Closed-shell Configuration Research Articles

Towards a low-spin configuration in extended metal atom chains. Theoretical study of trimetallic systems with 22 metal electrons

Different electronic configurations of a series of trinuclear heterometallic chains with 22 metallic electrons, MM'M(dpa)(4)X(2) (M = Co, Rh; M' = Ni, Pd; X = Cl, NCS), have been modelled in search of new systems with novel electrical properties. For this purpose, we explore the possibility of obtaining low-spin (extensively closed-shell) states by introducing chemical changes to the reference compound CoPdCo(dpa)(4)Cl(2) (1), isoelectronic to the herein studied systems, but possessing magnetically coupled localized electrons. The discussion is based on the orbital energies obtained by the DFT methodology. Among the systems herein analysed, CoNiCo(dpa)(4)(NCS)(2) has only two unpaired electrons vs. six in the case of 1, its closed-shell configuration appearing at high energies. For Rh(2)M-based chains, changes go a step further and the RhPdRh(dpa)(4)Cl(2) and RhPdRh(dpa)(4)(NCS)(2) molecules present a closed-shell ground state in close competition with the broken symmetry solution with S = ½ on each Rh(II). One-electron reduction of the latter compounds has been computed with marked structural changes. Our calculations show that the two lowest 23-electron states are separated by 7-8 kcal mol(-1) in favour of the state with an unpaired localized electron on the δ(Pd-N)* orbital instead of the delocalized one (σ(nb))(2)(σ*)(1).

A metastable He–O bond inside a ferroelectric molecular cavity: (HeO)(LiF)2

Elemental helium is a prototypical noble gas and its atom sets the records for many physicochemical properties. With its two electrons in the closed 1s shell, He is the smallest, the least polarizable, the most difficult to ionize, the hardest (in Pearson's sense) and the most electronegative atom known. Helium gas is considered to be as close to an 'ideal gas' as possible and used as a standard of compressibility and pressure. As a consequence of its closed-shell electronic configuration, helium also exhibits chemical inertness: non-charged species which contain chemically bound helium have not been synthesized to date. Here we are able to predict, using quantum mechanical methods, a small neutral molecule, (HeO)(LiF)(2), which contains a helium atom chemically bound to oxygen. The molecule is planar with the polarized He(δ+)O(δ-) unit embedded anti-parallel in-between two co-aligned LiF dipoles. We calculate its selected physicochemical properties as well as suggest possible decomposition and synthesis pathways. Successful preparation of neutral species, (HeO)(LiF)(2) or related molecules, would break down the paradigm of helium's inertness.

A Density Functional Theory Study of Raman Modes of Hydrogenated Cadmium Sulphide Nanoparticles

Raman scattering investigations based on density functional theory (DFT) calculations were performed to explore the vibrational modes of wurtzite structured CdS nanoparticles (NPs). The calculations were performed to obtain the Raman spectra for the CdS containing 2, 4, 8 and 12 atoms to study the size dependence. Several vibrational modes indicating stretching and bending features related to Cd and S atoms were observed. Modifications of the frequency and intensity of different Raman modes with an increase in number of atoms in NPs are discussed in detail. It is found that the frequency of the CdS symmetric stretching mode of vibration shows a consistent red shift and that of CdS anti-symmetric stretching shows a consistent blue shift with the increase in the number of atoms. Hydrogen atoms were added in order to make the closed shell configuration and saturate the NPs as per the requisite for calculating the Raman spectra. This produced some additional modes of vibration related to hydrogen atoms. The SH stretching mode showed a consistent red shift and the CdH stretching mode showed a consistent blue shift with an increase in the number of atoms in NPs. The results generated are found to be in close agreement with the literature. The observed red shift in different modes is assigned to stimulated Raman stretching and blue shift is ascribed to the coherent anti-stokes Raman scattering.

A DFT + U study of structure and reducibility of CenO2n−x (n ⩽ 4, 0 ⩽ x ⩽ n) nanoclusters

Equilibrium structures, stability and reducibility of CenO2n−x (n⩽4, x=0∼4) nanoclusters have been studied using first principles DFT+U method. The planar rhombus Ce2O2 structure is found to be the building block for the most stable CenO2n−x clusters. The normalized binding energy of the cluster decreases linearly with increasing cluster size. The most stable stoichiometric CenO2n clusters are electronically in closed-shell configuration (singlet), while the non-stoichiometric CenO2n−x clusters are in a high spin state (triplet or quintet). The reduction energy, i.e., the energy required to remove an oxygen atom from a cluster, increases with the size and the extent of reduction. On the other hand, per electron based reduction energy for the cluster to reach the same formal oxidation state is independent of the cluster size.

Theoretical Analysis of [5.7]nCyclacenes: Closed-Shell Cyclacene Isomers

The [5.7](n)cyclacenes represent a novel class of all sp(2)-hybridized carbon structures. In contrast to the isomeric [n]cyclacenes, [5.7](n)cyclacenes are predicted at the B3LYP/6-31G* level of theory to have stable, closed-shell singlet ground state configurations. Predicted geometries, electronic structures, band gaps, nucleus-independent chemical shift (NICS) values, and strain energies for this new family of cyclic conjugated molecules are presented.

Phenyloxenium Ions: More Like Phenylnitrenium Ions than Isoelectronic Phenylnitrenes?

The geometries and energies of the electronic states of phenyloxenium ion 1 (Ph-O(+)) were computed at the multireference CASPT2/pVTZ level of theory. Despite being isoelectronic to phenylnitrene 4, the phenyloxenium ion 1 has remarkably different energetic orderings of its electronic states. The closed-shell singlet configuration ((1)A(1)) is the ground state of the phenyloxenium ion 1, with a computed adiabatic energy gap of 22.1 kcal/mol to the lowest-energy triplet state ((3)A(2)). Open-shell singlet configurations ((1)A(2), (1)B(1), (1)B(2), 2(1)A(1)) are significantly higher in energy (>30 kcal/mol) than the closed-shell singlet configuration. These values suggest a revision to the current assignments of the ultraviolet photoelectron spectroscopy bands for the phenoxy radical to generate the phenyloxenium ion 1. For para-substituted phenyloxenium ions, the adiabatic singlet-triplet energy gap (ΔE(ST)) is found to have a positive linear free energy relationship with the Hammett-like σ(+)(R)/σ(+) substituent parameters; for meta substituents, the relationship is nonlinear and negatively correlated. CASPT2 analyses of the excited states of p-aminophenyloxenium ion 5 and p-cyanophenyloxenium ion 10 indicate that the relative orderings of the electronic states remain largely unperturbed for these para substitutions. In contrast, meta-donor-substituted phenyloxenium ions have low-energy open-shell states (open-shell singlet, triplet) due to stabilization of a π,π* diradical state by the donor substituent. However, all of the other phenyloxenium ions and larger aryloxenium ions (naphthyl, anthryl) included in this study have closed-shell singlet ground states. Consequently, ground-state reactions of phenyloxenium ions are anticipated to be more closely related to closed-shell singlet arylnitrenium ions (Ar-NH(+)) than their isoelectronic arylnitrene (Ar-N) counterparts.

Approximate theoretical model for the five electronic states ( Ω = 5/2, 3/2, 3/2, 1/2, 1/2) arising from the ground 3d 9 configuration in nickel halide molecules and for rotational levels of the two Ω = 1/2 states in that manifold

In the first part of this paper an effective Hamiltonian for a non-rotating diatomic molecule containing only crystal-field and spin–orbit operators is set up to describe the energies of the five spin–orbit components that arise in the ground electronic configuration of the nickel monohalides. The model assumes that bonding in the nickel halides has the approximate form Ni +X −, with an electronic 3d 9 configuration plus closed shells on the Ni + moiety and a closed shell configuration on the X − moiety. From a crystal-field point of view, interactions of the positive d-hole with the cylindrically symmetrical electric charge distribution of the hypothetical NiX − closed-shell core can then be parameterized by three terms in a traditional expansion in spherical harmonics: C 0 + C 2 Y 20( θ, ϕ) + C 4 Y 40( θ, ϕ). Interaction of the hole with the magnetic field generated by its own orbital motion can be parameterized by a traditional spin–orbit interaction operator A L · S . The Hamiltonian matrix is set up in a basis set consisting of the 10 Hund’s case (a) basis functions | L, Λ; S , Σ〉 that arise when L = 2 and S = 1/2. Least-squares fits of the observed five spin–orbit components of the three lowest electronic states in NiF and NiCl are then carried out in terms of the four parameters C 0, C 2, C 4, and A which lead to good agreement, except for the two | Ω| = 1/2 states. The large equal and opposite residuals of the | Ω| = 1/2 states can be reduced to values comparable with those for the | Ω| = 3/2 and | Ω| = 5/2 states by fixing A to its value in Ni + and then introducing an empirical correction factor for one off-diagonal orbital matrix element. In the second part of this paper the usual effective Hamiltonian B( J – L – S ) 2 for a rotating diatomic molecule is used to derive expressions for the Ω-type doubling parameter p in the two | Ω| = 1/2 states. These expressions show (for certain sign conventions) that the sum of the two p values should be −2 B, but that their difference can vary between −10 B and +10 B. These theoretical results are in good agreement with the two observed p values for both NiF and NiCl. The present formalism should in principle be applicable to NiBr and NiI, and to the halides of palladium, since Pd + has a well isolated 4d 9 electronic ground configuration. Extension to metal halides having d n configurations with n < 9, or to platinum halides may present difficulties, since manifolds from the d n and d n −1 s configurations may be heavily mixed, thus requiring “too many” parameters in the electronic part of the problem. Application to linear triatomic molecules may also present problems because of the large number of vibronic perturbations made possible by their four vibrational degrees of freedom.

ChemInform Abstract: Syntheses, Structures and Properties of Primarily Nanosized Homo/Heterometallic Palladium CO/PR3‐Ligated Clusters

AbstractReview: 58 refs.

Metal-organic charge transfer can produce biradical states and is mediated by conical intersections

The present paper illustrates key features of charge transfer between calcium atoms and prototype conjugated hydrocarbons (ethylene, benzene, and coronene) as elucidated by electronic structure calculations. One- and two-electron charge transfer is controlled by two sequential conical intersections. The two lowest electronic states that undergo a conical intersection have closed-shell and open-shell dominant configurations correlating with the 4 s 2 and 4 s 1 3 d 1 states of Ca, respectively. Unlike the neutral-ionic state crossing in, for example, hydrogen halides or alkali halides, the path from separated reactants to the conical intersection region is uphill and the charge-transferred state is a biradical. The lowest-energy adiabatic singlet state shows at least two minima along a single approach path of Ca to the π system: ( i ) a van der Waals complex with a doubly occupied highest molecular orbital, denoted , and a small negative charge on Ca and ( ii ) an open-shell singlet (biradical) at intermediate approach (Ca⋯C distance ≈2.5–2.7 Å ) with molecular orbital structure ϕ 1 ϕ 2 , where ϕ 2 is an orbital showing significant charge transfer form Ca to the π-system, leading to a one-electron multicentered bond. A third minimum ( iii ) at shorter distances along the same path corresponding to a closed-shell state with molecular orbital structure has also been found; however, it does not necessarily represent the ground state at a given Ca⋯C distance in all three systems. The topography of the lowest adiabatic singlet potential energy surface is due to the one- and two-electron bonding patterns in Ca-π complexes.

Adsorption, Oxidation State, and Diffusion of Pt Atoms on the CeO2(111) Surface

The interaction of atomic Pt with the regular CeO2(111) surface has been studied by periodic density functional calculations using exchange-correlation potentials with the inclusion of the on-site Hubbard correction (LDA+U and GGA+U) or employing the hybrid HSE06 functional. Various starting geometries have been explored, and two types of electronic configurations—with either no or two unpaired electrons in the unit cell—have been considered. All methods consistently predict that the most favorable interaction corresponds to adsorption on top of a site bridging two nearest-neighbor surface O atoms (O2 site) in a closed-shell electronic configuration with an essentially neutral or slightly positively charged Pt. However, for this site another slightly higher energy solution is found with two unpaired electrons in the unit cell. This other local minimum on the potential energy surface represents a completely different physical picture, where Pt has been oxidized to Pt+ with the concomitant reduction of a single Ce4+ to Ce3+. Calculations have also been carried out to estimate the energy barrier corresponding to diffusion of Pt atoms over the surface. The calculated values indicate that the energy cost required for this process is very small. This means that adsorbed Pt adatoms can easily nucleate into larger particles in Pt/CeO2 systems.

Na5GaGe2: A New Intermetallic Compound with an Isolated Linear Anion of GaGe25-

Because of large difference in electronegativity between two components, ionic model suit-ably represents the transfer of electrons from metal cations to Zintl anions. In order to satisfy closed-shell configuration, Zintle anions tend to form network structure or clusters. Binary or ternary intermetallic compounds of germanium provide ample examples of Zintl anions. A series of different geometries have been observed, such as 3D-networks,

The role of substrate electrons in the wetting of a metal surface

We address how the electronic and geometric structures of metal surfaces determine water-metal bonding by affecting the balance between Pauli repulsion and electrostatic attraction. We show how the rigid d-electrons and the softer s-electrons utilize different mechanisms for the redistribution of charge that enables surface wetting. On open d-shell Pt(111), the ligand field of water alters the distribution of metal d-electrons to reduce the repulsion. The closed-shell Cu d(10) configuration of isostructural Cu(111), however, does not afford this mechanism, resulting in a hydrophobic surface and three-dimensional ice cluster formation. On the geometrically corrugated Cu(110) surface, however, charge depletion involving the mobile sp-electrons at atomic rows reduces the exchange repulsion sufficiently such that formation of a two-dimensional wetting layer is still favored in spite of the d(10) electronic configuration.

Comment on “Brillouin scattering studies of liquid argon at high temperatures and high pressures” [J. Chem. Phys. 129, 154503 (2008)

In Ref. 1, the authors report on Brillouin scattering measurements on argon at high pressure P and temperature T conditions from which they obtain experimental data for the sound velocity and refractive index of the fluid phase along several isotherms up to 503 K and respective melting pressures. Starting from the refractive index data, they derive the density of fluid argon along the studied isotherms using the Lorentz–Lorenz relation, under the assumption that the electronic polarizability of argon does not vary with P-T conditions. Applying the same reasoning to the data obtained at the solid-fluid coexistence, they obtain values for the density of the fluid and solid phases at coexistence, from which they derive the volume jump Vm at melting or melting volume . This results in a highly unusual observation, that is, the increase in Vm with temperature along the melting line. In this comment I show that this finding is neither supported by available experimental data nor theoretical calculations, and that this is due to an erroneous determination of the fluid densities. Because it is an inert, weakly interacting system with a simple closed-shell electronic configuration, argon has longplayed the role of reference system in condensed-matter physics. In particular, its thermodynamic properties such as P-V-T equation of state EOS and melting curve have been often used as benchmark data for testing various theories. In parallel, a large number of experimental studies have been devoted to the accurate determination of thermodynamic data, which cover a large P-T range see, for example, Ref. 2 for a good number of references for the fluid phase and melting curve . Figure 1 shows the available literature data for the melting volume, obtained by direct volumetric measurements, and compare them to the values of Ref. 1. It is clear that all previous data sets show a regular decrease in Vm with Tm or Pm . In addition to experimental data, which are limited to 320 K, I have plotted the theoretical calculations of Zha et al. using the exponential-6 Buckingham pair potential fitted to shock and static high-pressure data. This potential is an “effective” pair potential as it includes the contribution from many-body interactions averaged over one pair. The calculations agree very well with experimental data, both for the melting curve and volume behaviors, and extend up to 3000 K–53 GPa for the sake of clarity, the T range of Fig. 1 was limited to 600 K . The solid line in Fig. 1 corresponds to the empirical law proposed by Cheng et al., who showed that the experimental data were very well reproduced by the form

Quantum Study of a Trapped Dipolar Fermi Gas

Using the time-dependent Hartree–Fock theory (TDHF), we study the ground states of dipolar fermion gases in a spherical trap. It is found that deformed configurations which have prolate shape both in real and momentum space are energetically favored and that when the dipolar interaction is not very strong, the deformation of the gases shows a prominent shell structure at closed oscillator-shell configurations. The quadrupole and breathing modes are also studied in TDHF and it is found that these modes are little affected by the dipolar interaction when the gases have the closed-shell configurations. The effects of the two-body correlations on the ground-state properties and also on the damping of the quadrupole mode are also investigated using a time-dependent density-matrix formalism. It is found that the effects of the two-body correlations are not significant for the closed-shell configurations.

Efficient and accurate approximations to the local coupled cluster singles doubles method using a truncated pair natural orbital basis

A production level implementation of the closed-shell local quadratic configuration interaction and coupled cluster methods with single and double excitations (QCISD and CCSD) based on the concept of pair natural orbitals [local pair natural orbital LPNO-QCISD and LPNO-CCSD) is reported, evaluated, and discussed. This work is an extension of the earlier developed LPNO coupled-electron pair approximation (LNPO-CEPA) method [F. Neese et al., Chem. Phys. 130, 114108 (2009)] and makes extended use of the resolution of the identity (RI) or density fitting (DF) approximation. Two variants of each method are compared. The less accurate approximations (LPNO2-QCISD/LPNO2-CCSD) still recover 98.7%-99.3% of the correlation energy in the given basis and have modest disk space requirements. The more accurate variants (LPNO1-QCISD/LPNO1-CCSD) typically recover 99.75%-99.95% of the correlation energy in the given basis but require the Coulomb and exchange operators with up to two-external indices to be stored on disk. Both variants have comparable computational efficiency. The convergence of the results with respect to the natural orbital truncation parameter (T(CutPNO)) has been studied. Extended numerical tests have been performed on absolute and relative correlation energies as function of basis set size and T(CutPNO) as well as on reaction energies, isomerization energies, and weak intermolecular interactions. The results indicate that the errors of the LPNO methods compared to the canonical QCISD and CCSD methods are below 1 kcal/mol with our default thresholds. Finally, some calculations on larger molecules are reported (ranging from 40-86 atoms) and it is shown that for medium sized molecules the total wall clock time required to complete the LPNO-CCSD calculations is only two to four times that of the preceding self-consistent field (SCF). Thus these methods are highly suitable for large-scale computational chemistry applications. Since there are only three thresholds involved that have been given conservative default values, the methods can be confidentially used in a "black-box" fashion in the same way as their canonical counterparts.

Passivation of oxygen vacancy states and suppression of Fermi pinning in HfO2 by La addition

We show that group III elements such as La, Y, Sc, and Al can passivate adjacent oxygen vacancies in HfO2 and ZrO2 by shifting the vacancy gap state up into the conduction band. The shift arises from the outward relaxation of the cations around the vacancy due to its positive charge, becoming a closed shell configuration. La substitution at Hf sites in HfO2 suppresses Fermi level pinning because its holes offer a deeper sink for electrons generated by the vacancy than transfer to the metal gate.

Aromaticity of neutral and doubly charged polyacenes

The aromatic character of neutral and doubly charged polyacenes was explored in terms of the harmonic oscillator model of aromaticity (HOMA) and bond resonance energy (BRE). Doubly charged species of polyacenes are different in global and local aromaticity from the neutral species. Neutral species are fairly uniform in local aromaticity, whereas doubly charged species are more aromatic in the edge rings than in the central rings. Higher polyacenes have been predicted to exist as singlet-state diradicals. It was found that these polyacene diradicals are quite similar in global and local aromaticity to the closed-shell molecular dications. The possible instability of higher polyacenes in closed-shell electronic configuration is associated with the high-lying highest occupied molecular orbital (HOMO) that contributes little to global aromaticity.

Synthesis, Structure, and Bonding of the Zintl Phase Ba3Cd2Sb4

Reported are the synthesis of the new ternary compound Ba3Cd2Sb4 and its structure determination by single-crystal X-ray diffraction. Ba3Cd2Sb4 crystallizes with the monoclinic space group C2/m (No. 12); unit cell parameters a = 17.835(2) A, b = 4.8675(5) A, c = 7.6837(7) A, and beta = 112.214(1) degrees; Z = 4. Its structure can be viewed as made of Ba2+ cations and [Cd2Sb4] double chains that are interconnected through Sb-Sb bonds to form 2D infinity2[Cd2Sb4]6- layers. The bonding arrangement in Ba3Cd2Sb4 can also be derived from other known structure types that feature similar fragments, such as TiNiSi, Ca3AlAs3, and Ca5Al2Sb6. Tight-binding linear muffin-tin-orbital band structure calculations are presented as well and show that the constituent elements have closed-shell configurations, indicative of Ba3Cd2Sb4 being a Zintl phase with poor metallic behavior. Crystal orbital Hamilton population analyses on selected atomic interactions in this structure are discussed within the context of the site preference, manifested in the mixed-cation compounds and Ba3-xAxCd2Sb4, where A = Ca, Sr, Eu, and Yb.

Surface modelling on heavy atom crystalline compounds: HfO 2 and UO 2 fluorite structures

The study of the bulk and surface properties of cubic (fluorite structure) HfO 2 and UO 2 was performed using the hybrid Hartree–Fock density functional theory linear combination of atomic orbitals simulations via the CRYSTAL06 computer code. The Stuttgart small-core pseudopotentials and corresponding basis sets were used for the core–valence interactions. The influence of relativistic effects on the structure and properties of the systems was studied. It was found that surface properties of Mott–Hubbard dielectric UO 2 differ from those found for other metal oxides with the closed-shell configuration of d-electrons.

Theoretical observation of hexaatomic molecules containing pentacoordinate planar carbon

The smallest molecules up to date containing a D5h pentacoordinate planar carbon (PPC) atom, CBe5 and CBe54−, are presented by means of ab initio calculations. To gain a better understanding about which electronic factors contribute to their stabilization, natural bond orbital (NBO) analysis and the nucleus independent chemical shifts (NICS) were calculated. The data reported here suggest that D5h CBe5 is σ aromaticity in nature, while in D5h CBe54− π aromaticity is dominating. The classical octet rule is well satisfied in both molecules, and is one of the fundamental reasons to understand the stability of the pentagon structures. The Be5 ring serves as σ donor in D5h CBe5, and π-acceptor in D5h CBe54−. The D5h CBe54− possessing 18 valence electrons with a closed-shell electron configuration is the most plausible candidate for experimental detection.