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  • 1
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    Load partition during hot deformation of AlSi12 and AlSi10Cu6Ni2 alloys: a quantitative evaluation of the stiffness of Si networks (2020)
    Springer
    Details
    Publication Date: 2020-07-09
    Print ISSN: 0022-2461
    Electronic ISSN: 1573-4803
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Springer
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  • 2
    Electronic Resource
    Electronic Resource
    Concerning the Structural Complexity of Hypericin: The Relative Stabilities of Some Neutral and Deprotonated Tautomers (1998)
    Springer
    Monatshefte für Chemie 129 (1998), S. 855-863 
    Details
    ISSN: 1434-4475
    Keywords: Keywords. Hypericin; Tautomerism; ab initio 6-31G calculations; Heats of formation; Hydrogen bonding.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung.  Die strukturelle Komplexität des Hypericins wurde in Hinblick auf seine drei relevantesten Tautomeren und deren bay-Deprotonierungsprodukte mit Hilfe von ab initio- Rechnungen mit 6-31G Basissätzen studiert. Die 1,7- und 1,6-Tautomeren sind gegenüber dem 7,14-Tautomeren um 45.3 und 83.0 kJ/Mol instabiler. Bei Deprotononierung bleibt dieselbe Stabilitätsreihenfolge erhalten. Kleine Differenzen, die hinsichtlich der absoluten Stabilitäten erhalten wurden, reichen nicht aus, um eventuelle Änderungen im Tautomeriegleichgewicht durch Deprotonierung zu verursachen. Geringfügige Differenzen der Dipolmomente sollten ebenfalls kaum zu Verschiebungen der Gleichgewichte durch Solvens/Solut Wechselwirkungen oder pH-Änderungen führen. Die Geometrie, die für die sieben Spezies errechnet wurde, wird in Hinblick auf Torsionsdeformation und Wasserstoffbrückenbindungen diskutiert.
    Notes: Summary.  The structural complexity of hypericin was investigated with respect to the three most relevant tautomers and their bay-deprotonation products using ab initio calculations at the 6-31G level. The 1,7- and 1,6-tautomers were found to be less stable than the 7,14-tautomer by 45.3 and 83.0 kJ/mol. Upon deprotonation, the same order of relative stabilities was found to be retained. The small differences of absolute stabilities thus obtained were judged to be too small to account for changes of the tautomeric equilibria upon deprotonation. The dipole moments were also found to be similar for these tautomers, making shifts in tautomeric equilibria induced by solvent/solute interactions and pH changes improbable. The geometrical features calculated for the seven species are discussed with respect to torsional deformation and hydrogen bonding.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00013493
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  • 3
    Electronic Resource
    Electronic Resource
    Concerning the Deprotonation of the Photooxidized 3-Hypericinate Ion (1999)
    Springer
    Monatshefte für Chemie 130 (1999), S. 1333-1339 
    Details
    ISSN: 1434-4475
    Keywords: Keywords. Hypericin radical; AM1 calculations; Acidity; Radical anions.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung.  Mit Hilfe von AM1 Rechnungen wurde abgeleitet, daß das Radikal aus der Photooxidation des 3-Hypericinat-Anions ((3) 1.) stark sauer ist – wenigstens so sauer wie das Hypericin selbst. Die Dissoziation von (3) 1 ˙ ergibt ein Proton, welches von der 6-peri- und/oder 4-bay-Hydroxylgruppe herrührt (diese dürften etwa gleiche Acidität aufweisen). Die peri-Hydroxylgruppen von (3) 1 ˙ in den Positionen 1, 8 und 13 unterliegen einer Deprotonierung weniger leicht. Dieses Ergebnis wirft neues Licht auf die Interpretation von kürzlich mitgeteilten experimentellen Ergebnissen über den intermolekularen Protonentransfer, der der Anregung von Hypericinat und einigen seiner Alkylderivate folgt. Auf der Basis des beobachteten graduellen Wechsels des Protonentransfers bei diesen Derivaten kann geschlossen werden, daß der Protonentransfer von den bay- und/oder peri-Hydroxylgruppen stammt.
    Notes: Summary.  By means of AM1 calculations it was established that the radical produced by photooxidation of the 3-hypericinate anion ((3) 1 ˙ ) possesses strong acidic properties and is at least as acidic as hypericin itself. Dissociation of (3) 1 ˙ yields a proton which stems from the 6-peri- and/or 4-bay-hydroxyl group (which seem to have comparable acidities). The peri-hydroxyl groups of (3) 1 ˙ at positions 1,8, are 13 are significantly less capable of deprotonation. This result sheds new light on the interpretation of a recent experimental finding involving intermolecular proton transfer following excitation of hypericinate and several of its alkyl derivatives. On the basis of observing a gradual change of proton transfer from these derivatives one could conclude that the proton might stem from the bay- and/or the peri-region hydroxyl groups.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00010193
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  • 4
    Electronic Resource
    Electronic Resource
    On the Photochemical Proton Expulsion Capability of Fringelite D – A Model of the Protist Photosensory Pigments of the Stentorin and Blepharismin Types (2000)
    Springer
    Monatshefte für Chemie 131 (2000), S. 1039-1045 
    Details
    ISSN: 1434-4475
    Keywords: Keywords. Two-photon excitation; Intermolecular proton transfer; Epifluorescence microscopy; Phenanthroperylene quinones.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  Using simultaneous two-photon excitation of fringelite D and a fluorescence indicator embedded in a vesicle system it was demonstrated that after excitation a proton was transferred from the pigment to the indicator similarly as recently documented for hypericin. Semiempirical AM1 calculations were used to show that the radical species formed by electron transfer from the excited pigment state constitutes an acid which is therefore well suited for intermolecular proton transfer. Accordingly, this process constitutes a suited candidate for the primary photoprocess in the signal transduction cascade of the photosensory pigments of the stentorin and blepharismin type.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s007060070035
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  • 5
    Electronic Resource
    Electronic Resource
    Concerning the Association of Hypericin Tautomers and their Hypericinate Ions (2000)
    Springer
    Monatshefte für Chemie 131 (2000), S. 333-340 
    Details
    ISSN: 1434-4475
    Keywords: Keywords. Hypericin; Hypericinates; Tautomers; Homoassociation; Heteroassociation.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  The association of the like and unlike configured propeller and butterfly conformers of unionized and bay-ionized 7,14-, 1,6-, and 1,7-tautomers of hypericin was investigated by means of semiempirical calculations of the MM3 type. From the various possible stack dimers it was found that the homodimer with like configurations of the propeller conformations of unionized hypericin was the most stable one, making them strongly stabilized over any coplanar arrangements. Oligomerization led to further stabilization of the stacked homoassociates up to five units. Adding more units did not lead to further stabilization. Accordingly, dispersion forces between the aromatic systems dominate in associate stabilization, with hydrogen bonding playing a marginal role only. These findings were compared with the rather scarce experimental evidence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s007060050314
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  • 6
    Electronic Resource
    Electronic Resource
    On the Ground State Energy Hypersurface of Blepharismins and Oxyblepharismins (2000)
    Springer
    Monatshefte für Chemie 131 (2000), S. 1115-1122 
    Details
    ISSN: 1434-4475
    Keywords: Keywords. Valence tautomerism; Tautomerism; Conformational analysis; AM1 semiempirical calculations.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  AM1 calculations on blepharismins and oxyblepharismins, which are related photosensory pigments of certain protists, revealed that the accessory substituents of the natural pigments do not lead to a change of the tautomerism and conformational states of the fundamental systems. The valence tautomerism possible in principle for the blepharismins yielding a cycloheptatriene–norcaradiene system was found to reside completely on the side of the cycloheptatriene. With respect to proton tautomerism, the strong predominance of the meso-type 7,15- and 7,14-dioxo tautomers was established in both series. Whereas the conformation of the fundamental condensed aromatic ring system of the oxyblepharismins remains comparable to that of hypericin, the conformational situation of the blepharismins was found to be unique with the phenyl group in an endo-position and dihedral twisting at the unperturbed bay-site only.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s007060070018
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