Showing papers on "Valence (chemistry) published in 2007"

Bond order and valence indices: a personal account.

Abstract: The paper accounts for the author's activity in developing bond order and valence indices since the early 80s. These indices represent an important conceptual link between the physical description of molecules as systems of electrons and nuclei and the chemical picture of molecules consisting of atoms kept together by bonds. They are also useful for a systematization and interpretation of the results obtained in the quantum chemical calculations, by permitting to extract from the wave function different pieces of information that may be assigned chemical significance. In some cases they can have some predictive power, too. Historically, the prototypes of such indices were introduced in the semiempirical quantum chemistry; the most important developments were Coulson's charge-bond order matrix in the simple Huckel theory and the Wiberg index in the CNDO framework. (Valence indices were also introduced in the semiempirical theory.) The definition of the ab initio bond order index emerged from the asymptotic term of the exchange energy component of the partitioning performed in the framework of the author's so-called "chemical Hamiltonian approach" using a "mixed" second quantization formalism for overlapping basis sets. They can also be introduced by studying the exchange part of the two-particle density (or of the second-order density matrix). Some properties of the bond order indices are discussed and the author's (until now unpublished) proof is also presented, showing the sufficient conditions under which the bond order index of a homonuclear diatomics is equal to the "chemist's bond order," i.e., the half of the difference between the number of electrons occupying bonding and antibonding orbitals. The ab initio valence indices are also introduced and discussed, and it is stressed that for correlated wave function the same "exchange only" definition of the bond order and valence indices should be used, which was introduced for the SCF case. The recent concept of the "atomic decomposition of identity" is also discussed and it is utilized for introducing bond orders and valences in the framework of the "3D analysis," when atoms are defined not by their basis orbitals but as regions of the three-dimensional (3D) physical space. Two versions of the 3D analysis are considered--the AIM (atoms in molecules)-type decomposing the space into disjunct atomic domains and the "fuzzy atoms" scheme in which there are no sharp boundaries between the atoms but they exhibit a continuous transition from one to another.

Valence band anticrossing in GaBixAs1−x

Abstract: The optical properties of GaBixAs1-x (004< x< 008) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T2 states of the Bi atoms

Critical Assessment of the Performance of Density Functional Methods for Several Atomic and Molecular Properties

Abstract: The reliable prediction of molecular properties is a vital task of computational chemistry. In recent years, density functional theory (DFT) has become a popular method for calculating molecular properties for a vast array of systems varying in size from small organic molecules to large biological compounds such as proteins. In this work we assess the ability of many DFT methods to accurately determine atomic and molecular properties for small molecules containing elements commonly found in proteins, DNA, and RNA. These properties include bond lengths, bond angles, ground state vibrational frequencies, electron affinities, ionization potentials, heats of formation, hydrogen bond interaction energies, conformational energies, and reaction barrier heights. Calculations are carried out with the 3-21G*, 6-31G*, 3-21+G*, 6-31+G*, 6-31++G*, cc-pVxZ, and aug-cc-pVxZ (x=D,T) basis sets, while bond distance and bond angle calculations are also done using the cc-pVQZ and aug-cc-pVQZ basis sets. Members of the popular functional classes, namely, LSDA, GGA, meta-GGA, hybrid-GGA, and hybrid-meta-GGA, are considered in this work. For the purpose of comparison, Hartree-Fock (HF) and second order many-body perturbation (MP2) methods are also assessed in terms of their ability to determine these physical properties. Ultimately, it is observed that the split valence bases of the 6-31G variety provide accuracies similar to those of the more computationally expensive Dunning type basis sets. Another conclusion from this survey is that the hybrid-meta-GGA functionals are typically among the most accurate functionals for all of the properties examined in this work.

The effect of valence state and site geometry on Ti L3,2 and O K electron energy-loss spectra of TixOy phases

Abstract: Titanium L 3,2 and O K electron energy loss near-edge structures (ELNES) of seven Ti oxides have been measured in a transmission electron microscope to obtain information on the valence state and site geometry of Ti. The coordination of Ti in all phases studied is octahedral, whereas the valence states occurring are Ti2+, Ti3+, and Ti4+. Effects of polyhedra distortions are particularly observed for two oxides with mixed Ti3+-Ti4+ valence state, i.e., the Magneli phases Ti4O7 and Ti5O9. A prominent pre-peak in the Ti L 3 edge is attributed to the orthorhombic polyhedra distortions in these compounds, leading to complex crystal field splitting. The effect of valence state manifests itself in a systematic chemical shift of Ti white lines by 2 eV per valence state. On the basis of collected Ti L 3,2 ELNES spectra we propose a new quantification technique for the determination of Ti4+/Ti3+ ratios. Complementary O K ELNES spectra were well reproduced by Density Functional Theory calculation, revealing that the O K -edge is sensitive to the covalent bonding in all analyzed oxides.

Ferromagnetism in 2p Light Element-Doped II-oxide and III-nitride Semiconductors

Abstract: II-oxide and III-nitride semiconductors doped by nonmagnetic 2p light elements are investigated as potential dilute magnetic semiconductors (DMS). Based on our first-principle calculations, nitrogen doped ZnO, carbon doped ZnO, and carbon doped AlN are predicted to be ferromagnetic. The ferromagnetism of such DMS materials can be attributed to a p-d exchange-like p-p coupling interaction which is derived from the similar symmetry and wave function between the impurity (p-like t_2) and valence (p) states. We also propose a co-doping mechanism, using beryllium and nitrogen as dopants in ZnO, to enhance the ferromagnetic coupling and to increase the solubility and activity.

Density functional theory optimized basis sets for gradient corrected functionals: 3d transition metal systems.

Abstract: Density functional theory optimized basis sets for gradient corrected functionals for 3d transition metal atoms are presented. Double zeta valence polarization and triple zeta valence polarization basis sets are optimized with the PW86 functional. The performance of the newly optimized basis sets is tested in atomic and molecular calculations. Excitation energies of 3d transition metal atoms, as well as electronic configurations, structural parameters, dissociation energies, and harmonic vibrational frequencies of a large number of molecules containing 3d transition metal elements, are presented. The obtained results are compared with available experimental data as well as with other theoretical data from the literature.

Fluctuating valence in a correlated solid and the anomalous properties of delta-plutonium.

Abstract: Although the nuclear properties of the late actinides (plutonium, americium and curium) are fully understood and widely applied to energy generation, their solid-state properties do not fit within standard models and are the subject of active research. Plutonium displays phases with enormous volume differences, and both its Pauli-like magnetic susceptibility and resistivity are an order of magnitude larger than those of simple metals. Curium is also highly resistive, but its susceptibility is Curie-like at high temperatures and orders antiferromagnetically at low temperatures. The anomalous properties of the late actinides stem from the competition between itinerancy and localization of their f-shell electrons, which makes these elements strongly correlated materials. A central problem in this field is to understand the mechanism by which these conflicting tendencies are resolved in such materials. Here we identify the electronic mechanisms responsible for the anomalous behaviour of late actinides, revisiting the concept of valence using a theoretical approach that treats magnetism, Kondo screening, atomic multiplet effects and crystal field splitting on the same footing. We find that the ground state in plutonium is a quantum superposition of two distinct atomic valences, whereas curium settles into a magnetically ordered single valence state at low temperatures. The f(7) configuration of curium is contrasted with the multiple valences of the plutonium ground state, which we characterize by a valence histogram. The balance between the Kondo screening and magnetism is controlled by the competition between spin-orbit coupling, the strength of atomic multiplets and the degree of itinerancy. Our approach highlights the electronic origin of the bonding anomalies in plutonium, and can be applied to predict generalized valences and the presence or absence of magnetism in other compounds starting from first principles.

Size dependence of the structures and energetic and electronic properties of gold clusters

Abstract: The structures and stabilities of gold clusters with up to 14 atoms have been determined by density-functional theory. The structure optimizations and frequency analysis are performed with the Perdew-Wang 1991 gradient-corrected functional combined with the effective core potential and corresponding valence basis set (LANL2DZ). The turnover point from two-dimensional to three-dimensional geometry for gold clusters occurs at Au12. The energetic and electronic properties of the small gold clusters are strongly dependent on sizes and structures, which are in good agreement with experiment and other theoretical calculations. The even-odd oscillation in cluster stability and electronic properties predicted that the clusters with even numbers of atoms were more stable than the neighboring clusters with odd numbers of atoms. The stability and electronic structure properties of gold clusters are also characterized by the maximum hardness principle of chemical reactivity and minimum polarizability principle.

Photo-induced valence tautomerism in Co complexes.

Abstract: A number of photofunctional molecular compounds have been developed recently. Typical examples of these are phototunable valence tautomeric compounds, which are now attracting great attention. When the charge-transfer bands of some Co valence tautomeric compounds are excited at low temperature, metastable redox isomers can be created after irradiation. The lifetimes of the metastable states can be more than several hours. These transformations can involve changes in the magnetic properties of the compounds, as well as their color. Hence, these compounds can be regarded as novel photomagnetic materials. The photoresponsive behaviors of these valence tautomeric compounds are similar to those of spin-crossover complexes (light-induced excited spin-state trapping effects).

Cation−Anion Interactions and Polar Structures in the Solid State

Abstract: Complicated structures where oxygen and fluorine are found together in one framework, where deviations from Pauling's second crystal rule (PSCR) are expected, often result in structures with important physical properties. The [NbOF5]2- anion and therefore all the individual Nb-O and Nb-F bonds are ordered in noncentrosymmetric KNaNbOF5 and centrosymmetric CsNaNbOF5. The Na/K- and Na/Cs-O/F interactions in these phases, in particular the expected deviations from PSCR and the bond valence model, reveal the essential role of the small potassium cations in the acentric packing of the [NbOF5]2- anion. KNaNbOF5 crystallizes in the orthorhombic and polar space group Pna21 (No. 33) with lattice constants a = 11.8653(11) A, b = 5.8826(6) A, c = 8.1258(8) A, and Z = 4, while CsNaNbOF5 crystallizes in the orthorhombic space group Pbcn (No. 60) with lattice constants a = 8.3155(7), b = 13.3176(11), c = 11.1314(9), and Z = 8.

Electronic structure of hybrid interfaces for polymer-based electronics

Abstract: The fundamentals of the energy level alignment at anode and cathode electrodes in organic electronics are described. We focus on two different models that treat weakly interacting organic/metal (and organic/organic) interfaces: the induced density of interfacial states model and the so-called integer charge transfer model. The two models are compared and evaluated, mainly using photoelectron spectroscopy data of the energy level alignment of conjugated polymers and molecules at various organic/metal and organic/organic interfaces. We show that two different alignment regimes are generally observed: (i) vacuum level alignment, which corresponds to the lack of vacuum level offsets (Schottky-Mott limit) and hence the lack of charge transfer across the interface, and (ii) Fermi level pinning where the resulting work function of an organic/metal and organic/organic bilayer is independent of the substrate work function and an interface dipole is formed due to charge transfer across the interface. We argue that the experimental results are best described by the integer charge transfer model which predicts the vacuum level alignment when the substrate work function is above the positive charge transfer level and below the negative charge transfer level of the conjugated material. The model further predicts Fermi level pinning to the positive (negative) charge transfer level when the substrate work function is below (above) the positive (negative) charge transfer level. The nature of the integer charge transfer levels depend on the materials system: for conjugated large molecules and polymers, the integer charge transfer states are polarons or bipolarons; for small molecules' highest occupied and lowest unoccupied molecular orbitals and for crystalline systems, the relevant levels are the valence and conduction band edges. Finally, limits and further improvements to the integer charge transfer model are discussed as well as the impact on device design.

Systematically convergent correlation consistent basis sets for molecular core-valence correlation effects: the third-row atoms gallium through krypton.

Abstract: The family of correlation consistent polarized valence basis sets has been extended in order to account for core−core and core−valence correlation effects within the third-row, main group atoms gallium through krypton. Construction of the basis sets is similar to that of the atoms boron through argon, where either the difference between core-correlated and valence-only correlation energies were calculated via configuration interaction (CISD) computations on the ground electronic states of the atoms (named cc-pCVnZ) or the sets were optimized with respect to the core−valence correlation energy and a small weight of core−core correlation energy (cc-pwCVnZ). Due to the correlation of 3d orbitals, added shells of higher angular momentum exponents compared to the valence sets are necessary to describe the core region. The pattern of added core-correlating functions is (1s1p1d1f) for double-ζ, (2s2p2d2f1g) for triple-ζ, (3s3p3d3f2g1h) for quadruple-ζ, and (4s4p4d4f3g2h1i) for quintuple-ζ. Atomic and molecular r...

Valence bond theory for chemical dynamics.

Abstract: This essay provides a perspective on several issues in valence bond theory: the physical significance of semilocal bonding orbitals, the capability of valence bond concepts to explain systems with multireferences character, the use of valence bond theory to provide analytical representations of potential energy surfaces for chemical dynamics by the method of semiempirical valence bond potential energy surfaces (an early example of specific reaction parameters), by multiconfiguration molecular mechanics, by the combined valence bond-molecular mechanics method, and by the use of valence bond states as coupled diabatic states for describing electronically nonadiabatic processes (photochemistry). The essay includes both ab initio and semiempirical approaches. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2007

Electronic structure and optical properties of lightweight metal hydrides

Abstract: We study the dielectric functions of the series of simple hydrides LiH, NaH, MgH2, and AlH3, and of the complex hydrides Li3AlH6, Na3AlH6, LiAlH4, NaAlH4, and Mg(AlH4)2, using first-principles density-functional theory and GW calculations. All compounds are large gap insulators with GW single-particle band gaps varying from 3.5 eV in AlH3 to 6.6 eV in LiAlH4. Despite considerable differences between the band structures and the band gaps of the various compounds, their optical responses are qualitatively similar. In most of the spectra the optical absorption rises sharply above 6 eV and has a strong peak around 8 eV. The quantitative differences in the optical spectra are interpreted in terms of the structure and the electronic structure of the compounds. In the simple hydrides the valence bands are dominated by the hydrogen atoms, whereas the conduction bands have mixed contributions from the hydrogens and the metal cations. The electronic structure of the aluminium compounds is determined mainly by aluminium hydride complexes and their mutual interactions.

Accurate evaluation of valence and low-lying Rydberg states with standard time-dependent density functional theory.

Abstract: Using a standard exchange-correlation functional, namely, PBE0, the basis set dependence of time-dependent density functional theory (TD-DFT) calculations has been explored using 33 different bases and five organic molecules as test cases. The results obtained show that this functional can provide accurate (i.e., at convergence) results for both valence and low-lying Rydberg excitations if at least one diffuse function for the heavy atoms is included in the basis set. Furthermore, these results are in fairly good agreement with the experimental data and with those delivered by other functionals specifically designed to yield correct asymptotic/long-range behavior. More generally, the PBE0 calculations show that a greater accuracy can be obtained for both Rydberg and valence excitations if they occur at energies below the epsilonHOMO + 1 eV threshold. This latter value is proposed as a thumb rule to verify the accuracy of TD-DFT/PBE0 applications.

Delta aromaticity in [Ta3O3]-.

Abstract: We report experimental and theoretical evidence of -aromaticity, which is discovered in the Ta3O3 – cluster via a combined photoelectron spectroscopy and ab initio study. Well-resolved low-lying electronic transitions are observed in the photoelectron spectra of Ta3O3 – and are compared with ab initio calculations, which show that the Ta3O3 – cluster possesses a planar D3h triangular structure. Chemical bonding analyses reveal that among the five valence molecular orbitals responsible for the multi-center metal-metal bonding there is a completely bonding delta and orbital from the 5d atomic orbitals of Ta. The totally delocalized multi-center bond renders -aromaticity for Ta3O3 – and represents a new mode of chemical bonding. Ta3O3 – is the first -aromatic molecule confirmed experimentally and theoretically, suggesting that -aromaticity may exist in many multi-nuclear, low oxidation state transition-metal compounds.

Reliable predictions of the thermochemistry of boron-nitrogen hydrogen storage compounds: BxNxHy, x = 2, 3.

Abstract: Thermochemical data calculated using ab initio molecular orbital theory are reported for 16 BxNxHy compounds with x = 2, 3 and y ≥ 2x. Accurate gas-phase heats of formation were obtained using coupled cluster with single and double excitations and perturbative triples (CCSD(T)) valence electron calculations extrapolated to the complete basis set (CBS) limit with additional corrections including core/valence, scalar relativistic, and spin−orbit corrections to predict the atomization energies and scaled harmonic frequencies to correct for zero point and thermal energies and estimate entropies. Computationally cheaper calculations were also performed using the G3MP2 and G3B3 variants of the Gaussian 03 method, as well as density functional theory (DFT) using the B3LYP functional. The G3MP2 heats of formation are too positive by up to ∼6 kcal/mol as compared with CCSD(T)/CBS values. The more expensive G3B3 method predicts heats of formation that are too negative as compared with the CCSD(T)/CBS values by up t...

δ Aromaticity in [Ta3O3]−

Abstract: The concept of aromaticity was introduced into organic chemistry to describe delocalized p bonding in planar, cyclic, and conjugate molecules possessing (4n+2) p electrons. In recent years, this concept has been advanced into main-group molecules including organometallic compounds with cyclic cores of metal atoms and, in particular, all-metal clusters. It has been shown that main-group clusters may exhibit multiple aromaticity (s and p), multiple antiaromaticity (s and p), and conflicting aromaticity (s aromaticity and p antiaromaticity or s antiaromaticity and p aromaticity). Here, we report experimental and theoretical evidence of d aromaticity, which is only possible in transition-metal systems. It is discovered in the [Ta3O3] cluster through combined photoelectron spectroscopy and ab initio studies. Well-resolved low-lying electronic transitions are observed in the photoelectron spectra of [Ta3O3] and are compared with ab initio calculations, which show that the [Ta3O3] cluster has a planar D3h triangular structure. Chemical-bonding analyses reveal that among the five valence molecular orbitals involved in the multicenter metal–metal bonding, there is a completely bonding d and p orbital formed from the 5d atomic orbitals of Ta. The totally delocalized multicenter d bond renders d aromaticity for [Ta3O3] and represents a new mode of chemical bonding. [Ta3O3] is the first d-aromatic molecule confirmed experimentally and theoretically, which suggests that d aromaticity may exist in many multinuclear, lowoxidation-state transition-metal compounds. In 1964, Cotton and co-workers published a milestone work on K2[Re2Cl8]·2H2O, [7] in which they showed the presence of a new type of chemical bond—a d bond between the two Re atoms. Since then, a branch of inorganic chemistry has been developed that involves multiple metal–metal bonding with bond orders higher than three, the maximum allowed for main-group systems. Power and co-workers recently reported the synthesis of a Cr2 compound with a quintuple bond (spd) between the two Cr atoms. This work, along with recent quantum chemical studies of multiple bonds in U2 and [Re2Cl8] 2 , has generated renewed interest in multiple metal–metal bonding. The presence of d bonds between two transition-metal atoms suggests that multicenter transition-metal species with a completely delocalized cyclic d bond may exist, thus raising the possibility of d aromaticity analogous to p or s aromaticity in main-group systems. We have been interested in understanding the electronic structure and chemical bonding of early transition-metal oxide clusters as a function of size and composition, and in using them as potential molecular models for oxide catalysts. During our investigation of tantalum oxide clusters, we found the presence of d aromaticity in the [Ta3O3] cluster, in which each Ta atom is in a low oxidation state of Ta and still possesses three electrons for Ta–Ta bonding. The experiment was conducted by using a magneticbottle-type photoelectron spectroscopy apparatus equipped with a laser vaporization cluster source. [TamOn] clusters with various compositions were produced by laser vaporization of a pure tantalum disk target in the presence of a helium carrier gas seeded with O2, and were size-separated by time-of-flight mass spectrometry. The [Ta3O3] species was mass-selected and decelerated before photodetachment by a pulsed laser beam. Photoelectron spectra were obtained at two relatively high photon energies, 193 nm (6.424 eV) and 157 nm (7.866 eV), to guarantee access to all valence electronic transitions (Figure 1). Three well-resolved bands (X, A, and B) were observed at the lower-binding-energy side. The X band is much more intense and shows a discernible splitting at 193 nm (Figure 1a). Surprisingly, no well-defined electronic transitions were observed beyond 3.7 eV, where continuous signals were present, probably as a result of multielectron transitions. The vertical detachment energies (VDEs) of the observed transitions at the low-bindingenergy side are given in Table 1, where they are compared with theoretical calculations by two different methods. [*] Dr. H. J. Zhai, Prof. Dr. L. S. Wang Department of Physics Washington State University 2710 University Drive, Richland, WA 99354 (USA) and Chemical & Materials Sciences Division Pacific Northwest National Laboratory MS K8–88, P.O. Box 999, Richland, WA 99352 (USA) Fax: (+1)509-376-6066 E-mail: ls.wang@pnl.gov

Investigation of the amino acids glycine, proline, and methionine by photoemission spectroscopy.

Abstract: The valence and core level photoelectron spectra of glycine, proline, and methionine in the gas phase have been investigated by VUV and soft X-ray radiation. The outer valence band photoemission spectra are similar to previously reported He I spectra, although relative peak intensities are different due to the different photon energy. We extended the spectral range to include the inner valence region. The carbon, nitrogen, and oxygen 1s as well as the sulfur 2p core level spectra of these amino acids have been measured and the states identified. Valence band spectra of proline have been recorded as a function of temperature, and they provide information about the relative populations of the lowest energy conformers.

Photoelectron spectrum of isolated ion-pairs in ionic liquid vapor.

Abstract: The gas-phase valence binding energy spectrum of isolated ion-pairs of the commonly used [1-ethyl-3-methylimidazolium][bis(trifluoromethylsulfonyl)imide)] room-temperature ionic liquid is obtained by photoionization of a molecular beam of ionic liquid vapor by extreme ultraviolet light. The isolated ion-pair nature of the ionic liquid vapor is corroborated by single photon ionization mass spectroscopy, complemented by computed vaporization energetics of ion-pairs and clusters of ion-pairs. The valence binding energy spectrum of the isolated ion-pairs is discussed in comparison with available liquid-phase data and theoretical density functional theory calculations.

Electronic structure of copper phthalocyanine: An experimental and theoretical study of occupied and unoccupied levels

Abstract: An experimental and theoretical study of the electronic structure of copper phthalocyanine (CuPc) molecule is presented. We performed x-ray photoemission spectroscopy (XPS) and photoabsorption [x-ray absorption near-edge structure (XANES)] gas phase experiments and we compared the results with self-consistent field, density functional theory (DFT), and static-exchange theoretical calculations. In addition, ultraviolet photoelectron spectra (UPS) allowed disentangling several outer molecular orbitals. A detailed study of the two highest occupied orbitals (having a(1u) and b(1g) symmetries) is presented: the high energy resolution available for UPS measurements allowed resolving an extra feature assigned to vibrational stretching in the pyrrole rings. This observation, together with the computed DFT electron density distributions of the outer valence orbitals, suggests that the a(1u) orbital (the highest occupied molecular orbital) is mainly localized on the carbon atoms of pyrrole rings and it is doubly occupied, while the b(1g) orbital, singly occupied, is mainly localized on the Cu atom. Ab initio calculations of XPS and XANES spectra at carbon K edge of CuPc are also presented. The comparison between experiment and theory revealed that, in spite of being formally not equivalent, carbon atoms of the benzene rings experience a similar electronic environment. Carbon K-edge absorption spectra were interpreted in terms of different contributions coming from chemically shifted C 1s orbitals of the nonequivalent carbon atoms on the inner ring of the molecule formed by the sequence of CN bonds and on the benzene rings, respectively, and also in terms of different electronic distributions of the excited lowest unoccupied molecular orbital (LUMO) and LUMO+1. In particular, the degenerate LUMO appears to be mostly localized on the inner pyrrole ring.

New trend of superconductivity in strongly correlated electron systems

Abstract: A new trend in the theory of heavy-fermion superconductivity is reviewed, paying attention to the role of critical valence fluctuations (CVFs). First of all, the trends of superconducting mechanisms of electronic (repulsive) origin are briefly summarized. Secondly, it is discussed that the pressure induced enhancement of the superconducting transition temperature Tc and associated anomalies observed in CeCu2(Ge, Si)2 can be understood in a unified way by a single assumption that the systems are subject to the critical valence transition of the Ce ion by the pressure tuning. Thirdly, it is shown that the extended periodic Anderson model with the repulsion between f and conduction electrons really has the potentiality of such a sharp valence transition of Ce ions containing f electrons. In particular, enhancement of the residual resistivity, the Sommerfeld constant and Tc ,a ndT -linear resistivity, are shown to be concluded from this model. Fourthly, the locality of this CVF is briefly discussed. Fifthly, it is argued that CVF is enhanced by the magnetic field when the system is located near the critical point of valence transition in relation to the case of CeCu6. Finally, it is argued that the CVF mechanism is a rather universal one which may be related to the physics of Ce115 and Pu115 compounds as well. (Some figures in this article are in colour only in the electronic version)

The role of radial nodes of atomic orbitals for chemical bonding and the periodic table.

Abstract: The role of radial nodes, or of their absence, in valence orbitals for chemical bonding and periodic trends is discussed from a unified viewpoint In particular, we emphasize the special role of the absence of a radial node whenever a shell with angular quantum number l is occupied for the first time (lack of “primogenic repulsion”), as with the 1s, 2p, 3d, and 4f shells Although the consequences of the very compact 2p shell (eg good isovalent hybridization, multiple bonding, high electronegativity, lone-pair repulsion, octet rule) are relatively well known, it seems that some of the aspects of the very compact 3d shell in transition-metal chemistry are less well appreciated, eg, the often weakened and stretched bonds at equilibrium structure, the frequently colored complexes, and the importance of nondynamical electron-correlation effects in bonding © 2006 Wiley Periodicals, Inc J Comput Chem 28: 320–325, 2006

Selective Adsorption of Ions with Different Diameter and Valence at Highly Charged Interfaces

Abstract: Monte Carlo simulation and density functional theoretical (DFT) results are reported for the selective adsorption of two competing cationic species at a highly charged planar interface. The two cations differ in both their diameter (2 and 4.25 A) and valence (mono- and divalent). Our results show that in general the smaller or the divalent cation is preferentially adsorbed at the electrode. In the case when the divalent ion is larger and the monovalent ion is smaller, we find a competitive situation: at lower surface charges the electrostatic advantage of the divalent ions dominates, whereas at higher surface charges the entropic advantage of the small ions dominates. We show results for the excess adsorption, density profiles, and mean electrical potential in various situations where charge inversion occurs when divalent ions are present. Using the DFT decomposition of the chemical potential into various terms (e.g., ideal, electrostatic, hard sphere), we demonstrate that the competition between ionic species of different sizes and valences originates in the balance of excluded volume and electrostatic terms.