Showing papers on "Valence (chemistry) published in 1997"

The crystal chemistry of hexavalent uranium; polyhedron geometries, bond-valence parameters, and polymerization of polyhedra

Abstract: The geomeffy, bond valences, and polymerization ofhexavalent uranium polyhedra from 105 well-refined structures are analyzed. The Utu cation is almost always present in crystal stnrctures as part of a nearly linear (UOr)z* uranyl ion that is coordinated by four, five or six equatorial anions in an approximately planar arangement perpendicular to the uranyl ion, giving square, pentagonal and hexagonal bipyramids, respectively. The Utu-O7\" bond length (Oy,: uranyl-ion O atom) is independent of the equatorial anions of the polyhedra;-averages of all polyhedra tlat contain uranyl ions ffs; I6lIJ6f-Or. = 1.79(3), mg0.-.9 a,= 1.79(4), and t8lu6+-Our = 1.78(3) A. Not a[ r6lu6+ polyhedra contain uranyl ions; there is a continuous series of coordiaation polyhedr4 from square bipyramidal polyhedra with uranyl ions to holosymmehic octahedral geometry. The mUo* and t8lu6+ polyhedra invariably contaitl a uranyl ion. The equatorial U6.-0 (0: O,-, OH-) bond-lengths of uranyl polyhedra depend upon coordhation number; averages for all polyhedra are t6lu6+-dq = 2,28(5), rlUot-$* = 2.37(9), afi t8tlJ6+-$q2.47 (12) A. Cunently available bond-valence parameters for U& are unsatisfactory for determining bond-valence sums. Coordination-specific bond-valence paxameters have been derived for U6|, together with parameters applicable to all coordination geometries. The parameters give bond-valence sums for Ue of -6 vlr and reasonable bond-valences for Uc,-Ou, bonds. The bond-valence paraneters facilitate the recognition of Ua, U5+ and U6| catiotrs in refined crystal structures. The crystal-chemical consfraints ofpolyhedral polymerization in uranyl phases are discussed.

The prediction of molecular equilibrium structures by the standard electronic wave functions

Abstract: A systematic investigation has been carried out of the accuracy of molecular equilibrium structures of 19 small closed-shell molecules containing first-row atoms as predicted by the following standard electronic ab initio models: Hartree–Fock (HF) theory, Mo/ller–Plesset theory to second, third, and fourth orders (MP2, MP3, and MP4), coupled-cluster singles and doubles (CCSD) theory; CCSD theory with perturbational triples corrections [CCSD(T)], and the configuration-interaction singles and doubles (CISD) model. For all models, calculations were carried out using the correlation-consistent polarized valence double-zeta (cc-pVDZ) basis, the correlation-consistent polarized valence triple-zeta (cc-pVTZ) basis, and the correlation-consistent polarized valence quadruple-zeta (cc-pVQZ) basis. Improvements in the basis sets shorten the bond distances at all levels. Going from cc-pVDZ to cc-pVTZ, bond distances are on the average reduced by 0.8 pm at the Hartree–Fock level and by 1.6 pm at the correlated levels. From cc-pVTZ to cc-pVQZ, the contractions are about ten times smaller and the cc-pVTZ basis set appears to yield bond distances close to the basis-set limit for all models. The models HF, MP2, and CCSD(T) give improved accuracy at increased computational cost. The accuracy of the Mo/ller–Plesset series oscillates, with MP3 being considerably less accurate than MP2 and MP4. The MP2 geometries are remarkably accurate, being only very slightly improved upon at the MP4 level for the cc-pVQZ basis. The CCSD equilibrium structures are only moderately accurate, being intermediate between MP2 and MP3. The accuracy of the CCSD(T) model, in contrast, is high and comparable to that observed in most experimental studies and it has been used to challenge the experimentally determined equilibrium structure of HNO. The CISD wave function provides structures of low quality.

Two-electron valence indices from the Kohn-Sham orbitals

Abstract: The recent Hartree-Fock (HF) difference approach to the chemical valence indices (ionic and covalent), formulated in the framework of the pair-density matrix, is implemented within the Kohn-Sham (KS) density functional theory (DFT). The valence numbers are quadratic in terms of displacements of the molecular spin-resolved charge-and-bond-order (CBO) matrix elements, relative to values in the separated atoms limit (SAL). It is shown that the global valence represents a generalized “distance” quantity measuring a degree of similarity between the two CBO matrices: the molecular and SAL. Numerical values for typical molecules exhibiting single and multiple bonds demonstrate that the KS orbitals give rise to these new bond valences in good agreement with both chemical and HF predictions. This KS bond multiplicity analysis is applied to the chemisorption system including the allyl radical and a model surface cluster of molybdenum oxide. It is concluded that the quadratic valence analysis represents a valuable procedure for extracting useful chemical information from standard DFT calculations. © 1997 John Wiley & Sons, Inc.

Biaxial strain dependence of exciton resonance energies in wurtzite GaN

Abstract: We have systematically studied the strain dependence of the free-exciton resonance energies in wurtzite GaN by photoreflectance measurements using well-characterized samples. The experimental data have been analyzed using the appropriate Hamiltonian for the valence bands in wurtzite GaN and determined the values of the crystal field splitting, the spin–orbit splitting, the shear deformation potential constants, and the energy gap in the unstrained crystal. Discussions are given on the strain dependence of the energy gaps, of the effective masses, and of the binding energies for the free-exciton ground states as well as on the valence-band parameters.

Cation disorder in ferroelectric Aurivillius phases of the typeBi2ANb2O9(A=Ba, Sr, Ca)

Abstract: The structures of the ferroelectric two-layer Aurivillius phases Bi 2 ANb 2 O 9 (A=Ba, Sr, Ca) have been refined using a combination of X-ray and neutron powder diffraction data. Bi 2 BaNb 2 O 9 is not significantly distorted from idealised symmetry and has been refined in tetragonal space group I4/mmm, a=3.9362(1) and c=25.6582(7) A. The Sr and Ca compounds have been refined in orthorhombic space group A2 1 am, with a=5.5193(3), b=5.5148(3), c=25.0857(6) A and a=5.4833(1), b=5.4423(1), c=24.8984(6) A, respectively. The orthorhombic distortion increases with decreasing A 2+ cation size and originates from bonding requirements at the perovskite A site, in agreement with previous work. However, in contrast to earlier work, we find a partial mixing of Bi and A cations on their respective sites, which increases in the order Ca

Ab initio lattice dynamics of BN and AlN: Covalent versus ionic forces

Abstract: We report first-principles calculations of the structural, lattice-dynamical, and dielectric properties for zinc-blende and wurtzite BN and AlN. The ground-state properties, i.e., the lattice constants, the bulk moduli, the ionicity factors of the chemical bonds, and the elastic constants, are calculated using a plane-wave-pseudopotential method within the density-functional theory. A linear-response approach to the density-functional theory is used to derive the Born effective charges, the high-frequency dielectric constants, and the phonon frequencies and eigenvectors. The different behavior of the structural and lattice-dynamical properties of BN and AlN is discussed in terms of the different ionicities, strengths of the covalent bonds, and the atomic masses. Our results are in excellent agreement with the experimental data available. {copyright} {ital 1997} {ital The American Physical Society}

Effects of Rapid Intramolecular Electron Transfer on Vibrational Spectra

Abstract: Single-electron reductions of linked triruthenium clusters of the general type Ru3-pyrazine-Ru3 produced mixed valence systems showing spectroscopic characteristics of rapid intramolecular electron transfer. Reflectance infrared spectroelectrochemistry was used to characterize the vibrational spectra of mixed valence systems that contained one carbon monoxide ligand on each Ru3 cluster. Infrared spectra in the CO stretching region showed two resolved, partially coalesced, and coalesced ν(CO) bands for clusters with rate constants for intramolecular electron transfer k e increasing from = 1 × 109 s– 1 up to 5 × 1011 and 9 × 1011s– 1, respectively. These data provide a strong correlation between rates of intramolecular electron transfer and infrared spectral bandshape.

Discrete coil-globule transition of single duplex dnas induced by polyamines

Abstract: Recently, it has become clear that single, long duplex DNAs exhibit a large discrete transition between elongated coil and compacted globule states. To obtain further insight into this phenomenon, in the present study we observed individual DNA chains in an aqueous environment by fluorescence microscopy. The long-axis lengths of individual T4DNA (166 kbp) were calibrated to obtain a size distribution. The main purpose of the present study was to determine the effect of the valence of cations on the coil−globule transition. We used the following multivalent cations to induce the compaction of long DNA chains: 1,3-diaminopropane (bivalent), spermidine (trivalent), and spermine (tetravalent). Our results showed that the collapse of isolated DNA chains induced by either bivalent or multivalent cations is discrete. The critical concentration of cation for inducing the transition was lowest for the tetravalent cation and highest for the bivalent cation. We also compare the properties of the transition observed...

Structural features of iron phosphate glasses

Abstract: The structures and valence states of iron ions in several iron phosphate glasses with batch compositions similar to 40Fe2O3-60P2O5 (mol%) have been investigated using Mossbauer spectroscopy, X-ray absorption fine-structure spectroscopy (XAFS), X-ray photoelectron spectroscopy (XPS), differential thermal (DTA) and thermo-gravimetric (TGA) analysis and X-ray and neutron diffraction. Mossbauer spectra show that a redox equilibria corresponding to an Fe(II)/[Fe(II) + Fe(III)] ratio of 0.2–0.4 is reached under processing conditions described in this paper. Even though the valence state of iron ions in the glass appears to be insensitive to the oxygen content in the melting atmosphere, the Fe(II) content can be increased within the observed range of redox equilibria by increasing the partial pressure of a reducing gas in the melting atmosphere. Large amounts of Fe(II), Fe(II)/[Fe(II) + Fe(III)] ≥ 0.4, appear to be detrimental to the glass-forming ability of the iron phosphate melts. The local structure of the iron phosphate glasses appears to be related to the short range structure of crystalline Fe3(P2O7)2 which consists of a network of (Fe3O12)−16 clusters. These clusters consist of one iron(II) ion and two iron(III) ions in sixfold coordination with near-neighbor oxygen ions. The (Fe3O12)−16 clusters are interconnected via (P2O7)−4 groups. Compared to other phosphate glasses, the proposed structure for iron phosphate glasses contain a smaller number of POP bonds, a feature which is believed to be responsible for the unusually good chemical durability of iron phosphate glasses.

Benchmark calculations with correlated molecular wave functions. X. Comparison with “exact” MP2 calculations on Ne, HF, H2O, and N2

Abstract: The convergence of the MP2 valence correlation energy and pair energies for the correlation consistent basis sets has been investigated. Ne, HF, H2O, and N2 were studied. For all of these molecules, accurate MP2 correlation and pair energies are available from the recent MP2-R12 calculations of W. Klopper [J. Chem. Phys. 102, 6168 (1995)]. The magnitudes of the calculated MP2 valence correlation and pair energies are found to increase systematically with increasing basis set size, with the cc-pV6Z basis set yielding 97.4%–98.3% of the MP2 valence correlation energy. A detailed analysis of the results for Ne reveals that the error due to truncation of the radial functions in the cc-pV6Z set is comparable to that due to neglect of higher angular momentum functions. Procedures for extrapolating the results to the complete basis set limit have also been investigated.

Dimers based on the α + α potential and chain states of carbon isotopes

Abstract: Using the well established binding energies of one and two valence neutrons in the two-center α+α system (forming the states in 9Be and 10Be*) the structure of these nuclear dimers and their rotational bands including those with more than 2 nucleons are discussed using published transfer reaction data for Be and Boron isotopes. Based on the 0 2 + state in 12C which is supposed to be an 3α particle chain at an excitation energy of 7.65 MeV and using the binding energy of these valence neutrons in 9Be and 10Be*, chain states in the system 12C* + x neutrons are constructed. The energy position of the lowest chain states are estimated and ways for their population in reactions on 9Be and using radioactive beams are proposed. It is expected that these states are metastable and could have appreciable branches for γ-decay. Further extrapolations to longer chain states (polymers) in neutron rich light isotopes are made.

Electron pairing and chemical bonds. Chemical structure, valences and structural similarities from the analysis of the Fermi holes

Abstract: A new method of visualisation of bonding in molecules is introduced. The method is based on the analysis of Fermi holes associated with conditional probabilities of finding one electron of the pair provided the second, reference, electron is localised in a certain molecular region. Based on this analysis it is possible to get a clear and highly visual insight into the structure of molecular fragments (functional groups) in molecules. In addition to this visualisation, the new approach opens the possibility of the new definition of atomic and group valence and, also, can be applied as a new means of the quantitative characterisation of similarity of structural fragments in the series of related molecules.

High-level electron correlation calculations on formamide and the resonance model

Abstract: The question of planarity and the validity of the amide resonance model have been investigated in formamide on the basis of high-level quantum chemical calculations. Complete geometry optimizations were performed for the equilibrium structure and for the 90°-rotated transition state at the MBPT(2), MBPT(4), CCSD, and CCSD(T) electron correlation levels, with basis sets up to cc-PVTZ. While electron correlation tends to give nonplanar equilibrium, the final result at the CCSD(T)/PVTZ level is an exactly planar structure, as proven by the absence of imaginary vibrational frequencies. The crucial parameter in the geometry, the C-N bond length is calculated at 1.354 A. For the barrier to internal rotation around the C-N bond our best estimate, including the zero-point-energy correction, is 15.2 ( 0.5 kcal/mol. To check predictions of the resonance model, we have analyzed geometric changes, charge shifts from Mulliken population analysis, and the nature of relevant valence orbitals and also calculated NMR chemical shieldings as a function of internal rotation. In contrast to recent suggestions by Wiberg et al .( J .Am. Chem. Soc. 1987, 109, 5935; 1992, 114, 831; Science 1991, 252, 1266) that ﷿-resonance would not play a significant role in explaining the rotational barrier in formamide, we have found no compelling evidence to doubt the validity of the amide resonance model.

Chemical Bonding between Cu and OxygenCopper Oxides vs O2 Complexes: A Study of CuOx (x = 0−6) Species by Anion Photoelectron Spectroscopy

Abstract: An extensive photoelectron spectroscopic study on the CuOx- (x = 0−6) species is presented. The photoelectron spectra of these species are obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The spectra of the copper atom are included to show the dependence of the detachment cross sections on the photon energies. An intense two-electron transition to the 2P excited state of Cu is also observed in the 6.42 eV spectrum of Cu-. For CuO-, we observe an excited state of the anion, as well as photodetachment transitions to charge transfer excited states of CuO (Cu2+O2-). Six transitions are observed for CuO2- at 6.42 eV, revealing all six valence molecular orbitals of the linear OCuO molecule. CuO3- is observed to undergo photodissociation at 3.49 eV to give an internally hot CuO- plus O2. It is shown to have an OCuO2 type of structure, and its electronic structure can be viewed to be due to that of CuO perturbed by an O2. For CuO4-, two isomers are observed. One of them undergoes photo...

Influence of core–valence separation of electron localization function

Abstract: The electron localization function (ELF) shows too-high values when computed from valence densities only (instead of using the total density). This effect is mainly found when d electrons are present in the outermost shell of the core. Although no pronounced qualitative differences could be noticed in the examples studied up to now, it is found that the quantitative differences between the values of ELF obtained from the valence densities only or from the total densities can be large. We also show, for the first time, an example (the Be atom) where ELF is obtained directly from the density. This exemplifies the possibility of computing ELF from highly accurate calculations (or from experimental data). © 1997 John Wiley & Sons, Inc. J Comput Chem18: 1431–1439, 1997

Influence of Chemical and Spatial Constraints on the Structures of Inorganic Compounds

Abstract: The restrictions imposed by both chemistry and three-dimensional space on the structures of inorganic crystals can be analysed using the bond-valence model and space-group theory. The bond-valence model is used to construct a bond graph (connectivity table) from which the multiplicities and possible site symmetries of each atom can be assigned. These are matched to Wyckoff positions of the three-dimensional space groups, selecting the matching space group with the highest possible symmetry. High-symmetry structures such as NaCl, perovskite and garnet are readily derived from the chemical formula and, with a little more effort, the same can be done for structures of intermediate symmetry such as wurtzite, corundum and rutile. For other compounds a relationship between the site symmetry and the multiplicity of an atom can severely restrict the possible structures.

Three Dimensionally Diluted Magnetic Semiconductor Clusters Cd1-yMnyS with a Range of Sizes and Compositions: Dependence of Spectroscopic Properties on the Synthesis Mode

Abstract: Absorption and photoluminescence spectroscopic measurements are presented for particles with sizes from 2 to 4 nm, composition from y = 0 to 0.3,where y is given by Cd1-yMnyS, and different preparation modes. No monotonous variation of bandgap energy with increasing composition is observed. The depth of the minimum increases with decreasing particle size. This is attributed to a marked increase in the interactions between the Mn2+ d electrons and the valence and conduction band electrons. Mn2+ photoluminescence is observed at 77 K, when the particles are aged in the solution. Otherwise only the trap emissions due to CdS defect states are seen. Certain preparation modes favor Mn2+ photoluminescence at room temperature.

Lattice distortion in Cu-based dilute alloys: A first-principles study by the KKR Green-function method

Abstract: The full-potential Korringa-Kohn-Rostoker Green function method is extended to treat the lattice distortion in the vicinity of a point defect. The method is applied to predict the atomic positions in the neighborhood of d and sp substitutional impurities in Cu. Both the total energy and the Hellmann-Feynman force are used for the calculation of the ground-state configuration, while the semicore states of the impurities are treated as valence states. Our results for the atomic displacements are in very good agreement with the experimental data from extended x-ray-absorption fine-structure and lattice-parameter measurements.

Can amorphous gan serve as a useful electronic material

Abstract: In this paper, we propose two structural models of amorphous GaN at different densities, obtained from approximate ab initio molecular dynamics. The network models we obtain are highly disordered but exhibit a large state-free optical gap, and have no homopolar bonds (or odd-membered rings). The radial distribution function, local bonding, and electronic density of states are described. We find that a model with many threefold atoms has weakly localized band tails, especially at the valence edge. This leads us to believe that amorphous GaN may have independent promise as a novel electronic material.

Solution and Solid State Properties of [N-(2-Hydroxyethyl)iminodiacetato]vanadium(IV), -(V), and -(IV/V) Complexes(1).

Abstract: A mononuclear vanadium(IV), a mononuclear vanadium(V), and a binuclear mixed valence vanadium(IV/V) complex with the ligand N-(2-hydroxyethyl)iminodiacetic acid (H(3)hida) have been structurally characterized. Crystal data for [VO(Hhida)(H(2)O)].CH(3)OH (1): orthorhombic; P2(1)2(1)2(1); a= 6.940(2), b = 9.745(3), c= 18.539(4) A; Z = 4. Crystal data for Na[V(O)(2)(Hhida)(2)].4H(2)O (2): monoclinic; P2(1)/c; a = 6.333(2), b = 18.796(2), c = 11.5040(10) A; beta = 102.53(2) degrees; Z = 4. Crystal data for (NH(4))[V(2)(O)(2)(m-O)(Hhida)(2)].H(2)O (3): monoclinic; C2/c; a = 18.880(2), b= 7.395(2), c = 16.010(2) A; beta = 106.33(2) degrees; Z = 4. The mononuclear vanadium(IV) and vanadium(V) complexes are formed from the monoprotonated Hhida(2)(-) ligand, and their structural and magnetic characteristics are as expected for six-coordinate vanadium complexes. An interesting structural feature in these complexes is the fact that the two carboxylate moieties are coordinated trans to one another, whereas the carboxylate moieties are coordinated in a cis fashion in previously characterized complexes. The aqueous solution properties of the vanadium(IV) and -(V) complexes are consistent with their structures. The vanadium(V) complex was previously characterized; in the current study structural characterization in the solid state is provided. X-ray crystallography and magnetic methods show that the mixed valence complex contains two indistinguishable vanadium atoms; the thermal ellipsoid of the bridging oxygen atom suggests a type III complex in the solid state. Magnetic methods show that the mixed valence complex contains a free electron. Characterization of aqueous solutions of the mixed valence complex by UV/vis and EPR spectroscopies suggests that the complex may be described as a type II complex. The Hhida(2)(-) complexes have some similarities, but also some significant differences, with complexes of related ligands, such as nitrilotriacetate (nta), N-(2-pyridylmethyl)iminodiacetate (pmida), and N-(S)-[1-(2-pyridyl)ethyl]iminodiacetate (s-peida). Perhaps most importantly, the mixed valence Hhida(2)(-) complex is significantly less stable than the corresponding pmida and s-peida complexes of similar overall charge but very similar in stability to the nta and V(2)O(3)(3+) complexes with higher charges. Thus, there is the potential for designing stable mixed valence dimers.

Shell model study of the neutron rich nuclei around N = 28

Abstract: We describe the properties of the neutron-rich nuclei around N=28 in the shell mode framework. The valence space includes the sd shell for protons and the pf shell for neutrons without any restriction. Good agreement is found with the available experimental data. The N=28 shell closure persists, even if eroded by the large neutron excess. The calculations predict that {sup 40}S and {sup 42}S are deformed with {beta}=0.29 and {beta}=0.32 respectively. {copyright} {ital 1997} {ital The American Physical Society}

Ab initio relativistic effective potentials with spin-orbit operators. VII. Am through element 118

Abstract: Ab initio averaged relativistic effective core potentials (AREP) and spin-orbit (SO) operators are reported for the elements Am through element 118. Two sets have been calculated for certain elements to provide AREPs with varying core/valence space definition, thereby permitting the treatment of core/valence correlation interactions. The AREPs and SO operators are tabulated as expansions in Gaussian-type functions (GTF). GTF valence basis sets are derived for the lowest energy state of each atom. The reliability of the AREPs and SO operators is gauged by comparing calculated atomic orbital eigenvalues and SO splitting energies with all-electron relativistic values.

Oxidation of TiAl based intermetallics

Abstract: The high temperature oxidation behaviour of the binary and ternary alloys of the Ti–48Al system was studied at different temperatures. The primary objectives of this work were the establishment of the activation energies, the migration tendencies of the alloy species, mechanism of oxidation and chemistry of the oxide scales. The ternary additions were Cr (1.5 at 19%), V (2.2 at%), W (0.2 at%) and Mn (1.4 at%). The addition of ternary additions did not play a significant role in the oxidation behaviour at 704°C. At 815°C the alloys with Cr and V exhibited linear oxidation behaviour with large weight gains while the base Ti–48Al alloys exhibited the best behaviour. At 982°C the Mn-containing alloy was the worst, exhibiting a linear oxidation behaviour while the alloy with V and W and the base alloy with 400 p.p.m. oxygen exhibited the best oxidation behaviour. At 982°C the outermost oxide layer in contact with air is always near stoichiometric TiO2. In all the alloys a layer of porosity is created just below the outer TiO2 layer by the Kirkendall mechanism due to the rapid outward diffusion of Ti atoms. The addition of trivalent atoms like Cr in small amounts appear to be detrimental to the oxidation process as they can generate additional oxygen vacancies while the addition of atoms with valence of 5, such as V, and 6, such as W, appear to have beneficial effect on the oxidation behaviour at 982°C by tying up oxygen vacancies.

Absorption spectra and optical transitions in inas/gaas self-assembled quantum dots

Abstract: We have applied the multiband effective mass/valence force field method to the calculation of optical transitions and absorption spectra in InAs/GaAs self-organized dots of different sizes. We have found that the apparently conflicting assignments of luminescence features to optical transitions in different experiments are in fact entirely compatible with each other. Whether the optical signature of a dot is constructed from transitions between states of the same quantum numbers, or via additional processes between the ground conduction state and a low-lying valence state depends on the aspect ratio of the quantum dot radius and height. The states involved can be predicted from a simple particle in a rigid rectangular box model.

Evidence for Incomplete Charge Transfer and La-Derived States in the Valence Bands of Endohedrally Doped La{at}C{sub 82}

Abstract: Photoelectron spectra from sublimed layers of chromatographically separated La{at}C{sub 82} are presented Using photon energies corresponding to the La 3d to 4f transition, a resonant enhancement of the La-derived valence states is observed From the calculated resonant intensities of La 5d and 5p states in a La atom we estimate that about (1)/(3) of an electron charge is left in the La-valence orbitals for La@C{sub 82} In contrast to previous interpretations of La core-level photoemission studies our results demonstrate that the La valence electrons are not completely delocalized on the fullerene cage {copyright} {ital 1997} {ital The American Physical Society}