Electronic Structure and Reactivity of a [1],[1]Disilamolybdenocenophane

The electronic structure of the two‐fold bridged [1],[1]disilamolybdenocenophane has been analyzed by means of density functional theory. As predicted, the relatively high charge at the metal center and, in particular, the highly strained geometry determine a noticeable reactivity towards unsaturated organic substrates. Thus, treatment with the nonpolar reagents 2‐butyne and azobenzene leads to side‐on coordination of the substrate to the metal center, whereas the reaction with polar tert‐butylisonitrile gives a highly unusual structural motif in the form of an ansa‐carbene.     

UC Pd: A New Form of Pd/C for Sonogashira Couplings

Screening of different sources of Pd/C shows reagents of highly variable nanoparticle sizes and oxidation states of the metal. Typically, catalysts with higher surface area are viewed as likely to be the more reactive. In this paper a new form of Pd/C, “UC Pd” is described that is shown to contain larger nanoparticles yet it is the most reactive catalyst of those sold commercially for Sonogashira coupling reactions. UC Pd functions efficiently in the absence of a copper co‐catalyst, under very mild and “green” conditions using inexpensive 95 % EtOH at 50 °C. It is also the only form of Pd/C that can be recycled. In side‐by‐side reactions with several commercially available forms of Pd/C, none compete successfully with UC Pd under standardized conditions. Physical data obtained from extensive surface analysis using TEM, XRD, XPS, and CO‐TPD measurements lead to an explanation behind the unique reactivity of this new recyclable form of Pd/C.     

Direct Electrochemistry of Cytochrome c at Modified Si(100) Electrodes

This paper demonstrates the direct electron transfer between the heme moiety of horse hearth cytochrome c and a pyridinyl group on self‐assembled‐monolayer‐modified Si(100) electrodes. Self‐assembled monolayers (SAMs) containing the putative receptor ligand were prepared by a step‐wise procedure using “click” reactions of acetylene‐terminated alkyl monolayers and isonicotinic acid azide derivatives. Unoxidized Si(100) electrodes, possessing either isonicotinate or isonicotinamide receptor ligands, were characterized using X‐ray photoelectron spectroscopy, contact‐angle goniometry, cyclic voltammetry, and electrochemical impedance spectroscopy. The ability of isonicotinic acid terminated layers to coordinatively bind the redox center of cytochrome c was found to be restricted to pyridinyl assemblies with a para‐ester linkage present. The protocol detailed here offers an experimentally simple modular approach to producing chemically well‐defined SAMs on silicon surfaces for direct electrochemistry of a well‐studied model redox protein.     

7‐Amino‐1‐(2‐deoxy‐β‐erythro‐pentofuranosyl)‐1H‐1,2,3‐triazolo[4,5‐d]pyrimidine: an 8‐azaadenine nucleoside with the nucleobase in a syn conformation

The title compound, C₉H₁₂N₆O₃, shows a syn‐glycosylic bond orientation [χ = 64.17 (16)°]. The 2′‐deoxyfuranosyl moiety exhibits an unusual C1′‐exo–O4′‐endo (₁T⁰; S‐type) sugar pucker, with P = 111.5 (1)° and τ_m = 40.3 (1)°. The conformation at the exocyclic C4′—C5′ bond is +sc (gauche), with γ = 64.4 (1)°. The two‐dimensional hydrogen‐bonded network is built from intermolecular N—H...O and O—H...N hydrogen bonds. An intramolecular bifurcated hydrogen bond, with an amino N—H group as hydrogen‐bond donor and the ring and hydroxymethyl O atoms of the sugar moiety as acceptors, constrains the overall conformation of the nucleoside.     

Weighted Fractal Networks

In this paper we define a new class of weighted complex networks sharing several properties with fractal sets, and whose topology can be completely analytically characterized in terms of the involved parameters and of the fractal dimension. General networks with fractal or hierarchical structures can be set in the proposed framework that moreover could be used to provide some answers to the widespread emergence of fractal structures in nature.     

Cleaning of artificially soiled paper using nanosecond, picosecond and femtosecond laser pulses

Cleaning of cultural assets, especially fragile organic materials like paper, is a part of the conservation process. Laser radiation as a non-contact tool offers prospects for that purpose. For the studies presented here, paper model samples were prepared using three different paper types (pure cellulose, rag paper, and wood-pulp paper). Pure cellulose serves as reference material. Rag and wood-pulp paper represent essential characteristics of the basic materials of real-world artworks. The papers were mechanically soiled employing pulverized charcoal. Pure and artificially soiled paper samples were treated with laser pulses of 28 fs (800 nm wavelength) and 8–12 ns (532 nm) duration in a multi pulse approach. Additionally, the cellulose reference material was processed with 30 ps (532 nm) laser pulses. Damage and cleaning thresholds of pure and soiled paper were determined for the different laser regimes. Laser working ranges allowing for removal of contamination and avoiding permanent modification to the substrate were found. The specimens prior and after laser illumination were characterized by light-optical microscopy (OM) and scanning electron microscopy (SEM) as well as multi spectral imaging analysis. The work extends previous nanosecond laser cleaning investigations on paper into the ultra-short pulse duration domain.