Structure and electronic properties of 3,3′-diamino-4,4′-azo-1,2,4-triazole nitrate and perchlorate

Based on density functional theory with regard to the dispersion interaction the crystal structure and electronic properties of C₄H₈N₁₂O₆ and C₄H₈N₁₀Cl₂O₈ are studied. Atomic structural parameters, bond populations, atomic charges, energy and spatial electron distributions are calculated. Differences in the studied characteristics caused by the non-equivalence of atoms are shown. A partially covalent nature of anion-cation bonds is revealed. The cationic nature of the lower unoccupied states is established, which results in a small band gap of ~1.7 eV as compared to other nitrates and perchlorates.     

First Principles Investigations of the Crystal Structure,Electron Spectra,and Chemical Bonding in the Low-Temperature Phase of Lithium Imide

The density functional method in the full-electron approximation is used to calculate optimized structural parameters, band spectrum, density of states, electron density maps and to study chemical bonding in the low-temperature phase of lithium imide with a crystal structure of the space group Ima2. The valence band width is 15.14 eV, and it consists of three subzones (slightly dispersed bottom subzone, medium subzone, and top subzone with a width of ~2.4 eV) due to dispersion cased by hybridization of hydrogen s states and nitrogen p states. The top of the valence band and the bottom of the conduction band are in the center of the Brillouin zone, the width of the band gap is 4.48 eV, the absorption edge is direct. The electron density maps demonstrate chains of N–H complexes bound by pairs of Li atoms. The Li–N bond is predominantly ionic, while the N–H bond is polar covalent.     

DFT calculations on the electronic structures of BiOX (X = F, Cl, Br, I) photocatalysts with and without semicore Bi 5d states

The electronic structures of BiOX (X = F, Cl, Br, I) photocatalysts have been calculated with and without Bi 5d states using the experimental lattice parameters, via the plane-wave pseudopotential method based on density functional theory (DFT). BiOF exhibits a direct band gap of 3.22 or 3.12 eV corresponding to the adoption of Bi 5d states or not. The indirect band gaps of BiOCl, BiOBr, and BiOI are 2.80, 2.36, and 1.75 eV, respectively, if calculated with Bi 5d states, whereas the absence of Bi 5d states reduces them to 2.59, 2.13, and 1.53 eV successively. The calculated gap characteristics and the falling trend of gap width with the increasing X atomic number agree with the experimental results, despite the common DFT underestimation of gap values. The shapes of valence-band tops and conduction-band bottoms are almost independent of the involvement of Bi 5d states. The indirect characteristic becomes more remarkable, and the conduction-band bottom flattens in the sequence of BiOCl, BiOBr, and BiOI. Both O 2p and X np (n = 2, 3, 4, and 5 for X = F, Cl, Br, and I, respectively) states dominate the valence bands, whereas Bi 6p states contribute the most to the conduction bands. With the growing X atomic number, the localized X np states shift closer toward the valence-band tops, and the valence and conduction bandwidths evolve in opposite trends. Atomic and bond populations have also been explored to elucidate the atomic interactions, along with the spatial distribution of orbital density.     

A First-Principles Calculation of Electronic Properties of LiNH<Subscript>2</Subscript> and NaNH<Subscript>2</Subscript>

Band spectra, densities of states, total and deformation densities of α-LiNH₂ and α-NaNH₂ are calculated from the first principles using the density functional method in the all-electron approximation. The upper valence band is formed mostly by nitrogen p-states with a small admixture of metal states, the lower conduction bands are formed by the states of all atoms in α-LiNH₂ and mainly by sodium and nitrogen states in α-NaNH₂. The bottom of the conduction band appears in both crystals in the center of the Brillouin zone. α-LiNH₂ exhibits indirect-gap transitions at the absorption edge and three valence band extrema at a short distance of ~0.15 eV from each other. The top of the valence band in α-NaNH₂ appears in the center of the Brillouin zone with the competing maximum at the lateral point at a distance of ~0.06 eV. The electron density distributions testify that polar covalent bonding occur inside the amide anion and ionic bonding occurs between the metal and the amide ion.     

DFT+U study of defects in bulk rutile TiO(2)

We present a systematic study of electronic gap states in defected titania using our implementation of the Hubbard-U approximation in the grid-based projector-augmented wave density functional theory code, GPAW. The defects considered are Ti interstitials, O vacancies, and H dopants in the rutile phase of bulk titanium dioxide. We find that by applying a sufficiently large value for the Hubbard-U parameter of the Ti 3d states, the excess electrons localize spatially at the Ti sites and appear as states in the band gap. At U=2.5?eV, the position in energy of these gap states are in fair agreement with the experimental observations. In calculations with several excess electrons and U=2.5?eV, all of these end up in gap states that are spatially localized around specific Ti atoms, thus effectively creating one Ti(3+) ion per excess electron. An important result of this investigation is that regardless of which structural defect is the origin of the gap states, at U=2.5?eV, these states are found to have their mean energies within a few hundredths of an eV from 0.94 eV below the conduction band minimum.     

Guanine crystals: a first principles study

We report first principles density functional theory studies on the basic ground state characteristics, dynamic properties, and the electronic structure of guanine crystals. The effect of water molecules within the crystal is studied in detail, and we discuss their influence on the structural, vibrational, and electronic properties. The geometries calculated for various crystal structures are compared with gas-phase calculations and available experimental data. Phonon frequencies and eigenvectors are predicted for intermolecular and intramolecular lattice vibrations. Vibrational and electronic density-of-states are presented and analyzed. The electronic band structure near the fundamental gap is calculated from the Kohn-Sham approach. We find that the former molecular HOMO states form a dispersive band in the pi-pi stacking direction upon condensation resulting in a large bandwidth of 0.83 eV. Consequences for the charge transport in layered van der Waals bonded organic molecular crystals are discussed.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of the pressure effect on the structural and electronic properties of crystalline hydrogen azide

Within density functional theory with regard to the dispersion interaction the crystal structure parameters of hydrogen azide are determined. The pressure effect on its structural and electronic properties is studied in the range of 0-10 GPa. By means of the Vinet equation of state the bulk modulus of compression is found to be 9.26 GPa. It is shown that with an increase in the pressure molecules approach each other in molecular layers and this is accompanied by an increase in the total electron density contours, which means the principal possibility for polymerization. The external pressure of 10 GPa leads to the broadening of the upper valence energy bands and a decrease in the band gap from 6.14 eV to 5.51 eV.     

Palladium and magnesium nitrates,a more universal modifier for graphite furnace atomic absorption spectrometry

A mixture of palladium and magnesium nitrates was found to be a very powerful modifier for the determination of As, Bi, In, Pb, Sb, Se, Sn, Te and Tl in graphite furnace atomic absorption spectrometry. Thermal pretreatment temperatures of 900-1400°C can be used with the proposed modifier. This is in most cases substantially more than what can be applied with the modifiers recommended up to now, so that separation of the analyte from the concomitants should be easier. This is shown to be true for the determination of lead in sea water and of selenium in biological materials. Optimum atomization temperatures are more uniform and typically around 2000°C for the investigated elements when the palladium and magnesium nitrates mixed modifier is used. This modifier therefore allows the use of common conditions for all the investigated elements with a minimum sacrifice in sensitivity, an important pre-requirement for multi-element furnace techniques. The proposed mixed modifier also minimizes the risk of contamination because palladium as well as magnesium nitrate can be obtained in high purity, and both elements are infrequently determined in the graphite furnace.     

A first‐principles investigation of the hydrogen bond interaction and the sensitive characters in cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole

A theoretical study of structural and electronic properties of cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole (BCHMX) crystal is performed using density functional theory. The band structure, the total density of states, the atomic orbit projected density of states (PDOS) of C, N, O, and H, and Mulliken population analysis are discussed. The study by analyzing the PDOS shows that the structure of BCHMX crystal possesses C? H···O intra‐ and intermolecular hydrogen bonding. There are hydrogen bonds between H₃‐1s and O₅‐2p orbits, H₂‐1s and O₆‐2p orbits of intramolecules and between H₂‐1s and O₁‐2p orbits of intermolecules. The reasons for the smaller impact sensitivity compared with β‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane and 1,3,5‐trinitro‐1,3,5‐triazinane are also explored from the band gap in the crystal and the weakest bond dissociation energy in single molecule. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

One-dimensional metallic conducting pathway of cyclohexyl-substituted spiro-biphenalenyl neutral radical molecular crystal

The unprecedented metallic character of the cyclohexyl-substituted spiro-biphenalenyl neutral radical molecular crystal (5) suggested by its Pauli paramagnetism [Science 2005, 309, 281] is contradicted by the thermally activated conduction measured along the needle axis of crystal 5 and by an optical gap of Eg = 0.34 eV. Herein we provide the first high quality ab initio electronic structure calculations using density functional theory to reconcile these properties. The calculations point toward 5 being a quasi one-dimensional (1-D) material, with a 1-D conducting pathway along the [101] pi-chain direction. Along any directions other than the pi-chain, conduction is impeded by the small interchain overlap. 5 has a quarter-filled band structure with a density of states of N(Ef) = 7.5 states eV-1 at the Fermi level, leading to a metallic character along the pi-chain.     

Density Functional Theory Studies on the High-pressure Behavior of Crystalline 5,7-Dinitrobenzo-1,2,3,4-tetrazine-1,3-dioxide

The structural and electronic properties of the solid 5,7-dinitrobenzo-1,2,3,4-tetrazine-1,3-dioxide(DNBTDO) under the hydrostatic pressure of 0~100 GPa were investigated using density functional theory method. The predicted crystal structure with the LDA/CA-PZ functional agrees well with the experimental data at the ambient pressure. The structural results show that the b axis is the most compressible, whereas the a and c axes both have slight variation with pressure. The band gap generally decreases with the increasing pressure, which shows that the DNBTDO molecular crystal undergoes an electronic phase transition from semiconductor to metallic system. Through the analysis of band gap, the title compound is most sensitive at 70 GPa. The density of states analysis indicates that the strong peaks split into some small peaks and become wider under compression, which shows the increase of charge overlap and delocalization among the bonded atoms in the system.     

Structure and electronic properties of the surface of alkali metal peroxides

The CRYSTAL09 program with the implemented B3PW hybrid density functional in a localized basis of atomic orbitals is used to determine the atomic and electronic structure of the surface of lithium, sodium, and potassium peroxides. Geometric parameters, surface energies, partial densities of states, electron density distributions, overlap populations, and atomic charges are calculated. It is found that the geometry relaxation has a characteristic depth up to ～10 Å, while the surface states are located in the upper layers at a depth up to ～2.5 Å. Structural displacements of atoms do not exceed ～ 0.2 Å; the charge of the upper surface layers is positive, whereas the energy state shifts relative to the bulk ones can reach ～1 eV. The surface energy of peroxides decreases with an increase in the atomic number of the cation.     

Electronic structures and optical properties of BiOX (X = F,Cl, Br,I) via DFT calculations

Based on the density functional theory (DFT), the lattice constants and atomic positions of BiOX (X = F, Cl, Br, I) species have been optimized, and the electronic and optical properties of the relaxed species have been calculated, with Bi 5d states considered or not. Relaxation generally results in the shrinkage in a and the expansion of c. Relaxed BiOCl, BiOBr, and BiOI present indirect band gaps, whereas BiOF exhibits a direct or somewhat indirect band‐gap feature corresponding to the relaxation and calculation with the Bi 5d states or not. The bottom of the conduction band is quite flat for relaxed BiOI, and apparently flat in BiOBr, and shows observable flatness in BiOCl as well when considering the Bi 5d states. The top of the valence band is rather even as well for some species. The obtained maximum gaps for relaxed BiOF, BiOCl, BiOBr, and BiOI are 3.34, 2.92, 2.65, and 1.75 eV, respectively. The density peak of X np states in the valence band shifts toward the valence band maximum with the increasing X atomic number. The bandwidths, atomic charges, bond orders, and orbital density have also been investigated along with some optical properties. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009     

Reactivity Analysis in Diamond Surfaces with a Density Functional Calculation

The surface states of different diamond surfaces are studied using total and partial density of states (DOS) curves, and are related qualitatively to the reactivity of these surfaces, which are important in the process of diamond growth. The calculations combined atomic and functional density approaches with MARVIN and LMTO–ASA codes, respectively. In the atomic calculation, the interatomic potentials are as follows: the b parameter in the Morse potential is 0.5523 Å, the A and B parameters in the nonbonding Lennards–Jonnes potential are 639.6258 eV Ź² and 3.632 eV Å⁶, and the three-body bending potential K₃, is 0.7797 eV rad². To validate these results, the elastic constants were evaluated, finding a good agreement with the experiment. With these potentials, a slab, for each of the diamond surfaces, of 40 carbon atoms with periodic conditions in two dimensions was optimized. The output coordinates were used for DOS calculations. These results were later verified with a surface-cluster calculation of HOMO and LUMO frontier orbitals. These were calculated using a 9-carbon cluster with the DGauss code. In a nonrelaxed surface, two surface states are identified: the first is an occupied state placed at the center of the gap, and the other, adjacent to a valence band maximum, is an empty state of p character and, therefore, potentially able to participate in chemical interactions. In the relaxation process of the surface, the surface states become narrower as the valences of the surface carbons are saturated; in this case the isolated p state participates in dangling bonds. With the monohydrogenation, the surface state placed at the center of the gap of the relaxed surface, becomes a subsurface state, that is, the highest density is not at the surface layer, but in inner layers. As a consequence, the reactivity diminishes. Therefore, it is possible to conclude that the study of surface states could give predictive information about the reactivity of surfaces.     

The Kohn-Sham density of states and band gap of water: from small clusters to liquid water

Electronic properties of water clusters (H2O)(n), with n=2, 4, 8, 10, 15, 20, and 30 molecules were investigated by sequential Monte Carlo/density-functional theory (DFT) calculations. DFT calculations were carried out over uncorrelated configurations generated by Monte Carlo simulations of liquid water with a reparametrized exchange-correlation functional that reproduces the experimental information on the electronic properties (first ionization energy and highest occupied molecular orbital-lowest unoccupied molecular orbital gap) of the water dimer. The dependence of electronic properties on the cluster size (n) shows that the density of states (DOS) of small water clusters (n>10) exhibits the same basic features that are typical of larger aggregates, such as the mixing of the 3a1 and 1b1 valence bands. When long-ranged polarization effects are taken into account by the introduction of embedding charges, the DOS associated with 3a1 orbitals is significantly enhanced. In agreement with valence-band photoelectron spectra of liquid water, the 1b1, 3a1, and 1b2 electron binding energies in water aggregates are redshifted by approximately 1 eV relative to the isolated molecule. By extrapolating the results for larger clusters the threshold energy for photoelectron emission is 9.6+/-0.15 eV (free clusters) and 10.58+/-0.10 eV (embedded clusters). Our results for the electron affinity (V0=-0.17+/-0.05 eV) and adiabatic band gap (E(G,Ad)=6.83+/-0.05 eV) of liquid water are in excellent agreement with recent information from theoretical and experimental works.     

Manipulating the electronic structures of silicon carbide nanotubes by selected hydrogenation

We show that the electronic and atomic structures of silicon carbide nanotubes (SiCNTs) undergo dramatic changes with hydrogenation from first-principles calculations based on density-functional theory. The exo-hydrogenation of a single C atom results in acceptor states close to the highest occupied valence band of pristine SiCNT, whereas donor states close to the lowest unoccupied conduction band appear as a Si atom being hydrogenated. Upon fully hydrogenating Si atoms, (8,0) and (6,6) SiCNTs become metallic with very high density of states at the Fermi level. The full hydrogenation of C atoms, on the other hand, increases the band gap to 2.6 eV for (8,0) SiCNT and decreases the band gap to 1.47 eV for (6,6) SiCNT, respectively. The band gap of SiCNTs can also be greatly increased through the hydrogenation of all the atoms.     

Theoretical studies of solid bicyclo-HMX: effects of hydrostatic pressure and temperature

On the basis of density functional theory (DFT) and molecular dynamics (MD), the structural, electronic, and mechanical properties of the energetic material bicyclo-HMX have been studied. The crystal structure optimized by the LDA/CA-PZ method compares well with the experimental data. Band structure and density of states calculations indicate that bicyclo-HMX is an insulator with the band gap of ca. 3.4 eV and the N-NO(2) bond is the reaction center. The pressure effect on the bulk structure and properties has been investigated in the range of 0-400 GPa. The crystal structure and electronic character change slightly as the pressure increases from 0 to 10 GPa; when the pressure is over 10 GPa, further increment of the pressure determines significant changes of the structures and large broadening of the electronic bands together with the band gap decreasing sharply. There is a larger compression along the c-axis than along the a- and b-axes. To investigate the influence of temperature on the bulk structure and properties, isothermal-isobaric MD simulations are performed on bicyclo-HMX in the temperature range of 5-400 K. It is found that the increase of temperature does not significantly change the crystal structure. The thermal expansion coefficients calculated for the model indicate anisotropic behavior with slightly larger expansion along the a- and c-axes than along the b-axis.     

Preparation and characterization of a novel single crystal of Th(Ⅳ) with cucurbit[6]uril

One single crystal based on Th~(4+) and cucurbit[6]uril(CB6) in nitric acid aqueous solutions was synthesized by slow evaporation method. The single crystal was characterized by elemental analysis,single crystal X-ray diffraction, XRD, FT-IR and TGA. The complexed cation of Th~(4+) is a ten coordinated structure, in which the central thorium ion is coordinated by six monodentate water molecules and two bidentate nitrates. While CB6, as a second-sphere ligand, coordinates with the water molecules of [Th(NO_3)_2(H_2 O)_6]~(2+) through the formation of hydrogen bonding. Two other nitrate ions act as the counter anions. Besides, there are two free water molecules in the crystal system. The formation of the Th~(4+)-CB6 complex can contribute to the study of the coordination of CB6 and the extraction of Th~(4+) in HNO_3 system