Biomedical subjects
Roald Hoffmann
Publications and source records attributed to Roald Hoffmann.
Origin of the Metal-to-Insulator Transition in H(0.33)MoO(3).
The electronic structure of the double octahedral layers present in H(0.33)MoO(3) has been studied. It is shown that, depending on structural details, three bands, two of them having a two-dimensional character and one having a one-dimensional character, can be in competition at the bottom of the t(2g)-block band structure. Both qualitative arguments and detailed computations show that the Fermi surface of the double octahedral layers has a two-dimensional character and does not exhibit nesting vectors. Consequently, the metal-to-insulator transition exhibited by H(0.33)MoO(3) cannot be a Fermi surface driven electronic instability, as recently proposed. An order-disorder transition of the protons is suggested as a more likely origin of this resistivity anomaly.
The Four-Connected Net in the CeCu(2) Structure and Its Ternary Derivatives. Its Electronic and Structural Properties.
The crystallochemistry of and the bonding in the orthorhombic four-connected nets of BaIn(2) (CeCu(2) structure) and of CaPtSn (TiNiSi structure, a derivative of the CeCu(2) structure) are analyzed with approximate molecular orbital calculations. Following the Zintl concept, in BaIn(2) the In(-) ions are isoelectronic with group IV tin and should adopt a four-connected structure. In contrast to alpha-tin, which has a cubic diamond structure, the indium ions in BaIn(2) build up an orthorhombic three-dimensional four-connected net containing distorted tetrahedra and ladder polymers of four-membered rings. In the CeCu(2) structure (space group Imma) two bond angles in these distorted tetrahedra are fixed at 90 degrees. The four-connected net in the CeCu(2) structure is topologically related to the layers in black phosphorus (space group Cmca). In CaPtSn (TiNiSi structure) the orthorhombic four-connected net is formed by (PtSn)(2)(-) ions in an ordered arrangement. Calculations on BaIn(2) and CaPtSn show that the four-connected nets are increasingly stabilized as the valence electron count is increased from 16 to 30 valence electrons per 4 formula units. For more than 30e, the nets are destabilized due to filling of M-E antibonding states. Structural data obtained by precise single crystal investigations for the TiNiSi series CaPdIn (20e), CaPdSn (24e), CaPdSb (28e), and CaAgSb (32e), confirm the results of the extended Hückel calculations. We find an interesting and understandable angular asymmetry of the tetrahedral sites in these ternary compounds.
Hypothetical C(60) Metal-Cluster Fullerides and General Aspects of Tetrahedral Cluster Bonding.
It is geometrically feasible to insert metal-metal-bonded M(4) tetrahedra and M(6) octahedra into the tetrahedral and octahedral holes, respectively, of the fcc C(60) lattice. The electronic structure of the hypothetical tetrahedral variants C(60)(M(4))(2), M = Rh, Co, is analyzed with approximate molecular orbital methods and band structure calculations. These compounds feature M-M and M-C(60) bonding and a variable degree of electron transfer to or from C(60). The C(60)(M(4))(2) phases should be metallic, but we have no way of predicting if they will be superconducting. A number of discrete molecular tetrahedral cluster compounds which serve as models for the solid state materials are analyzed. There is a clear indication that tetranuclear and even mononuclear Rh, Ir, and Co arene complexes should be relatively unstable.
Dioxygen Binding to Dinuclear Iron Centers on Methane Monooxygenase Models.
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Electronic Structure of [Ta(5)(NH)(4)Cl(17)](6-): A Cluster with a Distorted Square Pyramidal Ta(5) Core.
We report approximate molecular orbital calculations on the [Ta(5)(NH)(4)Cl(17)](6-) cluster synthesized by Simon and Meyer. The cluster is based on a "flattened" square pyramid of tantalum atoms, basal bridging imides, and terminal chlorides. This cluster was of interest to us due to the unusual presence of imide ligands, the distorted nature of the metal core, and the possible resemblance to B(5)H(9). Our calculations indicate that metal-metal bonding is limited to Ta(apical)-Ta(basal) bonding, with no significant bonding between the basal metal atoms. The imide ligands, which bridge the base of the pyramid, were found to have a significant amount of capping character. The metal-metal bonding orbitals have some unusual features due to the pyramid's distortion. Additionally, the flattened nature of the pyramid leads to an interesting energy ordering of the metal-metal bonding orbitals.
Compounds Containing Copper-Sulfur Layers: Electronic Structure, Conductivity, and Stability.
Compounds of the general formula MCu(2n)X(n)(+1), where M is a monovalent metal and X is a chalcogen, exhibit relatively high conductivity and an interesting structural pattern of copper-chalcogen layers. The electronic structure of a series of copper-sulfur layers with the Cu(2n)S(n)(+1) stoichiometry was studied using the extended Hückel method. Attention was focused on the unoccupied states at the top of the valence band. These states are Cu-S and Cu-Cu antibonding, which accounts for the observed contraction in the plane of the layers. The same states turn out to be strongly delocalized in the plane of the layers, with both copper and sulfur contribution; high mobility of holes in these states is responsible for the substantial conductivity observed in the corresponding materials. The idea of isodesmic reactions, borrowed from computational organic chemistry, was developed to address the relative stabilities of the copper-sulfur layers. We found the Cu(2)S(2)(-) layer to be less stable than the Cu(4)S(3)(-) layer, in accord with experiment.