Showing papers on "Valence (chemistry) published in 1996"

Population analysis of plane-wave electronic structure calculations of bulk materials

Abstract: Ab initio plane-wave electronic structure calculations are widely used in the study of bulk materials. A technique for the projection of plane-wave states onto a localized basis set is used to calculate atomic charges and bond populations by means of Mulliken analysis. We analyze a number of simple bulk crystals and find correlations of overlap population with covalency of bonding and bond strength, and effective valence charge with ionicity of bonding. Thus, we show that the techniques described in this paper may be usefully applied in the field of solid state physics. \textcopyright{}1996 The American Physical Society.

Intrinsic Proton Affinity of Reactive Surface Groups of Metal (Hydr)oxides: The Bond Valence Principle

Abstract: The proton affinity of individual surface groups has been calculated with a redefined version of the multi site complexation (MUSIC) model. In the new approach the proton affinity of an oxygen originates from the undersaturation of the oxygen valence. The factors valence and coordination number, which are the basis of Pauling's definition of bond valence, in combination with the number of coordinating (Me and H) ions, are dominant in determining the proton affinity. The neutralization of an oxygen by Me ion(s) is calculated on the basis of the actual bond valence, which accounts for structural details, resulting from an asymmetrical distribution of charge in the coordination environment. An important role in the new version of the MUSIC model is given to the H bonds. The model shows that the proton affinity is determined not only by the number of donating H bonds but also by the number of accepting H bonds. The proton affinity of surface groups and of solution complexes can be understood in one theoretical framework, on the basis of a different number of donating and accepting H bonds. The MUSIC model predicts the variation in proton affinity constants for surface groups in particular those with the same number of coordinating Me ions but with a different structural position. The model is able to predict on the basis of the proton affinity of the individual groups the correct PZC of Me hydroxides, oxohydroxides, and oxides, and explains previous exceptions. The model can also be applied in general to other minerals.

VALENCE: a program for calculating bond valences

Abstract: The DOS program VALENCE is designed to calculate bond valences from bond lengths and vice versa. It can also calculate bond-valence sums and average bond lengths, and can determine bond-valence parameters from the bonding environments of different cations.

Partial third‐order quasiparticle theory: Comparisons for closed‐shell ionization energies and an application to the Borazine photoelectron spectrum

Abstract: Valence ionization energies of a set closed‐shell molecules calculated in a partial third‐order (P3) quasiparticle approximation of the electron propagator have an average absolute error of 0.19 eV. Diagonal elements of the self‐energy matrix include all second‐order and some third‐order self‐energy diagrams. Because of its fifth power dependence on basis set size and its independence from electron repulsion integrals with four virtual indices, this method has considerable potential for large molecules. Formal and computational comparisons with other electron propagator techniques illustrate the advantages of the P3 procedure. Additional applications to benzene and borazine display the efficacy of the P3 propagator in assigning photoelectron spectra. In the borazine spectrum, 2E′ and 2A2′ final states are responsible for an observed feature at 14.76 eV. Another peak at 17.47 eV is assigned to a 2E′ final state.

Full configuration-interaction and state of the art correlation calculations on water in a valence double-zeta basis with polarization functions

Abstract: Using a valence double‐zeta polarization basis, full configuration–interaction (FCI) calculations are carried out on water at its equilibrium geometry and at geometries where the OH bond lengths are stretched until dissociation. At the same geometries and with the same basis set configuration interaction calculations at excitation levels up to hextuples, multireference singles doubles configuration interaction calculations, coupled cluster calculations at excitation levels up to quadruples, Mo/ller–Plesset perturbation theory calculations through order fifteen, and complete active space second‐order perturbation theory calculations are also carried out. The static correlation contribution increase with increasing bond length. The calculations show that the coupled cluster approach has a remarkable ability to describe even relatively large static correlation contributions. The single reference perturbation expansion breaks down for larger OH bond length, while the multireference approach preserves the accu...

Dipole bound, nucleic acid base anions studied via negative ion photoelectron spectroscopy

Abstract: The anions of the nucleic acid bases, uracil and thymine, were studied by negative ion photoelectron spectroscopy. Both monomer anions exhibit spectroscopic signatures that are indicative of dipole bound excess electrons. The adiabatic electron affinities of these molecules were found to be 93±7 meV for uracil and 69±7 meV for thymine. No conventional (valence) anions of these molecules were observed.

Intervalence Electron Transfer in Mixed Valence Diferrocenylpolyenes. Decay Law of the Metal-Metal Coupling with Distance.

Abstract: A series of diferrocenylpolyenes of general formula Fc(CHCH)nFc with n = 1−6 (Fc = ferrocenyl group) has been prepared and studied from the point of view of intervalence transitions in the mixed valence state. Well-resolved intervalence transitions are observed in dichloromethane upon partial electrolytic oxidation. Comproportionation constants have been determined from redox titration data and in some cases from electrochemical wave splitting. The corrected spectra of the mixed valence species have been deconvoluted to extract the parameters (position, intensity, width) of the intervalence bands, which allowed the determination of the metal−metal coupling (Vab) through the bridging unit using Hush's formula. The decay of Vab with distance is close to an exponential law with an exponent of 0.087 A-1, constituting one of the smallest attenuations reported so far. A small deviation to the exponential law is detected.

Theoretical Study of the Absorption and Emission Spectra of Indole in the Gas Phase and in a Solvent

Abstract: The complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2) have been used in a theoretical analysis of the electronic spectrum of indole. The calculations comprise a large number of singlet and triplet valence and Rydberg excited states with the aim to obtain a full understanding of the electronic spectrum. In addition to the gas-phase spectra, solvatochromic shifts have been computed at the CASPT2 level for the low-lying singlet valence states using a continuum representation of the solvent with the solute in a spherical cavity. The results support the assignments of two low-lying π → π* valence singlet excited states, 1Lb and 1La, computed at 4.43 and 4.73 eV in the gas phase. The location of the 1La band origin 0.23 eV above the 1Lb band origin is in agreement with the most recent experimental studies. The most intense feature of the spectrum is obtained at 5.84 eV as a π → π* singlet excited valence state. In total 25 singlet states have been computed...

On the binding of electrons to nitromethane: Dipole and valence bound anions

Abstract: ~Received 28 December 1995; accepted 9 May 1996! Conventional ~valence! and dipole-bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe( nf ) Rydberg atoms with CH 3 NO 2 exhibit a pronounced maximum at an effective quantum number of n*’1361 which is characteristic of the formation of dipole-bound anions @m~CH 3 NO 2 !53.46 D#. However, the breadth ~Dn’5, FWHM! of the n-dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ‘‘doorway’’ dipole anion state. Studies of the electric field detachment of CH 3NO2 formed through the Xe( nf ) reactions at various n values provide further evidence for the formation of both a dipole-bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 1263 meV. Photodetachment of CH3NO2 and CD3NO2 formed via a supersonic expansion nozzle ion source produces a photoelectron spectrum with a long vibrational progression indicative of a conventional ~valence bound! anion with a substantial difference in the equilibrium structure of the anion and its corresponding neutral. Assignment of the origin ~v850, v950! transitions in the photoelectron spectra of CH3NO2 and CD3NO2 yields adiabatic electron affinities of 0.2660.08 and 0.2460.08 eV, respectively. © 1996 American Institute of Physics.@S0021-9606~96!02831-0#

Pulsed Laser Photolysis and Thermodynamics Studies of Intramolecular Electron Transfer in Valence Tautomeric Cobalt o-Quinone Complexes

Abstract: Four valence tautomeric complexes with the general composition [Co(3,5-DTBSQ)2(N⌒N)] have been investigated, where 3,5-DTBSQ- is the semiquinonate form of 3,5-di-tert-butyl-o-quinone. The diiminium ligand N⌒N is one of the following ligands: phen (1,10-phenanthroline, complex 1); bpy (2,2‘-bipyridine, complex 2); dmbpy (4,4‘-dimethyl-2,2‘-bipyridine, complex 3); dpbpy (4,4‘-diphenyl-2,2‘-bipyridine, complex 4). All four of the complexes undergo a valence tautomerism converting from low-spin (ls) [CoIII(3,5-DTBSQ)(3,5-DTBCAT)(N⌒N)] at low temperatures to high-spin (hs) [CoII(3,5-DTBSQ)2(N⌒N)] at high temperatures, where the 3,5-DTBCAT2- ligand is the catecholate form of the o-quinone ligand. Variable temperature (150−340 K) magnetic susceptibility data are presented for toluene solutions of all four complexes. These data were least-squares fit to give ΔH and ΔS values that characterize the valence tautomerism equilibrium. The ΔH values range from 21.33 to 38.36 kJ mol-1, whereas the ΔS values range from 60...

Excited-State Contributions to Ground-State Properties of Mixed-Valence Dimers: Spectral and Electronic-Structural Studies of [Fe2(OH)3(tmtacn)2]2+ Related to the [Fe2S2]+ Active Sites of Plant-Type Ferredoxins

Abstract: The electronic transitions of the S = 9/2 class III mixed-valence dimer [Fe2(OH)3(tmtacn)2]2+ are assigned using absorption, magnetic circular dichroism (MCD), and resonance Raman (RR) spectroscopies. This set of excited-state assignments allows the relative contributions of metal−metal and metal−ligand interactions to the D3h molecular-orbital energy splittings to be estimated. From this analysis the pathway for valence delocalization in this dimer is determined to involve a significant direct Fe···Fe σ-bonding interaction. The spectroscopic analysis is supported by electronic-structure calculations, which predict a spectrum similar to that observed and provide descriptions of the dimer's molecular orbitals. These results are further supported by the observation of a significant increase in the Fe···Fe internuclear separation with σ−σ* excitation, determined by vibronic analysis of the σ−σ* absorption band shape and associated RR excitation profiles. Combined, these results provide a measure of the geome...

Small Multiply Charged Anions as Building Blocks in Chemistry

Abstract: Chemists view most if not all ionic crystals as composed of autonomously stable negative and positive ions. These building block ions can be singly or multiply charged (e.g., NaCl(s) consists of Na+ and Clions, and MgSO4(s) contains Mg2+ and SO4 ions), but they are expected to be stable to the extent that they can be characterized in the laboratory. Dividing crystals, melts, and ionic molecules into positive and negative ions is usually performed via the octet rule. For example, K2SO4 salt is considered to be composed of K+ cations and SO4 anions, within which all constituent atoms possess full octets of valence electrons. Within this point of view, chemists interpret the physical, reactivity, optical, and other properties of crystals, melts, and liquid ionic salts in terms of properties intrinsic to the corresponding positive and negative ions. For example, in NH4Cl(s), one interprets Raman spectra in terms of crystal phonons as well as internal N-H vibrations belonging to the NH4 cation. This widely accepted approach in chemistry is called into question when one recognizes that many multiply charged anions that occur frequently in salts do not exist as autonomously stable species in which the extra electrons occupy valence molecular orbitals. How then can one view the composition of an ionic material such as a salt in terms of building blocks that cannot be isolated and thus fully characterized? It is this difficulty that forms a primary focus of the present Account. It is very difficult to experimentally prove or disprove the electronic instability of isolated multiply charged anions. The absence of corresponding signals in mass spectral data does not necessarily mean that the ions do not exist; it could be that, under the experimental source conditions, these anions are not formed. Alternatively, mass spectral peaks cannot prove stability; the species may be metastable yet long-lived enough to survive to the ion detector. The significant increase in the reliability of electronic structure computer techniques allows the question of the instability of multiply charged anions to be addressed using ab initio methods. Moreover, in recent years, substantial progress has been made in the ab initio study of multiply charged species, specifically. Three very good reviews on theoretical and experimental studies have appeared in the last three years.1,2 However, because the reviews were devoted primarily to studies of electronically stable multiply charged anions, many issues related to unstable anions were not included and thus need to be addressed in this work. When we discuss the stability of multiply charged anions, we consider three types of stability. The first is electronic stability of the anion. If An-, at its own optimal geometry, is more stable than the corresponding A(n-1)at the same geometry, we consider Anto be vertically electronically stable. If Anat its optimal geometry is more stable than A(n-1)at its own optimal geometry, we consider Anto be adiabatically electronically stable. In addition, there is the issue of geometrical stability. If Anhas all real vibrational frequencies at its optimal geometry, it is locally geometrically stable. Finally, if Anis more stable than any possible dissociation fragment, it is thermodynamically stable. Clearly, if an anion is thermodynamically stable, it must also be electronically and geometrically stable. Most small multiply charged anions are not thermodynamically stable, although many are electronically and locally geometrically stable and may have large barriers to dissociation or to autodetachment. Such anions can be studied experimentally because they are long-lived. However, many multiply charged anions that are only vertically electronically stable are very difficult to probe experimentally because of their very short lifetimes.

Excitonic emissions from hexagonal GaN epitaxial layers

Abstract: Excitonic photoluminescence (PL) peaks from hexagonal GaN epilayers were investigated making a connection with the analysis of the photoreflectance spectra. Free exciton emissions associated with transitions from the conduction (Γ7c) band to the A (Γ9v) and B (Γ7uv) valence bands are dominant above 100 K. Values of the full widths at half maximum of them were smaller than the thermal energy kBT up to room temperature, which suggests the dominance of excitons in the PL spectra.

Electronic structure of the ferroelectric layered perovskite SrBi2Ta2O9

Abstract: The band structure of the layered perovskite SrBi2Ta2O9 (SBT) was calculated by tight binding and the valence band density of states was measured by x‐ray photoemission spectroscopy. We find both the valence and conduction band edges to consist of states primarily derived from the Bi–O layer rather than the perovskite Sr–Ta–O blocks. The valence band maximum arises from O p and some Bi s states, while the conduction band minimum consists of Bi p states, with a wide band gap of 5.1 eV. It is argued that the Bi–O layers largely control the electronic response whereas the ferroelectric response originates mainly from the perovskite Sr–Ta–O block. Bi and Ta centered traps are calculated to be shallow, which may account in part for its excellent fatigue properties.

Electronic structure and chemical-bonding mechanism of Cu 3 N, Cui 3 NPd, and related Cu(I) compounds

Abstract: The electronic structure and the chemical-bonding mechanism of ${\mathrm{Cu}}_{3}$N, ${\mathrm{Cu}}_{3}$NPd and related Cu(I) compounds, such as ${\mathrm{Cu}}_{2}$O, are studied on the basis of band-structure calculations, using both the linearized augmented plane wave and linear combination of atomic orbitals (LCAO) methods. In accordance with experimental observations, ${\mathrm{Cu}}_{3}$N is found to be a semiconductor, while ${\mathrm{Cu}}_{3}$NPd should exhibit a semimetallic conductivity. The chemical bonding is investigated using various methods, among them are the valence charge partitioning scheme of Bader and a basis set reduction technique built on the LCAO method. A partly ionic, partly covalent bonding is found. The admixture of the Cu (4s, 4p) states to the Cu 3d--N 2p bands resulted to be essential for the covalent bonding effect, since pure 3d-2p bands, with bonding and antibonding states fully occupied, do not lead to a covalent energy gain. This specific hybridization appears to be the origin of the twofold dumbbell like Cu(I) coordination observed in ${\mathrm{Cu}}_{3}$N and other Cu(I) compounds. In ${\mathrm{Cu}}_{3}$NPd, a covalent to metallic bonding between the ${\mathrm{Cu}}_{3}$N host crystal and the interstitial Pd atoms is found, which is mainly caused by Pd 5s and 5p states hybridizing to Cu 3d states. \textcopyright{} 1996 The American Physical Society.

Theoretical study of the valence π→π* excited states of polyacenes: Benzene and naphthalene

Abstract: Multireference perturbation theory with complete active space self‐consistent field (CASSCF) reference functions was applied to the study of the valence π→π* excited states of benzene and naphthalene. The eigenvectors and eigenvalues of CASSCF with valence π active orbitals satisfy pairing properties for the alternant hydrocarbons to a good approximation. The excited states of polyacenes are classified into the covalent minus states and ionic plus states with the use of the alternancy symmetry. The present theory satisfactorily describes the ordering of low‐lying valence π→π* excited states. The overall accuracy of the present approach is surprisingly high. We were able to predict the valence excitation energies with an accuracy of 0.27 eV for singlet u states and of 0.52 eV or better for singlet g states of naphthalene. Our predicted triplet states spectrum provides a consistent assignment of the triplet–triplet absorption spectrum of naphthalene. For benzene we were able to predict the valence excitatio...

Potential Functions for Silica and Zeolite Catalysts Based on ab Initio Calculations. 3. A Shell Model Ion Pair Potential for Silica and Aluminosilicates

Abstract: A shell model potential for silica and protonated zeolite catalysts is presented. The potential parameters are fitted exclusively to ab initio data generated by nonempirical quantum chemical calculations on small molecular models made of SiO4 and AlO4 tetrahedra. The Hartree−Fock method has been used with a basis set of double zeta + polarization quality on Si, Al, and H atoms and of valence triple zeta + polarization quality on O atoms. Comparison is made with an ab initio molecular mechanics force field previously derived from the same data and with an empirical parametrization of the shell model potential. The power of the new potential for predicting the crystal structures and the properties is demonstrated for a variety of silica and zeolite structure types. Cell parameters of dense and microporous silica are predicted with a mean error of 1.8%. Compared to earlier potentials, substantial progress is made in predicting dynamical properties. Examples are the phonon dispersion curves of α-quartz and th...

Third‐order nonlinear optical properties of sol–gel derived α‐Fe2O3, γ‐Fe2O3, and Fe3O4 thin films

Abstract: The third‐order nonlinear optical properties of sol–gel derived α‐Fe2O3, γ‐Fe2O3, and Fe3O4 thin films have been investigated by the third‐harmonic generation method. Especially, the effects of the valence and coordination number of Fe ions on the third‐order nonlinear optical susceptibility, χ(3), have been examined. The χ(3) values of α‐Fe2O3, γ‐Fe2O3, and Fe3O4 thin films were 5.8×10−11, 2.1×10−11, and 4.0×10−10 esu, respectively, which are the highest values among inorganic oxides reported so far. It was considered that χ(3) of α‐Fe2O3 and γ‐Fe2O3 was enhanced by the pair excitation process involving the simultaneous excitation of magnetically coupled two neighboring Fe3+ ions as well as by virtual transition from O 2p orbitals to Fe 3d orbitals while χ(3) of Fe3O4 was enhanced by both one‐ and three‐photon resonances. The higher second hyperpolarizability, γ(Fex/yO), was obtained when Fe ions are 3+ rather than 2+ and octahedrally rather than tetrahedrally coordinated by oxygens.

Terms linear in k in the band structure of wurtzite-type semiconductors

Abstract: Wurtzite has the space-group symmetry P${6}_{3}$mc. The absence of inversion symmetry allows linear-k terms in the electronic band structure when the spin-orbit interaction is included. Their existence has been confirmed in a number of experiments, but no microscopic calculations have been published. In the present paper, we discuss the origin of these linear-k terms using group theory and k\ensuremath{\cdot}p arguments. The various contributions to these terms are identified through band-structure models. We present an ab initio calculation, performed with the linear-muffin-tin-orbital method, of these spin splittings in CdS, CdSe, and ZnO. A renormalization of the valence-band spin-splitting coefficients obtained in the linear-muffin-tin-orbital calculations was found necessary to correct for errors in the relative energies of the uppermost valence bands as compared with the experimental values. We point out that a similar procedure should be used when evaluating masses and other band parameters from calculated local-density-approximation band structures. \textcopyright{} 1996 The American Physical Society.

Quantum chemical valence indices from the one-determinantal difference approach

Abstract: The recently introduced quadratic (two-electron) valence indices, ionic and covalent, derived from the Hartree–Fock finite-difference approach, are applied to selected organic and inorganic molecul...

Molecular Structure and Vibrational IR Spectra of Cytosine and Its Thio and Seleno Analogues by Density Functional Theory and Conventional ab Initio Calculations

Abstract: Full geometry optimizations (without any constraints on the planarity of the systems) were carried out for six tautomers of cytosine and their thio and seleno analogues by means of the density functional theory with the combined Becke3-LYP exchange-correlation energy functional (DFT(B3-LYP)) and conventional ab initio Hartree−Fock (HF) method. The valence double-ζ basis set augmented by polarization functions on all atoms was used in both approaches. At these geometries, the vibrational IR spectra of the tautomers were calculated at the DFT(B3-LYP) and HF levels with the same basis set. Additionally at the HF geometries of the tautomers, their dipole moments were computed at the MP2(fc) level and electronic energies at the MP2(fc), MP3(fc), and MP4(fc)(SDQ) levels to estimate relative tautomeric energies of the isolated tautomeric forms. The calculation results were compared with the corresponding experimental data when available. Calculations show that (a) the DFT(B3-LYP) method predicts better rotationa...

A theoretical study of the valence‐ and dipole‐bound states of the nitromethane anion

Abstract: The valence‐ and dipole‐bound states of CH3NO−2 are studied at the CCSD(T), HFDFT (B3LYP), and EA‐EOMCC levels of theory. At both CCSD(T) and HFDFT levels, we have found a positive valence EA in nice agreement with the experimental data. The binding energy of the dipole‐bound electron is about 13 meV according to the EA‐EOMCC calculations. Interaction of the valence‐ and dipole‐bound states (DBS) of CH3NO−2 is complicated, since the dipole‐bound state exists at the equilibrium geometry of the anion and corresponds to an excited state of the valence‐bound anion. Hence, excitations of the valence anionic state could lead to both the detachment of an electron or formation of a DBS, whose geometry is similar to the geometry of the neutral parent. At the equilibrium geometry of the anion, the energies of the dipole‐bound and valence states are close to each other. Since typical lifetimes of rovibrational excited states of a DBS are two orders of magnitude higher than the lifetimes of ordinary vibrationally exc...

Multiple Plasmon Satellites in Na and Al Spectral Functions from Ab Initio Cumulant Expansion.

Abstract: he valence photoemission spectra of alkali metals exhibit multiple plasmon satellite structure. The calculated spectral functions within the GW approximation show only one plasmon satellite at too large binding energy. In this Letter we use the cumulant expansion approach to obtain the spectral functions of Na and Al from ab initio calculations including the effects of band structure. The GW spectral functions are dramatically improved and the positions of the multiple plasmon satellites are in very good agreement with experiment while their intensities cannot be explained from intrinsic effects only.