Synthesis and properties of water-soluble gold colloids covalently derivatized with neutral polymer monolayers

Citrate-capped gold nanoparticles as well as planar gold surfaces can be efficiently grafted with a covalently attached polymer monolayer a few nanometers thick, by simple contact of the metal surface with dilute aqueous solutions of hydrophilic polymers that are end-capped with disulfide moieties, as shown by UV/vis absorption, dynamic light scattering, and surface plasmon resonance studies. The hydrophilic polymer-coated gold colloids can be freeze-dried and stored as powders that can be subsequently dissolved to yield stable aqueous dispersions, even at very large concentrations. They allow for applying filtrations, gel permeation chromatography, or centrifugation. They do not suffer from undesirable nonspecific adsorption of proteins while allowing the diffusion of small species within the hydrogel surface coating. In addition, specific properties of the original hydrophilic polymers are retained such as a lower critical solution temperature. The latter feature could be useful to enhance optical responses of functionalized gold surfaces toward interaction with various substrates.     

Multi-conformational compounds with two absorbing groups

Two methods are proposed for dealing with a three-membered ring which is incorporated into a large molecule. The first, an all-valence electron approach, is an iterative CNDO/2 which provides, through bond indices, self-consistent hybridizations and valency angles. The second is compatible with -electron approaches and aims at the study of conjugation and transmission of conjugation. The methods are illustrated by application to the unsubstituted cyclopropane molecule.

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Zusammenfassung Es werden zwei Methoden zur Behandlung von Molekülen, die einen Dreiring enthalten, vorgeschlagen. Die eine besteht in der iterativen Anwendung des CNDO/2-Verfahrens mit selbst-konsistenten Hybridisierungen und Valenzwinkeln. Die zweite entspricht etwa einem -Elektronenverfahren und dient besonders dem Studium von Konjugationseffekten. Illustriert werden beide am Beispiel des einfachen Cyclopropan.

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Résumé On propose deux méthodes pour étudier les molécules contenant une composante cyclopropanique. La première, qui tient compte de tous les électrons de valence, est une CNDO/2 itérative qui fournit — par l'intermédiaire des indices carrés de liaison — des valeurs auto-cohérentes pour l'hybridation et les angles de valence. L'autre, compatible avec les techniques -électroniques, a pour but l'étude de la conjugaison et de la transmission de conjugaison. Les méthodes sont illustrées par application au cyclopropane non-substitué.

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For Part V, see Ref. [1].     

Divalent Samarium: Synthesis and Crystal Structures of Sm4OCl6 and KSm2Cl5

Single crystals of Sm₄OCl₆ and KSm₂Cl₅ have been obtained by metallothermic reductions of SmCl₃ with lithium (in the presence of Sm₂O₃ or SmOCl) and potassium, respectively, at elevated temperatures in sealed tantalum containers. Sm₄OCl₆ (hexagonal, P6₃mc, Z = 2, a = 946.59(4), c = 717.88(4) pm) and KSm₂Cl₅ (monoclinic, P2₁/c, Z = 4, a = 888.06(6), b = 784.81(5), c = 1262.77(8) pm, ß = 90.085(6)°) are true divalent samarium compounds, Sm₄OCl₆ with remarkably short Sm²⁺–O^2? distances (236.0, 237.6 (3x) pm) within the [Sm₄O] tetrahedron.     

Möbius aromaticity in [12]annulene: cis-trans isomerization via twist-coupled bond shifting

Density functional and coupled cluster calculations show that facile thermal configuration change in [12]annulene occurs via a twist-coupled bond-shifting mechanism. The transition state for this process is highly aromatic with M?bius topology. At the CCSD(T)/cc-pVDZ//BH&HLYP/6-311+G** level, the isomerization of tri-trans-[12]annulene 1a (CTCTCT) to its di-trans isomer 2 (CCCTCT) via such a mechanism has a barrier of 18.0 kcal/mol, in good agreement with earlier experiments. Two other aromatic M?bius bond-shifting transition states were located that result in configuration change for other [12]annulene conformers. This mechanism contrasts sharply with diradical configuration change for acyclic polyenes and with planar bond-shifting mechanisms generally assumed for annulenes. This constitutes evidence that neutral M?bius aromatic annulenes play a role in the dynamic processes of neutral [4n]annulenes.     

Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.     

Longitudinal nuclear relaxation measurements in hcp H2

Measurements of T₁ in the hep phase of H₂, over the temperature range 2°–12°K and the ortho concentration range between 0.5 and 0.97 are presented. At temperatures below 10°K, the thermally activated self-diffusion is negligible and the mechanism for nuclear relaxation is that attributed by Moryia and Motizuki and by Harris to intramolecular dipolar interaction, modulated by intennolecular electric quadrupole-quadrupole (EQQ) interaction. The gaussian approximation for the correlation function was used by these authors to predict T₁. From the comparison between experiment and theory, we determine the EQQ parameter Γ/k_B to be 0.67°K. Above 10°K the effect of diffusion influences T₁, and the experimental results for an 88 per cent ortho H₂ sample up to the melting point suggest that the relaxation mechanisms resulting from EQQ interaction and diffusion are not independent of one another.     

Self-intersection points in classical area-minimizing surfaces

In this paper we study the structure at a point of tangential self-intersection in an area-minimizing classical minimal surface inR ⁿ. The result is that the lowest order homogeneous term in the splitting function, i.e. the difference of the height functions from the tangent plane, has the form