Showing papers in "Journal of the American Chemical Society in 1991"

A stable crystalline carbene

Abstract: Synthese, structure et caracterisation du (1,3-bis [1-adamantyl]-2,3-dihydro)-2-carbenoimidazole prepare par deprotonation du chlorure de (1,3-bis [1-adamantyl]) imidazolium

Comparison of the Structures and Wetting Properties of Self-Assembled Monolayers of n- Alkanethiols on the Coinage Metal Surfaces, Cu, Ag, Au'

Abstract: Long-chain alkanethiols, HS(CH2),CH3, adsorb from solution onto.the surfaces of gold, silver, and copper and form monolayers. Reflection infrared spectroscopy indicates that monolayers on silver and on copper (when carefully prepared) have the chains in well-defined molecular orientations and in crystalline-like phase states, as has been observed on gold. Monolayers on silver are structurally related to those formed by adsorption on gold, but different in details of orientation. The monolayers formed on copper are structurally more complex and show a pronounced sensitivity to the details of the sample preparation. Quantitative analysis of the IR data using numerical simulations based on an average single chain model suggests that the alkyl chains in monolayers on silver are all-trans zig-zag and canted by - 12' from the normal to the surface. The analysis also suggests a twist of the plane containing the carbon backbone of -45' from the plane defined by the tilt and surface normal vectors. For comparison, the monolayers that form on adsorption of alkanethiols on gold surfaces, as judged by their vibrational spectra, are also trans zig-zag extended but, when interpreted in the context of the same single chain model, have a cant angle of -27O and a twist of the plane of the carbon backbone of -53'. The monolayers formed on copper (when they are obtained in high quality) exhibit infrared spectra effectively indistinguishable from those on silver and thus appear to have the same structure. Films on copper are also commonly obtained that are structurally ill-defined and appear to contain significant densities of gauche conformations. These spectroscopically based interpretations are compatible with inferences from wetting and XPS measurements. The structure of the substrate-sulfur interface appears to control molecular orientations of the alkyl groups in these films. An improved structural model, incorporating a two-chain unit cell and allowing for the temperature-dependent population of gauche conformations, is presented and applied to the specific case of the structures formed on gold.

Semiconductor clusters in the sol-gel process: quantized aggregation, gelation, and crystal growth in concentrated zinc oxide colloids

Abstract: A new synthesis of ZnO wurtzite clusters (crystallite sizes 3-6 nm) is presented. This novel approach, employing ultrasound, allows one to produce relatively highly concentrated 0.1 M Q-ZnO colloids within a few minutes. These colloids remains in a dispersed state for weeks. They can be further concentrated into stable syruplike liquids (molarities of ∼1 M). Under the externe concentration conditions (10 M Q-ZnO colloids) employed, ZnO alcogels are formed, and self-induced crystal growth occurs in the alcogels (sizes ranging between 1 and 5 mm)

XPS studies of solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobalt-manganese particles on alumina and silica

Abstract: XPS studies of solvated metal atom dispersed (SMAD) catalysts coupled with detailed studies of reference compounds were carried out. It was shown that toluene-solvated cobalt atoms nucleate at surface OH groups, and the resultant cobalt oxide surface species serve as sites for nucleation of more cobalt atoms, leading to very small, reactive metallic clusters. The surface of these clusters/particles are partially oxidized by adventitious oxygen or water, but when cobalt loadings of over 4% are used, a major portion of the catalytic particle remains metallic. When solvated cobalt and manganese atoms are mixed together, the manganese atoms deposit first at surface OH sites, and manganese is highly dispersed in this way, but the majority ends up as surface MnO and Mn 2 O 3

Formation of self-assembled monolayers by chemisorption of derivatives of oligo(ethylene glycol) of structure HS(CH2)11(OCH2CH2)mOH on gold

Abstract: This paper describes the preparation of oligo(ethylene glycol)-terminated alkanethiols having structure HS- (CHz)'l(OCH2CHz),nOH (m = 3-7) and their use in the formation of self-assembled monolayers (SAMs) on gold. A combination of experimental evidence derived from X-ray photoelectron sp€ctroscopy (XPS), measurement of contact angles, and ellipsometry implies substantial disorder in the oligo(ethylene glycol)-containing segment. The order in the -(CHz)r- group is not defined by the available evidence. The SAMs are moderately hydrophilic: du(H2O) = 34-38o; d,(H2O) = 22-25". A study of monolayers containing mixtures of HS(CH2)rrCHr and HS(CH2)rr(OCH2CH 2)6OH suggests that the oligo(ethylene glycol) moieties are eflective at preventing underlying methylene groups from influencing wetting by water. A limited study demonstrates that these oligo(ethylene glycol)-containing SAMs resist the adsorption of protein from solution and suggests that SAMs will be a useful model system for studying the adsorption of proteins onto organic surfaces.

A molecular shuttle

Abstract: The molecular shuttle described in this communication is the prototype for the construction of more intricate molecular assemblies where the components will be designed to receive, store, transfer, and transmit information in a highly controllable manner, following their spontaneous self-assembly at the supramolecular level. Increasingly, we can look forward to a «bottom-up» approach to nanotechnology which is targeted toward the development of molecular-scale information processing systems

Atom sizes and bond lengths in molecules and crystals

Abstract: Atom size parameters are derived for 75 elements In conjunction with an «electronegativity» parameter, these can be used to determine bond valence parameters and hence to derive bond lengths for a wide variety of homopolar, heteropolar, and metallic bonds The size parameter of an atom is close to the distance from the nucleus at which the electrostatic potential is equal to the first ionization potential