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  • 1
    Electronic Resource
    Electronic Resource
    Hydrogen bonding and collective proton modes in clusters and periodic layers of squaric acid: A density functional study (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 6406-6417 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Hydrogen bonding in clusters and extended layers of squaric acid molecules has been investigated by density functional computations. Equilibrium geometries, harmonic vibrational frequencies, and energy barriers for proton transfer along hydrogen bonds have been determined using the Car–Parrinello method. The results provide crucial parameters for a first principles modeling of the potential energy surface, and highlight the role of collective modes in the low-energy proton dynamics. The importance of quantum effects in condensed squaric acid systems has been investigated, and shown to be negligible for the lowest-energy collective proton modes. This information provides a quantitative basis for improved atomistic models of the order–disorder and displacive transitions undergone by squaric acid crystals as a function of temperature and pressure. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1402166
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  • 2
    Electronic Resource
    Electronic Resource
    Factors governing the charge density wave patterns of layered transition-metal compounds of octahedral coordination with d2 and d3 electron counts (1993)
    s.l. : American Chemical Society
    Inorganic chemistry 32 (1993), S. 4094-4097 
    Details
    ISSN: 1520-510X
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ic00071a021
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  • 3
    Electronic Resource
    Electronic Resource
    A density functional study of crystalline acetic acid and its proton transfer polymorphic forms (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9208-9216 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a density functional study of the structure and dynamics of solid acetic acid. Our calculations are based on density functional theory combined with molecular dynamics, within the Car–Parrinello scheme. The computed structure of the acetic acid crystal, optimized without symmetry constraints, is in very good agreement with the experiment and reproduces the changes in the intramolecular structure when going from the gas phase to the solid. The cell parameters of the experimental structure are also well reproduced. Cooperative effects along the molecular chains building the crystal are found to be small (1.2 kcal/mol), although larger than it had been previously estimated. The anti conformation of the COOH fragment leads to a stable structure up to 250 K, with an energy of only 3 kcal/mol above that of the known syn form. The energy barrier associated with the most likely pathway for the syn to anti conformations, involving proton transfer along the OH(centered ellipsis)O units (〈5.8 kcal/mol) is much smaller than the experimental barrier for the syn/anti isomerism in gas phase and in solution. Intramolecular and intermolecular reorganizations upon change to the anti structure are analyzed. Overall, our results pinpoint the anti form as a good candidate for a possible acetic acid polymorph. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1319699
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  • 4
    Electronic Resource
    Electronic Resource
    Theoretical study of the structure and vibrational spectra of the (H2O)2…HF and H2O…(HF)2 molecular complexes (1994)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 52 (1994), S. 177-189 
    Details
    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: Using the Hartree-Fock and MP2 methods with bases of up to 6-31++G(2d, 2p) quality, the optimum geometry of the 1:2 and 2:1 (H2O)n… (HF)m complexes of water and hydrogen fluoride is searched in a systematic way. Two minimum-energy conformations are found for the 1:2 complex connected through a low-energy transition state. For the 2:1 complex, only one minimum-energy structure is obtained. The analysis of the geometries of the minima and their vibrational frequencies shows that none of them can be used to explain the existence of the H …F—H reverse complex detected experimentally. © 1994 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560520118
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  • 5
    Electronic Resource
    Electronic Resource
    A comparative study of O2, CO, and NO binding to iron-porphyrin (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 69 (1998), S. 31-35 
    Details
    ISSN: 0020-7608
    Keywords: iron-porphyrin ; density functional calculations ; oxyheme ; heme models ; Car-Parrinello molecular dynamics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Minimum-energy structures of O2, CO, and NO iron-porphyrin (FeP) complexes, computed with the Car-Parrinello molecular dynamics, agree well with the available experimental data for synthetic heme models. The diatomic molecule induces a 0.3-0.4 Å displacement of the Fe atom out of the porphyrin nitrogen (Np) plane and a doming of the overall porphyrin ring. The energy of the iron-diatomic bond increases in the order Fe(SINGLE BOND)O2 (9 kcal/mol) 〈 Fe(SINGLE BOND)CO (26 kcal/mol) 〈 Fe(SINGLE BOND)NO (35 kcal/mol). The presence of an imidazole axial ligand increases the strength of the Fe(SINGLE BOND)O2 and Fe(SINGLE BOND)CO bonds (15 and 35 kcal/mol, respectively), with few structural changes with respect to the FeP(CO) and FeP(O2) complexes. In contrast, the imidazole ligand does not affect the energy of the Fe(SINGLE BOND)NO bond, but induces significant structural changes with respect to the FeP(NO) complex. Similar variations in the iron-imidazole bond with respect to the addition of CO, O2, and NO are also discussed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 31-35, 1998
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxygen binding to iron-porphyrin: A density functional study using both LSD and LSD+GC schemes (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 387-394 
    Details
    ISSN: 0020-7608
    Keywords: iron-porphyrin ; density functional calculations ; oxyheme ; heme models ; Car-Parrinello molecular dynamics ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Equilibrium structures of iron-porphyrin (FeP) and its O2 complex, computed with Car-Parrinello molecular dynamics, are in excellent agreement with experimental data on synthetic heme models. Geometry optimization can affect significantly the relative energies among spin states of these systems. The Fe—O bond, best described as FeIII—O2-, increases its strength from 9 to 15 kcal/mol upon binding an imidazole axial ligand (Im). Our results are consistent with an open-shell singlet as the ground state of FeP(Im)(O2), in competition with a low-lying closed-shell singlet state and, as the FeOO angle increases, with a triplet state. The conformation in which the imidazole plane and the O—O axis projection lie in the same N—Fe=N porphyrin quadrant is found to be stable, although easy rotation of the O2 molecule around the Fe—O axis at room temperature is predicted. Comparison of the performance between the local spin density approximation (LSD) and the gradient-corrected scheme (LSD+GC) is provided.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 387-394, 1998
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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