Properties Of Transition Metal Clusters Research Articles

Computational Studies of Molybdenum-Containing Metal–Sulfur and Metal–Hydride Clusters

The development of transition metal clusters is an active area of research in inorganic chemistry, as they can be used as catalysts to perform chemically or biologically relevant reactions. Computational chemistry, employing density functional theory (DFT), plays a key role in rationalizing the electronic structure and properties of transition metal clusters. This article reviews recent quantum chemical studies of Mo3S4M clusters (M = Fe, Co, Ni), their CO- or N2-bound variants, and metal–hydride clusters. The ground state of the cluster systems was computed, and properties such as metal–metal bonding, orbital interactions, fluxional behavior of ligands, spectroscopy, and reaction mechanisms were rationalized and compared with available experimental results. Our research findings evidence that computational studies employing quantum chemical methods can guide experimental researchers to develop novel transition metal clusters for potential applications in catalysis.

The Effect of Global and Local Chemical Reactivity Descriptors in the Determination of Properties of Transition Metal Clusters

Recently, research on material properties has got lots of attention because of their promising technological applications. In this paper, the author’s focus is on the global and local chemical reactivity descriptors of some transition metal clusters. In addition, the author also include findings of chemical properties with reactivity descriptors. Furthermore, according to Thomas–Fermi approximation and from the exact formulation of Density Functional Theory by Hohenberg and Kohn’s theorem, the author introduce electronegativity and the theory of hardness and softness for further investigation of chemical properties of the clusters. As a part of investigations, the author also introduce the Fukui functions with an emphasis for the determinations of chemical reactivity descriptors. All the results are obtained by theoretical investigation of electronegativity), Chemical potential (), chemical hardness (), and polarizability () of the given transition metal clusters (Cr and Mn). From the results, the author observed that the ionization potential of the clusters increases with decreasing cluster size as the clusters become more stable and tend towards chemical inertness. In conclusion, determination of the reactivity descriptors of the transition metal clusters show results that are in line with previous experimental and theoretical findings.

Theoretical investigation of existence of meta-stability in iron and cobalt clusters

Nowadays considerable attention has been given for researches on magnetic properties of transition metal clusters (specifically FeN and CoN). This is because these clusters offer big hopes for the possibility of presenting significant magnetic anisotropy energy which is critical for technological applications. This study intends to find out the causes for the existence of the two states (ground and meta-stable) in Iron and Cobalt clusters. The study also explains the role of valence electrons for the existence of magnetism in the two states by using the concept of ionization potential, electron dipole polarizabilities, chemical hardness and softness of the clusters. Assuming that, when all itinerant electrons are at s-level and also at the d-level (ns=nandns→0.) the ground state and meta-stable state energies with distinct energy minima are (Egs=l/2n+εcn−2μBhnandEms=εdn−gμBhn) respectively. The findings also showed that polarizability of small cluster of the specified elements are increased compared with the bulk value, which means that there is an effective increase in the cluster radius due to the spilling out of the electronic charge. Furthermore, it is obvious that 4s electrons are more delocalized than the 3d electrons so that they spill out more than the 3d electrons. This leads to the conclusion that 4s electrons are primarily responsible for the enhanced polarizabilities and for shell structure effects. This indicates that polarizability at the meta-stable state is less than that of the ground state i.e. the meta-stable state loses its s electron. Therefore the two minima represent a ground state of configuration 3d↑53d↓2+δ4s2−δ with energy Egs and meta-stable state of configuration 3d↑53d↓3+δ4s1−δ with energy Ems for Co clusters and a ground state configuration 3d↑53d↓1+δ4s2−δ with energy Egs an meta-stable state of configuration 3d↑53d↓2+δ4s1−δ with energy Ems for Fe clusters. Hence, the existence of the two states (meta-stable & ground state) is due to the large disproportion in electronic configurations of the two clusters at their respective states. Furthermore, the chemical hardness and softness of the clusters also provide evidence for the existence of stability of the two states depending on the cluster size.

Size-dependent properties of transition metal clusters: from molecules to crystals and surfaces--computational studies with the program ParaGauss.

In the so-called scalable regime the size-dependent behavior of the physical and chemical properties of transition metal clusters is described by scaling relationships. For most quantities this scalable regime is reached for cluster sizes between a few tens and a few hundreds of atoms, hence for systems for which an accurate treatment by density functional theory is still feasible. Thus, by invoking scaling relations one is able to obtain properties of very large nanoparticles and even the bulk limit from the results of a series of smaller cluster models. In this invited review we illustrate this strategy by exploiting results from computational studies that mostly were carried out with the density functional theory software ParaGauss. We address mainly the size-dependent behavior of the properties of transition metal clusters. To this end, we first present benchmark studies probing various approximations that are used in such density functional calculations. Subsequently we show how physical insight may be gained by exploring less understood types of systems. These applications range from bare clusters to nanoislands and nanoalloys to adsorption complexes.

Tuning magnetic properties of Mn4 cluster with gold coating

The magnetic properties of transition metal clusters are a unique function of their size and differ from their bulk behavior due to quantum confinement. Here we show that surface modification provides another channel to tune their magnetic properties. This is demonstrated by taking Mn(4) as an example. Although Mn(4) carries a giant magnetic moment of 20 micro(B), the magnetic coupling can be tuned from ferromagnetic to ferrimagnetic by changing the number of gold atoms coated on its surface. We found that 26 gold atoms are needed to fully cover a Mn(4) cluster. When partially coated, the system exhibits ferromagnetic coupling with a total magnetic moment of 18 micro(B) and it becomes ferrimagnetic with a moment of 8 micro(B) when fully coated. This magnetic cross-over is caused by the shrinking of the Mn-Mn bond length, suggesting that the magnetic properties of a Mn(4) cluster can be tuned by controlling the surface coverage.

Second-order nonlinear optical properties of transition metal clusters [MoS4Cu4X2Py2] (M = Mo, W; X = Br, I)

We present in this paper the second-order nonlinear optical properties of a series of penta-nuclear metal clusters [MS4Cu4X2Py6] (M=Mo, W; X=Br, I) on the basis of the hyper-Rayleigh scattering experiments and the first-principle calculations (TDDFT). The measurements obtain the notably large dynamic quadratic hyperpolarizabilities at 1064 nm [beta(-2omega, omega, omega) values are around 200x10(-30) esu] and, by extrapolation, a large static values around 60x10(-30) esu. The computational results of the electronic excitation energies and quadratic hyperpolarizabilities by TDDFT method are in good agreement with the experimental values and by careful examination they are both dependent on the nature of the metals. The in-depth analysis of the mechanism for the second-order response unambiguously shows the evidence of the contribution of direct metal-metal interaction charge transfers. This provides a new tool to tune nonlinear optical properties in exploiting metal cluster materials and molecular devices.

Ligand-induced electronic effects in the physical properties of transition metal clusters: Modulation of Mössbauer parameters and magnetic exchange couplings in complexes [formula omitted] by the R substituent

The syntheses, crystallographic, 57Fe Mössbauer and magnetic studies of the diferric complexes [Fe 2O(O 2CR) 2(phen) 2(H 2O) 2](ClO 4) 2 [R = H ( 3), CH 2F ( 4)] are reported and the results are compared to those of the previously reported analogues with R = CCl 3 ( 1) and R = CMe 3 ( 2) [Boudalis et al., Polyhedron 25 (2006) 1391]. The Mössbauer spectra of 3 and 4 consist of symmetric quadrupole-split doublets [ δ = 0.392(1) mm s −1, Δ E Q = 1.726 mm s −1 ( 3) and δ = 0.398 mm s −1, Δ E Q = 1.691 mm s −1 ( 4), at 78 K]. The magnetic couplings within 2– 4 are antiferromagnetic [ J = −140 ( 2), −129 ( 3) and −116 ( 4) cm −1, H ˆ = - 2 J S ˆ 1 · S ˆ 2 ]. The results from Mössbauer spectroscopic and variable-temperature magnetic studies are discussed in terms of structural characteristics of the complexes and the electron donating strengths of the carboxylate R substituents. In addition, the syntheses and structures of complexes [Fe 3O(O 2CR) 6(H 2O) 3](ClO 4) [R = H ( 5), CH 2F ( 6)], which were derived as by-products from the syntheses of 3 and 4, are described.

Cubic magic clusters of rhodium stabilized with eight-center bonding: Magnetism and growth

Ab initio calculations on ${\mathrm{Rh}}_{n}\phantom{\rule{0.3em}{0ex}}(n=8--64)$ clusters surprisingly show simple cubic structures to be most favorable up to $n\ensuremath{\sim}27$. These are stabilized by eight center bonding and lie significantly lower in energy than the icosahedral isomers found in previous studies. With increasing size cuboctahedral, decahedral, and icosahedral isomers become preferred, showing a reversal of the growth behavior compared with the well-known $\text{icosahedral}\ensuremath{\rightarrow}\text{decahedral}\ensuremath{\rightarrow}\text{cuboctahedral}$ transitions. The low-coordination cubic structures have lower magnetic moments than the dense packed icosahedral isomers and which agree closely with experiments. Our results resolve a long standing discrepancy between theory and experiments and provide a new direction in the understanding of the properties of transition metal clusters.

On the anomalous temperature dependence of the magnetic moment of cobaltclusters

Cluster beam experiments using Stern–Gerlach type measurements yield information aboutthe magnetic properties of transition metal clusters. One of the intriguing results to havecome out of such measurements is the observation of a magnetic moment in cobalt clustersthat increases with temperature up to about 500 K, before decreasing as the bulk Curietemperature is approached. We argue in this paper that this behaviour can be understoodas an artifact of the method of extracting results from raw experimental data, and that theorigin of the anomalous behaviour lies in the neglect of the magnetic anisotropy in theanalysis.

Temperature evolution of structural and magnetic properties of transition metal clusters

We report an extension of our tight binding molecular dynamics method [Phys. Rev. B 57, 10069 (1998)] by incorporating the Nosé-bath and the multiple histogram approximations, so as to be applicable to cluster studies at finite temperatures in an efficient way. This generalization allows one to calculate the caloric curve for the cluster and use this to study the effect of temperature on the structural, electronic, and magnetic properties of clusters. The method is used to study the variation of structural and magnetic properties with temperature as well as to obtain the caloric curves of the Ni13 cluster. The results are compared with those obtained using classical potentials to describe the interatomic interactions.

Symmetry and magnetic properties of transition metal clusters

The magnetic properties of 55-atom Fe, Co and Ni clusters are studied using the local spin-density formalism. The dependence of magnetic moment on cluster symmetry is found to be also cluster size dependent. The symmetry of cluster plays an important role in determining the charge distribution. The surface magnetism enhancement are found to be decreased from Fe to Ni.

Structural and dynamical properties of transition metal clusters

Results of molecular dynamics simulation studies of structural and dynamical properties of 12-, 13-, and 14-atom transition metal clusters are presented. The calculations are carried out using a Gupta-like potential expressed in reduced units. The transformation to absolute units involves two size-dependent parameters which effectively convert the potential into a size-dependent one. The minimum energy geometries of the clusters are obtained through the technique of simulated thermal quenching. A melting-like transition is observed as the energy of the clusters is increased. A novel element of the transition is that it may involve a premelting state.