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  • 1
    Electronic Resource
    Electronic Resource
    The Lewis Basicity of Diaminocarbene - A Theoretical Study of Donor-Acceptor Complexes of C(NH2)2, NH3 and CO with the Lewis Acids EF3, ECl3 (E = B, Al, Ga, In), TiF4 and TiCl4 (1999)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1999 (1999), S. 2037-2045 
    Details
    ISSN: 1434-1948
    Keywords: Diaminocarbenes ; Lewis basicity ; Ab initio calculations ; Donor-acceptor complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum chemical calculations at the MP2 level using large valence basis sets up to TZ+2P quality have been carried out in order to predict the geometries and bond energies of the title compounds. The nature of the donor-acceptor bond has also been investigated. The calculations show clearly that diaminocarbenes are much stronger Lewis bases than amines. The complexation energies of C(NH2)2 have been calculated to be 14-27 kcal/mol higher than those of NH3. The most strongly bonded complex is Cl3Al-C(NH2)2, which has a theoretically predicted Al-C bond energy Do = 59.1 kcal/mol. In all the complexes, the strength of the Lewis bases is C(NH2)2 〉 NH3 〉 CO, but the ordering of Lewis acid strength of EX3 depends on the coordinated Lewis base. TiF4 and TiCl4 have similar Lewis acidities as BF3, but the titanium tetrahalides may bind one or two donor molecules with almost the same bond strength. The investigated donor-acceptor bonds have a high degree of ionic character. The largest covalent contributions are found for the diaminocarbene complexes. The covalent character of the X3E-CO bond increases on going from E = boron to the heavier Group 13 elements, while the opposite order is found for the X3E-NH3 and X3E-C(NH2)2 bonds.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Se2NBr3, Se2NCl5, Se2NCl-6: New Nitride Halides of Selenium(III) and Selenium(IV) (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1373-1378 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; crystal structure ; halogen compounds ; nitrides ; selenium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Se2NBr3, a planar molecule shaped liked dividers, was formed by the reaction of SeBr4 with N(SiMe3)3, which reduced the selenium to the trivalent state. Under the same conditions, Se2NCl5 was obtained by the reaction of SeCl4 with N(SiMe3)3, retaining the tetravalent state of selenium. The molecule is formed by two edge-sharing pseudotrigonal bipyramids with selenium as their central atoms. Se2NCl5 reacts with PPh4Cl to form the Se2NCl-6 ion by the addition of a Cl- ion to only one of the two selenium atoms, resulting in pseudooctahedral coordination of this atom. Ab initio calculations for all the compounds reported confirm the experimentally determined structures.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021108
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  • 3
    Electronic Resource
    Electronic Resource
    Synthese und Konformationsanalyse von Pyranophanonen und Pyrylophanium-Verbindungen mit intraannularen Substituenten (1993)
    New York, NY : Wiley-Blackwell
    Journal für Praktische Chemie/Chemiker-Zeitung 335 (1993), S. 503-514 
    Details
    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis and Conformational Analysis of Pyranophanones and Pyrylophanium Compounds with Intraannular SubstituentsThe synthesis of [3.3]dithiapyranophanone 6 and 11 is accomplished by use of the two-components-dilution-principle. Pyrolysis of their bis(sulfones) 7 and 12 gives the [2.2]pyranophanones 8, 9 and 13. Under preservation of conformation the intraannular carbonyl-function is used for the synthesis of methylpyranophanoles 14, 16 and 17. The synthesis of pyrylophanium compounds 15 and 18 is possible by elimination in trifluoroacetic acid.6 exhibits anti-conformation within its crystal-structure and like 7 reveals temperature-dependent behavior in solution. Using 6 as an example, a combination of 13C-NMR-spectroscopy, forcefield-calculation and computer-simulation is applied for the first time to give evidence for molecular-dynamic processes of cyclophanes.8 and 9 are the syn- and anti-conformers of the desired product, as shown by X-ray structural analysis. 13 reveals anti-conformation within its crystal structure as well as in solution. The conformational analysis of other new phanes described here is based on the 1H-NMR-spectra of these pyrolysis products.As expected the intraannular substituents of Pyrylophanium-lons 15 and 18 show the characteristic upfield-shift within their 1H-NMR-spectra.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/prac.19933350603
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  • 4
    Electronic Resource
    Electronic Resource
    The silaguanidinium cation and the search for a stable silylium cation in condensed phases (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 869-876 
    Details
    ISSN: 0947-6539
    Keywords: ab initio calculations ; silaguanidinium cations ; silylium cations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantium mechanical calculations at the MP2/6-31 G(d) level are reported for the silaguanidinium cation Si(NH2)3+ (1) and derivatives thereof. The equilibrium structure 1a has D3 symmetry with planar amino groups rotated out of the SiN3 plane by 19.60. The Si-N bond length of 1 a (1.658 Å) is intermediate between a single and a double bond. Isodesmic reactions show that the stabilization of the silylium cation 1 a by the amino groups (63.5 kcal mol-1) is about 40% of the resonance stabilization of the guanidinium cation (159.3 kcal mol-1), but 1 a is clearly better stabilized than alkyl-substituted silylium cations. The electronic stabilization of 1 a by the amino groups is also made obvious by the calculated complexation energy with one molecule of water. The calculated stabilization through complexation of water at HF/6- 31 G(d) is markedly lower for Si(NH2)3-(H2O)+ (6) (28.8 kcal mol-1) than for SiMe3(H2O)+ (40.6 kcal mol-1). The tris(dimethylamino) silylium cation Si(N-Me2)3+ (8) is even more stable than 1 a. The complexation energy of Si(NMe2)3-(H2O)+ (10) is only 17.3 kcal mol-1. IGLO calculations of the 29Si N M R chemical shifts predict that 1 a and 8 should not show the same extremely low shielding that is calculated for alkyl-sub-stituted silylium ions. The calculated 29Si resonances for 8 are in reasonable agreement with the experimental N M R spectrum of (Me2N)3 SiB(C6F5)4. AM 1 calculations predict that the substituted tripyrrolidino silylium cation 12 would be an even better candidate for a stable tricoordinate silylium cation in condensed phases. One of the pyrrolidine rings of 12 has tert-butyl groups in the 2 and 5 positions, which serve as a steric fence around the silicon atom.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020717
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  • 5
    Electronic Resource
    Electronic Resource
    100. Geburtstag von Erich Hückel (1997)
    Weinheim : Wiley-Blackwell
    Chemie in unserer Zeit 31 (1997), S. 27-31 
    Details
    ISSN: 0009-2851
    Keywords: Chemistry ; Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The theoretical physicist Erich Hückel, who would have become 100 years old on August 9, 1996, was one of the pioneers of quantum chemistry. The theory of conductivity of strong electrolytes (“Debye-Hückel theory”) and particularly his fundamental work about the electronic structure of benzene, which has unbroken actuality for modern chemistry, are the early highlights of his peculiar scientific career. His life was strongly characterized by the late recognition of his achievements in theoretical chemistry, which can partly be explained by the dominance of experimental chemistry in the history of the science, and by the long uncertainty of his academic position.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/ciuz.19970310106
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  • 6
    Electronic Resource
    Electronic Resource
    Für Ihre Dokumentation von Nr. 1/97 Chemie in unserer Zeit (1997)
    Weinheim : Wiley-Blackwell
    Chemie in unserer Zeit 31 (1997), S. 50-50 
    Details
    ISSN: 0009-2851
    Keywords: Chemistry ; Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/ciuz.19970310114
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  • 7
    Electronic Resource
    Electronic Resource
    Dissoziative Ringöffnung halogensubstituierter Methylcyclopropan-Radikalkationen in der Gasphase (1980)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 113 (1980), S. 1084-1094 
    Details
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Dissociative Ring Opening of Halogen Substituted Methylcyclopropane Cation Radicals in the Gas PhaseThe unimolecular gas phase reactions of several C4H7Br+· isomers are investigated. Rearrangement of the incipient C4H7+ ion is observed prior to Br· loss from all C4H7Br+· isomers except ionised 2- (9) and 3-methylallyl- (10) and cyclobutylbromides (6). This is evidenced by the increased average kinetic energy release associated with Br· elimination. In addition, the structure of the C4H7+ ions may be established using collisional activation spectroscopy. Ionised 1- and 2-methylcyclopropylbromides both yield 1-methylallyl cation rather than 2-methylallyl cation. MNDO calculations on the analogous C4H7Cl+· isomers lead to similar conclusions; moreover, these calculations shed light on the probable mechanism for both isomerization and decomposition of the molecular ions.
    Notes: Die unimolekularen Zerfälle einiger isomerer C4H7Br+·-Verbindungen werden in der Gasphase studiert. Die Br·-Abspaltung aus ionisiertem Cyclobutylbromid (6) und 2- bzw. 3-Methylallylbromid (9 bzw. 10) erfolgt ohne Isomerisierung des entstehenden C4H7+-Ions. Bei allen übrigen Verbindungen ist die dissoziative Ionisation mit einer Isomerisierung des Kations verbunden. Diese Umlagerung, die vor der eigentlichen Br·-Eliminierung eintritt, manifestiert sich in einer Linienverbreiterung der Übergangssignale („kinetic energy release“). Die resultierenden C4H7+-Kationen lassen sich anhand ihrer Stoßaktivierungsspektren charakterisieren. Aus den Radikalkationen der 1- und 2-Methylbromcyclopropane entsteht bei der Br·-Abspaltung das 1-Methylallylkation und nicht das 2-Methylallylkation. MNDO-Rechnungen für die analogen C4H7Cl+·-Radikalkationen unterstützen die bei den Bromiden gemachten experimentellen Befunde; außerdem enthüllen diese Rechnungen mechanistische Details der unimolekularen Reaktionen.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19801130327
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  • 8
    Electronic Resource
    Electronic Resource
    Röntgenstrukturuntersuchung von α-(Trimethylsilyl)benzyllithium·Tetramethylendiamin [C6H5CH(SiMe3)Li·TMEDA] und α-(Phenylthio)-benzyllithium·3 Tetrahydrofuran [C6H5CH(SPh)Li·(THF)3] - zwei zentral-chirale Benzyllithium-Verbindungen (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 543-549 
    Details
    ISSN: 0009-2940
    Keywords: Benzyllithium compounds ; Chirality, central ; Calculations, ab initio ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Crystal Structure of α-(Trimethylsilyl)benzyllithium·Tetramethylethylendiamin [C6H5CH(SiMe3)Li·TMEDA] und α-(Phenylthio)-benzyllithium·3 Tetrahydrofuran [C6H5CH(SPh)Li - (THF)3] - Two Benzyllithium Compounds with Central ChiralityThe crystal structure analyses of [C6H5CH(SiMe3)Li·TMEDA] (6) and of [C6H5CH(SPh)Li·(THF)3] (7, in which the positions of the benzylic hydrogen atoms have been determined experimentally, reveal compounds with pyramidal benzylic carbon atoms. In 6 lithium is λ2-bound to Cipso and α of the substituted benzylic anion which leads, together with the TMEDA molecule, to a tetra-coordinated lithium atom. In 7 only one Li - C bond is formed, namely that to Cα. Additional coordination with three THF molecules results also in a tetracoordinated lithium atom, The extent of the pyramidalization in 6 and 7 is qualitatively in agreement with quantum-mechanical ab initio calculations [G86, MP2/6-311+ + G(d,p)].
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911240321
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  • 9
    Electronic Resource
    Electronic Resource
    Stereoselective synthesis of alcohols, XXXIV. Towards an understanding of cram/anti-cram selectivity on addition of crotylboronates to α-methylbutyraldehyde (1990)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 123 (1990), S. 2387-2394 
    Details
    ISSN: 0009-2940
    Keywords: Calculations, force-field ; Transition-state models ; Allylboration reaction ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The diastereoselectivity on addition of γ-substituted allylboronates to α-methylbutyraldehyde (9) did not depend on the size of the substitutent in the reagent, but only on its location in the Z or E position. This finding required a reinterpretation of the reasons for the attendant reversal in diastereoselectivity. New hypotheses are presented based on force-field calculations on model structures for the transition states.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19901231222
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  • 10
    Electronic Resource
    Electronic Resource
    Cyclische C6S6-Isomere  -  Matrixspektroskopische und theoretische Untersuchungen (1992)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 125 (1992), S. 265-269 
    Details
    ISSN: 0009-2940
    Keywords: Matrix isolation ; Photochemistry ; Flash vacuum pyrolysis ; Calculations, ab initio ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cyclic C6S6 Isomers  -  Matrix-Spectroscopic and Theoretical InvestigationsA yellow C6S6 isomer, which can be interconverted photo-chemically into a colorless second C6S6 isomer, may be obtained upon matrix photolysis and flash pyrolysis of the precursor molecule 7. The structures of these isomers are most likely 3 and 5, as shown by comparison of their experimental with the calculated infrared spectra of the five possible cyclic C6S6 isomers 1 - 5.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19921250142
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