Biomedical subjects

Arndt Simon

Publications and source records attributed to Arndt Simon.

21 records · Page 2Linked to original sources

PubMed1999

Magic Electron Counts for Networks of Condensed Octahedral Niobium Clusters in Oxoniobates.

Chemical bonding in networks of octahedral oxoniobate clusters condensed through vertex-sharing is analyzed at the extended Hückel level of theory. Crystal or molecular orbitals of the condensed cluster networks are derived from the Nb-Nb bonding molecular orbitals of the monomeric Nb(6)O(18) cluster. Extended networks are treated by considering crystal orbitals at special points of the corresponding Brillouin zones. We find that cluster electron counts corresponding to optimal bonding in these networks decrease from 14 for the Nb(6)O(18) cluster to 12 per Nb octahedron for a dimer, 11 for a linear chain of clusters, 10-10(1)/(2) for a square network of clusters, and 7(3)/(4)-8(1)/(2) for cubic NbO in which all cluster vertices are shared. These results are in good agreement with both the counts derived from the experimentally observed structures and a previously devised counting scheme. The loss of Nb-Nb bonding at the shared cluster vertices and the Nb-O-Nb pi antibonding involving Nb atoms of neighboring octahedra are the reasons for the reduction of the magic cluster electron counts in the considered networks. It is proposed that similar counting schemes can be developed for other condensed cluster networks.

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PubMed1999

Y(21)I(18)C(14)B(7): Synthesis, Average Structure, and Structural Misfit.

The new compound Y(21)I(18)C(14)B(7) was prepared from YI(3), Y, C, and B at 1450 K. The average crystal structure was determined from single-crystal X-ray data. The structure consists of pseudoorthorhombic twisted metal atom double layers containing quasi-molecular CBC units. These are sandwiched by pseudohexagonal iodine layers. Owing to the symmetry and metric misfit the structure must be described with a large triclinic unit cell (space group P&onemacr;, a = 10.660(2) Å, b = 15.546(3) Å, c = 18.416(3) Å, alpha = 82.49(2) degrees, beta = 85.01(2) degrees, gamma = 82.92(2) degrees, Z = 2), which is probably only a good space-averaged approximation. Significant deviations from the averaged structure are indicated by unusual displacement parameters and the results of high-resolution electron microscopy.

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PubMed1996

Experimental and Theoretical Electronic Structure Investigations on alpha-Nb(3)Cl(8) and the Intercalated Phase beta'-NaNb(3)Cl(8).

The electronic structures of the cluster compound alpha-Nb(3)Cl(8) and the intercalated phase beta'-NaNb(3)Cl(8) have been studied by core level and valence band X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), diffuse reflectance spectroscopy, and charge-self-consistent molecular orbital (CSC-EH) and band structure (CSC-EH-TB) calculations. The crystal structures of the two compounds consist of layers of interconnected Nb(3)Cl(13) units. XP and UP valence band spectra as well as the band structure calculations show well separated sets of Cl 3p levels at lower energy (higher binding energy) and Nb 4d levels at higher energy (lower binding energy), indicative of mainly ionic Nb-Cl bonding. The UP spectra of alpha-Nb(3)Cl(8) reveal a triple-peak structure for the Nb 4d levels, corresponding to the 1a(1), 1e, and 2a(1) metal-metal bonding orbitals of a seven-electron Nb(3) cluster as suggested by theory. The valence band shapes are in good agreement with the theoretical density-of-states curve. The relative intensities in the XP valence band spectra of beta'-NaNb(3)Cl(8) evidence an additional electron in the Nb 4d orbitals. The better resolved UP spectra, however, show a broad pattern for the Nb 4d levels which is neither expected from a single cluster model nor given by the theoretical density-of-states curve. Possible origins for this discrepancy between experiment and theory are discussed. Electrostatic interactions between Na(+) and Cl(-) ions in beta'-NaNb(3)Cl(8) lead to a narrowing of the Cl 2p core level and Cl 3p valence band signals, the former being shifted to higher binding energy in comparison to alpha-Nb(3)Cl(8). Two rather narrow absorptions in the optical spectra of alpha-Nb(3)Cl(8) are assigned to the transitions from the 1a(1) and 1e levels into the singly occupied 2a(1) orbital. These absorptions are missing in the optical spectra of beta'-NaNb(3)Cl(8), in agreement with the theoretical expectations for an eight electron Nb(3) cluster.

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