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'Electronic Structure' in keywords Facet   Publication Year 1998  [X]
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1Author    Christian Kollmar, SighartF. Fischer, MichaelC. BöhmRequires cookie*
 Title    MO-Theoretical Studies on a Model Complex for Deoxymyoglobin  
 Abstract    The origin of the displacement of the Fe atom in deoxymyoglobin with respect to the porphyrin plane in the high-spin state is examined by a qualitative molecular orbital (MO) analysis on the extended Hückel level. We find that attachment of a fifth ligand (imidazole in our model complex) to Fe(II)porphyrin favors the out-of-plane shift due to a strengthening of the bonding interaction between Fe and the nitrogen of the imidazole ligand. This results in a high-spin (5 = 2) ground state with Fe shifted out-of-plane for the five-coordinate complex instead of an intermediate spin ground state (5=1) with Fe lying in the plane for four-coordinate Fe(II)porphyrin. The relative energies of the different spin states as a function of the distance between Fe and the porphyrin plane are evaluated using an ROHF (restricted open shell Hartree-Fock) version of an INDO (intermediate neglect of differential overlap) method. We observe a level crossing between high-spin and intermediate spin states whereas the low-spin (5 = 0) state remains always higher in energy. 
  Reference    Z. Naturforsch. 53a, 755—765 (1998); received July 1 1998 
  Published    1998 
  Keywords    Metalloporphyrines, Electronic Structure, Structural Preferences, Molecular Orbital Model 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0755.pdf 
 Identifier    ZNA-1998-53a-0755 
 Volume    53 
2Author    Andrea Gruß, MichaelC. Böhm, Joachim Schulte, Klaus-Peter DinseRequires cookie*
 Title    An ab initio Hartree-Fock Investigation of Endohedral Sc@C  
 Abstract    The electronic structure of endohedral Sc@C 82 with a C 82 cage of C 2v symmetry has been studied by ab initio Hartree-Fock (HF) calculations. The optimized position of Sc in the configuration of minimum energy is predicted to be on the two-fold axis of the fullerene cage. In the corresponding configuration Sc is above the center of a hexagon of site symmetry C 2 . This structure of C 2v symmetry is nearly degenerate with C s and C t structures with Sc slightly displaced from the center of the coordinated hexagon. The binding energy of the endohedral complex is larger than 3.55 eV. The ab initio HF data of the C 2v topoisomer of the fullerene unit are compared with new experimental findings and HF results derived for the 3(C 2) topoisomer of the C 82 cage. 
  Reference    Z. Naturforsch. 53a, 801—805 (1998); received June 12 1998 
  Published    1998 
  Keywords    Endohedral Fullerenes, Electronic Structure, ab initio Calculations 
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 TEI-XML for    default:Reihe_A/53/ZNA-1998-53a-0801.pdf 
 Identifier    ZNA-1998-53a-0801 
 Volume    53 
3Author    Z. NaturforschRequires cookie*
 Title    Bonding in an Unusual Nickel Carbide  
 Abstract    The bonding in an unusual nickel carbide polymer network synthesized by Musanke and Jeitschko is analyzed using qualitative, band-structure calculations in the framework of extended Hiickel tight-binding theory. The carbide features infinite, one-dimensional, vertex-sharing chains of Ni squares. Each square is centered by a carbon and flanked by C: units. -In this electron-rich polymer, the Ni atoms act as electron acceptors through their s and p orbitals. The orbital interaction schemes reveal that there is little Ni-Ni bonding and essentially no Ni to C2 back-donation. Molecular orbital interaction diagrams of some molecular models are used to describe the bonding in ^[N i^C s]8 -. We also discuss the stability of the planar building block of the polymer, (CNi.;)4"' (which resembles planar CH4) with respect to a "tetrahedral" alternative and we do so for the extended ^ [ N i3C]4-and ^[Ni.^C?]8-chains. In each model case the tetrahedral alternative is favored, and there is no sign of stabilization of a crucial C L4 orbital. Such stabilization is partially achieved through bonding to two Ca axial to the C in the center of the Ni4 ring, but it takes the entire inter-chain calcium network to effect the observed stabilization. 
  Reference    Z. Naturforsch. 53b, 322—332 (1998); received November 4 1997 
  Published    1998 
  Keywords    Carbides, Molecular Orbitals, Electronic Structure, Metal Carbides, Square-Planar Carbon 
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 TEI-XML for    default:Reihe_B/53/ZNB-1998-53b-0322.pdf 
 Identifier    ZNB-1998-53b-0322 
 Volume    53