Distribution by Scientific Domains
Distribution within Chemistry

Kinds of Orbitals

  • antibonding orbital
  • atomic orbital
  • bond orbital
  • d orbital
  • frontier molecular orbital
  • frontier orbital
  • hybrid orbital
  • lowest unoccupied molecular orbital
  • molecular orbital
  • muffin-tin orbital
  • natural bond orbital
  • natural orbital
  • p orbital
  • slater-type orbital
  • type orbital
  • unoccupied molecular orbital

  • Terms modified by Orbitals

  • orbital analysis
  • orbital angular momentum
  • orbital basis set
  • orbital calculation
  • orbital cellulitis
  • orbital cyst
  • orbital element
  • orbital energy
  • orbital energy level
  • orbital forcing
  • orbital implant
  • orbital inclination
  • orbital interaction
  • orbital involvement
  • orbital irradiation
  • orbital margin
  • orbital method
  • orbital methods
  • orbital motion
  • orbital ordering
  • orbital parameter
  • orbital period
  • orbital population
  • orbital radius
  • orbital region
  • orbital theory
  • orbital trauma
  • orbital vessel

  • Selected Abstracts

    The 15N-CPMAS spectra of simazine and its metabolites: measurements and quantum chemical calculations

    A. E. Berns
    Summary DFT calculations are a powerful tool to support NMR studies of xenobiotics such as decomposition studies in soil. They can help interpret spectra of bound residues, for example, by predicting shifts for possible model bonds. The described bound-residue models supported the hypothesis of a free amino side chain already suspected by comparison with the experimental data of the standards. No match was found between the calculated shifts of amide bondings of the amino side chains (free or substituted) and the experimental NMR shifts of a previous study. In the present paper, first-principles quantum chemical calculations were used to support and check the interpretation of the 15N cross polarization-magic angle spinning nuclear magnetic resonance (15N-CPMAS NMR) spectra of simazine and its metabolites. Density functional theory (DFT) calculations were performed using Gaussian 03 and the nuclear magnetic shielding tensors were calculated using the Gauge-Independent Atomic Orbital (GIAO) method and B3LYP/6,311+G(2d,p) model chemistry. Good agreement was reached between the calculated and measured chemical shifts of the core nitrogens and the lactam and lactim forms of the hydroxylated metabolites could be clearly distinguished. The calculated spectra showed that these metabolites exist preferentially in the lactam form, an important fact when considering the possible interactions of such hydroxylated metabolites with the soil matrix. Although the calculated bound-residue models in the present study only partly matched the experimental data, they were nevertheless useful in helping to interpret the experimental NMR results of a previous study. To get a better match between the calculated and the measured shifts of the side-chain nitrogens the calculations need to be further developed, taking into account the influence of neighbouring molecules in the solid state. Altogether, quantum chemical calculations are very helpful in the interpretation of NMR spectra. In the future, they can also be very useful for the prediction of NMR shifts, in particular when it is not possible to measure the metabolites due to a lack of material or in cases where practical experiments cannot be conducted. [source]

    Spontaneous radiative decay rates in Ga-like ions

    E. Charro
    Abstract The analysis of forbidden lines, such as E2 and M1, in the atomic spectra emitted by certain ions is important for the study of the plasma in astrophysical objects and fusion devices. Atomic data, such as wavelengths and transition rates for 4p3/2 , 4p1/2 emission lines in the gallium sequence have been calculated with the Relativistic Quantum Defect Orbital (RQDO) method. The present results are tested by comparison with other theoretical values available in the literature. The regularity of the transition intensities along the isoelectronic sequence for both (E2 and M1) lines, as well as their relative magnitude, are also analyzed. M1 transitions were found to dominate by at least a factor of ten times, being in many cases bigger that this. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

    A biometric study of the fetal orbit and lens in normal pregnancies

    Kanchapan Sukonpan MD
    Abstract Purpose. To construct nomograms of the size of the fetal orbit and lens and to evaluate the relationships between the gestational age and the biometry of the fetal orbit and lens. Method. Six hundred two normal pregnant women were evaluated from 15 to 40 weeks of gestation. Fetal orbital and lens measurements were added to routine biometric measurements for normal fetuses. Results. A total of 595 measurements were used for analyses. A strong linear correlation was observed between gestational age and orbital diameter, orbital circumference, and orbital surface. A linear correlation was also found between gestational age and lens diameter, lens circumference, and lens surface. A linear growth function was observed between biparietal diameter and both the orbital diameter and the lens diameter. Conclusion. Orbital and lens measurements provide data that correlate with fetal growth and development. These data may also help detect fetal ocular abnormalities. © 2008 Wiley Periodicals, Inc. J Clin Ultrasound, 2009 [source]

    Linear augmented Slater-type orbital method for free standing clusters

    K. S. Kang
    Abstract We have developed a Scalable Linear Augmented Slater-Type Orbital (LASTO) method for electronic-structure calculations on free-standing atomic clusters. As with other linear methods we solve the Schrödinger equation using a mixed basis set consisting of numerical functions inside atom-centered spheres and matched onto tail functions outside. The tail functions are Slater-type orbitals, which are localized, exponentially decaying functions. To solve the Poisson equation between spheres, we use a finite difference method replacing the rapidly varying charge density inside the spheres with a smoothed density with the same multipole moments. We use multigrid techniques on the mesh, which yields the Coulomb potential on the spheres and in turn defines the potential inside via a Dirichlet problem. To solve the linear eigen-problem, we use ScaLAPACK, a well-developed package to solve large eigensystems with dense matrices. We have tested the method on small clusters of palladium. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source]

    Synthesis and vibrational analysis of N-(2,-Furyl)-Imidazole

    A. E. Ledesma
    Abstract The N-(2,-furyl)-imidazole (1) has been prepared and characterized using infrared, Raman and multidimensional nuclear magnetic resonance spectroscopies. Theoretical calculations have been carried out by employing the Density Functional Theory (DFT) method, in order to optimize the geometry of their two conformers in the gas phase and to support the assignments of the vibrational bands of 1 to their normal modes. For a complete assignment of the compound, DFT calculations were combined with Scaled Quamtum Mecanic Force Field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental one. Furthermore, Natural Bond Orbital (NBO) and topological properties by Atoms In Molecules (AIM) calculations were performed to analyze the nature and magnitude of the intramolecular interactions. The result reveals that two conformers are expected in liquid phase. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    Orbital and physical parameters of eclipsing binaries from the All-Sky Automated Survey catalogue , I. A sample of systems with components' masses between 1 and 2 M,

    K. G. He, miniak
    ABSTRACT We derive the absolute physical and orbital parameters for a sample of 18 detached eclipsing binaries from the All-Sky Automated Survey (ASAS) data base based on the available photometry and our own radial velocity (RV) measurements. The RVs are computed using spectra we collected with the 3.9-m Anglo-Australian Telescope (AAT) and its University College London Echelle Spectrograph (UCLES), and the 1.9-m Radcliffe telescope and its Grating Instrument for Radiation Analysis with a Fibre-Fed Echelle (GIRAFFE) at the South African Astronomical Observatory (SAAO). In order to obtain as precise RVs as possible, most of the systems were observed with an iodine cell available at the AAT/UCLES and/or analysed using the two-dimensional cross-correlation technique (TODCOR). The RVs were measured with TODCOR using synthetic template spectra as references. However, for two objects we used our own approach to the tomographic disentangling of the binary spectra to provide observed template spectra for the RV measurements and to improve the RV precision even more. For one of these binaries, AI Phe, we were able to the obtain an orbital solution with an RV rms of 62 and 24 m s,1 for the primary and secondary, respectively. For this system, the precision in M sin3i is 0.08 per cent. For the analysis, we used the photometry available in the ASAS data base. We combined the RV and light curves using phoebe and jktebop codes to obtain the absolute physical parameters of the systems. Having precise RVs, we were able to reach ,0.2 per cent precision (or better) in masses in several cases but in radii, due to the limited precision of the ASAS photometry, we were able to reach a precision of only 1 per cent in one case and 3,5 per cent in a few more cases. For the majority of our objects, the orbital and physical analysis is presented for the first time. [source]

    Nucleophilie des Borzentrums in Base-stabilisierten Borol-Anionen

    ANGEWANDTE CHEMIE, Issue 14 2010
    Makoto Yamashita Dr.
    Ein Stück vom ,-System: Ein nucleophiles Carben-stabilisiertes Boryl-Anion wurde durch Reduktion erzeugt (siehe Schema). Die Reaktionen dieses Boracyclus deuten auf eine Nucleophilie über das ,-Orbital des Borzentrums hin. Diese wichtige Entdeckung wird vorgestellt und vor dem Hintergrund nucleophiler Borreagentien und Boracyclen diskutiert. [source]

    Lithium-Ammoniak-Lösungen: eine molekulare Betrachtung

    ANGEWANDTE CHEMIE, Issue 44 2009
    Eva Zurek Dr.
    Abstract Dieser Aufsatz gibt eine detaillierte Molekülorbitalanalyse der Strukturen und elektronischen Eigenschaften der vielfältigen Spezies, die in Lithium-Ammoniak-Lösungen auftreten. Als Spezies mit ungerader Elektronenzahl (Dublett-Zustände) betrachten wir: e,@(NH3)n (das solvatisierte Elektron, wahrscheinlich ein dynamisches Ensemble von Molekülen), das Li(NH3)4 -Monomer und die [Li(NH3)4+,,,e,@(NH3)n]-Ionenpaare. Das 2s-Elektron des Li besetzt ein diffuses Orbital, das hauptsächlich von den niedrigsten unbesetzten Molekülorbitalen (MOs) der Ammoniakmoleküle gebildet wird. Die einfach besetzten MOs sind zwischen den Wasserstoffatomen bindend; wir bezeichnen diese stabilisierende Wechselwirkung als H H-Brücke. Im e,@(NH3)n befindet sich das Überschusselektron nicht im Zentrum des von den Ammoniakmolekülen gebildeten Käfigs. Denkbare Spezies mit zwei oder mehr schwach wechselwirkenden Elektronen bilden ebenfalls H H-Brücken. Für diese Spezies finden wir, dass die Singulett-Zustände (S=0) bei etwas tieferer Energie liegen als die Zustände mit ungepaarten Spins (S=1, 2,). TD-DFT-Rechnungen an verschiedenen Ionenpaaren zeigen, dass die drei intensivsten elektronischen Anregungen von den Übergängen aus dem SOMO (mit s-Pseudosymmetrie) in die niedrigsten p-artigen Niveaus stammen. Die optischen Absorptionsspektren sind weitgehend unabhängig vom Metall, und wir können die in das Sichtbare reichende Absorptionsflanke gut erklären. Letztere verursacht die "schöne blaue Farbe", die Sir Humphry Davy vor 200 Jahren erstmals beobachtete. [source]

    Naphthalene-Mediated Electronic Communication in Tetrakis(imino)pyracene Complexes,

    ANGEWANDTE CHEMIE, Issue 44 2009
    Ein heißer Tip: Difunktionelle Tetrakis(imino)pyracen(Tip)-Liganden erfahren durch Kaliummetall, Germaniumdichlorid oder Decamethyleuropocen an beiden Diiminfunktionalitäten eine Einelektronenreduktion (siehe Schema, Cp*=C5Me5). Die übertragenen Elektronen besetzen gemeinsam ein über beide Diazabutadieneinheiten und die Naphthalinbrücke delokalisiertes Orbital. [source]

    Imaging of Orbital and Visual Pathway Pathology

    Elna-Marie Larsson
    No abstract is available for this article. [source]

    Crystal growth and structural refinement of NaMn7O12

    E. Gilioli
    Abstract We report the crystal growth and the structural refinement of NaMn7O12, a manganite having a double perovskite structure. As in many similar compounds, there is coexistence of Mn3+ and Mn4+ but in this material they orderly occupy different sites for crystallographic reasons. Therefore, this peculiar structure can be considered as a model system for studying complex mechanisms such as charge, orbital and spin ordering. High purity bulk samples and "large" single crystals are needed to study tiny modifications in the crystallographic and magnetic structures associated to the ordering phenomena. Almost single phase (more than 96% pure) and single crystals (up to about 150 µm) of NaMn7O12 were synthesized by solid state reaction under pressure in a multi-anvil apparatus. Single crystal x-ray diffraction and SEM analysis have been used to characterize the crystals. The structure refinement indicates that NaMn7O12 crystallizes in the cubic Im3 space group, with a = 7.312 Å and Z = 2. Further studies are in progress to optimize the synthesis conditions, in order to grow larger crystals. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Fractional Cryosurgery for Skin Cancer

    BACKGROUND Cryosurgical treatment of facial skin cancers 10 mm or larger in diameter can originate retractile scars that may alter physiognomic features. OBJECTIVES To treat skin cancers 10 mm or larger in diameter on the face with a cryosurgical method that prevents retractile scars. Also, to clarify the differences between this method and Zacarian's segmental cryosurgery. METHODS AND MATERIALS Fractional cryosurgery is performed in stages. First, the center of the lesion is frozen, reducing its size, then this procedure is repeated as necessary until the tumor diameter is smaller than 10 mm, at which point the standard cryosurgical procedure is performed. Eighty-seven basal cell carcinomas (BCCs) and nine squamous cell carcinomas (SCCs) of the face (65 of which were orbital or periocular) measuring between 9 and 45 mm were treated. RESULTS The cure rate of BCCs was related to tumor size. All SCCs were cured without recurrence. Global mean follow-up was 4.5 years. CONCLUSION Fractional cryosurgery does not cause deformity, and the final scar has no relation to the mass of the original tumor but instead corresponds to the size of the lesion preceding the last cryosurgical procedure. [source]

    Botulinum-A Toxin Treatment of the Lower Eyelid Improves Infraorbital Rhytides and Widens the Eye

    Timothy Corcoran Flynn MD
    Botulinum-A exotoxin (BTX-A) can be used cosmetically to improve rhytides, particularly of the upper one-third of the face. In this study, fifteen women had BTX-A (BOTOX, Allergan, Inc.) injected into the orbicularis oculi muscle. One lower eyelid received two units just subdermally in the midpupillary line three millimeters below the ciliary margin. The opposite periocular area received two units BTX-A in the lower eyelid with 12 units BTX-A injected into the lateral orbital ("crow's foot") area. Three injections of four units each were placed 1.5 cm from the lateral canthus, each 1 cm apart. Patients and physicians independently evaluated the degree of improvement (grade 0 = no improvement, grade 1 = mild improvement, grade 2 = moderate improvement, and grade 3 = dramatic improvement). An independent photographic analysis was performed. Patients reported a grade of 0.73 when two units were injected alone into the lower lid, and a grade of 1.9 when the lower eyelid and the lateral orbital areas were injected. Physician assessment was grade 0.7 with injection of the eyelid alone and grade 1.8 with injection of the lower eyelid and lateral orbital area. Single investigator photographic analysis demonstrated that 40% of the subjects who had injection of the lower eyelid alone had an increased palpebral aperture (IPA), while 86% of the subjects who had injection of the lower eyelid and lateral orbital area had an IPA. Subjects receiving two units alone had an average 0.5 mm IPA and a mean 1.3 mm IPA at full smile. Concomitant treatment of the lateral orbital area produced a mean 1.8 mm IPA at rest and a mean 2.9 mm IPA at full smile. The results were more notable in the Asian eye. Two units of BTX-A injected into the lower eyelid orbicularis oculi muscle improves infraorbital wrinkles, particularly when used in combination with BTX-A treatment of the lateral orbital area. [source]

    Toxicity assessment of mono-substituted benzenes and phenols using a Pseudomonas initial oxygen uptake assay

    Ded-Shih Huang
    Abstract A methodology is presented for assessing the toxicity of chemical substances through their inhibitory action toward the Pseudomonas initial oxygen uptake (PIOU) rate. The current studies reveal that the PIOU assay is rapid, cost-efficient, and easy to perform. The oxygen uptake rate was found to be associated with a putative benzoate transporter and highly dependent on benzoate concentration. The putative benzoate transporter has been shown to follow Michaelis,Menten kinetics. Most phenols were found to be noncompetitive inhibitors of the benzoate transporter. The inhibition constant (Ki) of these noncompetitive inhibitors can be related to the concentration causing 50% oxygen uptake inhibition in Pseudomonas putida. Modeling these data by using the response,surface approach leads to the development of a quantitative structure,activity relationship (QSAR) for the toxicity of phenols ((1/Ki) = ,0.435 (±0.038) lowest-unoccupied-molecular orbital + 0.517 (±0.027)log KOW ,2.340 (±0.068), n = 49, r2 = 0.930, s = 0.107, r2adj = 0.926, F = 303.1). A comparison of QSAR models derived from the Ki data of the PIOU method and the toxicity data of 40-h Tetrahymena pyrifomis growth inhibition assay (Tetratox) indicated that there was a high correlation between the two approaches (r2 = 0.925). [source]

    Magneto,Structural Correlations in Discrete MnII -WV Cyano-Bridged Assemblies with Polyimine Ligands

    Robert Podgajny
    Abstract We present the magneto,structural correlations for two novel discrete cyano-bridged assemblies based on cationic complexes of manganese(II) with diimine ligands and octacyanotungstate(V) ions. The crystal structure of [MnII(terpy)(dmf)(H2O)2][MnII(terpy)(H2O)(dmf)(,-NC)WV(CN)7]2·6H2O (1) (terpy = 2,2,;6,,2,-terpyridine, dmf = dimethylformamide) contains dinuclear {MnIIWV}, cyano-bridged anions, while the crystal structure of [MnII(phen)3]2[MnII(phen)2(,-NC)2WV(CN)6]2(ClO4)2·9H2O (2) (phen = 1,10-phenanthroline) is built of tetranuclear {MnII2WV2}2, square anions. Intramolecular Mn,W magnetic interactions through the cyano bridges are represented by magnetic coupling constants J = ,39 cm,1 for the {MnIIWV}, unit in 1 and J1 = ,25.7 and J2 = ,16.7 cm,1 for the {MnII2WV2}2, unit in 2. J and J1 represent relatively strong W,CN,Mn interactions and are ascribed to the bridges in b positions of TPRS-8 (trigonal prism square-face bicapped) of [W(CN)8]3, polyhedra, favoring the strongest electronic interactions between the d,d orbital of W and the ,* orbitals of CN,, whereas J2 is related to the m vertex of [W(CN)8]3,. The magnetic properties of 1 and 2 are compared with reference compounds and discussed in the context of the type of coordination polyhedra of [W(CN)8]3, as well as the metric parameters of cyano-bridged W,CN,Mn linkages. We found the type of coordination polyhedra and bridging mode of [W(CN)8]3, to be the most important factors influencing the magnitude of the Mn,W magnetic interaction. [source]

    Cubane-Like Bismuth-Iron Cluster: Synthesis, X-ray Crystal Structure and Theoretical Characterization of the [Bi4Fe8(CO)28]4, Anion

    Kirill Yu.
    Abstract The reaction of cyclo -Bi4[Si(SiMe3)3]4 (1) with Na2[Fe(CO)4] in the presence of nBu4NCl leads to the formation of the cage compound [nBu4N]4[Bi4Fe8(CO)28] (2). According to X-ray single-crystal structure analysis, the faces of the tetrahedral Bi4 core are capped by Fe(CO)3 moieties in a ,3 fashion to give a cubanoid Bi4Fe4 framework. The four Fe(CO)4 fragments are ,1 -coordinated to bismuth, each. With 12 skeletal electron pairs the [Bi4Fe8(CO)28]4, anion (2a) is a Bi4Fe4 cubane. The negative charge is localized within cluster 2a according to the NBO analysis of its derivatives. The strength of metal,ligand interactions Bi,,3 -Fe(CO)3 is responsible for the size of the cluster's cubic core. NICS computations at the cage centers of considered molecules show that 2a has paratropic character, whereas removal of four ,1 -Fe(CO)4 fragments from latter causes spherical aromaticity of the modified clusters [Bi4Fe4(CO)12]4, (2aa) and [Bi4Fe4(CO)12]2+ (2ab), mediated by a Bi4 cluster , orbital. [source]

    A Dinuclear Double-Stranded Oxido Complex of ReV with a Bis(benzene- o -dithiolato) Ligand

    Jorge S. Gancheff
    Abstract The reaction of [ReOCl3(PPh3)2] with 1,2-bis(2,3-dimercaptobenzamido)ethane (H4 - 1) in the presence of Na2CO3 in methanol under anaerobic conditions affords the dinuclear ReV oxido complex [PPh4]2[ReO(1)]2 containing two distorted square-pyramidal {ReVOS4} units bridged by the ligand strands in a double-stranded fashion. The coordinationgeometry around the metal centers is similar to the one observed for [ReO(bdt)2],. The ReS4 planes are arranged in a coplanar fashion and are not twisted around the metal,metal vector, which prevents the complex to adopt a helical structure. Luminescence studies show the presence of emission bands, which are assigned to singlet-singlet transitions exhibiting very fast decays (ca. 10 ns). Theoretical Density Functional (DFT) studies on geometry and electronic properties were performed employing the hybrid B3LYP and PBE1PBE functionals. While the general trends observed in the experimental data are well reproduced in all cases, a good agreement was obtained using PBE1PBE, in particular for the Re,S bonds. Natural Bond Orbitals (NBO) analysis indicates the presence of polarized Re,O and Re,S bonds, both of them polarized toward the non-metal. The calculation show that the molecular orbitals of the ReV are doubly degenerated, the occupied 5d orbital of rhenium lying beneath occupied sulfur-based MOs due to the rigid geometry imposed by the C,C backbone of the bis(benzene- o -dithiolato) ligands. The origin of all absorption bands is ascribed to a ligand-to-metal charge transfer (LMCT), in which occupied sulfur-based orbitals and unoccupied rhenium-centered orbitals are involved.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

    Fluoride-Free Hiyama and Copper- and Amine-Free Sonogashira Coupling in Air in a Mixed Aqueous Medium by a Series of PEPPSI-Themed Precatalysts,

    Chandrakanta Dash
    Abstract A new series of robust, user-friendly, and highly active PEPPSI-themed (pyridine-enhanced precatalyst preparation, stabilization and initiation) (NHC)PdX2(pyridine)-type (X = Cl, Br) precatalysts of C4,C5 saturated imidazole- (1,4) and triazole-based (5 and 6) N-heterocyclic carbenes for the Hiyama and Sonogashira couplings under amenable conditions are reported. Specifically 1,6 efficiently catalyze the fluoride-free Hiyama coupling of aryl halides with PhSi(OMe)3 and CH2=CHSi(OMe)3 in air in the presence of NaOH as a base in a mixed aqueous medium (dioxane/H2O, 2:1 v/v). Along the same lines, these 1,6 precatalysts also promote the Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with phenylacetylene in air and in a mixed aqueous medium (DMF/H2O, 3/1 v/v). The complexes 1,6 were synthesized by the direct reaction of the respective imidazolinium and triazolium halide salts with PdCl2 in pyridine in the presence of K2CO3 as a base. DFT studies on the catalytically relevant palladium(0) (NHC)Pd(pyridine) precursors 1a,6a reveal significant donation from the N-heterocyclic carbene lone pair onto the unfilled ,* orbital of the trans Pd,pyridine bond. This weakens the Pd-bound "throwaway" pyridine ligand, and its dissociation marks the initiation of the catalytic cycle.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

    The Role of Axial Ligation in Nitrate Reductase: A Model Study by DFT Calculations on the Mechanism of Nitrate Reduction

    Kuntal Pal
    Abstract The reactivity differences of the model anionic complexes [Mo(mnt)2(X)(PPh3)], [mnt2, = 1,2-dicyanoethylenedithiolate; X = SPh (1a), SEt (1b), Cl (1c), Br (1b)] towards oxygen atom transfer from nitrate, which is a key step performed by nitrate reductase, has been investigated by density functional theory calculations. Unlike complexes 1a and 1b, complexes 1c and 1d do not react with nitrate. Thermodynamically, all these complexes have a similar ability to generate the pentacoordinate active state [Mo(mnt)2(X)], by dissociation of PPh3, although the inaccessibility of the dxy orbital in 1c,d and the instability of the corresponding nitrate-bound enzyme substrate (ES) type complex contributes to their failure to reduce nitrate. The nature of the ES complex for 1a,b is described. The variation in the experimental data due to the change of axial ligation from SPh to SEt on the catalytic pathway has also been addressed. The gas-phase and solvent-corrected potential energy surface for the reaction of 1a,b with nitrate are established with fully optimized minima and transition states.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Hierarchical Structures in Tin(II) Oxalates

    Padmini Ramaswamy
    Abstract Six new SnII oxalates exhibiting a hierarchy of structures have been prepared employing hydrothermal methods. The compounds I [C10N2H10][Sn(C2O4)2], II [C10N2H10][Sn2(C2O4)3], and III [C8N4H26][Sn(C2O4)2]2·2H2O possess zero-dimensional molecular structures; IV [C10N2H8]2[Sn(C2O4)]2 and V [C12N2H8][SnC2O4] have one-dimensional chain structures; and compound VI [C5N2H14]2[Sn4(C2O4)6]·7H2O has a two-dimensional layer structure. The SnII ions have 4- and 6-coordination with square-pyramidal or pentagonal-bipyramidal geometry, in which the lone pair of electrons also occupies one of the vertices. Weak intermolecular forces such as hydrogen-bond interactions, ,···, interactions, and lone-pair,, interactions have been observed and appear to lendstructural stability. Theoretical studies indicate that the ,···, interaction energy between the bound 1,10-phenanthroline molecules is of the order of 5,6 kcal,mol,1 in V. Natural bond orbital (NBO) analysis on two model compounds, II and IV, indicates reasonable lone-pair,, interactions. The close structural relationship between all the compounds indicates that a building-up process from the zero-dimensional monomer can be considered. The present structures provide opportunities for evaluating the structure-directing role of the lone pair of electrons of SnII.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Multicomponent Supramolecular Devices: Synthesis, Optical, and Electronic Properties of Bridged Bis-dirhodium and -diruthenium Complexes,

    Anne Petitjean
    Abstract Four ruthenium- and rhodium-based metal,metal-bonded multicomponent systems have been synthesized, and their absorption, redox, spectroelectrochemical and structural properties have been studied. The absorption spectra of the four bis-dimetallic compounds M2LM2, where L is a bridging ligand and M is rhodium or ruthenium, exhibit very strong bands in the UV, visible and, for the diruthenium species, near-IR region. The low-energy absorption bands are assigned to charge-transfer transitions involving a metal,metal bonding orbital as the donor and an orbital centered on the bis-tetradentate aromatic ligands as the acceptor (metal,metal to ligand charge transfer, M2LCT). Each compound exhibits reversible bridging-ligand-centered reductions at mild potentials and several reversible oxidation processes. The oxidation signals of the two equivalent dimetallic centers of each bis-dimetallic compound are split, with the splitting , a measure of the electronic coupling , depending on both the metal and bridging ligand. The mixed-valence species of the dirhodium species was investigated, and the electronic coupling matrix element calculated from the experimental intervalence band parameters for one of them (86 cm,1) indicates a significant inter-component electronic interaction which compares well with good electron conducting anionic bridges such as cyanides. Although none of these compounds is luminescent, the M2LCT excited state of one of the bis-dirhodium complexes is relatively long-lived (about 6 ,s) in degassed acetonitrile at room temperature. The results presented here are promising for the development of linear poly-dimetallic complexes built on longer naphthyridine-based strands, with significant long-range electronic coupling and molecular-wire-like behavior. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

    A Theoretical Investigation of Substituent Effects on the Absorption and Emission Properties of a Series of Terpyridylplatinum(II) Acetylide Complexes

    Xiao-Juan Liu
    Abstract A comprehensive calculational investigation has been carried out on a series of complexes of the type [(terpyridyl-R1)Pt(C,C-R2)], where terpyridyl-R1 is a series of substituted 2,2':6',2"-terpyridyl ligands and C,C-R2 is a series of substituted acetylide ligands. In one series of complexes (I), the energy of the electronic excited state is varied by changing the substituents on the terpyridyl ligand (R1). In a second series of complexes (II), this electronic structure variation is obtained by changing the para substituents (R2) of the acetylide ligand. The effect of varying the substituents on the lowest-energy excited states of the complexes has been assessed by calculating their electronic structures and excitation energies. We anticipated that introduction of electron-withdrawing substituents on the terpyridyl ligand will benefit the LLCT (or MLCT) and prohibit the nonradiative pathways via d-d transitions in these complexes; introduction of electron-donating substituents on the acetylide ligand can also prohibit the nonradiative pathways by increasing the energy gaps between the HOMO,LUMO and d-d transitions. The results also reveal that the lowest-energy excitations of all complexes of series I and IIa,b complexes are dominated by a ,(C,C),,,,*(terp) (LLCT) transition mixed with some energetically d,(Pt),,,terpyridyl (MLCT) transition. However, for the complexes IIc,IId, in which phenyl rings are introduced on the acetylide ligand, the lowest-lying absorptions of IIc and IId are predominately LLCT in character, with less MLCT mixture, due to a lower contribution of the Pt(d) orbital to the HOMO, while for IIe, with a stronger donor on the acetylide, the lowest-lying absorption is completely LLCT in character. The absorption and emission calculations using the TDDFT method are based on the optimized geometries obtained at the B3LYP/LanL2DZ and CIS/LanL2DZ levels, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Pyrazolate-Based Dinucleating Ligands in L2M2 Scaffolds: Effects of Bulky Substituents and Coligands on Structures and M···H,C Interactions

    Jens C. Röder
    Abstract A series of nickel(II) and palladium(II) complexes [L2M2]2+ have been prepared and structurally characterized, where L is a pyrazolate ligand with bulky 2,6-dimethyl- or 2,6-di(isopropyl)anilinomethyl side arms. Coordinating counter anions such as chloride can bind to axial sites of the dinickel species in a solvent-dependent process, giving rise to five-coordinate high-spin metal ions. In the case of weakly coordinating anions, the metal ions are found in roughly square-planar environments, and the structures are governed by the tendency of the bulky aryl groups to avoid each other, which forces the methyl or isopropyl substituents in the aryl 2- and 6-positions to approach the metal ions from the axial directions. This leads to drastic low-field shifts of the respective 1H NMR signals, e.g. , = 7.86 ppm for the isopropyl ,CH which comes in close proximity to the low-spin nickel(II) center. The relevance of such low-field NMR resonances of protons close to the axial sites of d8 metal ions for possible three-center four-electron M···H,C hydrogen bonds involving the filled d orbital of the metal ion is discussed. In the present case, attractive M···H interactions are assumed to be of no major significance. This was corroborated by the structure of a further [L2Ni2]2+ type complex where the anilinomethyl side arms bear only a single 2-isopropyl group, which was found rotated away from the metal. Additional spectroscopic and electrochemical properties of the various complexes are reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    A Rigid Molecular Scaffold Affixing a (Polypyridine)ruthenium(II)- and a Nickel(II)-Containing Complex: Spectroscopic Evidence for a Weakly Coupled Bichromophoric System

    Yann Pellegrin
    Abstract The synthesis of DppztBuSalH2 (7), a rigid conjugated ditopic ligand containing a Dppz (dipyrido[3,2- a:2,,3,- c]phenazine) skeleton and a salophen-type chelate, is reported. The complexes DppztBuSalNi (10), [Ru(bpy)2(DppztBuSalH2)]2+ (11), and [Ru(bpy)2(DppztBuSalNi)]2+ (12) have been prepared and characterised using common spectroscopic methods. Electrochemical, UV/Vis spectroelectrochemical and EPR studies were conducted on compounds 7, 10, 11, and 12. The singly reduced radical forms of 7 and 10 can be generated electrochemically, with the lone electron located on the low-lying phenazine ,*-molecular orbital. Complexes 11 and 12 show several reduction waves and electronic and EPR data obtained for the electrogenerated singly reduced species show them to be closely related to the radical species 7·, and 10·,, respectively. The presence of nickel(II) in compound 12 renders the addition of the second electron on the phenazine group reversible. Both 11 and 12 show common features on the cathodic side of their cyclic voltammograms, with reversible one-electron ruthenium-centred oxidation. An additional low-potential reversible-oxidation wave is observed for 12, and this is ascribed to oxidation of the nickel(II) ion. The combined spectroscopic data best describe the ruthenium-containing complexes as weakly coupled bichromophoric systems. Photophysical studies attest to the formation of a charge-separated state for 11, whereas a strong quenching is detected for 12. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

    Identification of brain neurons expressing the dopamine D4 receptor gene using BAC transgenic mice

    Daniela Noaín
    Abstract The dopamine D4 receptor (D4R) has received considerable interest because of its higher affinity for atypical antipsychotics, the extremely polymorphic nature of the human gene and the genetic association with attention deficit and hyperactivity disorder (ADHD). Several efforts have been undertaken to determine the D4R expression pattern in the brain using immunohistochemistry, binding autoradiography and in situ hybridization, but the overall published results present large discrepancies. Here, we have explored an alternative genetic approach by studying bacterial artificial chromosome (BAC) transgenic mice that express enhanced green fluorescent protein (EGFP) under the transcriptional control of the mouse dopamine D4 receptor gene (Drd4). Immunohistochemical analysis performed in brain sections of Drd4 -EGFP transgenic mice using an anti-EGFP polyclonal antibody showed that transgenic expression was predominant in deep layer neurons of the prefrontal cortex, particularly in the orbital, prelimbic, cingulate and rostral agranular portions. In addition, discrete groups of Drd4 -EGFP labelled neurons were observed in the anterior olfactory nucleus, ventral pallidum, and lateral parabrachial nucleus. EGFP was not detected in the striatum, hippocampus or midbrain as described using other techniques. Given the fine specificity of EGFP expression in BAC transgenic mice and the high sensitivity of the EGFP antibody used in this study, our results indicate that Drd4 expression in the adult mouse brain is limited to a more restricted number of areas than previously reported. Its leading expression in the prefrontal cortex supports the importance of the D4R in complex behaviours depending on cortical dopamine (DA) transmission and its possible role in the etiopathophysiology of ADHD. [source]

    Synthesis, Morphology, and Properties of Poly(3-hexylthiophene)- block -Poly(vinylphenyl oxadiazole) Donor,Acceptor Rod,Coil Block Copolymers and Their Memory Device Applications

    Yi-Kai Fang
    Abstract Novel donor,acceptor rod,coil diblock copolymers of regioregular poly(3-hexylthiophene) (P3HT)- block -poly(2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiaz-ole) (POXD) are successfully synthesized by the combination of a modified Grignard metathesis reaction (GRIM) and atom transfer radical polymerization (ATRP). The effects of the block ratios of the P3HT donor and POXD pendant acceptor blocks on the morphology, field effect transistor mobility, and memory device characteristics are explored. The TEM, SAXS, WAXS, and AFM results suggest that the coil block fraction significantly affects the chain packing of the P3HT block and depresses its crystallinity. The optical absorption spectra indicate that the intramolecular charge transfer between the main chain P3HT donor and the side chain POXD acceptor is relatively weak and the level of order of P3HT chains is reduced by the incorporation of the POXD acceptor. The field effect transistor (FET) hole mobility of the system exhibits a similar trend on the optical properties, which are also decreased with the reduced ordered P3HT crystallinity. The low-lying highest occupied molecular orbital (HOMO) energy level (,6.08 eV) of POXD is employed as charge trap for the electrical switching memory devices. P3HT- b -POXD exhibits a non-volatile bistable memory or insulator behavior depending on the P3HT/POXD block ratio and the resulting morphology. The ITO/P3HT44 - b - POXD18/Al memory device shows a non-volatile switching characteristic with negative differential resistance (NDR) effect due to the charge trapped POXD block. These experimental results provide the new strategies for the design of donor-acceptor rod-coil block copolymers for controlling morphology and physical properties as well as advanced memory device applications. [source]

    Triplet Formation in Fullerene Multi-Adduct Blends for Organic Solar Cells and Its Influence on Device Performance

    Clare Dyer-Smith
    Abstract In organic solar cells, high open circuit voltages may be obtained by choosing materials with a high offset between the donor highest occupied molecular orbital (HOMO) and acceptor lowest unoccupied molecular orbital (LUMO). However, increasing this energy offset can also lead to photophysical processes that compete with charge separation. In this paper the formation of triplet states is addressed in blends of polyfluorene polymers with a series of PCBM multi-adducts. Specifically, it is demonstrated that the formation of such triplets occurs when the offset energy between donor ionization potential and acceptor electron affinity is ,1.6 eV or greater. Spectroscopic measurements support a mechanism of resonance energy transfer for triplet formation, influenced by the energy levels of the materials, but also demonstrate that the competition between processes at the donor,acceptor interface is strongly influenced by morphology. [source]

    Conjugated Polymer Based on Polycyclic Aromatics for Bulk Heterojunction Organic Solar Cells: A Case Study of Quadrathienonaphthalene Polymers with 2% Efficiency

    Shengqiang Xiao
    Abstract Polycyclic aromatics offer great flexibility in tuning the energy levels and bandgaps of resulting conjugated polymers. These features have been exploited in the recent examples of benzo[2,1- b:3,4- b']dithiophene (BDT)-based polymers for bulk heterojunction (BHJ) photovoltaics (ACS Appl. Mater. Interfaces2009, 1, 1613). Taking one step further, a simple oxidative photocyclization is used here to convert the BDT with two pendent thiophene units into an enlarged planar polycyclic aromatic ring,quadrathienonaphthalene (QTN). The reduced steric hindrance and more planar structure promotes the intermolecular interaction of QTN- based polymers, leading to increased hole mobility in related polymers. As-synthesized homopolymer (HMPQTN) and donor,acceptor polymer (PQTN - BT) maintain a low highest occupied molecular orbital (HOMO) energy level, ascribable to the polycyclic aromatic (QTN) moiety, which leads to a good open-circuit voltage in BHJ devices of these polymers blended with PCBM ([6,6]-phenyl-C61 -butyric acid methyl ester; HMPQTN: 0.76,V, PQTN - BT: 0.72,V). The donor,acceptor polymer (PQTN - BT) has a smaller optical bandgap (1.6,eV) than that of HMPQTN (2.0,eV), which explains its current (5.69,mA,cm,2) being slightly higher than that of HMPQTN (5.02,mA,cm,2). Overall efficiencies over 2% are achieved for BHJ devices fabricated from either polymer with PCBM as the acceptor. [source]

    Electron Trapping in Higher Adduct Fullerene-Based Solar Cells

    Martijn Lenes
    Abstract Here, the performance of bulk-heterojunction solar cells based on a series of bisadduct analogues of commonly used derivatives of C60 and C70, such PCBMs and their thienyl versions, is investigated. Due to their higher lowest unoccupied molecular orbital an increase in open-circuit voltage and thus performance is expected. It is shown that the occurrence of a multitude of different isomers results in a decrease in the electron transport for some of the materials. Surprisingly, the solar-cell characteristics are very similar for all materials. This apparent discrepancy is explained by a significant amount of shallow trapping occurring in the fullerene phase that does not hamper the solar cell performance due the filling of these shallow traps during illumination. Furthermore, the trisadduct analogue of [60]PCBM has been investigated, which, despite an even further increase in open-circuit voltage, results in a significantly reduced device performance due to a strong deterioration of the electron mobility in the fullerene phase. [source]

    Effect of Electric Field on Coulomb-Stabilized Excitons in Host/Guest Systems for Deep-Blue Electrophosphorescence

    Stephan Haneder
    Abstract Here, a study of the electric field induced quenching on the phosphorescence intensity of a deep-blue triplet emitter dispersed in different host materials is presented. The hosts are characterized by a higher triplet excitonic level with respect to the emitter, ensuring efficient energy transfer and exciton confinement, whereas they differ in the highest occupied molecular orbital (HOMO) alignment, forming type I and type II host/guest heterostructures. While the type I structure shows negligible electric field induced quenching, a quenching up to 25% for the type II at a field of 2,MV/cm is reported. A similar quenching behaviour is also reported for thin films of the pure emitter, revealing an important luminescence loss mechanism for aggregated emitter molecules. These results are interpreted by considering Coulomb stabilized excitons in the type II heterostructure and in the pure emitter, that become very sensitive to dissociation upon application of the field. These results clarify the role of external electric field quenching on the phosphorescence of triplet emitters and provide useful insights for the design of deep-blue electrophosphorescent devices with a reduced efficiency roll-off. [source]