Nanoclusters of Silicon and Germanium

Synthetic routes for the preparation of Si or Ge nanoclusters as gaseous species, colloids, supported composites, or as unsupported powders are reviewed along with selected characterization data. The optical properties of these and related materials, such as porous Si, are summarized with particular emphasis on photo- or electroluminescence phenomena. Research opportunities related to Si and Ge cluster chemistry are suggested.     

Antiferromagnetic three-dimensional order induced by carboxylate bridges in a two-dimensional network of [Cu3(dcp)2(H2O)4] trimers

A new Cu(II) complex, [Cu(3)(dcp)(2)(H(2)O)(4)](n), with the ligand 3,5-pyrazoledicarboxylic acid monohydrate (H(3)dcp) has been prepared by hydrothermal synthesis, and it crystallizes in the monoclinic space group P2(1)/c with a = 11.633(2) A, b = 9.6005(14) A, c = 6.9230(17) A, beta = 106.01(2) degrees, and Z = 2. In the solid state structure of [Cu(3)(dcp)(2)(H(2)O)(4)](n), trinuclear [Cu(3)(dcp)(2)(H(2)O)(4)] repeating units in which two dcp(3-) ligands chelate the three Cu(II) ions with the central Cu(II) ion, Cu(1) (on an inversion center), link to form infinite 2D sheets via syn-anti equatorial-equatorial carboxylate bridges between Cu(2) atoms in adjacent trimers. These layers are further linked by syn-anti axial-equatorial carboxylate bridging between Cu(1) atoms in adjacent sheets resulting in the formation of a crystallographic 3D network. A detailed analysis of the magnetic properties of [Cu(3)(dcp)(2)(H(2)O)(4)](n) reveals that the dcp(3-) ligand acts to link Cu(II) centers in three different ways with coupling constants orders of magnitude apart in value. In the high temperature region above 50 K, the dominant interaction is strongly antiferromagnetic (J/k(B) = -32 K) within the trimer units mediated by the pyrazolate bridges. Below 20 K, the trimer motif can be modeled as an S = 1/2 unit. These units are coupled to their neighbors by a ferromagnetic interaction mediated by the syn-anti equatorial-equatorial carboxylate bridge. This interaction has been estimated at J(2D)/k(B) = +2.8 K on the basis of a 2D square lattice Heisenberg model. Finally, below 3.2 K a weak antiferromagnetic coupling (J(3D)/k(B) = -0.1 K) which is mediated by the syn-anti axial-equatorial carboxylate bridges between the 2D layers becomes relevant to describe the magnetic (T, H) phase diagram of this material.     

The configurational dependence of binding free energies: A Poisson–Boltzmann study of Neuraminidase inhibitors

The linear finite difference Poisson-Boltzmann (FDPB) equation is applied to the calculation of the electrostatic binding free energies of a group of inhibitors to the Neuraminidase enzyme. An ensemble of enzyme-inhibitor complex conformations was generated using Monte Carlo simulations and the electrostatic binding free energies of subtly different configurations of the enzyme-inhibitor complexes were calculated. It was seen that the binding free energies calculated using FDPB depend strongly on the configuration of the complex taken from the ensemble. This configurational dependence was investigated in detail in the electrostatic hydration free energies of the inhibitors. Differences in hydration energies of up to 7 kcal mol^–1 were obtained for root mean square (RMS) structural deviations of only 0.5 Å. To verify the result, the grid size and parameter dependence of the calculated hydration free energies were systematically investigated. This showed that the absolute hydration free energies calculated using the FDPB equation were very sensitive to the values of key parameters, but that the configurational dependence of the free energies was independent of the parameters chosen. Thus just as molecular mechanics energies are very sensitive to configuration, and single-structure values are not typically used to score binding free energies, single FDPB energies should be treated with the same caution.     

Changes in calmodulin main-chain dynamics upon ligand binding revealed by cross-correlated NMR relaxation measurements

The fast dynamics of protein backbones are often investigated by nuclear magnetic relaxation experiments that report on the degree of spatial restriction of the amide bond vector. By comparing calmodulin in the peptide-bound and peptide-free states with these classical methods, we observe little difference in the dynamics of the polypeptide main chain (average order parameter decrease of 0.01 unit upon binding). However, when using NMR methods that monitor the mobility of the CO-Calpha bond vector, we reveal a significant reduction of dynamics of the protein main chain (average order parameter decrease of 0.048 units). Previous investigations have suggested that the side-chain dynamics is reduced by an average of 0.07 order parameter units upon ligand binding (Lee, A. L.; Kinnear, S. A.; Wand, A. J. Nat. Struct. Biol. 2000, 7, 72-77). The current findings suggest that the change of the CO-Calpha bond vector dynamics is intermediate between the changes in NH and side-chain dynamics and report a previously undetected loss of main-chain entropy. Weak site-to-site correlations between the different motional indicators are also observed.     

Fully imidized soluble polyimides based on a novel dianhydride: 2,2′-dichloro-4,4′,5,5′-benzophenone tetracarboxylic dianhydride

We have synthesized a novel dianhydride, 2,2′-dichloro-4,4′,5,5′-benzophenone tetracarboxylic dianhydride (DCBTDA). Polyimides were synthesized with DCBTDA or 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and several relatively rigid meta- and para- substituted mononuclear diamines. The BTDA based systems were insoluble in dipolar, aprotic solvents whereas the DCBTDA based polymers displayed enhanced solubility in these solvents. The thermal stability of these polyimides was excellent as measured by 5% weight loss decomposition. The T_g's of the polymers were all above 290°C.     

Novel species in the electrochemical reduction of a metalmetal bond bridged by a bidentate fluorophosphine

The purple bridged bimetallic complex [CH₃N(PF₂)₂]₃Co₂(CO)₂ undergoes successive chemically and electrochemically reversible one-electron reductions to the corresponding green radical anion and pale-yellow dianion. The radical anion is relatively unreactive towards oxygen and methyl iodide. The dianion is not only reactive towards oxygen and methyl iodide but also captures small positively charged species (e.g. Li⁺ and H⁺) with significant alteration of its chemical properties.     

Clar valence bond representation of pi-bonding in carbon nanotubes

The application of the Clar aromatic sextet valence bond (VB) model to extended, defect-free single-walled carbon nanotubes (CNTs) with roll-up vectors (m, n) provides a real space model of their electronic structure. If m - n = 3k, where k is an integer, then all pi-electrons can be represented by aromatic sextets, and the CNT is fully benzenoid; the converse is also true. Since m - n = 3k is known to be a necessary criterion for conductivity in CNTs, only fully benzenoid CNTs are metallic, and only potentially metallic CNTs are fully benzenoid. This behavior contrasts with that of planar polycyclic aromatic hydrocarbons, in which the fully benzenoid structures are known to have large HOMO-LUMO gaps. For CNTs that are not fully benzenoid, e.g., m - n = 3k + l, where l = 1 or 2 and k is an integer, a seam of double bonds wraps about an otherwise benzenoid CNT at the chiral angle - 60 degrees or the chiral angle, respectively. Nucleus-independent chemical shift calculations on hydrogen-terminated CNT segments support this, and show that the magnetic manifestation of aromatic sextets is not due to electron correlation. The resonance hybrid of the Clar VB structures corresponds to patterns occasionally observed in scanning tunneling microscopy images of CNTs.     

Gradient corrections in density functional theory calculations for surfaces: Co on Pd{110}

Ab initio total energy calculations have been performed for CO chemisorption on Pd{110}. Local density approximation (LDA) calculations yield chemisorption energies which are significantly higher than experimental values but inclusion of the generalised gradient approximation (GGA) gives better agreement. In general, sites with higher coordination of the adsorbate to surface atoms lead to a larger degree of overbinding with LDA, and give larger corrections with GGA. The reason is discussed using a first-order perturbation approximation. It is concluded that this may be a general failure of LDA for chemisorption energy calculations. This conclusion may be extended to many surface calculations, such as potential energy surfaces for diffusion.     

Thermal analysis of PMMA/gel silica glass composites

Thermal analysis of poly-methylmethacrylate (PMMA) impregnated porous gel silica glasses confirms that the PMMA chains form hydrogen bonds with the pore surface silanol groups. The adopted conditions for the insitu polymerisation result in about 4% of residual monomers trapped in the polymer, most of them in the amorphous structure. The polymer and monomer mixture takes up the whole of the free pore volume. Most of the residual monomer polymerises during the DSC scans above the glass transition temperature providing an excellent probe for the weak glass transition. Polymerisation in the gel silica glass medium affects the glass transition temperature, the length of polymer chains, and the degree of polymerisation.