe₇S₈C cluster composition. The role of the carbide ligand, a unique motif in protein active sites, remains poorly understood. Toward addressing its function, we isolated synthetic models of subsite MFe₃S₃C displaying sulfides and a carbyne ligand. We developed synthetic protocols for structurally related clusters, [Tp*MFe₃S₃X]^n-, where M = Mo or W, the bridging ligand X = CR, N, NR, S, and Tp* = tris(3,5-dimethyl-1-pyrazolyl)hydroborate, to study the effects of the identity of the heterometal and the bridging X group on structure and electrochemistry. While the nature of M results in minor changes, the μ₃-bridging ligand X has a large impact on reduction potentials, with differences higher than 1 V, even for the same formal charge, the most reducing clusters being supported by the carbyne ligand. </p>

Cluster Models of FeMoco with Sulfide and Carbyne Ligands: Effect of Interstitial Atom in Nitrogenase Active Site

<p>Nitrogen-fixing organisms perform dinitrogen reduction to ammonia at an iron-M (M = Mo, Fe, or V) cofactor (FeMco) of nitrogenase. FeMoco displays eight metal centers bridged by sulfides and a carbide having the MoFe₇S₈C cluster composition. The role of the carbide ligand, a unique motif in protein active sites, remains poorly understood. Toward addressing its function, we isolated synthetic models of subsite MFe₃S₃C displaying sulfides and a carbyne ligand. We developed synthetic protocols for structurally related clusters, [Tp*MFe₃S₃X]^n-, where M = Mo or W, the bridging ligand X = CR, N, NR, S, and Tp* = tris(3,5-dimethyl-1-pyrazolyl)hydroborate, to study the effects of the identity of the heterometal and the bridging X group on structure and electrochemistry. While the nature of M results in minor changes, the μ₃-bridging ligand X has a large impact on reduction potentials, with differences higher than 1 V, even for the same formal charge, the most reducing clusters being supported by the carbyne ligand. </p>

Molecular Dynamics Study of Interaction of Carbon, Nitrogen, and Oxygen Impurity Atoms with Self-Interstitial Atoms in Nickel, Silver and Aluminum

The interaction of impurity atoms of carbon, nitrogen, and oxygen with self-interstitial atoms in FCC metals like nickel, silver, and aluminum is studied using the molecular dynamics method. It is found that the self-interstitial atom migration in the crystal lattice follows two mechanisms: dumbbell and crowdion. In this case, the first mechanism that includes one interatomic distance displacement and the rotation of the <001> dumbbell is characterized by broken paths of atomic migration. The second mechanism is described by straight paths along the close-packed directions <011> in the crystal. The binding energies between impurity atoms and selfinterstitial atoms in Ni, Ag, and Al are calculated in the paper. It is shown that impurity atoms are effective “traps” for interstitial atoms that migrate relatively quickly in a crystal. During the interaction of an interstitial and an impurity atom, the interstitial atom forms a dumbbell configuration with an axis along the <001> direction, and the impurity atom is located in the nearest octahedral pore. It is found that the mobility of interstitial atoms is significantly reduced due to the presence of impurities in the metal. The introduction of 10 % impurity atoms leads to a severalfold increase in the migration energy of interstitial atoms. At the same time, the contribution of the crowdion mechanism is noticeably reduced while the dumbbell mechanism contribution is increased.

Interstitial Atom Engineering in Magnetic Materials

Interstitial light elements play an important role in magnetic materials by improving the magnetic properties through changes of the unit cell volume or through orbital hybridization between the magnetic and interstitial atoms. In this review focusing on the effects of interstitial atoms in Mn-based compounds, which are not well researched, the studies of interstitial atoms in three kinds of magnetic materials (rare-earth Fe-, Mn-, and rare-earth-based compounds) are surveyed. The prominent features of Mn-based compounds are interstitial-atom-induced changes or additional formation of magnetism—either a change from antiferromagnetism (paramagnetism) to ferromagnetism or an additional formation of ferromagnetism. It is noted that in some cases, ferromagnetic coupling can be abruptly caused by a small number of interstitial atoms, which has been overlooked in previous research on rare-earth Fe-based compounds. We also present candidates of Mn compounds, which enable changes of the magnetic state. The Mn-based compounds are particularly important for the easy fabrication of highly functional magnetic devices, as they allow on-demand control of magnetism without causing a large lattice mismatch, among other advantages.

PENTAMER WITH INTERSTITIAL ATOM AS THE STRUCTURAL BLOCK OF RECONSTRUCTED Si(331) AND Ge(331) SURFACES

Atomic structure models of the Si(331) and Ge(331) surfaces are developed based on unified structural block, containing pentamer with interstitial atom. The work is performed using density functional calculations.

THE DIFFUSION IN FCC BINARY INTERSTITIAL ALLOY

We build the theory of diffusion for FCC binary interstitial alloy under pressure based on the statistical moment method, where there are the analytic expressions of the jumping frequency of interstitial atom, the effective jumping length, the correlation factor, the diffusion coefficient, and the activated energy. In limit cases, we can obtain the diffusion theory for FCC metal A under pressure.