Semiempirical study of electronic and bonding properties of cobalt silicide clusters

The electronic structure of cobalt silicide clusters Co₇Si₇ and Si₇Co₇ was studied in comparison to that of Co₁₉ and Si₁₇ clusters under the scope of the MINDO/SR method. Clusters Co₇Si₇ and Si₇Co₇ represent the environment of a cobalt atom and that of a silicon atom in the cobalt monosilicide bulk, respectively. It is found that the Co? Si bond is essentially sp in character with an indirect participation (by electrostatic interaction) of the cobalt d orbitals. Our calculations show a charge transfer from silicon to the d orbitals of cobalt via sp–sp interaction with an internal sp–d hybridization. The theoretical density of states for cobalt silicide clusters are reported and compared with experimental results of surface spectroscopies. © 1992 by John Wiley & Sons, Inc.     

Energetics and bonding in beryllium metallized carbon clusters

Small carbon clusters C, C₂ and C₃ metallized with beryllium were studied by first principles within the hybrid density functional approach with generalized gradient correction. Cluster isomer structures for the ground state and several excited states where systematically calculated for C_xBe_y with x = 1–3 and y = 1–4 including the vibrational analysis of all states. Ionization potentials and electron affinities are calculated for the ground state cluster isomers. The thermal stability of the ground state isomers was verified within the harmonic approximation of the Helmholtz free energy up to temperatures of about 1000 K. Within the family of clusters studied, C₂Be₃ undergoes a solid-like structural transition at about 260 K changing from a planar structure at low temperatures to a linear isomer at high temperatures.     

First-principles study of structural and vibrational properties of crystalline silver azide under high pressure

A detailed first-principles study of the structural and vibrational properties of crystalline silver azide under hydrostatic pressure of 0–500 GPa has been performed with density functional theory in the generalized gradient approximation. The crystal structure is relaxed to allow ionic configurations, cell shape, and volume to change without any symmetry constraints. It is found that the silver azide crystal remains orthorhombic structure with Ibam space group for pressures up to 7 GPa, where there is a transition to an I4/mcm tetragonal symmetry. The lattice parameter and electronic structure are investigated as functions of pressure. The calculated vibrational frequencies at ambient pressure are in agreement with available experimental data. We also discuss the pressure-induced frequency shifts for the internal and lattice modes of silver azide crystal upon compression.     

气相中La催化乙烯脱氢及C-C键耦合的理论研究

采用密度泛函理论中的B3LYP方法研究了气相中过渡金属La在二、四重态势能面上催化C2H4的反应机理。全参数优化了二、四重态势能面上各个驻点的几何构型,同时对过渡态进行了频率分析,使用内禀反应坐标（IRC）方法验证了过渡态的准确性,通过AIM理论和NBO分析方法对主要的驻点进行了键分析,并对2IM1、2IM3进行了态密度分析。结果表明：La与C2H4的反应存在两种可能的路径,反应在二重态势能面上进行且均为放热反应。键分析表明初始复合物中La与C2H4分子之间为共价作用。     

The novel silicide Eu3Si4: structure, chemical bonding, magnetic behavior and electrical resistivity

The novel binary europium silicide Eu₃Si₄ was synthesized from the elements. Its crystal structure is a derivative of the Ta₃B₄ type: space group Immm, a=4.6164(4) Å, b=3.9583(3) Å, c=18.229(1) Å, Z=2. In the structure, the silicon atoms form one-dimensional bands of condensed hexagons. Deviating from the prototype structure, a partial corrugation of the initially planar bands may be concluded from the analysis of the experimental electron density in the vicinity of the Si1 atoms. In the paramagnetic region, Eu₃Si₄ shows a 4f⁷ electronic configuration for the europium atoms. Two consecutive magnetic ordering transitions were found at 117 and 40 K. The first one is attributed to a ferromagnetic ordering of the Eu2 atoms; the second one is caused by a ferromagnetic ordering of the Eu1 atoms resulting in a ferrimagnetic ground state with a net magnetization of 7 μ_B at 1.8 K. The temperature dependence of the electrical resistivity reflects the metallic character of the investigated compound. Furthermore, the pronounced changes of the dρ/dT slope confirm the magnetic transitions. From bonding analysis with the electron localization function, Eu₃Si₄ shows a Zintl-like character and its electronic count balance can be written as (Eu^1.83+)₃(Si1^0.95−)₂(Si2^1.8−)₂, in good agreement with its magnetic behavior in the paramagnetic region.     

Structural and electronic properties of hydrated MgO nanotube clusters

Hydrated MgO nanotube clusters are constructed and studied by the density functional theory at the B3LYP/6-31G(d) level. A strong exothermicity chemisorption reactivity of MgO nanotube clusters with water, which releases 137.5–171.8 kJ/mol. The averaged charge of Mg ions is steady, and presents a stronger ionic bonding. Mg ions are more sensitive to the coordination number. For the reaction of water onto clusters, electronic properties of hydrated clusters have remarkable change compared with anhydrous clusters.     

MINDO/SR calculations of nickel surface properties as a function of hydrogen coverage

The MINDO/SR method is used to study surface properties such as the work function, heat of adsorption and magnetic moment as a function of hydrogen coverage. A good correlation between theory and experiment is found. Furthermore, a qualitative analysis of surface geometry changes due to hydrogen chemisorption is presented.     

First-principles study of crystalline mono-amino-2,4,6-trinitrobenzene, 1,3-diamino-2,4,6-trinitrobenzene, and 1,3,5-triamino-2,4,6-trinitrobenzene

The electronic structure and thermodynamic properties of crystalline mono-amino-2,4,6-trinitrobenzene (MATB), 1,3-diamino-2,4,6-trinitrobenzene (DATB), and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been comparatively studied using density functional theory in the local density approximation. An analysis of electronic structure shows that the CNO₂ bonds in the three solids are easier to be broken in the thermal decomposition than the CNH₂ bonds. The calculated thermodynamic properties show that the order of their thermodynamic stability is TATB > DATB > MATB and their decomposition reactions are favorable under high temperature. Finally, an attempt is made to correlate the impact sensitivity of the three solids with their band gap. The result shows that there is the relationship between the band gap and impact sensitivity.     

First-principles study of structural,elastic, electronic and vibrational properties of BiCoO3

We used density functional theory (DFT) to study the structural, elastic, electronic, and lattice dynamical properties of tetragonal BiCoO₃ applying the “norm-conserving” pseudopotentials within the local spin density approximation (LSDA). The calculated equilibrium lattice parameters and atomic displacements are in agreement with the available experimental and theoretical results. Moreover, the structural stability of tetragonal BiCoO₃ were confirmed by the calculated elastic constants. In addition, the elastic properties of polycrystalline aggregates including bulk, shear and Young's moduli, and Poisson's ratio are also determined. The electronic band structure, total and partial density of states (DOS and PDOS) with ferromagnetic spin configuration are obtained. The results show that tetragonal BiCoO₃ has an indirect band gap with both up- and down-spin configurations and its bonding behavior is of covalent nature. We compute Born effective charge (BEC) which is found to be quite anisotropic of Bi, Co and O atoms. The infrared and Raman active phonon mode frequencies at the Г point are found. The phonon dispersion curves exhibit imaginary frequencies which lead from the high-symmetry tetragonal phase to low-symmetry rhombohedral phase in BiCoO₃. The six independent elastic constants, including bulk, shear and Young's moduli, and Poisson's ratio, complete BEC tensor and phonon dispersion relations in tetragonal BiCoO₃ are predicted for the first time. Results of the calculations are compared with the existing experimental and theoretical data.     

Density functional study of structural, electronic, and optical properties of small bimetallic ruthenium-copper clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Cu(n)Ru(m)](+/0/-) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 nonlocal hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Cu(n Ru(m)](+/0/-) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and assignments of all principal electronic transitions were made and interpreted in terms of contribution from specific molecular orbital excitations.     

On the electronic density of states of copolymers of silole and pyrrole

The electronic density of states of various periodic and random copolymers of silole and pyrrole of different compositions have been determined numerically using the negative factor counting method in tight binding approximation. The trends in the electronic structures and conduction properties of these copolymers are discussed.     

Theoretical studies of structural and electronic properties of AlnAsn clusters

We present theoretical results of size dependent structural, electronic, and optical properties of ligand‐free stoichiometric Al_nAs_n clusters of zinc‐blende modification. The investigation is done using a simplified parametrized linear combination of atomic orbital–density functional theory‐local density approximation–tight‐binding (LCAO–DFT–LDA–TB) method and consider clusters with n up to around 100. Initial structures have assumed as spherical parts of infinite zinc‐blende structure and then allowed to relax to the closest local‐energy‐minimum structure. We analyze the radial distributions of atoms, Mulliken populations, electronic energy levels (in particular, HOMO and LUMO), bandgap, and stability as a function of size and composition. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Melting of palladium clusters – density of states determination by Monte Carlo simulation

The density of states (DOS) has been calculated for the metal clusters Pd₁₃, Pd₅₅ and Pd₁₄₇ using the recently proposed reference system equilibration (RSE) method. The interaction within the clusters was described by a many-body alloy potential. Using this DOS, the caloric curve of Pd₁₃ has been calculated and excellent agreement with canonical Monte Carlo simulations is obtained. For Pd₅₅ and Pd₁₄₇, the solid and one molten isomers have been isolated in order to calculate the DOS for the isomers separately. The melting of the clusters occurs when the DOS for the solid and the molten isomers are equal. Comparison with previous microcanonical Monte Carlo simulations shows that the number of statistically equivalent molten isomers are 1.1×10¹⁸ for Pd₅₅ and 4.1×10⁴¹ for Pd₁₄₇.     

A theoretical study of electronic and vibrational properties of neutral,cationic, and anionic B24 clusters

The equilibrium geometries, electronic and vibrational properties, and static polarizability of B₂₄, B, and B clusters are reported here. First‐principles calculations based on density functional theory predict the staggered double‐ring configuration to be the ground state for B₂₄, B, and B, in contrast to the quasi‐planar structure observed in small neutral and ionized B_n clusters with n ≤ 15. Furthermore, the (4 × B₆) tubular structure is found to be relatively stable in comparison to the 3D cage structure. The presence of delocalized π and multicentered σ bonds appears to be the cause of the stability of the double‐ring and tubular isomers. For the ground state of B₂₄, the lower and upper bound of the electron affinity is 2.67 and 2.81 eV, respectively, and the vertical ionization potential is 6.88 eV. Analysis of the frequency spectrum of the double‐ring and tubular isomers reveals the characteristic vibrational modes typically observed in carbon nanotubes. The corresponding IR spectrum also reflects the presence of some of these characteristic modes in the neutral and ionized B₂₄, suggesting that double‐ring and tubular structures can be considered as the building blocks of boron nanotubes. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Bonding analysis and electronic structure of transition metal-benzoquinoline complexes: A theoretical study

The geometric and electronic structures of a series of hypothetical compounds of the types CpM(C₁₃H₉N) and (CO)₃M(C₁₃H₉N) (M = first row transition metal and C₁₃H₉N = 7,8-benzoquinoline) have been investigated by means of density functional theory (DFT). The benzoquinoline ligand can bind to the metal through η¹-η⁶ coordination modes, adopting structures of types a, b and c, in agreement with the electron count and the nature of the metal. In the investigated species, the most favored closed-shell count is 18-MVE, except for the Ti and V models which prefer the open-shell 16-MVE configuration. This study has shown the difference in the coordination ability of this heteropolycyclic ligand and coordination of the inner C₆ ring is less favored than the outer C₆ and C₅N rings, in agreement with the π-electron density localization.     

Drastic changes of electronic structure and crystal chemistry upon oxidation of SnII2TiO4E2 into SnIV2TiO6: An ab initio study

From DFT based calculations establishing energy-volume equations of state and electron localization mapping, the electronic structure and crystal chemistry changes from Sn₂TiO₄ to Sn₂TiO₆ by oxidation are rationalized; the key effect being the destabilization of divalent tin Sn^II towards tetravalent state Sn^IV leading to rutile Sn₂TiO₆ as experimentally observed. The subsequent electronic structure change is highlighted in the relative change of the electronic band gap which increases from ∼1 eV up to 2.2 eV and the 1.5 times increase of the bulk modulus assigned to the change from covalently Sn^II based compound to the more ionic Sn^IV one. Such trends are also confronted with the relevant properties of black Sn^IIO.     

Ab initio study of electronic structure and optical properties of heavy-metal azides: TlN3, AgN3, and CuN3

Detailed ab initio studies on the electronic structure and optical properties of the heavy-metal azides have been performed using density functional theory within the generalized gradient approximation. An analysis of band structure, density of states, charge transfer, and bond order shows that the heavy-metal azides are ionic compounds but have covalent character. The valence bands of AgN3 and CuN3 are strongly dominated by Ag- and Cu-d, respectively, but that of TlN3 arises from the contributions of Tl-s and terminal N-p and not from Tl-d. The real and imaginary parts of the dielectric function, adsorption coefficient, and electron energy-loss spectra are calculated and compared with available experimental data.     

Use of phosphine gold acetylides [(R3P)AuCCR] to form phosphine gold derivatives of tetra-iron and penta-iron clusters

Five new gold acetylides, [AuCCR], with hydroxyl or amino functions in the organic radical R have been prepared. From these, nine phosphine complexes [(R₃P)AuCCR] with R = Ph or Cy were synthesised. Reactions between the phosphine gold acetylides [(Ph₃P)AuCCC(Me)(OH)Et] or [(Cy₃P)AuCCC(Me)(OH)Et] and the iron carbonyl cluster [Et₄N][Fe₄N(CO)₁₂] gave both neutral [(R₃P)AuFe₄N(CO)₁₂] and ionic compounds [(R₃P)₂Au][Fe₄N(CO)₁₂]. Reaction with the penta-iron cluster [Et₄N][Fe₅N(CO)₁₄] afforded [(R₃P)₂Au][Fe₅N(CO)₁₄], [(R₃P)₂Au][Fe₄N(CO)₁₂] and [(R₃P)AuFe₄N(CO)₁₂]. The gold-iron clusters were characterised with spectroscopic methods (IR, NMR and Mössbauer) and in the case of [(Cy₃P)AuFe₄N(CO)₁₂] a single-crystal X-ray analysis.     

Synthesis and characterization of new heterometallic iron sulfur nitrosyl clusters

The reactivity of the (PPN)₂[Fe₈S₆(NO)₈] and (PPN)₂[Fe₆S₆(NO)₆] clusters is explored and new derivative clusters have been synthesized and structurally characterized. The unique (PPN)₂Fe₄S₄(NO)₆ “open-cubane” cluster with a chair like Fe₄S₄ core is obtained along with the mixed metal pentandite-like clusters (PPN)₂[Mo₂Fe₆S₆(NO)₆(CO)₆], (PPr₃)₂Cu₂Fe₆S₆(NO)₆, (PPr₃)₄Cu₄Fe₄S₆(NO)₄, (PPr₃)₂Ni₂Fe₆S₆(NO)₆, (PPr₃)₃Ni₃Fe₄S₆(NO)₄. The rich electrochemistry of the mixed metal clusters is presented as well.