Tl-based superconducting films prepared by aerosol spray deposition and thallinated in an open system

Superconducting Tl-based films were prepared on a LaAlO₃ single crystal substrate. Spray pyrolysis of Ba, Ca and Cu nitrate solutions was used for deposition of the precursor films. They were subsequently ex-situ thallinated in flowing oxygen (open system). While the superconducting Tl-2212 phase formed at an annealing temperature of 880°C, thallination at 900°C led to the formation of a Tl-2223 /Tl-2212 mixture. The amount of Tl-2223 increased with prolonged thallination, whereas the Tl-2212 phase progressively disappeared. Films prepared in such a manner were c-oriented and contained only low amounts of non-superconducting impurities. The resulting samples were characterized by XRD and SEM and their T _C values were determined by resistive four-point measurements. They showed critical temperatures in the range of T _ON = 125–135 K, T _C0 = 91–93 K. Differences between the composition and properties of the obtained films and those thallinated in closed systems under 50 kPa of oxygen are discussed in this paper. Results show that the Tl-2212 → Tl-2223 transformation proceeds at a slower rate under flowing oxygen than in a closed system. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia.     

An efficient approach towards the convergent synthesis of "fully-carbohydrate" mannodendrimers

Glycosylation of sugar trityl ethers with sugar 1,2-O-(1-cyano)ethylidene derivatives (the trityl-cyanoethylidene condensation) has been applied to the synthesis of highly branched (dendritic) mannooligosaccharides incorporating a Manalpha1-->3(Manalpha1-->6)Man structural motif. The convergent synthetic strategy used to assemble these oligosaccharides was based on the use of glycosyl acceptors and/or a glycosyl donor already bearing this structural motif. The former were represented by mono- and ditrityl ethers of ManalphaOMe, Manalpha1-->3ManalphaOMe, and Manalpha1-->3(Manalpha1-->6)ManalphaX, where X=OMe or SEt. The pivotal glycosyl donor was the peracetylated 1,2-O-(1-cyano)ethylidene-3,6-di-O-(alpha-D-mannopyranosyl)-beta-D-mannopyranose (1), prepared by orthogonal Helferich glycosylation of the known 1,2-O-(1-cyano)ethylidene-beta-D-mannopyranose with tetra-O-acetyl-alpha-D-mannopyranosyl bromide followed by O-acetylation. Glycosylation of acetates of methyl 6-O-trityl-alpha-D-mannopyranoside and methyl 3,6-di-O-trityl-alpha-D-mannopyranoside with one equivalent of the donor 1 gave rise to the isomeric tetrasaccharide derivatives, Manalpha1-->3(Manalpha1-->6)Manalpha1-->6ManalphaOMe and Manalpha1-->3(Manalpha1-->6)Manalpha1-->3ManalphaOMe, respectively. The latter derivative was further mannosylated at the remaining 6-O-trityl acceptor site to give the protected pentasaccharide Manalpha1-->3(Manalpha1-->6)Manalpha1-->3(Manalpha1-->6)ManalphaOMe. The isomeric pentasaccharide, Manalpha1-->3(Manalpha1-->6)Manalpha1-->6(Manalpha1-->3)ManalphaOMe, was prepared by reaction of 1 with the 6-O-trityl derivative of (Manalpha1-->3)ManalphaOMe. In a similar fashion, 6'- and 6"-O-trityl derivatives of the branched trisaccharide Manalpha1-->3(Manalpha1-->6)ManalphaOMe served as precursors for two isomeric mannohexaosides. The 3,6-di-O-trityl ether of ManalphaOMe and the 6',6"-di-O-trityl ether of Manalpha1-->3(Manalpha1-->6)ManalphaX (X=OMe or SEt) were efficiently bis-glycosylated with the donor 1 to give the corresponding protected mannoheptaoside and mannononaoside. The yields of these glycosylations with the donor 1 ranged from 50 to 66 %. Final deprotection of all the oligosaccharides was straightforward and afforded the target products in high yields. Both the acylated and deprotected products were characterized, and the intersaccharide connectivities were elucidated by extensive one- and two-dimensional NMR spectroscopy. The described blockwise convergent approach allows assembly of a variety of 3,6-branched mannooligosaccharides.     

Structure of V(2)O(5)*nH(2)O xerogel solved by the atomic pair distribution function technique

A long-standing issue regarding the local and long-range structure of V(2)O(5)*nH(2)O xerogel has been successfully addressed. The full three-dimensional structure of the lamellar turbostratic V(2)O(5)*nH(2)O xerogel was determined by the atomic pair distribution function technique. We show that on the atomic scale the slabs of the xerogel can be described well as almost perfect pairs (i.e., bilayers) of single V(2)O(5) layers made of square pyramidal VO(5) units. These slabs are separated by water molecules and stack along the z-axis of a monoclinic unit cell (space group C2/m) with parameters a = 11.722(3) A, b = 3.570(3) A, c = 11.520(3) A, and beta = 88.65 degrees. The stacking sequence shows signatures of turbostratic disorder and a structural coherence limited to 50 A.     

Gate‐Induced Modification of Water Adsorption on Dielectrics Probed by EFM and Carbon Nanotube FETs

Humidity plays an important role in molecular electronics. It facilitates charge movement on top of dielectric layers and modifies the device transfer characteristics. Using two different methods to probe temporal charge redistribution on the surface of dielectrics, we were able to extract the surface humidity for the first time. The first method is based on the relaxation time constants of the current through carbon nanotube field‐effect transistors (CNTFETs), and the second is based on electric force microscopy (EFM) measurements. Moreover, we found that applying external gate biases modifies the surface humidity. A theoretical model based on dielectrophoretic attraction between the water molecules and the substrate is introduced to explain this observation, and the results support our hypothesis. Furthermore, it is found that upon the adsorption of two to three layers of water the surface conductivity saturates.     

Alkali Metal Triphenyl- and Trihydridosilanides Stabilized by a Macrocyclic Polyamine Ligand

Potassium silanide [KSiH₃]_∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me₄TACD (1,4,7,10-tetramethyl-1,4,7,10-tetraaminocyclododecane, L ) stabilizes the full range of triphenylsilyl complexes [( L )MSiPh₃]_n (M=Li–Cs), which react with H₂ or PhSiH₃ to form molecular [( L )MSiH₃]_n that can be isolated in soluble form and fully characterized.     

tert-Butylthiacalix[4]arene monolayers as a biomimetic model for the oxidation of antioxidants with cytochrome c

The possibility of designing a model of sensor to antioxidants based on thiacalix[4]arene monolayers with immobilized cytochrome c was shown. The molecular surface area S ₀ of thiacalix[4]arene in the monolayers and the surface pressure coefficient ?dπ/dS (elasticity) reflect changes in the redox state of cytochrome c in the presence of dihydroquercetin and ascorbic acid in the aqueous subphase. The absorption spectra in the visible and UV ranges of solutions of the subphase and transferred thiacalix[4]arene monolayers with immobilized cytochrome c confirm the oxidation of the antioxidants to quinones and formation of the reduced form of cytochrome c.     

Electronic structure and stability of fullerene C82 isolated-pentagon-rule isomers

All nine isolated-pentagon-rule isomers of fullerene C(82) were investigated by the DFT method with the B3LYP functional at the 6-31G, 6-31G*, and 6-31+G* levels. The distribution of single, double, and delocalized π-bonds in the molecules of these isomers is shown for the first time. The obtained results are fully supported by DFT quantum-chemical calculations of electronic and geometrical structures of these isomers. The molecules of isomers 7 (C(3v)), 8 (C(3v)), and 9 (C(2v)) contain some radical substructures (such as the phenalenyl-radical substructure), which indicates that they are unstable and cannot be obtained as empty molecules. Thus, there is a possibility of obtaining them only as endohedral metallofullerenes or exohedral derivatives. Isomers 1 (C(2)), 2 (C(s)), 4 (C(s)), 5 (C(2)), and 6 (C(s)) with closed electronic shell are supposed to be stable, resembling isomer 3 (C(2)), which has just been extracted experimentally as an empty fullerene. We assume they can be produced as empty molecules.     

Calcium-mediated dearomatization, C-H bond activation, and allylation of alkylated and benzannulated pyridine derivatives

A facile and general synthetic pathway for the production of dearomatized, allylated, and C? H bond activated pyridine derivatives is presented. Reaction of the corresponding derivative with the previously reported reagent bis(allyl)calcium, [Ca(C₃H₅)₂] ( 1 ), cleanly affords the product in high yield. The range of N‐heterocyclic compounds studied comprised 2‐picoline ( 2 ), 4‐picoline ( 3 ), 2,6‐lutidine ( 4 ), 4‐tert‐butylpyridine ( 5 ), 2,2′‐bipyridine ( 6 ), acridine ( 7 ), quinoline ( 8 ), and isoquinoline ( 9 ). Depending on the substitution pattern of the pyridine derivative, either carbometalation or C? H bond activation products are obtained. In the absence of methyl groups ortho or para to the nitrogen atom, carbometalation leads to dearomatized products. C(sp³)? H bond activation occurs at ortho and para situated methyl groups. Steric shielding of the 4‐position in pyridine yields the ring‐metalated product through C(sp²)? H bond activation instead. The isolated compounds [Ca(2‐CH₂‐C₅H₄N)₂(THF)] ( 2 b ?(THF)), [Ca(4‐CH₂‐C₅H₄N)₂(THF)₂] ( 3 b ?(THF)₂), [Ca(2‐CH₂‐C₅H₃N‐6‐CH₃)₂(THF)_n] ( 4 b ?(THF)_n; n=0, 0.75), [Ca{2‐C₅H₃N‐4‐C(CH₃)₃}₂(THF)₂] ( 5 c ?(THF)₂), [Ca{4,4′‐(C₃H₅)₂‐(C₁₀H₈N₂)}(THF)] ( 6 a ?(THF)), [Ca(NC₁₃H₉‐9‐C₃H₅)₂(THF)] ( 7 a ?(THF)), [Ca(4‐C₃H₅‐C₉H₇N)₂(THF)] ( 8 b ?(THF)), and [Ca(1‐C₃H₅‐C₉H₇N)₂(THF)₃] ( 9 a ?(THF)₃) have been characterized by NMR spectroscopy and metal analysis. 9 a ?(THF)₄ and 4 b ?(THF)₃ were additionally characterized in the solid state by X‐ray diffraction experiments. 4 b ?(THF)₃ shows an aza‐allyl coordination mode in the solid state. Based on the results, mechanistic aspects are discussed in the context of previous findings.