ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    ISSN: 0947-6539
    Keywords: biphenyls ; bipyridinium salts ; conformation ; semiempirical calculations ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxidation of substituted N-aminopyridinium compounds yields bipyridinium salts. After anion exchange Br- → CIO4-, it was possible to grow single crystals from the 4,4′-di(tert-butyl) derivative. The structure determined at 100 K shows a dihedral angle of 84° between the molecular halves connected by a shortened N—N bond of 143 pm in length. This result is contrary to that expected from the isoelectronic correspondence 〉C—C〈↔〉N—N〈, which would suggest planarity, as has been observed experimentally both in solid biphenyl, and in tetraalkylhydrazine dications and radical cations. Lattice packing analysis, however, reveals the presence of hydrogen bonds C(H)…O between the phenyl rings and the perchlorate anions. AM 1 enthalpy hypersurface calculations for the isoelectronic series H5C5X-YC5H5 (X-Y = C—C, +N—C, +N—N+, -B—N+, B—C, and B—B-) predict single-minimum potentials for both the dication 〉N—N〈 as well as the dianion 〉B—B〈 with the molecular halves twisted perpendicular to each other. In detailed model calculations, counteracting effects of π electron density delocalization vs. H/H repulsion of the ortho ring hydrogens adjacent to the central bond seem to dictate the delicate balance of the biphenyl twisting. For further experimental confirmation, the structure of the isosteric molecule 4,4′-di(tert-butyl)biphenyl has been determined: in contrast to the unsubstituted π hydrocarbon with a latticeenforced dihedral angle of 0°, close to the calculated value, the two molecular halves are twisted by 40° - as in biphenyl in the gas phase.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 2
    ISSN: 0947-6539
    Keywords: ab initio calculations ; clusters ; electronic structure ; gallium ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present idealised geometrical models for the α-Ga and β-Ga structures based on two differently distorted, corrugated 36 nets. We investigated the structural stability of two sets of two-dimensional and three-dimensional model structures consisting of these corrugated nets as a function of the net puckering. We used the simple tight-binding (TB) Hückel model with the structural energy difference theorem and the advanced full-potential linear muffin-tin orbital (FP-LMTO) method in the framework of local-density functional theory. Both methods show the existence of an optimum puckering angle that corresponds well to the situation in the experimental α-Ga structure. The geometrical model of the α-Ga structure follows the building principle of terminally coordinated deltahedral clusters, extended to two-dimensional structures. The chemical bonding in α-Ga is interpreted in terms of multicentre bonding within the corrugated 36 nets and two-electron-two-centre bonds connecting these nets.
    Additional Material: 12 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 3
    ISSN: 0947-6539
    Keywords: nonlinear optics ; quadrupolar compounds ; quantum-chemical calculations ; semiempirical calculations ; squaraines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this theoretical study, we investigate the molecular and electronic structures of symmetrical and unsymmetrical squaraine dyes. Such dyes can be represented by the formula D-A-D, where D is a donor group and A an acceptor moiety. We analyze the evolution in geometric structure that results from changing both donor substituents simultaneously or from varying only one donor group to produce an asymmetrical system. The changes in geometric and electronic structures are compared and found to be consistent. The trends in linear and nonlinear optical properties, and in particular in second-order polarizabilities, are investigated in several ways. The two-state model appears to be inadequate in describing the second-order polarizability, β. Consequently, we are unable to deduce simple structure/property relationships that might help in the design of quadrupolar compounds for nonlinear optics. Finally, a series of unsymmetrical squaraines with OH substitution and enol-ketone isomerism are investigated; the calculated nonlinear properties follow a similar trend to the experimental results: the OH substitution and isomerization contribute to increasing the ground-state polarization.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 4
    ISSN: 0947-6539
    Keywords: ab initio calculations ; biosynthesis ; porphyrinogens ; rearrangements ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The [1,5]-sigmatropic rearrangements of hydrogen and other groups, such as methyl, tert-butyl, allyl, benzyl, and azafulvenium, in pyrroles and, to some extent, in furans and thiophenes, has been studied by using semiempirical and ab initio methods. These systems are used as models to explain the ring D inversion in the biosynthesis of uroporphyrinogen III and the stereoselective shift of a methyl group in the biosynthesis of vitamin B12. The difference in energy between the competing pathways of hydrogen and methyl shifts is significantly lower than for cyclopentadiene. The rearrangements are usually concerted, except for in strongly resonance-stabilized systems, such as azafulvenium cations. Furthermore, ab initio calculations of the 1,3-allylic strain for a range of substituted pyrroles has been performed, and the results compared with semiempirical data.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 5
    ISSN: 0947-6539
    Keywords: ab initio calculations ; annulenes ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interconversion between the valence tautomers 1,6-methano[10]-annulene (1) and dinorcaradiene (2) was computationally investigated by qualitative extended Hückel molecular orbital methods (with CACAO visualization) and quantitative semiempirical and ab initio methods. The fundamental bonding interactions are described in terms of perturbation theory arguments for both tautomers, the influence of π-acceptor or π-donor substituents at the C11 position is rationalized, and bonding changes during the interconversion are monitored. The electronic molecular structural preferences can be modified by through-space interactions, and residual C1-C6 σ bonding remains even at the 〉2.1 å separations of the annulenic structures. Hitherto unprecedented calculations of the geometries of C11 homo-disubstituted derivatives of 1 and 2 have been carried out at semiempirical and ab initio levels to identify the more stable tautomer and find whether two stable minima may exist for each derivative. Searches at the AM 1 level to determine transition-state structures for the interconversion of 1 to 2 and of their C11 homo-disubstituted derivatives are also reported.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 390-397 
    ISSN: 0947-6539
    Keywords: band structures ; hypervalent bonding ; semiempirical calculations ; tellurium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bonding in the recently reported Cs3 Te22 phase, which contains both Te8 rings and remarkable Te6 sheets, is studied by approximate molecular orbital theory. Our focus is on the geometric and electronic features of the unique 2,3-connected Te net found as a substructure in this phase. The calculations show that both the linear and T-shaped Te geometries in the 2,3-connected Te net of Cs3 Te22 are determined by their particular electron count. Both types of tellurium atoms are hypervalent; we make connections to other well known hypervalent molecules, such as XeF2, I3-, and BrF3. Several possible variations and distortions of this net are discussed, all of which are found to be less stable. The discrete crown-shaped Te8 units that appear in the phase show normal covalent bonding and should occur in smaller molecular entities, too. According to our computations, Cs3 Te22 should be metallic. Two structurally related phases, CsTe7 and Cs2 Te15, are suggested.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 7
    ISSN: 0947-6539
    Keywords: additions ; fullerenes ; regioselectivity ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three series of regioisomeric bisadducts of C60, namely, C62(anisyl)4 and the mixed systems C62(anisyl)2-(COOEt)2, and C61(COOEt)2(NCOOEt), were synthesized starting from the 1,2-monoadducts C61(COOEt)2 (1), C61-(anisyl)2 (2), and C60(NCOOEt) (4) by using the Bingel and Bamford-Stevens reactions, and nitrene additions. In the case of C61(COOEt)2(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis-1 isomers of C62(anisyl)4 and C62(anisyl)2(COOEt)2 were not formed. The transannular [6,6] bonds in the cis-1 isomer 42 of C61(COOEt)2(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C62-(COOEt)4 and C60(NCOOEt)2, which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans-3 for sterically demanding addends (e.g., combinations of C(anisyl)2 and C(COOEt)2) and cis-1, e, and trans-3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)2). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend-independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 8
    ISSN: 0947-6539
    Keywords: acidity ; adenosinephosphates ; adenosinethiophosphates ; protonated isomers ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The acidity constants of H2-(AMPS)± were determined by potentiometric pH titrations in aqueous solution at 25°C and I=0.1M (NaNO3). Titrations with a combined single-junction glass electrode were hampered in the presence of AMPS by a “poisoning” effect; the problem could be avoided by use of two separated electrodes. The values of the acidity constants PKHH2(AMPS) = 3.72 ± 0.03 and pKHH2(AMPS) = 4.83 ± 0.02 are relatively close to each other; the buffer regions of the two equilibria overlap, and therefore a micro acidity constant scheme was developed and the constants for the various sites calculated. It is concluded that the thiophosphateprotonated species (AMPS⋅H)- dominates at about 75% occurrence, while the form (H⋅AMPS)-, with the proton at the N1 site of the adenine residue, occurs at about 25%. Semiempirical AM1 and PM3 calculations including water as a solvent locate the proton in (AMPS⋅H)- mainly on the terminal oxygen atoms rather than the sulfur. The acid-base properties of H2(AMPS)± are considerably more complicated than those of the parent nucleotide, H2(AMP)±; for the latter the two (intrinsic) acidity constants are well separated and consequently practically all protons have left the N1 site before deprotonation at the monoprotonated phosphate group occurs. Finally, an estimate for the acidity constants of H2(ATPγS)2- is given.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 9
    ISSN: 0947-6539
    Keywords: light-emitting diodes ; polymers ; semiempirical calculations ; electroluminescence ; polymer/metal interface ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The early stages of metal/polymer interface formation between aluminum and poly(2,5,2′,5′-tetrahexyloxy-8,7′-dicyanodi-p-phenylenevinylene) or their ring-substituted derivatives have been studied theoretically by using quantum-chemical calculations as well as experimentally by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. This class of conjugated polymer is of interest in the development of organic light-emitting diodes. The theoretical and experimental results indicate that aluminum preferentially reacts with the polymer by forming covalent bonds with the nitrogen and carbon atoms of the cyano groups. When the side chains of the phenylene rings include carbonyl groups, however, the theoretical results indicate that the carbonyl moiety is another preferred site of interaction.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
  • 10
    ISSN: 0947-6539
    Keywords: amino acids ; biosynthesis ; hydroxylations ; nitric oxides ; semiempirical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure (charge distribution, bond indices, character of the frontier orbitals) and geometry (bond distances and angles) of L-arginine and N-methyl-L-arginine were determined by means of the INDO procedure. The method was also adopted to model the conversion of L-arginine into N-hydroxy-L-arginine in biological systems. This revealed that the approach of diatomic O species does not result in reaction, whereas the approach of either an O atom or an O2- ion leads to insertion of oxygen and formation of hydroxy-L-arginine. The insertion of oxygen between the nitrogen and hydrogen atoms leads to more stable products than insertion into the C-H bond. The same results were obtained for N-methyl-L-arginine, and are consistent with the hypothesis that the inhibitive effect of N-substitution in L-arginine is of no importance for the first step in the biosynthesis of NO (hydroxylation process). The mechanistic considerations based on the charge distribution and frontier orbital characteristics led to the conclusion that the most probable mechanism of L-arginine hydroxylation consists in electrophilic attack of [FeO]3+ at the Nω-H bond, initiated by the reduction of L-arginine+, followed by insertion of oxygen and product oxidation.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...