Electrochemical behavior of silver thin films interfaced with yttria-stabilized zirconia

Thin silver films (100–800 nm) were deposited by physical vapor deposition (PVD) on yttria-stabilized zirconia solid electrolyte. The electric percolation as a function of the film thickness was studied during deposition and annealing using a two-electrode in-situ resistance measurement technique. Electrical percolation was achieved in as-deposited films greater than 5.4?±?0.4 nm; however, thermal treatment (550 °C in air) resulted in film dewetting for Ag films as thick as 500 nm and formation of electronically isolated Ag nanoparticles, as was confirmed by SEM and XPS. In thermally treated samples, stable electronic conductivity associated with a continuous percolated network was only observed in samples greater than 600 nm in thickness. The effect of polarization on the electrochemical reactions at the three-phase (electrode-gas-electrolyte) and two-phase (electrode-electrolyte) boundaries of the electrode was investigated by solid electrolyte cyclic voltammetry (SECV) at 350 °C and P _O2?=?6 kPa. With the application of positive potential, silver oxide (Ag₂O) was found to form along the three-phase boundary and then extends within the bulk of the electrode with increasing anodic potentials. By changing the hold time at positive potential, passivating oxide layers are formed which results in a shift in favor of the oxygen evolution reaction at the working electrode. This oxide forms according to a logarithmic rate expression with thick oxides being associated with decrease in current efficiency for subsequent oxide formation.     

Synthesis of Selectively 15N‐Labeled 2′‐O‐{[(Triisopropylsilyl)oxy]methyl}(=tom)‐Protected Ribonucleoside Phosphoramidites and Their Incorporation into a Bistable 32Mer RNA Sequence

We present optimized reaction conditions for the conversion of 2′‐O‐{[(triisopropylsilyl)oxy]methyl}(=tom) protected uridine and adenosine nucleosides into the corresponding protected (3‐¹⁵N)‐labeled uridine and cytidine and (1‐¹⁵N)‐labeled adenosine and guanosine nucleosides 4, 6, 12 , and 18 , respectively (Schemes 1–4). On a DNA synthesizer, the resulting ¹⁵N‐labeled 2′‐O‐tom‐protected phosphoramidite building blocks 19 – 22 were efficiently incorporated into five selected positions of a bistable 32mer RNA sequence 23 (known to adopt two different structures) (Fig. 1). By 2D‐HSQC and HNN‐COSY experiments in H₂O/D₂O 9 : 1, the ¹⁵N‐signals of all base‐paired ¹⁵N‐labeled nucleotides could be identified and attributed to one of the two coexisting structures of 23 .     

Carbonyl- and carboxyl-substituted enediynes: synthesis, computations, and thermal reactivity

The influence of electron-withdrawing groups (carbonyl and carboxyl) at the alkyne termini on the reactivity of enediynes was investigated by a combination of experimental and computational techniques. While the general chemical reactivity of such enediynes, especially if non-benzannelated, is increased markedly, the thermal cyclization, giving rise to Bergman cyclization products, is changed little relative to the parent enediyne system. This is evident from kinetic measurements and from density functional theory (DFT, BLYP/6-31G + thermal corrections) computations of the experimental systems which show that the Bergman cyclization barriers slightly (3-4 kcal/mol) increase, in contrast to earlier theoretical predictions. The effect on the endothermicities is large (DeltaDeltaH(r) = 7-12 kcal/mol). Hence, the increased reactivity of the substituted enediynes is entirely due to nucleophiles or radicals present in solution. This was demonstrated by quantitative experiments with diethylamine and tetramethyl piperidyl oxide (TEMPO) which both give fulvenes through 5-exo-dig cyclizations.     

Über die Reaktion des käfigartigen Kieselsäurederivats [(CH3)2HSi]8Si8O20 mit ungesättigten organischen Verbindungen

Reaction of the Cage-like Silicic Acid Derivative [(CH₃)₂HSi]₈Si₈O₂₀ with Unsaturated Organic Compounds By ²⁹Si, ¹H, and ¹³C NMR investigations were shown that the eight HSi?groups of the double four-ring silicic acid derivative [(CH₃)₂HSi]₈Si₈O₂₀ react with the following unsaturated compounds: vinylcyclohexene, allyl glycidyl ether, methyl methacrylate, octadecene-1, and styrene. The resulting oily products are soluble in organic solvents. The compounds were characterized by the chemical shifts of the ²⁹Si, ¹H, and ¹³C NMR signals. Their formulae are [C₆H₉(CH₂)₂Si(CH₃)₂]₈Si₈O₂₀, [CH₃OOCCH(CH₃)CH₂Si(CH₃)₂]₈Si₈O₂₀, [CH₃(CH₂)₁₇Si(CH₃)₂]₈Si₈O₂₀ and [C₆H₅(CH₂)₂Si(CH₃)₂]₈Si₈O₂₀, and [C₆H₅CH(CH₃)Si(CH₃)₂]₈ Si₈O₂₀, respectively. Mainly the addition reactions do not follow the Markovnikov rule.     

Synthesis of pyrrolidinone PNA: a novel conformationally restricted PNA analogue

To preorganize PNA for duplex formation, a new cyclic pyrrolidinone PNA analogue has been designed. In this analogue the aminoethylglycine backbone and the methylenecarbonyl linker are connected, introducing two chiral centers compared to PNA. The four stereoisomers of the adenine analogue were synthesized, and the hybridization properties of PNA decamers containing one analogue were measured against complementary DNA, RNA, and PNA strands. The (3S,5R) isomer was shown to have the highest affinity toward RNA, and to recognize RNA and PNA better than DNA. The (3S,5R) isomer was used to prepare a fully modified decamer which bound to rU10 with only a small decrease in Tm (delta Tm/mod = 1 degree C) relative to aminoethylglycine PNA.     

Adsorptionsuntersuchungen an Organokieselsäurepolymeren

Adsorption Studies on Organosilic Acid Polymers We present four organosilic acid polymers containing double-4-ring silicate units cross-linked at different degree by different organosilicon bridges. BET surfaces were determined and adsorption isotherms of n-hexane, benzene, nitrogen and water were measured. All polymers are hydrophobic, one of them behaves microporously, the other are unporous. With organic adsorptives, swelling and adsorption occur simultaneously. Possible relations of microporosity and structure are discussed.     

Synthese,Aufbau und Eigenschaften Käfigartiger vinyl- und allylsilylierter Kieselsäuren

Synthesis, Constitution and Properties of Cage-like Vinyl- and Allylsilylated Silicic Acids By silyation of tetramethylammonium silicate [N(CH₃)₄]₈Si₈O₂₀ · 69 H₂O with vinyldimethylchlorosilane ( I ) and divinyltetramethyldisiloxane, respectively, or allyldimethylchlorosilane there were synthesized the crystalline silicic esters [CH₂?CH(CH₃)₂Si]₈Si₈O₂₀ and[CH₂?CH? CH₂(CH₃)₂Si]₈Si₈O₂₀. By means of gas chromatography, mass spectrometry, ¹H and ²⁹Si NMR the two compounds were identified to be cage-like double four-ring(D4R)-silicic esters containing eight vinyldimethylsilyl- or allyldimethylsilyl groups, Silylation with a mixture of I and trimethylchlorosilane yields in dependence on the ratio of silanes vinyldimethylsilyltrimethylsilyl D4R silicic esters with average numbers of unsaturated groups < 8.     

Antisymmetric exchange in [2Fe-2S]1+ clusters: EPR of the Rieske protein from Thermus thermophilus at pH 14

[2Fe-2S] clusters found in the xanthine oxidase family of proteins exhibit an S = 1/2 EPR feature, called signal II, for which one g-value is significantly above g = 2.0. The g-values of signal II cannot be explained with the standard spin coupling model that has been so successful in describing the g = 1.94 signals of [2Fe-2S] ferredoxins. We have studied the EPR spectra of the Rieske protein from Thermus thermophilus at pH 14 and observed a signal II-type EPR spectrum, with g-values at 1.81, 1.94, and 2.14. It is shown that the g-values of signal II can be explained by including an antisymmetric exchange term, d.S1xS2, in the spin Hamiltonian. The presence of this term is sensed by EPR if the isotropic exchange coupling constant J is sufficiently small. For the Rieske protein we determined J = 43 cm-1 which is at least 4 times smaller than the J values reported for [2Fe-2S] clusters that yield standard g = 1.94 signals.