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  • Computational Chemistry and Molecular Modeling  (3)
  • Organic Chemistry  (3)
  • 31.20.Sy  (2)
  • 1
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The configuration of several keto-cyclolignans related to podophyllotoxin has been reviewed. Under basic catalysis, the configuration at the C-atom in α-position to the lactone carbonyl group in podophyllotoxone is inverted instead of the C-atom in α-position to the ketone group, as it has been reported.
    Additional Material: 1 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 19 (1991), S. 141-143 
    ISSN: 1434-6079
    Keywords: 31.20.Sy ; 36.40. +d
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A method which combines density functional theory and the use of pseudopotentials is applied to obtain ground state and low-lying metastable geometries of Na n clusters (7≤n≤40). The large variation in the magnitude of energy gaps between isomers suggests that the melting temperature is not a simple monotonous function of size. A detailed study of the differences between electronically stabilized (n=8, 20, 40) and structurally stabilized (n=13) clusters suggests some clues to understand the intriguing behaviour of Na13.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 20 (1991), S. 119-122 
    ISSN: 1434-6079
    Keywords: 36.40. + d ; 31.20.Sy ; 65.50. + m ; 63.70. + h
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A scheme based in density functional theory with pseudopotentials is used to obtain the normal modes of vibration of Na n clusters (4 ≤n ≤ 22). The monomer and dimer evaporation rates from thermally excited clusters are obtained in this harmonic approximation. The time evolution of the abundance spectra from an initial uniform mass distribution of hot clusters is studied and its influence in the experimentally observed spectra is discussed.
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  • 4
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The vibrational frequencies of NaN clusters (2 ≤ N ≤ 72) are calculated by direct diagonalization of the dynamical matrix. Density functional theory with a spherically averaged pseudopotential is used to compute the total energy. The geometry is optimized by the simulated annealing technique. Contributions to the Hessian matrix due to electron relaxation following the ionic displacements are calculated in linear response theory. The frequencies are in the range 0-220 cm-1 and the electron relaxation strongly modifies those of the modes dominated by radial oscillations, particularly the breathing mode frequencies that are proportional to N-1/3. The filling of atomic shells produces a stepwise behavior of the highest frequencies. The giant dipole resonance energies are obtained as a byproduct of the calculation. © 1995 John Wiley & Sons, Inc.
    Additional Material: 9 Ill.
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  • 5
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Rate, endo/exo and diastereofacial selectivities of the Diels-Alder reaction between cyclopentadiene and ( - )-menthyl acrylate were measured in a series of organic solvents and organic-aqueous mixtures. Regression analyses show that a model with the α empirical solvent parameter accounts for most of the changes in rates and diastereofacial selectivities, whereas in the case of the endo/exo selectivity, inclusion of the π* and δ parameters is needed. Theoretical calculations carried out on the model reaction between methyl acrylate and cyclopentadiene using dielectric continuum and supermolecule models do not clearly show the effect of the different solvation mechanisms on rate. However, they agree with the above-mentioned influence of α and π* on the endo/exo and diastereofacial selectivities.
    Additional Material: 1 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1988 (1988), S. 737-742 
    ISSN: 0170-2041
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Synthese von Pyrimidin-Analoga des Nucleosid-Antibiotikums AscamycinVier Analoga des Ascamycins 〈5′-O′-O-[N-(L-alanyl)sulfamoyl]-2-chloradenosin〉(1), bei welchen der 2-Chloradenosinrest durch Uridin, Thymidin, Cytidin und 2′-Deoxy-5-methylcytidin ersetzt wurde, sind durch selektive Aminoacylierung der 5′-O-Sulfamoyl-Derivate 2, 9, 14 und 21 des 2′,3′-Di-O-acetyluridins, 3′-O-Acetylthymidins, 2′,3′-O-Isopropylidencytidins, bzw. des 3′-O-Acetyl-2′-deoxy-5-methylcytidins mit Boc-L-Ala-OSu in DMF und in Anwesenheit von DBU, mit nachfolgender Abspaltung der Schutzgruppen, erhalten worden. Ähnlich wurde 5′-O-[N-(D-Alanyl)sulfamoyl]uridin (8) ausgehend von Boc-D-Ala-OSu und 2 hergestellt. Die Verbindungen 14 und 21 wurden direkt durch Sulfamoylierung der 4-N-(Dimethylamino)methylen-Derivate 13 und 20 von 2′,3′-O-Isopropylidencytidin bzw. 3′-O-Acetyl-2′-deoxy-5-methylcytidin über die 5′-O-Tributylzinn-Zwischenverbindungen dargestellt. Verbindung 20 wurde aus 5′-O-Isopropylidencytidin bzw. 3′-O-Acetyl-2′-deoxy-5-methylcytidin über die 5′-O-Tributylzinn-Zwischenverbindungen dargestellt. Verbindung 20 wurde aus 5′-O-(tert-Bu-tyldimethylsilyl)thymidin (17) mittels Acetylierung und nachfolgende Überführung des Thymidinrestes in 5-Methylcytosin über das entsprechende Triazolylpyrimidinon unter Schutz der Amidingruppe erhalten.
    Notes: Four analogues of ascamycin 〈5′-O-[N-(L-alanyl)sulfamoyl]-2-chloroadenosine〉 (1) in which the 2-chloroadenosine moiety has been replaced by uridine, thymidine, cytidine, and 2′-deoxy-5-methylcytidine have been synthesized by selective aminoacylation of the 5′-O-sulfamoyl derivatives 2, 9, 14, and 21 of 2′,3′-di-O-acetyluridine, 3′-O-acetylthymidine, 2′,3′-O-isopropylidenecytidine, and 3′-O-acetyl-2′-deoxy-5-methylcytidine, respectively, with Boc-L-Ala-OSu in DMF and in the presence of DBU, followed by removal of the protecting groups. Similarly, 5′-O-[N-(D-alanyl)sulfamoyl]uridine (8) has been prepared from Boc-D-Ala-OSu and 2. Compounds 14 and 21 were directly prepared by sulfamoylation of the 4-N-(dimethylamino)methylene derivatives 13 and 20 of 2′,3′-O-isopropylidenecytidine and 3′-O-acetyl-2′-deoxy-5-methylcytidine, respectively, via the intermediate 5′-O-tributyltin derivatives. Compound 20 was obtained from 5′-O-(tert-butyldimethylsilyl)thymidine (17) by a route involving acetylation, conversion of the thymidine moiety into 5-methylcytosine through the corresponding triazolylpyrimidinone, and amidine protection.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 52 (1994), S. 1039-1049 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Local and nonlocal density functional computations have been carried out to study the electronic structure and the equilibrium geometry of the isoelectronic series Cr(NC)4, Mn(NO)3(CO), Fe(NO)2(CO)2, and Co(NO)(CO)3 and model compounds Fe(NO)2L2 (L = Cl, HCN, NH3, PH3, and C2H4). The structure of Fe(NO)2(C4H6) is also described. The discussion is focused on structural modifications through a change of ligand, in particular those concerning the metal-nitrosyl conformation (linear vs. bent). Though this is a preliminary study of metal-nitrosyl properties by DFT methods and more computations are required to analyze the mechanism of homogeneous catalysis processes, our results support the hypothesis that structural reorganization from linear to bent metal-nitrosyl plays a key role in some reactions, such as in butadiene dimerization. © 1994 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 15 (1994), S. 479-487 
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The effect of correlation energy, basis set size, zero-point energy (ZPE) correction, and solvation on the reaction mechanism of the ketene-imine cycloaddition reaction has been investigated. The electrostatic solvent effect was studied with a self-consistent reaction field method in which the solvation energy is obtained using a multipole expansion of the molecular charge distribution. The ab initio results have been analyzed by means of a theoretical method based on the expansion of the MOs of the supermolecule in terms of those of the reactants and the performance of the configuration analysis. In gas phase, due to the correlation energy and/or the ZPE corrections, the reaction is predicted to be a one-step process. In solution, the stabilization of the charge-transferred configurations results in the occurrence of a very stable, Zwitterionic intermediate giving a two-step mechanism. © 1994 by John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
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