Electronic structure,Compton profiles and cohesive properties of Cs-halides

We have reported energy bands, density of states, valence electron charge densities and Compton profiles of CsCl, CsBr and CsI using linear combination of atomic orbitals with Hartree–Fock and density functional theories. We have also computed these properties, except the momentum densities, using full potential linearized augmented plane wave method. The general features of the energy bands and the density of states in these halides are found to be almost similar. To interpret the theoretical data on Compton line shapes, we have also measured the Compton profiles using our 20 Ci ¹³⁷Cs spectrometer. It is seen that the Hartree–Fock calculations give relatively a better agreement with the experimental momentum densities. On the basis of equal-valence-electron-density profiles, a comparison of relative nature of bonding is made which is in agreement with the valence charge densities and atomic charges by means of Mulliken analysis. Using our experimental and theoretical Compton profiles, we have also computed the cohesive energy of the halides.     

Electronic structure,optical properties and Compton profiles of Bi2S3 and Bi2Se3

In this paper, we present the Compton profiles of Bi₂S₃ and Bi₂Se₃ using our 20 Ci ¹³⁷Cs Compton spectrometer. To compare our experimental data, we have computed the Compton profiles, energy bands and density of states using linear combination of atomic orbitals with density functional theory (DFT) and Hartree-Fock (HF) scheme. It is seen that hybrid functional involving HF and DFT approximations gives a relatively better agreement with experimental momentum densities than other approximations of DFT. We have also reported the band structure, density of states, valence charge densities, dielectric functions and electron energy loss spectra using full potential linearized augmented plane wave scheme. On the basis of charge densities, Mulliken’s population data and equal-valence-electron-density profiles, Bi₂S₃ is found to be more ionic than Bi₂Se₃. The calculated dielectric functions for the parallel and perpendicular polarizations show a small anisotropic effect. The electron energy loss spectrum for Bi₂Se₃ is found to be in good agreement with the available experimental data.     

Electron momentum density and band structure calculations of α- and β-GeTe

We have measured isotropic experimental Compton profile of α-GeTe by employing high energy (662 keV) γ-radiation from a ¹³⁷Cs isotope. To compare our experiment, we have also computed energy bands, density of states, electron momentum densities and Compton profiles of α- and β-phases of GeTe using the linear combination of atomic orbitals method. The electron momentum density is found to play a major role in understanding the topology of bands in the vicinity of the Fermi level. It is seen that the density functional theory (DFT) with generalised gradient approximation is relatively in better agreement with the experiment than the local density approximation and hybrid Hartree–Fock/DFT.     

Electron momentum density in ZnSe: Theory and experiment

We present experimental Compton profiles of ZnSe along [1 0 0] and [1 1 0] directions using our 740 GBq ¹³⁷Cs Compton spectrometer. We have also computed the momentum densities, energy bands, density of states (DOS) and band gaps using density functional theory (local density and generalized gradient approximations) and pseudopotential (PP) approach. The anisotropy in the momentum density is well reproduced by the density functional calculations. The energy bands and bond length are interpreted in terms of the anisotropies.     

Molecular momentum distributions and compton profiles. Effects of bonding on some small molecules

Electronic momentum distributions and Compton profiles have been calculated from minimum basis set SCF wavefunctions for H₂O, H₂O₂, CO, CO₂ and H₂CO. Radial distributions and profiles have also been estimated for these molecules from localized molecular orbitals. The results suggest that (a) the height of the Compton peak, <p^?1>, may be as sensitive to the effects of chemical bonding as the kinetic energy, <p²>/2, and that (b) the virial theorem may provide a more useful criterion than energy minimization in assessing the accuracy of calculated bonding effects and Compton profiles.     

Electronic structure,Compton profiles and optical properties of TaC and TaN

Isotropic Compton profiles of TaC and TaN have been measured for the first time, at an intermediate resolution, using 662 keV γ-radiation. Energy bands, density of states and Fermi surface topology of TaC and TaN have been computed using linear combination of atomic orbitals with density functional theory and full potential linearised augmented plane wave method. Both band structure calculations predict the metallic character of TaC and TaN. The electron momentum densities calculated using various approaches of density functional theory are compared with the present measurements. On the basis of Mulliken’s population, it is also seen that TaC has more covalent bonding than TaN. The optical properties computed using full potential linearised augmented plane wave method are explained in terms of intraband transitions.     

Valence electronic structure of CH3F and CH3Cl: electron momentum distributions and separation energies

Fluoromethane (CH₃F) has been studied by binary (e,2e) coincidence spectroscopy at 1200 eV using non-coplanar symmetric kinematics. Separation energy spectra have been determined in the energy range up to 50 eV at azimuthal angles of 0° and 9°. The separation energy spectra and electron momentum distributions measured for the valence orbitals of CH₃F and CH₃Cl are compared with the results of calculations employing SCF wavefunctions and outer valence as well as extended 2ph—TDA Green function methods. Electron density and momentum density maps have been generated for all valence orbitals of both molecules using the SCF wavefunctions and are used to explain differences in the bonding properties of the halomethanes investigated here.     

Valence electronic structure of CH3Br and CH3I: Electron momentum distributions and separation energies

Bromomethane (CH₃Br) and iodomethane (CH₃I) have been studied by binary (e,2e) coincidence spectroscopy at 1200 eV using non-coplanar symmetric kinematics. Separation energy spectra have been determined in the energy range up to 47 eV at azimuthal angles of 0° and 8° for CH₃Br and 0° and 6° for CH₃I. The separation energy spectra and the electron momentum distributions measured for each of the valence orbitals are compared with theoretical predictions employing SCF wavefunctions and outer valence type and extended 2 ph-TDA Green function calculations. Electron density and momentum density maps have been calculated for all the valence orbitals using the SCF wavefunctions, and they are used to explain trends and contrasts in the electronic structure and bonding properties of these halomethanes in both position and momentum space.     

Electron momentum distributions from valence-bond wave functions

Momentum densities obtained from the Heitler-London (HL) wave functions for diatomic molecules and those from the corresponding valence-bond (VB) wave functions including ionic terms are compared. In each case they shown maxima in the direction perpendicular to the bond. However, the dependence of momentum densities on mutual orientations of the two electronic momenta is quite complex in the latter case. The improvement in the Compton profile on including the ionic terms is illustrated with the example of H₂. The momentum denmsities obtained from the VB wave function constructed from orthogonalized atomic orbitals (OAO) have also been examined. The HL wave function with OAOS leads to the same momentum distribution as the repulsive state HL wave function constructed from overlapping AOS.     

Electronic structure and bonding of the hydrides Mg3TH7 (T = Mn,Re) from first principles

From pseudo-potentials and all-electrons computations within density functional theory, desorption energies within range of MgH₂ and covalent like hydrogenated intermetallic compounds are identified for hydrogen rich Mg₃TH₇, (T = Mn, Re). The rhenium based compound is found with a lower desorption energy which has been quantified from the analysis of the Bader charges within the {TH₆}^5- complex anion as related with a decreasingly ionic charge on hydrogen from Mn to Re. The electronic densities of states show insulating compounds in agreement with literature relevant to this class of salt-like hydrides with a larger band gap for the Re compound. From chemical bonding analyses stronger Mn–H bonding versus Re–H is identified in agreement with desorption energies magnitudes favoring Mg₃ReH₇.     

A momentum-space view of the chemical bond. I. The first-row Homonuclear diatomics

The electronic probability distribution in momentum space or electron momentum density (EMD) is studied in detail for the first-row homonuclear diatomics. The total density difference (molecule minus constituting atoms)is analyzed in terms of the separate orbital contributions. The nodal structure shown by the orbital EMD is characteristic for the various types of orbital (σ,σ*,=,=*), and is affected, by the amount of s-p hybridization. Directional and isotropic Compton profiles are used to study the bond-oscillation and bond-directional principles. The bond- directional principle does not hold for pe bonding. Spherically averaged EMD differences (SA Δ EMDs) are related to the changes in kinetic energy (ΔT) upon bond formation. The SA ΔEMDs and ΔT are rationalized by considering the different ranges of internuclear distance that are optimal for 2s-2s, 2p_o-2p_o and 2p_o-2p_o interaction. This leads to a reassessment of the role of the various orbitals in bonding complementing the picture based on orbital Hellmann- Feynman forces.     

On the nature of interactions in the F2OXe…NCCH3 complex: Is there the Xe(IV)?N bond?

Nature of the bonding in isolated XeOF₂ molecule and F₂OXe^…NCCH₃ complexes have been studied in the gas phase (0 K) using Quantum Chemical Topology methods. The wave functions have been approximated at the MP2 and DFT levels of calculations, using the APFD, B3LYP, M062X, and B2PLYP functionals with the GD3 dispersion correction. The nature of the formal Xe?O bond in the XeOF₂ monomer depends on the basis set used (all‐electron vs. the ecp‐28 approximation for Xe). Within the all‐electron basis set approach the bond is represented by two bonding attractors, V_{i = 1,2}(Xe,O), with total population of about 1.06e and highly delocalized electron density in both bonding basins. No bonding basins are observed using the ecp‐28 approximation. These results shows that the nature of xenon–oxygen is complicated and may be described with mesomeric equilibrium of the Lewis representations: Xe⁽⁺⁾O^(?) and Xe^(–)O⁽⁺⁾. For both the xenon–oxygen and xenon–fluorine interactions the charge‐shift model can be applied. The F₂OXe^…NCCH₃ complex exists in two structures: “parallel,” stabilized by non‐covalent C^…O and Xe^…N interactions and “linear” stabilized by the Xe^…N interaction. Topological analysis of ELF shows that the F₂OXe^…NCCH₃ molecule appears as a weakly bound intermolecular complex. Intermolecular interaction energy components have also been studied using Symmetry Adapted Perturbation Theory. © 2016 Wiley Periodicals, Inc.     

Structural,elastic, electronic and magnetic properties of perovskite-like Co3WC,Rh3WC and Ir3WC from first principles calculations

We have performed ab initio total energy calculations using the full-potential linearized augmented plane wave method (FP-LAPW) with the generalized gradient approximation (GGA) for the exchange-correlation potential to predict the structural, elastic, cohesive, electronic and magnetic properties of perovskite-like phases Co₃WC, Rh₃WC and Ir₃WC. The optimized lattice parameters, density, independent elastic constants (C_ij), bulk moduli, shear moduli, tetragonal shear moduli, compressibility, and Cauchy pressure, as well as electronic densities of states, cohesive and formation energies, atomic magnetic moments have been obtained and analyzed for the first time.     

Outer valence orbital momentum distributions of carbon dioxide by binary (e,2e) spectroscopy

The outer valence orbital momentum distributions of CO₂ have been reinvestigated using a high momentum resolution (0.1 a_o^?1 fwhm) binary (e,2e) spectrometer operated at 1200 eV impact energy under the non-coplanar symmetric scattering condition. Generally good agreement of the measured momentum distributions with theoretical momentum distributions calculated using literature SCF double-zeta quality wavefunctions has been obtained for the 1π_g, (1π_u + 3σ_u) and 4σ_g orbitals. Although there is a reasonable agreement of the measured momentum distributions with earlier low momentum resolution (0.4 a_o^?1 fwhm) non-coplanar measurements at 400 eV impact energy reported by Cook and Brion, given the large differences in the momentum resolutions much more definitive results are obtained in the present study. In particular, the significantly higher momentum resolution clearly shows the mixed s-p character of the 4σ_g orbital. The present study also gives a much better agreement with theory in the case of the 4σ_g momentum distribution. For each orbital the calculated and where possible the experimentally determined spherically averaged momentum distributions are compared and contrasted with their respective two-dimensional momentum and position density maps. These together with three-dimensional surface plots at selected constant density values of the four outermost orbitals are used to provide a detailed comparison of momentum-space bonding and orbital properties with their more familiar position-space counterparts in the CO₂ triatomic molecule. The calculated momentum-space density contour maps of the core orbitals exhibit rather large density oscillations and the feasibility of future experiments is discussed.     

Transition-element hexafluoride systems in ionic lattices. A Suhf molecular orbital study

An unrestricted Hartree-Fock SCFMO method, based on the MCZDO method of Brown and Roby, suitable for computing spin densities in transition-element compounds, is described. The method is used to study spin densities on fluorine in Cs₂MnF₆, K₂NaCrF₆, K₂MnF₆, K₂NaFeF₆, KMnF₃, RbMnF₃ and KNiF₃ using a “cluster” approximation in which a MF ₆ ^n? unit is explicitly considered. Excellent agreement is obtained between calculated and experimental spin parameters. The effect of the lattice is incorporated by using the electrostatic approximation of Brown, O'Dwyer and Roby. The lattice potential for these highly symmetric systems is found to have little effect on spin densities and charge distributions, but it effects substantial stabilization of the anion molecular orbitals. A general feature of the results is that central-atom 4p orbitals are scarcely involved in bonding, this being confined to the 3d and to some extent the 4s orbitals. Comments are offered on the lack of spin symmetry in the unrestricted Hartree-Fock wavefunctions of these systems, and the need to evaluate the core hamiltonian as accurately as possible.     

Experimental and calculated momentum densities for the valence orbitals of carbon tetrafluoride

Carbon tetraflouoride has been investigated by binary (e,2e) spectroscopy at 1200 eV impact energy. Binding energy spectra (10–60 eV) at azimuthal angles of 0° and 8° are reported and are found to be in quantitative agreement with a previous Green's function calculated spectrum. Momentum distributions corresponding to individual orbitals are also reported and compared with theoretical momentum distributions evaluated using double-zeta quality SCF wavefunctions. Excellent agreement between experimental and theories is found for the strongly bonding 3t₂ orbital and the antibonding 4a₁ orbital but agreement is less good for the outermost non-bonding orbitals. Intense structure due to molecular density (bond) oscillation is observed experimentally in the region above 1.0 a_o^?1 in the case of the non-bonding 4t₂ orbital. It is also notable that the measured 4a₁ momentum distribution exhibits an extremely well-defined “p” character with clear separation between the s and p components. Contour maps of the position-space and momentum-space orbital densities in the F-C-F plane of the molecule are used to provide a qualitative interpretation of the features observed in the momentum distribution. In order to further extend momentum-space chemical concepts to three-dimensional systems, constant density surface plots are also used to give a more comprehensive view of the density functions of the CF₈ molecule.     

Some new members of MAX family including light-elements: Nanolayered Hf2XY (X= Al,Si, P and Y=B,C, N)

The structural, electronic, mechanical and dynamical properties of new members of MAX family (Hf₂XY, X=Al, Si, P and Y= B, C, N compounds) with Cr₂AlC-type structure have been investigated by first-principles density functional plane-wave pseudopotential calculations within generalized gradient approximation. From calculated cohesive energies, all compounds are energetically stable. And, from calculated elastic constants and phonon dispersion curves, it is shown that all compounds are mechanically stable, while the boron including ones are dynamically unstable except for Hf₂PB. At the same time, related mechanical properties such as bulk and shear moduli are calculated. For further mechanical characterization, hardnesses of the compounds are determined theoretically. It is observed from electronic structure calculations including band structure and partial density of states, all stable compounds are metallic. Additionally, bonding nature of the compounds are analyzed by using 3D and 2D electron density maps, Mulliken atomic charges and bond overlap populations.     

Theoretical investigations on the structure, density, thermodynamic and performance properties of amino-, methyl-, and nitroimidazoles and their N-oxides

Density functional theory calculations at the B3LYP/aug-cc-pVDZ level have been performed to explore the structure, stability, heat of explosion, density, and the performance properties of amino-, methyl-, and nitroimidazoles. N-Nitroimidazoles have shown lower densities compared with those of C-nitroimidazoles. Detonation properties of title compounds were evaluated by using Kamlet–Jacob semi-empirical equations based on the predicted densities and the calculated heats of detonation. It has been found that some compounds with the calculated densities 2.0 g/cm³, detonation velocities over 9.10 km/s and detonation pressures of about 45 GPa (some even over 50 GPa) may be novel potential high energy materials. The higher performance of nitroimidazole-N-oxides is apparently due to their higher densities (2.0–2.515 g/cm³). Heat of explosion, stability, density, and performance properties are related to the number and relative positions of –NO₂, –NH₂, and –CH₃ groups of the imidazole ring. The designed nitroimidazoles satisfy the criteria of high energy materials.     

Periodic trends in metal–metal bonding in edge-shared [M2Cl10] systems

Periodic trends in metal–metal interactions in edge-shared [M₂Cl₁₀]⁴⁻ systems, involving the transition metals from groups 4 through 8 and electronic configurations ranging from d¹d¹ through d⁵d⁵, have been investigated by calculating metal–metal bonding and spin-polarization (exchange) effects using density functional theory. The trends found in this study are compared with those for the analogous face-shared [M₂Cl₉]³⁻ systems reported in earlier work. Strong linear correlations between the metal–metal bonding and spin-polarization terms have been obtained for all groups considered. In general, spin polarization and electron localization are predominant in 3d–3d species whereas electron delocalization and metal–metal bonding are favoured in 5d–5d species, with more variable results observed for 4d–4d systems. As previously found for face-shared [M₂Cl₉]³⁻ systems, the strong correlations between the metal–metal bonding and spin polarization energy terms can be related to the fact that both properties appear to be similarly affected by the changes in the metal orbital properties and electron density occurring within the dⁿdⁿ groups. A significant difference between the face-shared and edge-shared systems is that while the 4d metals in the former show a strong tendency for delocalized metal–metal bonded structures, the edge-shared counterparts display much greater variation with both metal–metal bonded and weakly coupled complexes observed. The tendency for weaker metal–metal interactions can be traced to the inability of the edge-shared bridging structure to accommodate the smaller metal–metal distances required for strong metal–metal bonding.