Showing papers on "Valence (chemistry) published in 2001"

6-31G* basis set for third-row atoms

Abstract: Medium basis sets based upon contractions of Gaussian primitives are developed for the third-row elements Ga through Kr. The basis functions generalize the 6-31G and 6-31G* sets commonly used for atoms up to Ar. A reexamination of the 6-31G* basis set for K and Ca developed earlier leads to the inclusion of 3d orbitals into the valence space for these atoms. Now the 6-31G basis for the whole third-row K through Kr has six primitive Gaussians for 1s, 2s, 2p, 3s, and 3p orbitals, and a split-valence pair of three and one primitives for valence orbitals, which are 4s, 4p, and 3d. The nature of the polarization functions for third-row atoms is reexamined as well. The polarization functions for K, Ca, and Ga through Kr are single set of Cartesian d-type primitives. The polarization functions for transition metals are defined to be a single 7f set of uncontracted primitives. Comparison with experimental data shows good agreement with bond lengths and angles for representative vapor-phase metal complexes. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 976–984, 2001

Correlation consistent valence basis sets for use with the Stuttgart–Dresden–Bonn relativistic effective core potentials: The atoms Ga–Kr and In–Xe

Abstract: We propose large-core correlation-consistent (cc) pseudopotential basis sets for the heavy p-block elements Ga–Kr and In–Xe. The basis sets are of cc-pVTZ and cc-pVQZ quality, and have been optimized for use with the large-core (valence-electrons only) Stuttgart–Dresden–Bonn (SDB) relativistic pseudopotentials. Validation calculations on a variety of third-row and fourth-row diatomics suggest them to be comparable in quality to the all-electron cc-pVTZ and cc-pVQZ basis sets for lighter elements. Especially the SDB-cc-pVQZ basis set in conjunction with a core polarization potential (CPP) yields excellent agreement with experiment for compounds of the later heavy p-block elements. For accurate calculations on Ga (and, to a lesser extent, Ge) compounds, explicit treatment of 13 valence electrons appears to be desirable, while it seems inevitable for In compounds. For Ga and Ge, we propose correlation consistent basis sets extended for (3d) correlation. For accurate calculations on organometallic complexes o...

Bulk electronic structure of SrTiO3: Experiment and theory

Abstract: Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3 Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states The experimentally determined indirect band gap energy is 325 eV, while the direct band gap energy is 375 eV The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 325 eV up to 15 eV The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV Interband transitions from the O 2s band to the conduction bands appear at 26 eV A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons

Electronic structure and bonding in metal phthalocyanines, Metal=Fe, Co, Ni, Cu, Zn, Mg

Abstract: Electronic structure and bonding in metal phthalocyanines (Metal=Fe, Co, Ni, Cu, Zn, Mg) is investigated in detail using a density functional method. The metal atoms are strongly bound to the phthalocyanine ring in each case, by as much as 10 eV. The calculated orbital energy levels and relative total energies of these D4h structures indicate that Fe and Co phthalocyanines have 3A2g and 2Eg ground states, respectively, but that these states are changed upon interaction with strong-field axial ligands. The valence electronic structures of Fe and Co phthalocyanines differ significantly from those of the others. The HOMOs in Fe, Co, and Cu phthalocyanine are metal 3d-like, whereas in Ni and Zn phthalocyanines, the HOMO is localized on the phthalocyanine ring. The first ionization removes an electron from the phthalocyanine a1u orbital in all cases, with very little sensitivity of the ionization energy to the identity of the metal. Whereas the first reduction in Fe and Co phthalocyanine occurs at the metal, i...

Crystal and defect chemistry of rare earth cations in BaTiO3

Abstract: This study revisits the issue of rare earth cation substitutions into barium titanate. Analysis based upon crystal chemistry, defect chemistry and metastable states is presented to aid interpretation of experimental data. Recent detailed and highly precise X-ray powder diffraction and Electron Paramagnetic Resonance experiments performed on samples produced with different A/B ratios and fired under different oxygen partial pressure conditions give rise to new insights into the material. Specifically, the site occupancy and the valence states for the rare-earth dopants in barium titanate are considered. Earlier work is also reviewed and compared to the studies performed here. Collectively a classification of the various types of behavior observed for the rare-earth series in barium titanate is presented.

Modeling CMOS tunneling currents through ultrathin gate oxide due to conduction- and valence-band electron and hole tunneling

Abstract: A semi-empirical model is proposed to quantify the tunneling currents through ultrathin gate oxides (1-3.6 nm). As a multiplier to a simple analytical model, a correction function is introduced to achieve universal applicability to all different combinations of bias polarities (inversion and accumulation), gate materials (N/sup +/, P/sup +/, Si, SiGe) and tunneling processes. Each coefficient of the correction function is given a physical meaning and determined by empirical fitting. This new model can accurately predict all the current components that can be observed: electron tunneling from the conduction band (ECB), electron tunneling from the valence band (EVB), and hole tunneling from the valence hand (HVB) in dual-gate poly-Si/sub 1-x/Ge/sub x/-gated (x=0 or 0.25) CMOS devices for various gate oxide thicknesses. In addition, this model ran also be employed to determine the physical oxide thickness from I-V data with high sensitivity. It is particularly sensitive in the very-thin-oxide regime, where C-V extraction happens to be difficult or impossible (because of the presence of the large tunneling current).

Hydrogen produced from hydrohalic acid solutions by a two-electron mixed-valence photocatalyst.

Abstract: Energy conversion cycles are aimed at driving unfavorable, small-molecule activation reactions with a photon harnessed by a transition metal complex. A challenge that has occupied researchers for several decades is to create molecular photocatalysts to promote the production of hydrogen from homogeneous solution. We now report the use of a two-electron mixed-valence dirhodium compound to photocatalyze the reduction of hydrohalic acid to hydrogen. In this cycle, photons break two RhII–X bonds of a LRh0–RhIIX2 core in the presence of a halogen trap to regenerate the active LRh0–Rh0 catalyst, which reacts with hydrohalic acid to produce hydrogen.

Experimental realization of a 2D fractional quantum spin liquid.

Abstract: The ground-state ordering and dynamics of the two-dimensional S = 1/2 frustrated Heisenberg antiferromagnet Cs(2)CuCl(4) are explored using neutron scattering in high magnetic fields. We find that the dynamic correlations show a highly dispersive continuum of excited states, characteristic of the resonating valence bond state, arising from pairs of S = 1/2 spinons. Quantum renormalization factors for the excitation energies (1.65) and incommensuration (0.56) are large.

Relationship between bond valence and bond softness of alkali halides and chalcogenides

Abstract: Established bond-valence parameter tables rely on the assumption that the bond-valence sum of a central atom is fully determined by interactions to atoms in its first coordination shell. In this work the influence of higher coordination shells is tested in detail for bonds between lithium and oxygen. It is demonstrated that the sum of the weak interactions with atoms of the second coordination shells significantly contributes to the valence sum and should therefore not be neglected. Since the independent refinement of the two parameters R0 and b is hardly possible from the limited range of bond lengths occurring in the first coordination shell, the restriction of bond-valence sums to contributions from nearest neighbours implicated another far-reaching simplification: the postulation of a universally fixed value of the bond-valence parameter b which characterizes the shape of the bond-valence pseudopotential for the respective atom pair. However, recent more sophisticated applications of the bond-valence concept, e.g. to model ion-transport pathways in solid electrolytes, demand sensible estimates of the bond-valence sums for mobile ions not only at their equilibrium sites but also at interstitial sites and bottle-necks of transport pathways. Calculations of bond valences at these non-equilibrium sites require the knowledge of the actual shape of the bond-valence pseudopotential. A systematic route to a more realistic estimate of b for alkali halides and chalcogenides is developed in this work from an empirical correlation between b and the absolute softnesses of the interacting particles.

Metallophilic interactions in closed-shell copper(I) compounds--a theoretical study.

Abstract: Cuprophilic interactions in neutral perpendicular model dimers of the type (CH 3 CuX) 2 (X = OH 2 , NH 3 , SH 2 , PH 3 , N 2 , CO, CS, CNH, CNLi) were analyzed by ab initio quantum-chemical methods. The basis set superposition error for the weakly interacting CH 3 CuX subunits is significant and is discussed in detail. A new correlation-consistent pseudopotential valence basis set for Cu, derived at the second-order Moller - Plesset level suppresses considerably the basis set superposition error in Cu-Cu interactions compared to the standard Hartree-Fock optimized valence basis set. This allowed the first accurate predictions of cuprophilicity, which has been the subject of considerable debate in the past. The dependence of the strength of cuprophilic interactions on the nature of the ligand X was addressed. The Cu-Cu interaction increases with increasing o-donor and π-acceptor capability of the ligand and is approximately one-third of the well-documented aurophilic interactions. By fitting our potential-energy data to the Hershbach-Laurie equation, we determined a relation between the Cu-Cu bond length and the Cu-Cu force constant; this is important for future studies on vibrational behaviour. The role of relativistic effects on the structure and the interaction energy is also discussed. Finally we investigated cuprophilic interactions in (CH 3 Cu) 4 as a model species for compounds isolated and characterized by X-ray diffraction.

Ferromagnetism in magnetically doped III-V semiconductors.

Abstract: The origin of ferromagnetism in semimagnetic III-V materials is discussed. The indirect exchange interaction caused by virtual electron excitations from magnetic impurity acceptor levels to the valence band can explain ferromagnetism in GaAs(Mn) in both degenerate and nondegenerate samples. Formation of ferromagnetic clusters and the percolation picture of phase transition describes well all available experimental data and allows us to predict the Mn-composition dependence of transition temperature in wurtzite (Ga,In,Al)N epitaxial layers.

Are the compressive effects of encapsulation an artifact of the bond valence parameters

Abstract: The large bond valence sums found for O 2- encapsulated by Pb 2+ ions are shown to result from the use of inappropriate bond valence parameters. New values of r 0 = 1.963 A and b = 0.49 A are recommended for Pb 2+ -O bonds.

Synthesis and Anion Exchange of Tunnel Structure Akaganeite

Abstract: Akaganeite (β-FeOOH) has been prepared by the hydrolysis of FeCl3 solutions. The effects of microwave heating, pH of the solution, and addition of manganese with different valence states have been ...

(V2O5)n Gas-Phase Clusters (n = 1−12) Compared to V2O5 Crystal: DFT Calculations

Abstract: Stable structures of neutral (V2O5)n clusters (n = 1−5, 8, 10, and 12) are determined by density functional calculations (BP86 functional with a double-ζ (V)/triple-ζ (O) valence basis set augmented by polarization functions). Comparison is made with calculations for the periodic structure of solid V2O5. The most stable structure of the smallest cluster is doubly O-bridged, OV−O2−VO2, and by 184 kJ/mol VO2.5 less stable than the periodic bulk structure. From the tetrahedral V4O10 structure on (41 kJ/mol VO2.5 above the crystal energy) polyhedral cage structures are the most stable isomers: trigonal prism (V6O12), cube (V8O20), pentagonal prism (V10O25), 16-hedron (V16O40), dodecahedron (V20O50), and truncated octahedron (V24O60). The polyhedra have vanadyl groups at the apexes and bridging oxygen atoms on the edges. Differently from the crystal structure, vanadium is 4-fold coordinated and 3-fold coordinated oxygen is avoided. The energies relative to the periodic solid are 22.1, 12.4, 9.4, 5.5, 3.3, and...

Real and Hypothetical Intermediate-Valence AgII/AgIII and AgII/AgI Fluoride Systems as Potential Superconductors

Abstract: With the aim of gauging their potential as conducting or superconducting materials, we examine the crystal structures and magnetic properties of the roughly one hundred binary, ternary, and quaternary Ag(II) and Ag(III) fluorides in the solid state reported up to date. The Ag(II) cation appears in these species usually in a distorted octahedral environment, either in an [AgF](+) infinite chain or as [AgF(2)] sheets. Sometimes one finds discrete square-planar [AgF(4)](2-) ions. The Ag(III) cation occurs usually in the form of isolated square-planar [AgF(4)](-) ions. Systems containing Ag(III) (d(8)) centers are typically diamagnetic. On the other hand, the rich spectrum of Ag(II) (d(9)) environments in binary and ternary fluorides leads to most diverse magnetic properties, ranging from paramagnetism, through temperature-independent paramagnetism (characteristic for half-filled band and metallic behavior) and antiferromagnetism, to weak ferromagnetism. Ag(II) and Ag(III) have the same d-electron count as Cu(II) (d(9)) and Cu(III) (d(8)), respectively. F(-) and O(2-) ions are isoelectronic, closed-shell (s(2)p(6)) species; both are weak-field ligands. Led by these similarities, and by some experimental evidence, we examine analogies between the superconducting cuprates (Cu(II)/Cu(III)-O(2-) and Cu(II)/Cu(I)-O(2-) systems) and the formally mixed-valence Ag(II)/Ag(III)-F(-) and Ag(II)/Ag(I)-F(-) phases. For this purpose we perform electronic-structure computations for a number of structurally characterized binary and ternary Ag(I), Ag(II), and Ag(III) fluorides and compare the results with similar calculations for oxocuprate superconductors. Electronic levels in the vicinity of the Fermi level (x(2)-y(2) or z(2)) have usually strongly mixed Ag(d)/F(p) character and are Ag-F antibonding, thus providing the potential of efficient vibronic coupling (typical for d(9) systems with substantially covalent bonds). According to our computations this is the result not only of a coincidence in orbital energies; surprisingly the Ag-F bonding is substantially covalent in Ag(II) and Ag(III) fluorides. The electron density of state at the Fermi level (DOS(F)) for silver fluoride materials and frequencies of the metal-ligand stretching modes have values close to those for copper oxides. The above features suggest that properly hole- or electron-doped Ag(II) fluorides might be good BCS-type superconductors. We analyze a comproportionation/disproportionation equilibrium in the hole-doped Ag(II) fluorides, and the possible appearance of holes in the F(p) band. It seems that there is a chance of generating an Ag(III)-F(-)/Ag(II)-F(0) "ionic/covalent" curve crossing in the hole-doped Ag(II)-F(-) fluorides, significantly increasing vibronic coupling.

The Theoretical Prediction of Molecular Radical Species: a Systematic Study of Equilibrium Geometries and Harmonic Vibrational Frequencies

Abstract: A systematic study of the accuracy of structures and frequencies of 33 small radical molecules is presented as predicted by Hartree−Fock (HF) theory, second-order Moller−Plesset (MP2) theory, coupled-cluster singles and doubles (CCSD) theory, coupled-cluster singles and doubles with perturbational triples correction [CCSD(T)] theory, and gradient-corrected density functional theory with 3-parameter exact exchange mixing (B3LYP). For all methods, calculations were carried out using the Pople 6-31G**, the correlation-consistent polarized valence double-ζ (cc-pVDZ), and the correlation-consistent polarized valence triple-ζ (cc-pVTZ) basis sets. While basis set effects were moderate, large differences in the performance of the different methods were found. Due primarily to artifactual symmetry breaking and orbital instabilities, both restricted and unrestricted HF and MP2 perform too erratically to be acceptable. CCSD with either restricted or unrestricted orbitals yields results in generally good agreement w...

Mixed-Valence Tetranuclear Manganese Single-Molecule Magnets

Abstract: The preparations, X-ray structures, and detailed physical characterizations are presented for two new mixed-valence tetranuclear manganese complexes that function as single-molecule magnets (SMM's): [Mn4(hmp)6Br2(H2O)2]Br2.4H2O and [Mn4(6-me-hmp)6Cl4].4H2O, where hmp- is the anion of 2-hydroxymethylpyridine and 6-me-hmp- is the anion of 6-methyl-2-hydroxymethylpyridine.

A contribution to borate crystal chemistry: Rules for the occurrence of polyborate anion types

Abstract: The structural features of over 460 anhydrous oxoborates that are described in literature were analysed. Lewis acid strengths were calculated for all cations other than B 3+ that were found in these structures, using the valences of the cations and their determined average coordination numbers. From the observation of a restriction of the occurrence of complex polyborate anions to specific ranges of cation Lewis acid strength in existing oxoborate compounds, the Lewis base strengths of the complex polyborate anions (not of the oxygen ligands) were inferred, using the idea that is at the bottom of the valence matching principle, that in the most stable compounds the Lewis acid strength of the cation nearly equals the Lewis base strength of the anion. From this, general inherent rules for the occurrence of the miscellaneous polyborate anions with respect to the ratio cation: boron (A:B), the Lewis acid strength(s) L a of the cation(s) other than B 3+ and the cation radii were developed These rules offer the possibility for a prediction of the stoichiometry and the most probable polyborate structural group that will be adopted by a new anhydrous borate compound.

Relationship between the Bond Valence and the Temperature Coefficient of the Resonant Frequency in the Complex Perovskite (Pb1−xCax)[Fe0.5(Nb1−yTay)0.5]O3

Abstract: The temperature coefficient of the resonant frequency (TCF) of complex perovskite (Pb 1-x Ca x )[Fe 0.5 (Nb 1-y Ta y ) 0.5 ]O 3 ceramics (x = 0.5, 0.55; 0.0 ≤ y ≤ 1.0) was investigated, relative to the bond valence of the A- and B-site ions in the ABO 3 perovskite structure (such as the barium-, strontium-, and calcium-based complex perovskites). The TCF of these complex perovskite compounds varied with the bond valence of the A- and B-sites and the tolerance factor (t) in the perovskite structure. In the tilted region (t < 1.0), the tilting of the oxygen octahedra increased and the TCF decreased, because of the increased bond valence of the B-site. Also, the dependence of TCF on the bond valence of the A-site was similar to its dependence on t.

Beyond Classical Stoichiometry: Experiment and Theory

Abstract: Most known molecules and compounds follow fixed stoichiometry and can be rationalized on the basis of classical valence theories. However, nonstoichiometric species, particularly in the gas phase, are common. These species cannot be easily understood by classical valence considerations because they do not have the full octet of valence electronssthey are valence unsaturated molecules with dangling bonds. We consider nonstoichiometric molecules consisting of only four or five atoms and show the great variety of molecules and bonding that can be derived from this class of seemingly simple species. We demonstrate that gas-phase photodetachment photoelectron spectroscopy using a laser vaporization source and ab initio quantum calculations provide an ideal approach to characterize and understand the structure and bonding of nonstoichiometric molecular and cluster species. Specifically, we review our recent progress in the design and characterization of the first pentaatomic tetracoordinate planar carbon molecules, CAl 4 - , CAl3Si - , CAl3Ge - , and a salt complex, Na + [CAl4 2- ] containing a planar carbon building block. We also review our recent discovery of an all-metal aromatic species, Al4 2- , in a series of bimetallic clusters, M + [Al4 2- ]( M) Cu, Li, Na), as well as the Ga4 2- and In4 2- analogues. We also show the existence of aromaticity in a series of isoelectronic singly charged anions, MAl3 - (M ) Si, Ge, Sn, Pb), and how aromaticity helps stabilize the heterocyclic structure over a pyramidal isomer. We show how, by pursuing and understanding the concept of nonstoichiometry, one can extend the classical valence theory and discover new structures and new types of bonding.

CuMn2O4: properties and the high-pressure induced Jahn-Teller phase transition

Abstract: Single crystal x-ray diffraction, x-ray photoelectron spectroscopy and magnetic susceptibility measurements at normal pressure have shown that, in spite of two Jahn-Teller active ions in CuMn2O4, the crystal is cubic with partly inverse spinel structure, the inversion parameter being \mbox{$x = 0.8$}. The cation configuration at normal pressure was determined as Cu0.2+Mn2+0.8[Cu2+0.8Mn3+0.2Mn4+1.0]O4. The high-pressure behaviour of the crystal was investigated up to 30 GPa using the energy dispersive x-ray diffraction technique and synchrotron radiation. A first-order phase transition connected with a tetragonal distortion takes place at Pc = 12.5 GPa, the c/a ratio being 0.94 at P = 30 GPa. The high-pressure phase has been described in terms of ligand field theory and explained by the changes to the valence and electronic configuration of the metal ions, leading to the formula Cu2+0.2Mn3+0.8[Cu2+0.8Mn3+1.2]O4. The electron configuration of the tetrahedrally coordinated Cu2+ and Mn3+ is (e4)t5 and e2t2, respectively. On the other hand, the electron configuration of Cu2+ located at octahedral sites is (t62g)e3g. While six electrons with antiparallely aligned spins occupy the triplet (t62g), three electrons on the orbital eg can be distributed in two ways (double degeneracy): (dx2-y2)1(dz2)2 and (dx2-y2)2(dz2)1. The first alternative leads to an axially elongated octahedron; the second one causes flattening of the octahedron. The contraction of the c axis indicates, that in the high-pressure phase the second configuration with unpaired electron on the dz2 orbital occurs. A similar effect of the octahedral contraction brings the orbital degeneracy of Mn3+ with the t32ge1g distribution. It follows that at high pressure the ligand field forces the two metals to take the valences that they show in the parent oxides CuO and Mn2O3.

Valence tautomerism and metal-mediated catechol oxidation for complexes of copper prepared with 9,10-phenanthrenequinone.

Abstract: Bis(pyridine)(9,10-phenanthrenequinone)(9,10-phenanthrenediolato)copper(II), Cu(py)(2)(PhenCat)(PhenBQ), has been prepared by treating copper metal with 9,10-phenanthrenequinone in pyridine solution. In dilute solution, both Cu(py)(2)(PhenCat)(PhenBQ) and the related complex Cu(tmeda)(PhenCat)(PhenBQ) lose PhenBQ to form Cu(II)L(2)(PhenCat), where L(2)= tmeda, 2 py. EPR spectra recorded at temperatures between 300 and 77 K reveal the presence of species with radical and metal localized spins together at equilibrium. Equilibria between Cu(II)L(2)(PhenCat) and Cu(I)L(2)(PhenSQ) redox isomers are solvent dependent, with a shift to higher temperature for polar solvents. Both complexes are oxygen sensitive, reacting with dioxygen to give complexes of diphenic acid. Structural characterization on products obtained with tmeda show that dioxygen insertion across the C-C bond within the chelate ring leads to dimeric products with adjacent Cu(II) ions bridged by diphenate ligands. The addition of O(2) to Cu(tmeda)(PhenCat) in acetonitrile solution at 0 degrees C appears to form a peroxo complex, tentatively identified as Cu(tmeda)(O(2))(PhenQ) on the basis of iodometric titration, as the precursor to the diphenate complex.

Electronic structure of SiC(0001) surfaces studied by two-photon photoemission

Abstract: The electronic structure of the valence and conduction bands at the Si(100) surface has been studied by two-photon photoemission over a wide photon-energy range. The ionization energy was determined to 5.40 ′ 0.03 eV. Theoccupied surface state at r is placed 0.15 ′ 0.06 below the valence-band maximum. Several other spectral features are assigned to transitions involving surface states and between bulk bands including backfolded bands due to the surface reconstruction. The moderate agreement between experimental data and band-structure calculations calls for an improved theoretical description of the two-photon photoemission process at semiconductor surfaces incorporating, e.g., a one-step model and excitonic effects.