Potential energy surfaces for BeH2 and BH2

The diatomics-in-molecules method has been used to obtain potential energy surfaces (PES) for the molecules BeH₂ and BH₂. The method is used in a way proposed by Tully [1]. The present paper contains an analysis of some features of PES in the vicinity of their crossing near linear configurations of the molecules.     

Ab initio electronic analysis of the hydride transfer in the [CH3-H-CH3]+ system

Summary The electronic aspects of the hydride transfer process between CH₄ and CH ₃ ⁺ fragments, are studied theoretically withab initio molecular orbital methods, subject to the constraint of maintaining a fix distance between both fragments. Mulliken and Natural population analyses are performed to gain an insight into the hydride character of the atom being transferred. From these analyses, charge migrating diagrams are depicted to obtain a more visual information. Further analysis is performed from the contour maps of the electronic charge density, together with the analysis of its gradient and laplacian. Basis set and electronic correlation effects are also discussed. Finally, the effect of applying a uniform electric field is assessed.     

Electronic structures of the boron cage molecules B4H4, B4Cl4 and B4F4

Ab initio calculations have been performed on B₄H₄, B₄Cl₄ and B₄F₄ in order to aid our understanding of the bonding in these compounds, which is presumably based on a tetrahedral boron cage. This cage has only 8 electrons and so is less than that expected on the basis of the usual framework electron counting rules. Basis sets with polarisation functions were used at the SCF, CI and CPF levels of theory to confirm that the T _d structures are indeed more stable than the D _4h ones. Davidson-Roby population analyses were able to show that many factors, including 3-centre 2-electron bonding and backbonding from the ligand to the boron cage, are of importance in determining the relative stability of the three compounds, of which B₄Cl₄ is the only one that has yet been observed experimentally.     

The crystal chemistry of L-Ta2O5 and related structures

Single crystals of a new form of L-Ta₂O₅ with a 19×b superstructure have been synthesised by flux growth. The phase is most likely stabilised by the incorporation of a small amount of lithium (0.14 wt% Li) from the flux. The phase has C-centred monoclinic symmetry with , (), , γ=90.00(1)°. The structure was refined in space group C112/m to R₁=0.044 for 814 unique reflections with F>4σ(F). The structure can be described as comprising chains of edge-shared TaO₇ pentagonal bipyramids that are regularly folded at (010) planes to give sinusoidal chains along [010]. These chains are interconnected along [100] and [001] by corner sharing, creating inter-chain regions that are occupied by isolated TaO₆ octahedra and pairs of corner-shared octahedra. A comparison with published data for high-quality refinements of related structures has led to the development of a general model that can explain the structural chemistry variations in the known L-Ta₂O₅-related structures. A shorthand notation is presented for representing the structures, based on the sequence along [010] of the interchain octahedra.     

Influence of partial substitution of zinc for copper on electronic structures of YBa2Cu3O y

Summary By use of an approximate band-structure treatment based on the EHMO approach, the energy band structures for the Zn-doped superconductor YBa₂Cu_3–x Zn _x O _y were calculated in the present paper and the influence of partial substitution of zinc for copper on the electronic structures for orthorhombic YBa₂Cu₃O_y was studied. From analysis of the band structures and the densities of states for YBa₂Cu_3–x Zn _x O _y , it was demonstrated that the 2D Cu-O planes in the Y-Ba-Cu-O superconducting system have a direct and dominant influence on superconductivity, whereas the role of the 1D Cu-O ribbons and the O(4) atoms is also of some importance.     

A CNDO/2 study of the polymerization of beryllium chloride

The LCGO-CNDO/2 method has been applied in a study of polymer molecules of the type (BeCl₂) _n for values of n between 2 and 6. The calculations predict chain stretching and increasing. covalent character of the Be-Cl ring bonds as polymerization proceeds.     

Nanoparticle Halos Mechanism in Stabilizing of Colloidal Suspensions: Polymeric Binder and Dispersant Effects

Dispersion behavior of colloidal Al₂O₃ aqueous suspensions was investigated in the presence of highly charged CeO₂ nanoparticles and polymeric additives. It was found that among the investigated parameters, ceria nanoparticles concentration had the highest significant effect on the stability of Al₂O₃-CeO₂ suspensions. However, the low influence of dispersant concentration may be due to significant role of ceria nanoparticles stabilizing alumina microspheres by “nanoparticle halo” formation. The stabilization of the bidispersed suspensions was also evidenced by scanning electron microscopy and elemental analysis of the sediment layers after three weeks.     

Syntheses,crystal structures,and some spectroscopic properties of zinc(II) complexes with N2O2 ligands derived from m-phenylenediamine and m-aminobenzylamine

The reaction of bis(salicylidene)-m-phenylenediamine with zinc(II) ion affords a 2?:?2 dinuclear zinc(II) complex formulated as [Zn₂(L¹)₂]. A similar 2?:?2 dinuclear zinc(II) complex, [Zn₂(L²)₂], can be obtained by reaction of bis(salicylidene)-m-aminobenzylamine with zinc(II) ion. These two dinuclear complexes slightly differ in their crystal structures, especially coordination environments around the zinc(II) centers, depending on the dissimilar flexibilities of the two ligands. The differences between the two complexes are reflected in their diffuse reflectance and photoluminescence behaviors.     

Magnetic and electronic structure of TlCo2S2

An extensive investigation of the ferromagnetic compound TlCo₂S₂ has resulted in new information on the electronic and magnetic structure. Electronic structure calculations showed that magnetic ordering is energetically favorable with a clear driving force for ferromagnetic coupling within the cobalt layers. TlCo₂S₂ is metallic and the conductivity is due to holes in the valence band. XPS single crystal measurements did not show evidence of mixed oxidation states of cobalt. Neutron powder diffraction resulted in a ferromagnetic structure with the magnetic moment in the ab-plane. The derived magnetic moment of the cobalt atom is at 10 K and is in very good agreement with the value, at 10 K, inferred from the magnetic hysteresis curve.     

Influence of support acidity on electronic state of platinum in oxide systems promoted by SO4 2− anions

The electronic state of platinum supported on SO₄/ZrO₂, SO₄/TiO₂, SO₄/Al₂O₃, and SO₄/SiO₂ systems and on systems unpromoted by sulfur was investigated by diffuse-reflectance IR spectroscopy using CO as the probe molecule. The introduction of SO₄ ²⁻ anions increases the electron deficit on platinum particles. This suppresses the formation of bridging CO complexes with the metal, leads to the high-frequency shift of absorption maxima of CO adsorbed in the linear form, and stabilizes positively charged metal species (Pt^δ+ and Pt⁺) during the reduction process. The formation of the positively charged species includes the interaction between the acidic protons and the metal particles yielding [Pt−H]^δ+ adducts. The extent of the influence of the support on the electronic state of the metal increases in the series SO₄/SiO₂<SO₄/Al₂O₃<SO₄/TiO₂<SO₄/ZrO₂ in parallel with an increase in the strength of the acid sites in the system. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1094–1099, June, 1998.     

Stereoselective total synthesis of muconin

An antitumor acetogenin, muconin, was synthesized through a coupling reaction of a THF-THP segment and a terminal butenolide. The key reactions include successive ether-ring formation reaction under acidic and basic conditions or one-pot double cyclization promoted by TBAF and stereoselective reduction of acyclic ketones adjacent to the 2,6-cis THP with Zn(BH₄)₂.     

Structure and electronic structure of [(CF3)2PN]2NVCl2, a molecule involving a planar six-membered ring (N3P2V)

Summary The SCF method is applied to determine the (gas phase) structure of [(CF₃)₂PN]₂NVCl₂, which agrees with the solid-state X-ray structure within typical errors of 2 pm and 2° in bond distances and angles. The electronic structure of atoms forming the ring is best described in terms of divalent N^– and tetravlent P⁺ with appreciable declocalization of nitrogen lone pairs into low-lying empty orbitals of neighbouring atoms P and V. No evidence for aromaticity of the ring system is found.     

Solid-state metal hydride batteries using ZrO2·nH2O as an electrolyte

ZrO₂·1.5H₂O or ZrO₂·1.5H₂O-KOH composite was used as an electrolyte in order to develop a solid-state nickel-metal hydride battery. The battery using the ZrO₂·1.5H₂O-KOH composite had rechargeability, but had a very low discharge efficiency, even at low current density. However, the performance of the battery was prominently improved by enlarging the electrode-electrolyte interface area. The resultant battery exhibited the remarkably longer cycle life, the higher discharge efficiency, and the lower polarization: it was able to operate over 150 cycles at 10 mA/g alloy.     

X-ray/neutron diffraction studies and ab initio electronic structure of CeMgNi4 and its hydride

The crystal structure of CeMgNi₄ intermetallic compound has been studied by both X-ray and neutron diffraction. Rietveld refinement shows that both 4a and 4c sites are occupied by Ce and Mg. The exchange has been evaluated to be about 15%. The hydrogenation of the sample leads to a decomposition and to the formation of CeH_2.52. Ab initio calculations using pseudo-potential and all-electron DFT methods are performed to explain such an unexpected behaviour. They predict a larger stability of the hydride system in the orthorhombic structure rather than in the cubic one. Anti-bonding Ce–H interactions within the hydride are proposed to assess the observed easy decomposition. Moreover, the metastability introduced by mechanosynthesis (i.e. exchange between Ce and Mg) was also evaluated.     

Solid-state metal hydride secondary batteries using heteropolyacid hydrate as an electrolyte

In order to enhance the performance of a solid-state MnO₂-metal hydride battery using H₃PMo₁₂O₄₀ · 20H₂O as an electrolyte, a moderate amount of the electrolyte was added to both positive and negative electrodes. The high rate characteristics of the battery were improved significantly by optimizing the electrolyte content in the electrodes; the resulting battery was able to operate over 140 cycles, even at a current density of 20 mA/g alloy, which is large enough for the batteries using inorganic solid electrolytes, and keep the discharge efficiency about 90%. The improvement of battery performance appears to be caused by an increase in electrode-electrolyte interface area. The AC impedance analyses revealed that the resistance of interface is decreased by the addition of a suitable amount of the electrolyte, suggesting an increase in the interface area.     

Response calculations of electronic and vibrational transitions in molecular oxygen induced by interaction with noble gases

The Einstein coefficient for the singlet oxygen emission a1Deltag-->X3Sigmag- at lambda=1270 nm and b1Sigmag+-->X3Sigmag- emission at lambda=750 nm were calculated by quadratic response (QR) multiconfiguration self-consisted field (MCSCF) method for a number of collision complexes O2+M, where M=He, Ne, Ar. Interaction with He clusters was studied in order to simulate cooperative effect of the environment on the oxygen emission. Calculations of the dipole transition moment for the Noxon band, b1Sigmag+-a1Deltag, by linear response (LR) MCSCF method were also performed for a number of collision complexes. Spin-orbit coupling (SOC) between the b1Sigmag+ and X3Sigmag- (MS=0) states does not change much upon collisions, thus the a-X transition borrows intensity mostly from the collision-induced Noxon band b-a. The a-X intensity borrowing from the Schumann-Runge transition is negligible. The calculations show that the b-a and a-X transition probabilities are enhanced approximately by 10(5) and 10(3) times by O2+M collisions. An order of magnitude differences occur for both transitions for noble gases with large difference in polarizability. A strong cooperative effect is obtained when few He atoms perturb the oxygen molecule. Depending on mutual orientation of the partners it can be a complete quenching of the a-->X emission or strong non-additive enhancement. Collision-induced infrared vibrational transitions in a number of molecular oxygen excited states were studied and shown to be state selective.     

Crystal structures of chloroform solvates of fullerenes

The chloroform solvates of C₆₀ and C₇₀ fullerenes and of the C₆₀/C₇₀ mixture were synthesized and investigated by X-ray powder diffraction.     

The electronic and molecular structure of carbon clusters: C8 and C10

Summary The equilibrium geometries of C₈ and C₁₀ have been determined from electronic structure calculations, using a variety of correlated methods and large basis sets of atomic natural orbitals. For C₈, a cyclic form withC _4h symmetry (¹ A _g) and a linear, cumulene-like form (³ _g ^– ) are isoenergetic candidates for the electronic ground state. For C₁₀, the ground-state equilibrium structure is definitely monocyclic. Three different cyclic structures have been considered here, i.e. cumulenicD _10h , distorted-cumulenicD _5h and acetylenicD _5h . These are all essentially isoenergetic, and are about 50 kcal/mol below the linear³ _g ^– state. The choice of basis sets and methods used has a strong impact on the predicted ground-state structures.Dedicated to Prof. Klaus Ruedenberg     

Evaluation of polymeric nanocomposites for the detection of toxic gas analytes

Four different metal oxide nanoparticles, copper oxide (CuO), aluminum oxide (Al₂O₃), nickel oxide (NiO), and titanium dioxide (TiO₂), were added to poly (2,5-dimethyl aniline) (P25DMA) during synthesis to create different polymer nanocomposites. These polymer nanocomposites were evaluated as potential sensing materials for six different gas analytes (acetaldehyde, acetone, benzene, ethanol, formaldehyde, and methanol). It was found that CuO did not incorporate into the P25DMA and only a small percentage of Al₂O₃ was incorporated. However, both NiO and TiO₂ were incorporated into the P25DMA at the same concentration as during the synthesis step. Overall, the type of metal oxide significantly affected the morphology of the sensing material and the amount of each analyte sorbed. For example, P25DMA doped with 5 wt% Al₂O₃ had high selectivity towards ethanol, whereas P25DMA doped with 20 wt% TiO₂ sorbed the most ethanol. However, P25DMA doped with 20 wt% TiO₂ also sorbed a high amount of formaldehyde, making P25DMA doped with 20 wt% TiO₂ less selective than P25DMA doped with 5 wt% Al₂O₃ towards ethanol with respect to formaldehyde.     

Structural and spectroscopic characterization of Al2−x Cr x O3 powders obtained by polymeric precursor method

Al₂O₃ and Al_2−x Cr _x O₃ (x = 0.01, 0.02 and 0.04) powders have been synthesized by the polymeric precursors method. A study of the structural evolution of crystalline phases corresponding to the obtained powders was accomplished through X-Ray Diffraction and UV-vis spectroscopy (reflectance spectra and CIEL^*a^*b^* color data). The obtained results allow to identify the γ-Al₂O₃ to α-Al₂O₃ phase transition. The single-phase α-Al₂O₃ powder was obtained after heat treatment at 1050 °C for 2 h. The results show that the green to red color transition and ruby luminescence lines observed for the powders of Al_2−x Cr _x O₃ are related to the γ to α-Al₂O₃ phase transition and the temperature and time range for such transition depends on the chromium content.