Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Surface modification of soft-glass capillaries for gas chromatography by treatment with water vapour

“Leaching” or “etching” by strong mineral acids seems to be a necessary pretreatment step for the most commonly used deactivation procedures of glass capillaries by reaction with either polyethylene glycol or silylation reagents. The acidic sites which are formed on the surface during this acid treatment cannot be completely removed by the subsequent deactivation process. This drawback can be overcome by performing the leaching with water vapour, resulting in an accumulation of cations at the surface and a decrease in the number of silanol groups. Capillaries of this type show excellent properties for the chromatography of strongly basic compounds. After the wash-out of the alkaline surface layer, the acidity of the support is suited for the chromatography of strongly basic as well as strongly acidic compounds. Due to a lack of reactive acidic sites, special deactivation procedures have to be applied to capillaries produced in this way.     

Vitamin B12 as an allosteric cofactor; dual fluorescence, hysteresis, oscillations and the selection of corrin over porphyrin

Studies of the emission spectra of four Co(III) cobinamides (diaquo-, aquohydroxo-, dihydroxo- and dicyano-) show (1) that the excited states corresponding to the alphabeta and epsilon absorption bands behave like the S(1) and S(2) levels in the non-alternant hydrocarbon azulene (with emission from S(2)> S(1) in violation of Kasha's rule) and (2) that the excited states include a TICT (twisted intramolecular charge transfer) mechanism, as in the simpler cyanines, but where the TICT state gives rise to dual fluorescence instead of cis-trans isomerisation. Combined with the previously reported dual fluorescence from the S(1) level in synthetic metal corrinoids and in the naturally-occurring metal-free corrin, this provides evidence that the existence of an additional (metastable) ground state with a significantly different vibronic splitting and nuclear configuration is an intrinsic property of the basic corrin ligand (irrespective of the nature of the side-chains and the metal ion or even the absence of a metal) which distinguishes it from porphyrin. The occurrence of hysteresis (and its associated oscillations) in redox reactions of the cobinamides involving both the Co(III/II) and Co(II/I) couples indicates that the corrin ligand also has an intrinsic ability to exist in different conformations or "allosteric" forms with differing redox potential, which further distinguishes it from the porphyrin ligand. Possible links between the existence of an additional metastable ground state and of allosteric changes and the likely reasons for the selection of corrin over a porphyrin for the vitamin B(12)-dependent enzymes are discussed.     

Kubische Berechnung der Retentionsindices ohne Kenntnis der Totzeit tm

Zusammenfassung In der vorliegenden Arbeit wird eine Methode zur Retentionsindex-Bestimmung beschrieben, die von einem kubischen Zusammenhang zwischen Bruttoretentionszeit-Differenzen der Referenzhomologen und der Kohlenstoffzahl ausgeht. Hieraus ergeben sich direkt die Nettoretentionszeiten. Der Fehler der Totzeitbestimmung entfällt bei dieser Methode. Mit den so gewonnenen Nettoretentionszeiten erhält man über einen kubischen Zusammenhang zwischen 1g t_s=f(C) die Retentionsindices. Extrapolationen und Interpolationen sind über 300 Retentionsindexeinheiten mit einem mittleren Fehler von ±0,02 Retentionsindexeinheiten möglich. Das Verfahren bietet sich für eine automatische Berechnung der I-Werte mittels on-line-Datenverarbeitung an.

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Cubic calculation of retention indices without determining the dead-time t_m

Summary The method for the calculation of retention indices described here is based on a third order relationship between the logarithm of differences of unadjusted retention times of homologues and the carbon number. From this adjusted retention times are directly calculated. A determination of the dead-time is not necessary thus avoiding the errors connected with this factor. A cubic equation for the logarithm of the adjusted retention time lg t_s as a function of carbon number C_n is used for the retention index calculation. Extrapolations and interpolations can be done over a range of 300 index units with an average deviation of ±0.02 i.u.. The method offers the possibility of an automated on-line calculation of retention indices by computer merely on the basis of unadjusted retention times.

     

Effect of conformational constraints on gated electron-transfer kinetics. 3. Copper(II/I) complexes with cis- and trans-cyclopentanediyl-1,4,8,11-tetrathiacyclotetradecane

Previous kinetic and electrochemical studies of copper complexes with macrocyclic tetrathiaethers-such as 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4)-have indicated that electron transfer and the accompanying conformational change occur sequentially to give rise to a dual-pathway mechanism. Under appropriate conditions, the conformational change itself may become rate-limiting, a condition known as "gated" electron transfer. We have recently hypothesized that the controlling conformational change involves inversion of two donor atoms, which suggests that "gated" behavior should be affected by appropriate steric constraints. In the current work, two derivatives of [14]aneS4 have been synthesized in which one of the ethylene bridges has been replaced by either cis- or trans-1,2-cyclopentane. The resulting copper systems have been characterized in terms of their Cu(II/I)L potentials, the stabilities of their oxidized and reduced complexes, and their crystal structures. The electron self-exchange rate constants have been determined both by NMR line-broadening and by kinetic measurements of their rates of reduction and oxidation with six or seven counter reagents. All studies have been carried out at 25 degrees C, mu = 0.10 M (NaClO4 and/or Cu(ClO4)2), in aqueous solution. Both Cu(II/I) systems show evidence of a dual-pathway mechanism, and the electron self-exchange rate constants representative of both mechanistic pathways have been determined. The first-order rate constant for gated behavior has also been resolved for the Cu(I)(trans-cyclopentane-[14]aneS4) complex, but only a limiting value can be established for the corresponding cis-cyclopentane system. The rate constants for both systems investigated in this work are compared to values previously determined for the Cu(II/I) systems with the parent [14]aneS4 macrocycle and its derivatives involving phenylene and cis- or trans-cyclohexane substituents. The results are discussed in terms of the influence of the fused rings on the probable conformational changes accompanying the electron-transfer process.     

Scalable bottom‐up assembly of suspended carbon nanotube and graphene devices by dielectrophoresis

Bottom‐up assembly by dielectrophoresis (DEP) has emerged in recent years as a viable alternative to conventional top–down fabrication of electronic devices from nanomaterials, particularly carbon nanotubes and graphene. Here, we demonstrate how this technique can be extended to fabricate devices containing carbon nanotubes and graphene suspended between two electrodes over a back‐gate electrode. The suspended device geometry is critical for the development of nano‐electromechanical devices and to extract maximum performance out of electronic and optoelectronic devices. This technique allows for parallel assembly of devices over large scale. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)