Showing papers on "Valence (chemistry) published in 1992"

The 2p absorption spectra of 3d transition metal compounds in tetrahedral and octahedral symmetry

Abstract: The authors present the 2p (L2,3) absorption spectra of first-row transition metal ions in tetrahedral and octahedral crystal field symmetry. These have been calculated using a localized description for the 3dn to 2p53dn+1 excitation including electrostatic and spin-orbit interactions. The spectra are significantly different from those already presented where the 3d spin-orbit interaction was neglected. The spectral shape provides a valence- and symmetry-selective probe. Whereas it changes gradually with the crystal field, abrupt changes in the spectra are indicative of high-spin to low-spin transitions. These spin transitions are accompanied by a strong decrease in the 2p branching ratio. The calculated spectra provide a basis for the use of L2,3 absorption spectroscopy in materials science and biological science. The limitations of these calculations and the use of configuration interaction are discussed.

Uranium Stabilization of C28: A Tetravalent Fullerene

Abstract: Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C28. Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C28 in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C28 with large highest occupied molecular orbital—lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C28 cage to make, for example, C28H4, or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C28. An example of this second, inside route to C28 stabilization is reported here: the laser and carbon-arc production of U@C28.

Bond valence sum analysis of metal-ligand bond lengths in metalloenzymes and model complexes

Abstract: Bond valence sums (BVS's) have been calculated for a series of model complexes for important metalloenzymes. Values calculated from crystallographic distances predict the correct oxidation state of the metal center in a wide range of structural types. Many of the selected complexes have previously been studied by X-ray absorption spectroscopy, and first-shell metal-ligand bond distances have been determined by analysis of extended X-ray absorption fine structure (EXAFS)

Electronic structure and bonding in actinyl ions

Abstract: The actinyl ions exhibit an unusually robust covalent bond which has a profound influence on their chemistry. Their electronic structure has been unravelled by the use of a variety of optical measurements and by photoelectron spectroscopy, which together establish the composition and role of the valence orbitals. The experimental energy level scheme can be compared with the results of molecular orbital calculations of varying degrees of sophistication, the most successful being SCF calculations incorporating the effects of relativity. An important contribution to the bonding comes from the pseudo-core 6p shell, which is important in determining the linearity of the ions. A new concept, the Inverse trans Influence, is introduced in this context. There is some evidence that (d-orbital π-bonding is more important than σ-bonding. The remarkable strength of the actinyl bond can be attributed to the presence in the valence shell of both f and d metal orbitals, giving each actinide-oxygen bond a formal bond order of three.

The abinitio model potential method. Cowan–Griffin relativistic core potentials and valence basis sets from Li (Z = 3) to La (Z = 57)

Abstract: A comprehensive tabulation of Cowan–Griffin relativistic abinitio core model potentials and valence basis sets corresponding to the Cowan–Griffin abinitio model potential method is presented. It includes those for the elements from Li (Z = 3) to La (Z = 57), the alkaline M+ and alkaline earth M2+ cations, and for the halogen X− anions. Molecular results are presented in order to test the potentials and basis sets and to estimate the extent of the mass–velocity and Darwin relativistic effects considered within the method, which lie within the expected margins. Keywords: relativistic, model potential, effective core potential, basis sets, abinitio.

Bond-valence parameters for anion-anion bonds in solids

Abstract: Calcul des parametres ds liaisons de valence reliant les valences aux longueurs des liaisons dans une paire d'atomes, chaque atome etant l'un des elements suivants: H, B, C, Si, Ge, N, P, As, Sb, O, S, Se, Te, F, Cl, Br, and I. Suggestions d'utilisation des tables de donnees

Dissociative chemisorption of CH4 on Ni(111)

Abstract: The dissociative chemisorption of methane at an atop‐atom site on a (111) surface of nickel is treated using a many‐electron embedding theory to describe bonding, modeling the lattice as a 41‐atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. Ni 3d orbitals are explicitly included on seven nickel atoms on the surface. The calculated activation energy of CH4 adsorbed at an atop Ni site to produce CH3 and H coadsorbed at separated threefold sites is 17 kcal/mol. The dissociation of CH4 to CH3(ads)+H (ads) is predicted to be 2.8 kcal/mol exothermic. The Ni 3d orbitals contribute to the bonding by directly mixing with methane C–H orbitals during the dissociation process and through a direct interaction of 3d9 and 3d10 configurations at the transition state. The dissociation pathway and the bonding properties of adsorbed CH4 and coadsorbed CH3 and H are disc...

Periodic ab initio Hartree-Fock calculations of the low-symmetry mineral kaolinite

Abstract: Periodic ab initio Hartree-Fock LCAO calculations have been carried out on the P1 symmetry clay mineral kaolinite, Al{sub 2}Si{sub 2}O{sub 9}H{sub 4} using STO-3G and modified 6-21G basis sets. The three structural degrees of freedom associated with the inner hydrogen atom of this compound have been optimized using the STO-3G basis. The equilibrium position of the inner hydrogen (described relative to the adjoining O-H bond) is predicted to be approximately parallel to the (ab) plane (forming an angle of +3.1{degrees}) with an O-H bond length of 0.99 {angstrom} and the two Al-O-H angles of 107.7{degrees} and 107.4{degrees}, respectively. The physical properties of the system were obtained using the 6-21G basis at the geometry deduced from the STO-3G calculations. The total valence density of states has been computed. Projected densities of states have been evaluated for the chemically distinct sets of elements. It is found that the majority of the valence states are composed of oxygen 2s and 2p atomic orbitals with overlap from Si and Al 3s and 3p atomic orbitals. Crystal charge densities and density deformation maps have been computed in the basal plane of the silicate ring system. The calculations have revealed small charge densities in the centermore » of the ditrigonal cavity (10{sup {minus}4}-10{sup {minus}5} e/bohr{sup 3}) and an oblate distortion of the oxygen charge density directed toward the center of the ring system. Electrostatic potential maps have been computed in the chemically accessible regions of the structure near the ditrigonal cavities. These maps indicate that the center of the cavity is at a negative potential relative to nearby silicon positions. Analogous calculations have been carried out perpendicular to the layers of the material. The results of these calculations reveal extensive interlayer hydrogen bonding. 21 refs., 11 figs., 2 tabs.« less

Magnetic properties of mixed-valence heteropoly blues. Interactions within complexes containing paramagnetic atoms in various sites as well as "blue" electrons delocalized over polytungstate frameworks

Abstract: Magnetic behaviors are reported for 16 heteropoly blue tungstateanions and for their oxidized parent heteropoly complexes (isomorphs of the corresponding blue species). Ten of the reduced species and one of the oxidized ones are new compounds. Each blue complex contained either one or two added «blue» electrons delocalized over numerous W and O atoms of the heteropoly framework. Most of the species contained one or two paramagnetic atoms, substituted and/or tetrahedral sites into the heteropoly frameworks as integral parts thereof

Defect energetics in MgO treated by first-principles methods

Abstract: Ab initio total-energy calculations have been performed on a parallel computer to study the formation and migration energies of cation and anion vacancies in MgO. The calculations are made in the framework of density functional and pseudopotential theory, using the supercell method, with the valence orbitals expanded in plane waves. The relaxed ground state is determined by conjugate-gradients minimization of the total-energy functional with respect to the plane-wave coefficients of occupied orbitals. The calculated defect energies are shown to be in remarkably close agreement with experiment and with values obtained from empirical modeling. We present results for the electron distribution surrounding the vacancies that show that the distortion induced in the oxygen ions is more complex than has previously been thought.

On the performance of large Gaussian basis sets for the computation of total atomization energies

Abstract: The total atomization energies of a number of molecules have been computed using an augmented coupled-cluster method and (5s4p3d2f1g) and 4s3p2d1f) atomic natural orbital (ANO) basis sets, as well as the correlation consistent valence triple zeta plus polarization (cc-pVTZ) correlation consistent valence quadrupole zeta plus polarization (cc-pVQZ) basis sets. The performance of ANO and correlation consistent basis sets is comparable throughout, although the latter can result in significant CPU time savings. Whereas the inclusion of g functions has significant effects on the computed Sigma D(e) values, chemical accuracy is still not reached for molecules involving multiple bonds. A Gaussian-1 (G) type correction lowers the error, but not much beyond the accuracy of the G1 model itself. Using separate corrections for sigma bonds, pi bonds, and valence pairs brings down the mean absolute error to less than 1 kcal/mol for the spdf basis sets, and about 0.5 kcal/mol for the spdfg basis sets. Some conclusions on the success of the Gaussian-1 and Gaussian-2 models are drawn.

Ab initio study of the trans-butadiene π-valence states using the effective valence shell Hamiltonian method

Abstract: Low‐lying π‐electron vertical excitation energies of trans‐butadiene are calculated using the effective valence shell Hamiltonian method. The results are compared with previous experimental and theoretical analyses of this molecule’s congested electronic spectra. The computations employ a large basis set (126 functions) which includes both diffuse functions on the carbon atoms and polarization functions on all atoms. Good agreement is obtained with the experimentally well known vertical excitation energies to the 1 3Bu, 1 3Ag, and 1 1Bu states where deviations from experiment are only 0.01, 0.01, and 0.22 eV, respectively. We confirm the experimental assignment of a valence like 1Ag state around 7.4 eV (calculated at 7.49 eV). Likewise, a member of a symmetry allowed 3p Rydberg series (of Au or Bu symmetry) in the electron impact spectrum with origin at 7.07 eV is assigned as the 2 1Bu state (with calculated vertical excitation energy of 7.00 eV). Most experiments place the 2 1Ag state above the 1 1Bu sta...

Theoretical study of the activation of carbon-carbon bonds by transition metal atoms

Abstract: Quantum chemical model studies have been performed for the transition metal activation of C-C bonds in ethane, cyclopropane, and cyclobutane. Both the ethane and cyclobutane reactions have been studied for the entire second row of transition metal atoms, for both equilibrium states and transition states. For cyclobutane the first transition metal series has also been studied. The cyclopropane reaction has only been fully studied for rhodium and palladium. The quantum chemical calculations include a size-consistent treatment of electron correlation of all the valence electrons with fairly large basis sets including f functions on the metal. The geometries have been fully optimized. Palladium is found to have the smallest barriers for the C-C bond breaking reaction, and the C-C bond in cyclopropane is easiest to break, in line with general experimental experience for transition metal complexes. 38 refs., 6 figs., 11 tabs.

Ar-ion bombardment effects on ZrO2 surfaces

Abstract: Chemical, compositional, and electronic changes induced by 3-keV Ar{sup +} sputtering in ZrO{sub 2} have been quantitatively studied by use of x-ray photoemission spectroscopy. It is shown that 3-keV Ar{sup +} bombardment leads to a gradual buildup of an oxygen depletion layer which has been quantitatively characterized by its thickness ({similar to}1.2 nm), averaged composition ({similar to}ZrO), and in-depth distribution of the different oxide phases that originate during the process. In addition, the electronic distribution at the valence band of both thermally grown and Ar{sup +}-bombarded ZrO{sub 2} was also determined by x-ray photoemission spectroscopy. Thermally grown ZrO{sub 2} was shown to exhibit significant photoemission in the band gap, probably due to defects. Ar{sup +} bombardment of those surfaces caused a broadband emission at 2.3 eV above the top of the valence band as well as significant changes in the O 2{ital p} region of the valence band which are associated with the above-mentioned changes in surface composition. The Zr 4{ital d} contribution to the valence band of the altered layer could also be estimated. This contribution increases with Ar{sup +} sputtering and accounts not only for the whole emission band in the gap but also significantly for the photoemision more » in the O 2{ital p} energy range. « less

Structural and magnetic characterization of trinuclear, mixed-valence manganese acetates

Abstract: Four mixed valence, trinuclear manganesecomplexes of general stoichiometry Mn II Mn III 2 L 2 (carboxylate) 4 X 2 , where L is a tridentate Schiff base ligand and X is a neutral monodentate donor, have been structurally characterized by using X-ray crystallography. The solid-state magnetic behavior of these compounds and two additional isostructural complexes are defined by examining the variable-temperature magnetic susceptibilities of each compound

Decomposition of N2O on Perovskite-Related Oxides

Abstract: Mixed metal oxides crystalizing in a perovskite-related structure have long been of interest to solid state chemists and physicists because of their technologically important physical properties. The ready availability of a family of isomorphic solids with controllable physical properties makes these oxides suitable for basic research in catalysis. These mixed metal oxides are more advantageous and are better catalytic materials than simple oxides because: (i) the crystal structure can accomodate various metal ions and can stabilize unusual and mixed valence states of active metal ion; (ii) appropriate formulation of these oxides leads to easy tailoring of many desirable properties such as valence state of transition metal ion, distance between active sites, binding energy, diffusion of oxygen in the lattice, magnetic and conducting properties of the solid; (iii) the catalytic activity can be correlated to solid state properties since many of their solid state properties are thoroughly understood...

Core‐valence separation in the spin‐coupled wave function: A fully variational treatment based on a second‐order constrained optimization procedure

Abstract: The theory of the spin‐coupled (SC) wave function with core‐valence separation, in which the core electrons are confined to a closed shell of doubly‐occupied orbitals and the valence electrons are described with the complete set of features of the SC formalism, is developed to produce an efficient approach which makes possible its fully variational determination. The simultaneous optimization of the core orbitals, valence orbitals, and spin‐coupling coefficients is achieved through a second‐order nonlinear elimination constrained minimization algorithm which exhibits excellent convergence properties. It is no longer necessary to introduce an ad hoc preselection of core and valence orbitals−this is carried out by the minimization procedure itself which makes an optimum choice from a variational point of view. The only important item left to personal judgment is the selection of the number of core and valence electrons in the problem under investigation. Simplifications such as ‘‘freezing’’ of a part of the core orbitals are discussed alongside with the verification of the theoretical work and program code by emulating, under an appropriately modified set of constraints, the generalized valence‐bond (GVB) wave function with perfect pairing and strong‐orthogonality restrictions. It is demonstrated that it is possible, similar to the Hartree–Fock (HF) method, to transform the core orbitals into canonical form and to associate with each one of them an energetical quantity analogous to the HF orbital energy. The essential features of the approach are illustrated by a SC study of the process H2CO(1A1)→CH2(X 3B1)+O(3P), involving the breaking of the C■O double bond. The results prove that only a fully variational SC wave function with core‐valence separation is capable of providing a uniform description of the change in the physical properties of the system upon dissociation, which can be achieved by including in the SC part of the wave function just the four electrons immediately involved in the carbon–oxygen double bond.

Valence isomers of benzene and their relationship to isomers of isoelectronic P6

Abstract: We report the results of geometry optimized ab initio SCF MO calculations with 3-12G, 4-31G*, and 6-31G* basis sets for neutral P 6 in forms corresponding to the five valence isomers of benzene, (CH 6 ), with which P 6 is valence isoelectronic. At the 6-31G* level benzene, Dewar benzene, benzyvalene, prismane, and bicyclopropenyl structures all correspond to real minima on the P 6 energy surface. The order of energies of P 6 structures is quite different from that for the (CH 6 ) isomers. For P 6 , benvalence and prismane have low energies and the planar hexagon has the highest. Bond distances are easily classified as single, double, or aromatic. The energy of 2P 6 lies only 30-35 kcal/mol above that of 3P 4 and 60-100 kcal/mol below 6P 2