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1993 (2)
1Author    Tanja Asthalter, Wolf WeyrichRequires cookie*
 Title    The Anisotropy of the Reciprocal Form Factor and the Electronic Structure of Crystalline Lithium Hydride  
 Abstract    New high-precision measurements of the isotropic as well as of three directional Compton spectra of lithium hydride have been carried out using 24 'Am as a y-ray source. In order to account for the extreme sensitivity of LiH powder to atmospheric moisture, the final data (i.e. the reciprocal form factor) were corrected for the LiOH content determined by titrimetric analysis. For the interpreta-tion of the data, theoretical calculations were carried out using a Hartree-Fock program for periodic systems (CRYSTAL). Basis sets published by Dovesi et al. were used, one of which allows for polarisation of both the hydride and lithium ions. Comparison of the theoretical data with the experiment shows much better agreement of the results of complete solid-state calculations that take into account higher-order effects (polarisation and covalency) than those obtained by Löwdin orthogonalisation of free-ion wave functions (which assumes pure ionicity, neglecting all but first-order effects). The influence of further polarisation functions on the reciprocal form factor is inves-tigated and discussed. The remaining discrepancies are attributed to electron-electron correlation. 
  Reference    Z. Naturforsch. 48a, 303—309 (1993); received January 13 1993 
  Published    1993 
  Keywords    Compton spectroscopy, Reciprocal form factor, Ionic crystal, Electronic structure, Lithium hydride 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0303.pdf 
 Identifier    ZNA-1993-48a-0303 
 Volume    48 
2Author    M. Weisser, W. WeyrichRequires cookie*
 Title    The Electronic Structure of Hydrogen Bonding in a-Oxalic Acid Dihydrate  
 Abstract    Compton spectra of a-oxalic acid dihydrate single-crystal slices and powder samples were mea-sured in a helium atmosphere using 59.537 keV 241 Am radiation. In the anisotropic case, the scattering vector was oriented parallel to the shortest hydrogen bond, in the direction of the C = O bond, parallel to the C — O bond and perpendicular to the (a, c)-plane. The comparison of theoretical reciprocal form factors, calculated with a local density-approximation (LDA) method for an a-(COOH) 2 • 2H 2 0 cluster in triple-zeta basis-set quality, with the experimental data has demon-strated that there is a considerable influence of intermolecular interactions on the electronic struc-ture. Further calculations including the intermolecular interactions of the first coordination sphere by symmetrical orthogonalisation of the LDA wave functions clearly improved the agreement of experiment and theory. A theoretical analysis of the reciprocal form factor B a (s) in the direction of the three different hydrogen bonds of a-oxalic acid dihydrate on the basis of the cluster calculations has shown that in the region above s = 1.9 Ä all these bonds are dominated by electrostatic attrac-tion and exchange repulsion of the donor and the acceptor system. 
  Reference    Z. Naturforsch. 48a, 315—324 (1993); received January 28 1993 
  Published    1993 
  Keywords    a-Oxalic acid dihydrate, Compton spectroscopy, Hydrogen bonding, Reciprocal form factor, Density functional calculations 
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 TEI-XML for    default:Reihe_A/48/ZNA-1993-48a-0315.pdf 
 Identifier    ZNA-1993-48a-0315 
 Volume    48