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The novel bridging ligand 3,6-bis(phenylcyanamido)-1,2,4,5-tetrazine (tdpc[H.sub.2]) and its dinuclear complex [[{Ru(ttpy)(bpy)}.sub.2] (µ-tdpc)][[P[F.sub.6]].sub.2] were prepared and characterized by elemental analysis and ¹H NMR spectroscopy. Cyclic voltammetry and vis-NIR and IR spectroelectrochemistry of [[[{Ru(ttpy)(bpy)}.sub.2](µ-tdpc)].sup.2+] showed that [[[{Ru(ttpy)(bpy)}.sub.2](µ-tdpc)].sup.3+] is a Class II mixed-valence system with metal-metal coupling of 400 [cm.sup.-1] assuming a transition dipole moment length of 21 Å. DFT calculations of [tdpc.sup.2-] suggested that the stability of the HOMO results in weak metal-metal coupling via the hole-superexchange mechanism. Key words: mixed-valence, ruthenium, spectroelectrochemistry. Resume: Le nouveau ligand pontant 3,6-bis(phenylcyanamido)-1,2,4,5-tetrazine (tdpc[H.sub.2]) et son complexe dinucleaire [[[{Ru(ttpy)(bpy)}.sub.2] (µ-tdpc)][[P[F.sub.6]].sub.2] ont ete synthetises et caracterises par analyse elementaire et spectroscopie RMN ¹H. Les analyses par voltamperometrie cyclique et par spectroelectrochimie visible-proche infrarouge et infrarouge de l'ion [[[{Ru(ttpy)(bpy)}.sub.2](µ-tdpc)].sup.2+] ont revele que l'ion [[[{Ru(ttpy)(bpy)}.sub.2](µ-tdpc)].sup.3+] est un systeme a valence mixte de classe II qui presente uncouplage metal-metal a 400 [cm.sup.-1], si l'on considere un moment dipolaire de transition d'une longueur de 21 Å. Les calculs de la theorie de la fontionnelle de la densite du ligand [tdpc.sup.2-] semblent indiquer que la stabilite de l'orbitale moleculaire haute occupee est a l'origine du faible couplage metal-metal qui se produit par un mecanisme de superechange par transfert de trou. [Traduit par la Redaction] Mots-cles: valence mixte, ruthenium, spectroelectrochimie., Introduction The nature of the bridging ligand plays a significant role in determining the magnitude of metal-metal coupling in mixed-valence complexes. For mixed-valence complexes with π-acid bridging ligands such as [...]

The density of defect states near the valence band of the hole blocking layer (commonly called the n-layer) is determined by analyzing the transient dark current behavior of multilayer amorphous selenium (a-Se) X-ray image detectors. The previous transient dark current model (Mahmood et al. Appl. Phys. Lett. 92, 223506 (2008)) is modified and compared with recently published experimental transient dark currents on commercial n-i-p and cold deposited n-i a-Se detector structures to determine the energy distributed deep defect densities in these two types of n-layer. The peak defect state exists at 0.75 and 0.78 eV from the valence band mobility edge in alkaline doped and cold deposited n-layers, respectively. The peak trap density in these n-layers varies in the range of 5 x [10.sup.16] - 5 x [10.sup.17] [cm.sup.-3][eV.sup.-1]. The energy depths of the trap centers should be ~(0.75-0.8) eV from the valence band mobility edge for a requirement of less transient time to reach a plateau. The shallower trap levels are unable to retain sufficient trapped charge to reduce the dark current, and the deeper trap centers create longer transient times to reach a steady level of dark current. PACS Nos.: 42.79.Pw, 71.55.Jv, 72.20.Jv, 73.40.Sx. Nous determinons la densite d'etats defauts pres de la bande de valence de la couche bloquant les trous (generalement appelee couche n) par l'analyse du comportement du courant d'obscurite transitoire d'un detecteur a multicouches de selenium amorphe (a-Se) pour les images X. Nous modifions le precedent modele de courant d'obscurite transitoire (Mahmood et al. Appl. Phys. Lett. 92, 223506 (2008)) et comparons avec les resultats recemment publies sur le courant d'obscurite transitoire dans des detecteurs (a-Se), commerciaux n-i-p et depose a froid n-i, afin de determiner les densites de defauts profonds distribues en energie dans ces deux types de couche n. Le pic de l'etat defaut se retrouve a 0.75 et 0.78 eV du seuil de mobilite de la bande de valence dans les couches n a dopage alcalin ou deposees a froid respectivement. La densite de pics pieges dans ces couches n varie entre 5 x [10.sup.16] et 5 x [10.sup.17] [cm.sup.-3][eV.sup.-1]. La profondeur en energie du centre de ces pieges devrait etre a ~(0.75-0.8) eV du seuil de mobilite de la bande de valence si on demande moins de temps transitoire pour rejoindre un plateau. Les pieges moins profonds sont incapables de retenir assez de charges pour reduire le courant d'obscurite et les pieges plus profonds generent des temps transitoires plus longs pour rejoindre un niveau stationnaire de courant d'obscurite. [Traduit par la Redaction], 1. Introduction Amorphous selenium (a-Se) based detector structures are increasingly used in various applications, such as X-ray image detectors [1], UV sensitive photodetectors [2], and avalanche solid state imaging devices [...]

We have calculated the states associated with valence alternation pair defects in a Te-rich amorphous GeTe random network. Two-fold Te sites and four-fold Ge sites are found to introduce states that are resonant with the valence and conduction bands, respectively. The system has a band gap, but these defect states are in the same energy range as those found by modulated photocurrent experiments. The absence of electron spin resonance signals is still not explained. PACS Nos.: 71.15.Mb, 71.23.Cq, 73.61.Jc. Nous calculons les etats associes a des defauts de paires de valence alternee dans un reseau aleatoire de GeTe amorphe, riche en Te. Nous trouvons que les etats doubles de Te et les etats quadruples du Te introduisent des etats qui sont resonants avec la bande de valence et la bande de conduction respectivement. Le systeme a une bande interdite, mais ces etats defauts/lacunaires sont dans le meme domaine d'energie que ceux trouves par experience de photo-courant module. L'absence de signal de resonance de spin electronique n'est toujours pas eclaircie. [Traduit par la Redaction], Introduction Phase change materials based on Ge, Sb, and Te alloys are used as the optical memory material in rewritable DVD (RW-DVD) [1]. This relies on the contrast in optical [...]

The EPR parameters of a series of dinuclear manganese(III,IV) complexes with mono(μ-oxo), bis(μ-oxo), (μ-oxo)(μ-carboxylato), bis(μ-oxo)(μ-carboxylato), and (μ-oxo)bis(μ-carboxylato) bridges were studied by broken-symmetry density functional (DFT) methods. The influence of the exchange-correlation functional on the agreement with experiment has been evaluated systematically for g tensors; [sup.55.Mn], [sup.14.N], and [sup.1.H] hyperfine coupling tensors; and Heisenberg exchange couplings. [sup.14.N] and [sup.1.H] hyperfine couplings, [sup.55.Mn] hyperfine anisotropies, g tensors, and exchange couplings are well described by hybrid functionals with moderate exact-exchange admixtures such as B3LYP. The isotropic [sup.55.Mn] hyperfine couplings require larger exact-exchange admixtures. However, the errors of the B3LYP calculations are systematic and may be corrected by a constant scaling factor, providing good predictive power for a wide range of EPR parameters with broken-symmetry DFT and standard functionals. The influence of terminal and bridging ligands on structure, spin-density distributions, and EPR parameters are evaluated systematically. Computed hyperfine and g tensors are not covariant to each other. This may have consequences for spectra simulations. The nature of the broken-symmetry state and the origin of its spin contamination were analyzed by an expansion into restricted determinants, based on paired orbitals. Key words: broken-symmetry density functional theory, electron paramagnetic resonance, g tensor, Heisenberg exchange coupling constant, hyperfine tensor, dinuclear manganese complex, paired orbital analysis. Faisant appel aux methodes de la theorie de la fonctionnelle de la densite (TFD) a symetrie brisee, on a etudie les parametres de resonance paramagnetique electronique (RPE) d'une serie de complexes dinucleaires du manganese(III,IV) comportant des ponts mono(μ-oxo), bis(μ-oxo), (μ-oxo)(μ-carboxylato), bis(μ-oxo)(μ-carboxylato) et (μ-oxo)bis(μ-carboxylato). On a evalue d'une facon systematique l'influence de la fonctionnelle d'echange de correlation sur l'accord entre les valeurs experimentales et calculees des tenseurs-g, des tenseurs de couplage hyperfin du [sup.55.Mn], du [sup.14.N], et du [sup.1.H] ainsi que des couplages d'echange d'Heisenberg. Les couplages hyperfins du [sup.14.N] et du [sup.1.H] hyperfine, les anisotropies hyperfines du [sup.55.Mn], les tenseurs-g et les couplages d'echange sont bien decrits par les fonctionnelles hybrides avec un melange d'echange exact mode re du type B3LYP. Les couplages hyperfins isotrope du [sup.55.Mn] necessitent des melanges d'echange exact plus importants. Toutefois les erreurs decoulant d'une utilisation de calculs B3LYP sont systematiques et peuvent etre corrigees par un facteur d'echelle constant, ce qui permet de profiter de bons pouvoirs de prediction pour une grande variete de parametres RPE avec la TFD a symetrie brisees et des fonctionnelles standard. On a evalue d'une facon systematique l'influence des ligands terminaux et ceux agissant comme pont sur la structure, les distributions de la densite de spin et les parametres RPE. Les valeurs calculees pour les tenseurs-g et les proprietes hyperfines ne sont pas covariantes l'une par rapport a l'autre. Cette situation peut avoir des consequences pour les simulations de spectres. On a analyse la nature de l'etat de la symetrie brisee et l'origine de sa contamination du spin par une expansion dans des determinants restreints, basee sur des orbitales en paires. Mots-cles : theorie de la fonctionnelle de densite a symetrie brisee, resonance paramagnetique electronique, tenseur-g, constante de couplage d'echange d'Heisenberg, tenseur hyperfin, complexe dinucleaire du manganese, analyse d'orbitales par paires. [Traduit par la Redaction], Introduction The structure and electronic properties of multinuclear manganese complexes are of substantial importance in such diverse fields as molecular magnetism (1) and bioinorganic chemistry. Prominent examples for the latter [...]

It is theorized that multivalent interaction can result in better affinity and selectivity than monovalent interaction in the design of high-performance ligands. Accordingly, biomolecular engineers are increasingly taking advantage of multivalent interactions to fabricate novel molecular assemblies, resulting in new functions for ligands or enhanced performance of existing ligands. Substantial efforts have been expended in using small molecules or epitopes of antibodies for designing multifunctional or betterperforming ligands. However, few attempts to use nucleic acid aptamers as functional domains have been reported. In this study, we explore the design of bivalent nucleic acid ligands by using thrombin and its aptamers as the model by which to evaluate its functions. By assembling two thrombin-binding aptamers with optimized design parameters, this assembly has resulted in the successful development of a nucleic acid-based high-performance bivalent protein inhibitor. Our experimentation proved (11 that the simultaneous binding of two aptamers after linkage achieved 16.6-fold better inhibition efficiency than binding of the monovalent ligand and (ii) that such an improvement originated from changes in the kinetics of the binding interactions, with a [k.sub.off] rate ~1/50 as fast. In addition, the newly generated aptamer assembly is an excellent anticoagulant reagent when tested with different samples. Because this optimized ligand design offers a simple and noninvasive means of accomplishing higher performance from known functional aptamers, it holds promise as a potent antithrombin agent in the treatment of various diseases related to abnormal thrombin activities. anticogulation | aptamers | multivalent binding | thrombin

A general interpretation is provided for the valency interaction formula (VIF), which applies for VIF pictures as one-electron Hamiltonian operators as well as VIF pictures as one-electron density operators. The VIF method has provided the accurate VIF pictures for hydrocarbons and it can also be used to predict thermal barriers to chemical reactions.

Density functional theory (DFT)/ab initio/symmetry adapted cluster-configuration interaction (SAC-CI) techniques are used to investigate the oxyallyl substructure and the near-infrared (NIR) transition. The results have shown that the mixture of an intermediate diradical character and some zwitterionic character in the molecule along with a large coupling between these two valence band (VB) resonance forms, are responsible for large [gamma] values.

The synthesis and properties of metal--metal (MM) quadruply bonded compounds of molybdenum and tungsten in liquid crystalline phases are described. Covalently linked MM quadruply bonded complexes, dimers of dimers, are shown to give rise to mixed valence complexes upon oxidation and the degree of electron delocalization is shown to be correlated with the [M.sub.2][delta] bridge [pi] interactions. Class III behavior or complete delocalization is observed to [M.sub.2] to [M.sub.2] distances of up to 14 [Angstrom]. When the [M.sub.2] unit is attached to oligothiophenes via a carboxylate tether, the photophysical properties of the thienyl units are greatly modified due to [M.sub.2][delta]-thienyl [pi] conjugation and spin--orbit coupling. This leads to long-lived emissive states and electroluminescence in solid-state devices. mesogens | mixed valency

Electron energy loss spectroscopy (EELS) was used with scanning transmission electron microscopy (STEM) to quantify the average Mn valence in natural minerals at the nanometer scale. A method was developed to calibrate the energy-loss scale accurately, providing a comparison between STEM-EELS and the X-ray absorption spectroscopy methods that investigate the L-edge chemical shift as Mn valence changes. The chemical-shift measurements were consistent with data reported by previous researchers from both X-ray and electron energy-loss spectroscopy. The [L.sub.3]/[L.sub.2] white-line intensity ratios also were consistent with previous work. A calibration curve for Mn valence was produced using the [L.sub.3]/[L.sub.2] white-line intensity ratios from measurements of synthetic standards. The average Mn valence was determined because it is not possible to distinguish [Mn.sup.3+] from mixtures of [Mn.sup.2+] and [Mn.sup.4+] using either method. The white-line intensity method was implemented in automated software that allows for rapid processing of point spectra, and I-D and 2-D spectrum images. Point analyses of two natural pyrolusite samples indicated a Mn valence of 4.0, and point analyses of romanechite and manganite gave values of 3.8 and 3.4, respectively. An interface between braunite and bementite was used to illustrate 1-D and 2-D spectrum-imaging capabilities. The measured valence of Mn in the braunite and bementite was 2.9 and 2.0, respectively; both consistent with theoretical values. The braunitebementite sample demonstrated the heterogeneity of Mn valence common to natural minerals and the advantages of acquiring quantitative valence information in a known spatial context. Keywords: Electron microscopy, electron energy loss spectroscopy (EELS), scanning transmission electron microscopy (STEM), spectrum imaging, valence mapping, manganese, Mn valence, pyrolusite, manganite, romanechite, braunite, bementite

A series of di-copper(I) complexes has been prepared via the reaction of copper(I) tetrafluoroborate, 2,6-diformylpyridine, 8-aminoquinoline, and a series of aliphatic diamines and 4-substituted anilines. To avoid a 'valence-frustrated' state, involving a mismatch between the number of ligand donor atoms and the number of metal acceptor sites, the product structures formed selectively: One of the formyl groups of the diformylpyridine reacted specifically with the aminoquinoline, whereas the other formyl group reacted with the diamine or aniline. The observed selectivity was demonstrated to be thermodynamic in nature: When two dicopper complexes that were stable yet 'valence-frustrated' were mixed, an imine metathesis reaction was observed to occur spontaneously to generate a 'valence-satisfied' structure. In addition to control over the constitution of the ligands, we were able to exercise control over their relative orientations within the complex. Diamines exclusively gave structures in which the ligand exhibited a head-to-head orientation along the copper-copper axis to avoid stretching. Anilines gave predominantly head-to-tail structures, with the proportion of head-to-head isomer decreasing in complexes that incorporate more electron-deficient anilines and disappearing in less polar solvents. We also demonstrated the removal of the metals and the hydrogenation of the imine bonds to generate a molecule containing nonexchanging secondary amines, suggesting potential uses of this technique in the domain of organic synthesis. coordination chemistry | dynamic combinatorial chemistry | synthesis | self-assembly

The valence state of vanadium ([V.sup.2+], [V.sup.3+], [V.sup.4+], and [V.sup.5+]) is highly sensitive to variations in redox conditions of basaltic magmas. Differences in valence state will influence its partitioning behavior between minerals and basaltic liquid. Using partitioning behavior of V between olivine and basaltic liquid precisely calibrated for martian basalts, we determined the oxidation state of a primitive (olivine-rich, high Mg no.) martian basalt (Y980459) near its liquidus. The behavior of V in the olivine from other martian olivine-phyric basalts (SaU005, DaG476, and NWA1110) was documented. The combination of oxidation state and incompatible-element characteristics determined from early olivine indicates that correlations among geochemical characteristics such as [fo.sub.2], LREE/HREE, initial [sup.87]Sr/[sup.86]Sr, and initial [[epsilon].sub.Nd] observed in many martian basalts is also a fundamental characteristic of these primitive magmas. These observations are interpreted as indicating that the mantle sources for these magmas have a limited variation in [fo.sub.2] from IW to IW + 1 and are incompatible-element depleted. Moreover, these mantle-derived magmas assimilated a more oxidizing (>IW+3), incompatible-element enriched, lower-crustal component as they ponded at the base of the martian crust. Keywords: Shergottite, olivine, vanadium, valence state partitioning

An automated method for systematic hierarchical fragmentation of large molecules is presented utilizing isodesmic reactions. It is shown that by taking appropriate linear combination of fragment energies, it is possible to obtain highly accurate total energies when compared to total energy of full molecule.

We have undertaken a detailed analysis of the X-ray photoelectron spectra obtained from the three polymorphs of [Al.sub.2]Si[O.sub.5]; andalusite, sillimanite, and kyanite. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each polymorph. The spectra for Si(2p) for all three polymorphs are nearly identical, consistent with the fact that all the Si atoms are in 4-fold (tetrahedral) coordination, whereas the binding energies, peak shapes, and peak widths for Al(2p) vary depending on the type of polymorph. The upper-valence band for all three polymorphs is characterized by four main features derived from O(2p), Al(3s), Al(2p), Si(3s), and Si(3p), and the differences in their contributions are observed. The density of state of the [Al.sub.2]Si[O.sub.5] polymorphs is relatively featureless compared to those observed from [alpha]-Si[O.sub.2] and [alpha]-[Al.sub.2][O.sub.3], suggesting that the orbital overlaps span a greater range in energy. The observed band gap energy for [Al.sub.2]Si[O.sub.5] (sillimanite) was ~9.1 eV, a value in between those for [alpha]-Si[O.sub.2] (~8.6eV) and [alpha] - [Al.sub.2][O.sub.3] (-9.6eV). The conduction band feature of [Al.sub.2]Si[O.sub.5] was experimentally compared to those of [alpha]-Si[O.sub.2] and [alpha]-[Al.sub.2][O.sub.3], and shown that it is indeed intermediate between the [alpha]-Si[O.sub.2] and [alpha]-[Al.sub.2][O.sub.3] phases. Keywords: [Al.sub.2]Si[O.sub.5] polymorphs, X-ray photoelectron spectroscopy, low electron energy loss spectroscopy, valence- and conduction band structures, andalusite, sillimanite, kyanite

A method for expressing the wave function in terms of Lewis structures is proposed and tested on the allyl cation. This method gives results that are consistent with the breathing orbital valence bond (BOVB) calculations for the energy and with the natural resonance theory (NRT) analysis for the weights of the Lewis structures.

The evaluation of ab inito QM/MM free-energy surfaces and activation free energies of reactions in enzymes and in solutions is carried out by using the empirical valence bond (EVB) method as a reference potential and then using linear response approximation (LRA). This new method evaluates the free energy associated with both the substrate and the solvent and helps in obtaining a free-energy barrier that reflects the solute entropy properly.

[Fe.sup.2]/[Fe.sup.3+] charge ordering in the contact layers of lamellar magnetism in the hematite-ilmenite series is not a postulate of the lamellar magnetism hypothesis. Such ordering is possible, however, and a model was suggested earlier in which contact layer [Fe.sup.3+] octahedra share faces with ilmenite layer [Ti.sup.4+] octahedra, and contact layer [Fe.sup.3+] octahedra share faces with hematite layer [Fe.sup.3+] octahedra, thus copying the shared-face configurations of ilmenite and hematite, respectively. This model and related charge-balance matters could be explored using bond-valence theory, a simplified picture of complex bonding which takes into account the relationship between bond strength and bond distance, but ignores magnetism. This has now been done, and shows that local oxygen charge satisfaction is strongly favored by a different charge-ordering scheme, in which contact layer [Fe.sup.3+] octahedra share faces with ilmenite layer [Ti.sup.4+] octahedra, and contact layer [Fe.sup.3+] octahedra share faces with hematite layer [Fe.sup.3+] octahedra. A new, more sophisticated, Monte Carlo simulation of the interface, including electrostatic and magnetic interaction parameters of cations, reported in detail elsewhere, independently shows the same charge-ordering scheme. For the cation ordered metastable ilmenite 50 composition, bond-valence theory indicates that, unlike the contact layers, the favored charge ordering scheme in Fe layers would have the same shared-face configurations as ilmenite and hematite. Keywords: Crystal chemistry, bond valence, hematite, ilmenite, magnetic properties, lamellar magnetism, order-disorder, Fe charge ordering

The perfect pairing approximation from generalized valence bond theory is formulated in an unrestricted fashion for both closed- and open-shell systems using a coupled cluster ansatz. Active electron pairs are correlated but the unpaired or radical electrons remain uncorrelated, leading to a linear number of decoupled cluster amplitudes that can be solved for analytically.

The applicability of the scaled quantum mechanical force field (SQM FF) method to the vibrational spectra of a charged molecule is studied by the example of acetate ion (CH3CO2)(super -)) in sodium acetate for which an empirical valence force field (SVFF) based on IR spectra of six isotopomers of sodium acetate. SQM calculations were done by simple structural models of sodium acetate testing using different QM methods.

The existing uncertainties concerning the beta band of NO as well as to test the adequacy of the molecular quantum defect orbital (MQDO) procedure to describe the vibronic levels of a valence state that is subject to the interaction with Rydberg state (3ppi(C(sub 2)pi)--B(super 2)pi) or of the Rydberg--valence state of NO is studied. The high ratio between the quality of results and the computational effort of this procedure is undoubtedly linked to its analyticity.

An efficient algorithm for the computation of density functional-based orbital reactivity indices, such as the orbitally resolved hardness and softness tensors, total hardness and softness, and Fukui indices, is proposed and implemented in the deMon-2003 code. The theory accounts for all valence orbitals in the computations of the hardness, thus allowing direct and relatively easy computations of the local reactivity indices.

The valence bond self-consistent field (VBSCF) calculations explain the qualitative properties of the complexes and transition structures for proton transfer processes. The VBSCF calculations also provide satisfactory analyses of the elements contributing to the barrier heights.

The computation of orbital Fukui indices was considered where all valence orbitals are taken into account. The atomic Fukui Function (FF) and atom in molecule (AIM) softness values were useful in rationalizing the regioselectivity and direction effects, whereas the indicators in orbital resolutions were needed to explain reactivity effects.

A valence bond (VB) is presented and applied to calculate the hidden excited states, 2(super 1)A(sub g) and other covalent excited states of polyenes from C4H6 to C28H30. The VBDFTs method presented enables to gain an insight into a variety of properties of various covalent states of polyene, such as excitation energies and their convergence.

The singlet-singlet electronic spectrum of the anti conformer of n-tetrasilane is studied using multiconfigurational wave functions (CASSCF), second-order perturbation theory (CASPT2), and its multi-state extension (MS-CASPT2) in conjunction with large ANO-type basis sets including Rydberg functions. The results are consistent with the available experimental data and are easily understood in terms of a simple Huckel model of sigma-delocalization.

It is demonstrated that for each of N2O and HCNO, with minimal basis sets, one-electron delocalizations of oxygen 2ppi(sub x) and 2ppi(sub y) electrons into bonding pi(sub x)(ON) and pi(sub y)(ON) LMOs are preferred energetically to electron-pair [2ppi(sub x)](super 2) - -> [pi(sub x)(ON)](super 2) and [2ppi(sub y)](super 2) - -> [pi(sub y)(ON)](super 2) and [2ppi(sub y)](super 2) - -> [pi'(sub y)(ON)](super 1)[pi'(sub y)(ON)](super 1) delocalizations.

Issues concerning a quantitative relationship (interference effect) between basis set truncation errors in MP2 energies and basis set truncation errors in CCSD(T) energies are discussed. It is clear that a new generation of model chemistry methods will no longer need significant error cancellation.

An all-electron STO-6G valence bond investigation of the source of the long, weak N-N bond of asym-N2O3 is presented. Further evidence is found to support the view that the lengthening and weakening of the N-N bond of N2O3 is linked mainly with the nature of the hybridization of the NO-moiety nitrogen atomic orbitals, and the delocalization of the odd electron of the NO2 moiety.

Density functional, broken symmetry techniques have been used to assess exchange coupling constants for di-mu-oxo-bridged copper(II) dimers with varying Cu-O-Cu bond angles. It is demonstrated that O. Kahn's antiferromagnetic term can be ferromagnetic.

Valence bond theory has been used to investigate the transition metal hydride cations TMH+. It was established that modern valence bond methods are valuable for studies involving transition metals.

An examination of the ability of a number of computational methods to forecast the vertical excitation and perhaps also symmetric relaxation energies for 14 excited electronic states of pyrazine is presented. It seems that the most reliable results are provided by B3LYP, EOM-CCSD(T) and CASPT2 methods.

Ultraviolet photoelectron spectroscopy has been used to study the work function and density of states of multiwall carbon nanotubes. Results indicate that oxidative treatments increase the work function and influence the valence bands DOS.

A study of a heterosupermolecule assembly by covalently linking a TiO (sub)2 ruthenium complex and a viologen has been investigated. Long-lived light-induced charge separation function and its potentiostatic modulation have been demonstrated.

The predictions of the two-valence bond state model for molecular nonlinear optical properties are compared with experimental data on polarizabilities. The values of micro(sub g)(super 2)alpha(sub zz)(0) and micro(g)beta(sub zzz)(0) in different solvents are calculated from the model and compared to experimental measurements for a variety of push-pull polyene molecules in different solvents. Calculations from the two-valence bond state model are shown to be in qualitative agreement with experimental results, although quantitatively the results are mixed.

The influence of doping on the host crystal structure of yttrium aluminum garnet (YAG) was investigated using x-ray photoelectron spectroscopy. The consequences of doping were observed indirectly through an analysis of the peaks and variations of the chemical shifts of the main ions forming the YAG crystal. The presence of only one independent oxygen ion was proven, even though the O(1s) spectra of all YAG systems exhibit two readily discerned peaks. This dichotomy is linked to the possible polarization of the oxygen valence electron density between aluminum and yttrium.

Research was conducted to examine the photoionization of SiBr4 in the valence shell and the Br(3d) and Si(2p) inner shell excitation/ionization regions using a time-of-flight mass spectrometer and synchrotron radiation over the range 30-133 eV. Photoelectron-photoion coincidence and photoion-photoion coincidence techniques were employed to investigate the dissociation processes of SiBr4. Results provide information on energy dissipation processes of the core-excited states.

The structural properties of a series of compounds based on Sb(sub 3) as a nuclear unit were investigated using ab initio analysis. Specifically, the suitability of the Zintl-Klemm-Busmann principle on alkali-antimony clusters of the form A(sub 3)Sb(sub 3) and A(sub 4)Sb(sub 3)(super +) (A = Na, K, Rb) was analyzed. It was found that the structure of the 18 valence electron units pertaining to bonding, geometry and stability is analogous to the respective properties of the isoelectronic ozone molecule.

The different methods for calculating the transfer integral in mixed valence compounds were evaluated. The different physical properties which influence the electron transfer such as overlap, through bond delocalization, relaxation and polarization of molecular orbutals were described. The ability of the various methods of calculations to integrate these physical effects has been evaluated.

Hartree-Fock, density functional and second-order Moller-Plesset perturbation theory were used to calculate relaxed potential energy surface scans of 1.6-methano(10)annulene and its derivatives to explore nonclassical structures conforming to a very long C1-C6 bond (~1.8 A). The findings indicate that methanofullerenes may exist and that constraints regarding the bookfolding angle were strong in methano(10)annulenes but moderate in methanofullerenes.

The magnetic properties of LaT(sub 13-x)Al(sub x) were measured using the empirical electron theory. The hybridization in various atomic states were examined. A conformity between the experimental findings and calculated data on the magnetic properties of the compounds were observed. Meanwhile, the valence electronic structures revealed a correlation between the electronic structures and magnetic properties of the samples. The electronic properties of the cobalt atoms and the magnetic properties of the whole compound were influenced by the aluminum content of LaCo(Sub 13-x)Al(sub x).

The effects of inter-diffusion on the gain, differential gain, linewidth enhancement factor and the injection current density of In(sub 0.2)Ga(sub 0.8)As-GaAs and Al(sub 0.3)Ga(sub 0.7)As-GaAs quantum-well (QW) lasers are examined theoretically. Findings reveal that the wavelengths can be shifted significantly without a tremendous increase in the operation current of the laser. Results indicate that interdiffusion can be utilized to make devices for multiwavelength applications.

A preliminary report detailing the successful realization of a two-stage covalent connection strategy in a model system is presented. A model 1-naphthoic acid bearing a peri-positioned cinnamic acid residue crystallizes as a closed H-bonded dimer. The irradiation of the crystalline solid produces an alpha-truxillate-type cyclobutane photodimer in quantitative yield. This model shows the feasibility of designing a covalent molecular connector whose orientation is encoded by its H-bonding pattern.

Spatial electron ordering and uniform mixed-valence behavior, experimentally observed in many transition and rare-earth metals and compounds with narrow d and f bands, can be interpreted in terms of a generalized Hubbard-like two-band model. In the strong interaction limit (U [approaches][infinity]) and exactly at half-filling, an effective Hamiltonian for such a model becomes strictly equivalent to an anisotropic XYZ model in an external field. The problem of forming either an 'antiferromagnetic' orbital crystal with valence density waves (VDW) or a liquid-like mixed valence state (MVS) with strong excitonic correlations is then analogous to the problem of finding the magnetization of the corresponding Heisenberg antiferromagnet. The variation of the orbital occupation number versus the energy difference between the centers of two bands and the conditions for smooth and first-order transitions at half-filling from VDW into MVS have been found using a generalized mean field approximation at large U. The observed anomalies of the valence behavior and compressibility in mixed-valence compounds can be viewed in this context as a diamagnetic response an applied external field. We demonstrate strong evidence for the existence in the ground state of a sequence of spatially modulated phases with periodic arrangement of d or f electrons (commensurate VDW) in the presence of an external field.

The long predicted existence of an excitonic condensed phase - i.e. the excitonic insulator - has first been verified experimentally under pressure and low temperature on the intermediate valent system Tm[Se.sub.0.45][Te.sub.0.55] and Tm[Se.sub.0.32][Te.sub.0.68] by Neuenschwander and Wachter and Bucher, Steiner and Wachter. Now additional compounds such as [Sm.sub.0.75][La.sub.0.25]S and YbO and YbS have been shown to fall under pressure into the category of excitonic insulators. The problem whether magnetism plays a role in the condensation will be discussed.

A new model for evaluating optical nonlinear response in quantum-well structures under conditions of quasi-thermal equilibrium incorporates Coulomb effects between carriers and valence-band mixing. The numerical solution to the problem is rendered possible by the introduction of approximations. The excitonic effects are pronounced even at room temperatures. The valence-band states are described using a variational technique which takes into account both heavy- and light-hole mixing.

Quantum Confinement and Host/Guest Chemistry: Probing a New Dimension THE EVOLUTION OF ELECTRONIC TRANSPORT MEDIA TOwards the nanosize regime has revolutionized electronic device development in terms of response times and [...]

The Valence Bond (VB) model is a physically correct model whereas the Z-model ignores the true origins of the barrier. Even if its quantitative performance appears good within a limited set of reactions, Shaikh et al. prefers to use a correct model and continue to improve its quantitative aspects.

In reply to Jan Florian's comment, it is noted that some methods actually use quantum mechanical valence bond formulas in detail, whereas other methods simply use the concept of configuration interaction with diagonal matrix elements corresponding to states with definite bonding charact er.

It has been possible to use ab initio electronic-structure methods to make a systematic theoretical analysis of the rare-earth elements and their sulphides. This approach allows both the bonding s-d electrons and the f electrons to be described on an equal basis. It was established that both elemental rare earths and their sulphides progress in a similar way as a function of atomic number. There seems to be a direct connection between the difference in the number of the less well localized f electrons and the difference in energy between the two valence states.