Showing papers on "Valence (chemistry) published in 1993"

Effective core potential methods for the lanthanides

Abstract: In this paper a complete set of effective core potentials (ECPs) and valence basis sets for the lanthanides (Ce to Lu) are derived. These ECPs are consistent not only within the lanthanide series, but also with the third‐row transition metals which bracket them. A 46‐electron core was chosen to provide the best compromise between computational savings and chemical accuracy. Thus, the 5s and 5p are included as ‘‘outer’’ core while all lower energy atomic orbitals (AOs) are replaced with the ECP. Generator states were chosen from the most chemically relevant +3 and +2 oxidation states. The results of atomic calculations indicate that the greatest error vs highly accurate numerical potential/large, even‐tempered basis set calculations results from replacement of the large, even‐tempered basis sets with more compact representations. However, the agreement among atomic calculations remains excellent with both basis set sizes, for a variety of spin and oxidation states, with a significant savings in time for the optimized valence basis set. It is expected that the compact representation of the ECPs and valence basis sets will eventually encourage their use by computational chemists to further explore the bonding and reactivity of lanthanide complexes.

High-Tc Molecular-Based Magnets: Ferrimagnetic Mixed-Valence Chromium(III)-Chromium(II) Cyanides with Tc at 240 and 190 Kelvin

Abstract: Molecular-based magnets with high magnetic-ordering temperatures, T(c), can be obtained by mild chemistry methods by focusing on the bimetallic and mixed-valence transition metal micro-cyanide of the Prussian blue family. A simple orbital model was used to predict the electronic structure of the metal ions required to achieve a high ordering temperature. The synthesis and magnetic properties of two compounds, [Cr(5)(CN)(12)].10H(2)O and Cs(0.75) [Cr(2.125)(CN)(6)].5H(2)O, which exhibited magnetic-ordering temperatures of 240 and 190 kelvin, respectively, are reported, together with the strategy for further work.

Bond valence sum analysis of metal-ligand bond lengths in metalloenzymes and model complexes. 2. Refined distances and other enzymes

Abstract: The bond valence sum (BVS) method is further applied to metalloenzyme active sites. When a particular coordination model is assumed, the BVS method allows for oxidation states of metal ions in metalloproteins to be determined from metal-ligand bond distances measured using extended X-ray absorption fine structure (EXAFS) analysis. Thus, the BVS can be used to determine the compatibility between a given coordination model and a particular oxidation state. A new procedure for calculating r 0 values on which BVS's are based is presented. This procedure allows for calculation of r 0 values on heteroleptic complexes and was used to determine a new set of r 0 distances using complexes that more closely model the active sites of interest

Effects of altervalent cation doping of titania on its performance as a photocatalyst for water cleavage

Abstract: The influence of altervalent cation doping of TiO[sub 2] on its performance as a photocatalyst in water cleavage is investigated. It is shown that under illumination in the near-UV region (250--400 nm), platinized anatase TiO[sub 2] exhibits H[sub 2] production rates which are significantly higher than those of the rutile form. Photocatalytic efficiency is shown to be independent of the specific surface area of the semiconductor Incorporation of cations of valence higher than that of the parent cation (W[sup 6+], Ta[sup 5+], Nb[sup 5+]) into the crystal matrix of TiO[sub 2] results in enhanced rates of water cleavage while the opposite is observed upon doping with cations of lower valence (In[sup 3+], Zn[sup 2+], Li[sup +]). The enhancement or reduction of photocatalytic activity is found to be dependent on the concentration and valence of the doping cations. These results do not correlate with observed alterations in the light-absorption capacity of the doped semiconductor. They are explained in term of alteration of the bulk electronic structure of the semiconductor, which influences its electron-hole generation and separation capacity, under illumination. 28 refs., 7 figs., 2 tabs.

XPS studies of the oxygen 1S and 2s levels in a wide range of functional polymers

Abstract: Over 70 oxygen-containing polymers have been studied using high-resolution monochromated XPS. O 1s binding energies (BEs) are reported and compared with previous data from much smaller sample sets. The separation of the two components from O-C=O groups is quite sensitive to structural variation. O 1s peak widths and the effect of shake-up on peak areas are also discussed. For about a representative one-third of these polymers the O 2s region of the valence band has been studied in detail. O 2s BEs are reported and a nen parameter is introduced: the O Is-O 2s energy separation (oxygen valence parameter). This parameter has a greater range than the O Is chemical shift, is independent of any surface charging, and appears to have interesting diagnostic potential

Identification of strained silicon layers at Si-SiO 2 interfaces and clean Si surfaces by nonlinear optical spectroscopy

Abstract: Optical second-harmonic and sum-frequency spectra of clean and oxidized Si(100) and Si(111) samples reveal a strong resonance band at 3.3 eV photon energy. It is concluded that the resonance arises from direct transitions between valence and conduction band states in a few monolayers of strained silicon at the Si-${\mathrm{SiO}}_{2}$ interface and at the selvedge of clean reconstructed silicon surfaces.

Large second-order optical polarizabilities in mixed-valency metal complexes

Abstract: THE potential development of optoelectronic devices based on the nonlinear polarization of molecular materials has aroused much recent interest1,2. The search for large second-order electric susceptibilities (that is, proportional to the square of an applied electric field) has concentrated on acentric organic or organometallic chromophores with an organic π-electron system coupling electron donor and acceptor groups3–6. It is conceivable that mixed-valence compounds characterized by an intervalence charge-transfer (IVCT) transition7, in which the donor and acceptor centres are both metal atoms, might also have the potential to provide a large second-order response8, but this possibility has not been widely explored. Here we report the first hyperpolarizability, β, of a bimetallic complex ion, [(CN)5Ru-μCN-Ru(NH3)5]- (I in Fig. 1), and a novel organometallic analogue, [(η5-C5H5)Ru(PPh3)2-μ-CN-Ru(NH3)5]3+ (II). Measurements of β (which is related to the bulk second-order response) in solution at a wavelength of 1,064 nm using the newly developed hyper-Rayleigh scattering technique9,10 give values greater than 10–27 e.s.u., which are among the largest reported for solution species. The ease with which the energy of the IVCT transition can be modified suggests that there may be considerable potential for this class of chromophore in nonlinear optical devices.

Femtosecond experiments and absolute rate calculations on intervalence electron transfer of mixed‐valence compounds

Abstract: New experimental results and theoretical calculations are reported for the optical electron transfer (ET) and subsequent reverse radiationless ET for the molecules (NH3)5RuIIINCRuII(CN)−5 and (NH3)5FeIIINCRuII(CN)−5. A procedure is presented for extracting many of the key parameters in ET theory from a combined analysis of resonance Raman data and the optical absorption ET band shape. Using these parameters, ET rates have been calculated using contemporary ET models. The experimental and theoretical rates agree within the uncertainty of the theoretical predictions, which results from an uncertainty in some of the parameters. The results demonstrate that inertial solute/solvent interactions and intramolecular sources of fast energy fluctuations play an important role in ultrafast ET kinetics for these compounds.

Theoretical Study of the Electronic Spectra of Cyclopentadiene, Pyrrole, and Furan

Abstract: The electronic spectra of the title molecules have been studied using a newly proposed quantum chemical approach for ab initio calculations of dynamic electron correlation effects in molecular systems: multiconfigurational second-order perturbation theory (CASPT2). For cyclopentadiene and furan, the calculations comprise three valence excited singlet states and, in addition, the la2 - 3s, 3p, and 3d Rydberg states, thus providing a full assignment of the spectra in the energy range below 8.0 eV. For pyrrole, the 2bl - 3s. 3p, and 3d components of the Rydberg series have been added. The four lowest triplet states have also been studied in all three molecules. The computed excitation energies deviate from experiment by less than 0.17 eV in all cases where an assignment is possible. It is shown that the two main features in the spectra are caused by the valence excited states 1Bz (5.27, 5.92,6.04 eV) and 'A: (7.89, 7.46, 7.74 eV), where the calculated energies for the two states in the three molecules are given in parentheses. In addition, the 'A; state has been determined to appear near 6 eV in all three molecules. These results differ drastically from earlier theoretical predictions but are in agreement with experimental data. A number of new assignments of the Rydberg states are suggested.

Ab initio theoretical predictions of C28, C28H4, C28F4, (Ti@C28)H4, and M@C28 (M=Mg, Al, Si, S, Ca, Sc, Ti, Ge, Zr, and Sn)

Abstract: Recent experiments have demonstrated that C28 is the smallest fullerene cage that successfully traps elements in its inside. In this work, we have studied the electronic structures, equilibrium geometries, and binding energies of the title molecules at the self‐consistent field (SCF) Hartree–Fock level of theory employing basis sets of double‐zeta quality. The empty C28 fullerene is found to have a 5A2 open‐shell ground state and behaves as a sort of hollow superatom with an effective valence of 4, both toward the outside and inside of the carbon cage. The theoretical evidence suggests that C28H4 and C28F4 should be stable molecules. The possibility of simultaneous bonding from the inside and outside of the C28 shell, as in (Ti@C28)H4, is also explored. Our calculations show that the binding energy of the M@C28 species is a good indicator of the success in experimentally trapping the metal atoms (M) inside the fullerene cage. Based on these results, we propose that elements with electronegativities smaller than 1.54 should form endohedral fullerenes larger than a minimum size which depends on the ionic radius of the trapped atom. This qualitative model, correctly reproduces the available experimental evidence on endohedral fullerenes.

First-principles study of graphite monofluoride (CF)n

Abstract: The structure and the electronic properties of graphite monofluoride (CF${)}_{\mathit{n}}$ have been studied within the framework of the density-functional technique, using nonlocal ionic pseudopotentials and a large number of plane waves. The chair conformation is shown to be energetically favored with respect to the boat conformation by 0.145 eV per CF unit. The transition between these two states has been investigated: the boat conformation is metastable, with an estimation of the transition barrier on the order of 2.72 eV. This indicates that the material actually synthesized could depend on the kinetics of the intercalation or could as well be a mixing of both conformations. The equilibrium geometries are compared with experimental data. We also present the valence charge density and the band structure of the chair conformation.

Use of L-edge X-ray absorption spectroscopy to characterize multiple valence states of 3d transition metals; a new probe for mineralogical and geochemical research

Abstract: 2p (L2,3) X-ray absorption spectra are presented for a range of minerals to demonstrate the usefulness of L-edge spectroscopy as a symmetry- and valenceselective probe. 2p XAS provides a sensitive fingerprint of the electronic states of 3 d transition metals and can be applied to phases containing mixtures of such elements. Calculated spectra for 3dn → 2p5 3dn+1 transitions provide a basis for the interpretation of the measured spectra. Thus, in principle, multiple valence states of a particular 3 d metal can be precisely characterized from a single L-edge spectrum. Examples of vanadium L-edge spectra are presented for a range of minerals; these complex spectra hold information concerning the presence of vanadium in multiple valence states. The Cu L-edge spectrum of sulvanite (Cu3 VS4) indicates the presence of both Cu+ and Cu2+; the V L-edge spectrum of the same sample shows that both V2+ and V5+ are present. Spectral simulations representing mixtures of Fe d5 and Fe d6 states are used to quantify Fe3+/∑Fe in a spinel, a glass, and an amphibole, all of which contain Fe as a major component. To illustrate the sensitivity of 2p XAS in a dilute system, the Fe L-edge spectrum of amethyst (α-SiO2: Fe) has been recorded; this spectrum shows that ∼68% of the Fe in amethyst is Fe2+, and ∼32% is Fe3+. Although previous studies on amethyst using other spectroscopic methods cite evidence for Fe4+, there is no indication in the L-edge spectrum for Fe4+ in amethyst. Comparison of theoretical and experimental spectra not only allows the valence states of 3 d ions to be recognised, but also provides site-symmetry information and crystal field parameters for each ion site.

Theoretical study of the electronic spectrum of all-trans-1,3,5,7-octatetraene

Abstract: Ab initio calculations have been performed for the electronic spectrum of all-trans-1,3,5,7-octatetraene including geometry optimizations for the ground and low-lying valence excited singlet states at the complete active space self-consistent-field level. Excitation energies have been calculated using multiconfigurational second-order perturbation theory (CASPT2). Computed excitation energies for the valence and a number of low-lying Rydberg excited singlet and triplet states make possible confident assignments of the main features reported in the experimental spectra. The computed vertical and nonvertical excitations and the fluorescence maxima confirm the suggested presence of the 2(1)A(g) valence state below the 1 1B(u) optically allowed state, as is the characteristic for the longer polyenes. The agreement is within 0.05 eV for all excitation energies where comparison with experimental data is possible.

Excited states of carbonyl compounds. 1. Formaldehyde and acetaldehyde

Abstract: The electronically excited states of formaldehyde and acetaldehyde are examined with the configuration interaction with all single excitations (CIS) method. Vertical and adiabatic transition energies for singlet and triplet states are calculated, and generally good agreement with experiment is obtained for valence states at the CIS level and for both valence and Rydberg states at the CIS-MP2 level. Analysis of the charge density distribution, via difference plots with respect to the ground-state distribution, clearly revealed the nature of the excitation and allowed for an unambiguous assignment of the valence and atomic-orbital-like Rydberg states. The adiabatic geometries of the singlet and triplet states are calculated, and they agree well with experiment

Electronic and structural properties of GaN by the full-potential linear muffin-tin orbitals method: The role of the d electrons.

Abstract: The structural and electronic properties of cubic GaN are studied within the local-density approximation by the full-potential linear muffin-tin orbitals method. The Ga 3d electrons are treated as band states, and no shape approximation is made to the potential and charge density. The influence of d electrons on the band structure, charge density, and bonding properties is analyzed. Due to the energy resonance of Ga 3d states with nitrogen 2s states, the cation d bands are not inert, and features unusual for a III-V compound are found in the lower part of the valence band and in the valence charge density and density of states. To clarify the influence of the d states on the cohesive properties, additional full- and frozen-overlapped-core calculations were performed for GaN, cubic ZnS, GaAs, and Si. The results show, in addition to the known importance of core-valence exchange-correlation nonlinearity, that an explicit description of closed-shell interaction has a noticeable effect on the cohesive properties of GaN. Since its band structure and cohesive properties are sensitive to a proper treatment of the cation d bands, GaN appears to be somewhat exceptional among the III-V compounds and reminiscent of II-VI materials.

Emission of electrons from a clean gold surface induced by slow, very highly charged ions at the image charge acceleration limit.

Abstract: Total low-energy electron yields for the normal incidence interaction of slow, very highly charged ions ${(}^{136}$${\mathrm{Xe}}^{\mathit{q}+}$, 21\ensuremath{\le}q\ensuremath{\le}51; $^{232}\mathrm{Th}^{\mathit{q}+}$, 51\ensuremath{\le}q\ensuremath{\le}80) with a clean gold surface have been determined from the related measured electron emission statistics. The projectile impact energies could be reduced down to the image charge acceleration limit. The electron emission yield was found to increase proportionally with the increasing projectile charge state in all cases studied, suggesting no saturation in the ability of the Au target to provide necessary electrons within the above surface interaction time (e.g., about 280 electrons/projectile for ${\mathrm{Th}}^{79+}$ in less than ${10}^{\mathrm{\ensuremath{-}}13}$ s). Because of the relatively narrow energy distribution of the incident ions, as well as their very high charge states, the first clear measurements of the image charge acceleration from electron emission yields could be performed. Results of a quantitative study of this acceleration are in good agreement with those of a theoretical model based on classical over the barrier transitions (recently developed by J. Burgd\"orfer et al.).

Electronic structure and its dependence on local order for H/Si(111)-(1×1) surfaces

Abstract: The valence and core level spectra of chemically prepared, ideally H-terminated Si(111) surfaces are characterized by remarkably sharp features. The valence band levels and their dispersion are well described by first-principles calculations using a quasiparticle self-energy approach within the GW approximation. From the ${\mathrm{Si}}_{2\mathit{p}}$ spectra, an upper limit of 35\ifmmode\pm\else\textpm\fi{}10 meV is derived from the core hole lifetime broadening, a value substantially lower than previously measured.

Photoelectron spectroscopic study of the valence and core-level electronic structure of BaTiO3.

Abstract: We present valence and core-level photoemission measurements from vacuum-fractured, single-crystal barium titanate. These results resolve contradictory measurements in the literature which have employed other methods of sample surface preparation. The valence-shell electronic structure is compared with previously published results of band structure and cluster calculations. Resonant photoemission is used to probe the covalent coupling between titanium and oxygen in the cubic and tetragonal phases of this ionic compound. Photoelectron spectra of the Ti 2p and O 1s core levels reveal the valence of these two ions to be ${\mathrm{TiO}}_{2}$-like. Valence, core, satellite, and Auger transitions are also assigned and tabulated.

Valency and Structure of Iridium in Anodic Iridium Oxide Films

Abstract: The valence of iridium in anodic iridium oxide films has been determined by x-ray absorption spectroscopy and varies between 3.0 and 4. 8 in a voltammogram cycle. When the valency v starts to increase the first coordination shell around an iridium ion begins to shrink from 0.203 at v=3.0 to 0.191 nm at v=4.8. At the same time a strong spread in the interatomic distances in this shell is observed. Our data give support to the model proposed by Kotz et al. for the charge exchange reaction during a voltammogram cycle

Ultrafast electron transfer and coupled vibrational dynamics in cyanide bridged mixed-valence transition-metal dimers

Abstract: Picosecond infrared spectroscopy has been used to investigate electron transfer and vibrational excitation and relaxation in the mixed-valence transition-metal dimer [(NC) 5 M II CNM III (NH 3 ) 5 ] - (M=Ru, Os). Optical excitation into the metal-to-metal charge transfer band results in formation of the excited state redox isomer [(NC) 5 M III CNM II (NH 3 ) 5 ] -

GW quasiparticle calculations in atoms.

Abstract: We present quasiparticle calculations done in atoms using various versions of Hedin's GW approximation. The performance of GW in major-group elements is illustrated by calculations of first and second ionization energies from the same initial configuration, along with various promotion energies. The results significantly improve Hartree-Fock eigenvalues and are comparable, in accuracy, to Hartree-Fock, total-energy differences. In the 3d (iron-series) transition metals, there are also significant improvements in the 4s- and 3d-state electron addition energies in the first half of the series. In the second half of the series, we find that strong correlations among 3d electrons of opposite spin are not accounted for by GW. We also study the screening effects by cores on valence-electron addition and removal energies. In particular, we examine the ``alkalilike'' problem of one valence electron bound to a closed-shell core. Extensive tests show that here the generalized GW approximation describes quantitatively important effects of core-relexation and core-valence correlation on the valence electrons. In the following paper it is shown that these core-relaxation and core-valence correlation effects can be incorporated into an effective potential suitable for use in calculations of valence properties of atoms, molecules, and solids.

Chemical valence from the two-particle density matrix

Abstract: The two-particle approach to the covalent and ionic valence indices is presented within the restricted and unrestricted Hartree-Fock theories (RHF and UHF). It is based on the analysis of contributions from the two-particle density matrix Γ in the orthogonal atomic orbital (OAO) representation. The atomic and diatomic valence indices are identified with respect to the reference states of separated atoms or ions and their physical interpretation is given. It is found that ionic indices originate from the Coulomb part of Γ, while the covalent indices are related to its exchange part. They are shown to be related to the differences, with respect to the reference state values, of the condensed Γ-matrix elements, measuring a total probability of simultaneously finding two electrons on atoms A and B. An interpretation of a second-order Taylor expansion around the reference states of the proposed electron pair valence indices is given. Illustrative valence diagrams for the two atomic orbital model are presented for both the A+B and A−+B+ reference limits, and their implications for the bond description are briefly discussed. The valence indices for simple diatomics and polyatomics are generated within the RHF and UHF schemes. The problem of the residual valence in the RHF approach is examined and the RHF predictions for alternative reference states are compared.

Steric variation of the cerium valence in Ce2Fe14B and related compounds

Abstract: Through measurements of the near‐edge Ce L3 x‐ray absorption structure we have investigated the influence of site volume on the cerium valence in Ce2Fe14B, Ce2Fe14BHx, and Ce2Fe17. The crystal structures of these compounds are isomorphic with those of technologically important permanent magnet materials. Confirming other experiments, we find that the Ce ion in Ce2Fe14B is in a strongly mixed‐valent α‐like state which is incompatible with a local 4f moment. Comparison of the spectroscopically determined valences of the three materials demonstrates a shift toward a γ‐like cerium state, which supports a magnetic moment, as the steric volume of the Ce site(s) increases. Our results strongly suggest that the volume of the rare‐earth site is the principal factor controlling the Ce chemical valence in these systems.

Electron-transfer reactions in proteins: an artificial intelligence approach to electronic coupling

Abstract: The electronic interactions which are responsible for electron transfer in proteins are treated using an artificial intelligence (AI) approach and a quantum mechanical formulation of superexchange. An AI search procedure is devised to select the most important amino acid residues which mediate long-range electron transfer. All the valence orbitals of these amino acid residues are used in a diagonalization of the "bridge" orbitals. The electronic coupling matrix element is then calculated by using second-order perturbation theory to couple the bridge orbitals to the donor and acceptor orbitals. The relative values of the electronic coupling elements obtained with this model are shown to be in good agreement with experimental results for cytochrome c derivatives, without use of adjustable parameters. Further, an optimum path calculation in which the path consists of several amino acids is also presented and compared with the many amino acid calculation. The various results show that not merely the separation distance but also the nature of the protein medium separating the redox centers is an important factor in controlling the rate of these electron-transfer reactions.

Chapter 5 Magnetic properties of binary rare-earth 3d-transition-metal intermetallic compounds

Abstract: Publisher Summary This chapter discusses the magnetic properties of binary rare-earth 3d-transition-metal intermetallic compounds The basic concepts related to the intrinsic magnetic properties of the 3d-rich R n T m (R = rare earths; T = 3d heavy transition metals Mn, Fe, Co, Ni) intermetallic compounds are discussed The study of rare-earth transition-metal intermetallic compounds has a number of interesting aspects Owing to the wide range of intermetallics and their different stoichiometries and variable rare-earth elements, modifications of magnetic properties of 3d transition-metal and 4f rare-earth ions can be investigated systematically These investigations illuminate the complex interactions in which the 3d and 4f electrons are involved The rare-earth metals in their ‘normal’ state where the magnetic properties of the ion cores are well defined The 4f electrons are positioned within the ion cores and hybridization with the conduction-band-electron states is negligible This situation is realized for most iron- and cobalt-based compounds with 4f elements The Ce and Yb ions, which sometimes demonstrate unusual properties connected with valence fluctuations, tend to behave quite normally in the compounds with iron or cobalt Nevertheless, there are some anomalies in the Ce compounds that can be ascribed to a mixed-valence state of the cerium ion