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.     

A redox-active ionic liquid manifesting charge-transfer interaction between a viologen and carbazole and its effect on the viscosity,ionic conductivity,and redox process of the viologen

Redox-active ionic liquids (RAILs) are gaining attention as a material that can create a wide range of functions. We herein propose a charge-transfer (CT) RAIL by mixing two RAILs, specifically a carbazole-based ionic liquid ([CzC₄ImC₁][TFSI]) as a donor and a viologen-based ionic liquid ([C₄VC₇][TFSI]₂) as an acceptor. We investigated the effect of CT interaction on the physicochemical properties of the CT ionic liquid (CT-IL) using the results of temperature-dependent measurements of UV-vis absorption, viscosity, and ionic conductivity as well as cyclic voltammograms. We employed the Walden analysis and the Grunberg–Nissan model to elucidate the effect of the CT interaction on the viscosity and ionic conductivity. The CT interaction reduces the viscosity by reducing the electrostatic attraction between the dicationic viologen and TFSI anion. It also reduces the ionic conductivity by the CT association of the dicationic viologen and carbazole. The electrochemically reversible responses of the viologens in [C₄VC₇][TFSI]₂ and CT-IL are consistent with the Nernstian and the interacting two-redox site models. Notably, the transport and electrochemical properties are modulated by CT interaction, leading to unique features that are not present in individual component ILs. The inclusion of CT interaction in RAILs thus provides a powerful means to expand the scope of functionalized ionic liquids.

A redox-active ionic liquid (RAIL) consisting of a carbazole and viologen shows charge transfer (CT) interaction. The physicochemical properties are modulated by the CT interaction by comparison with the individual RAILs.     

Plasma quantification of quazepam and its 2-oxo and N-desmethyl metabolites by capillary gas chromatography

The authors have developed a gas chromatographic method for the simultaneous quantification of quazepam in plasma and its two main metabolites, 2-oxoquazepam and N-desmethylquazepam. This method involves an extraction from plasma using butyl acetate, and an analysis by electron-capture detection on a CP-Sil 5 WSCOT capillary column. Intra- and inter-day precision and accuracy were better than 10% for each of these three compounds, even near their detection limit estimated at 0.2 ng/ml. Linearity proved satisfactory between 0.2 and 60-70 ng/ml. For endogenous plasma components, adequate specificity was achieved. Despite some inconveniences, a long analysis time, a progressive saturation of the column owing to a low oven temperature, and a relatively short life-span of the CP-Sil 5 columns, this method was the only one available in the literature for the quantification of quazepam and its metabolites from the same plasma sample. It was successfully applied to phase I studies in healthy volunteers.     

MAO‐free and extremely active catalytic system for ethylene tetramerization

The original Sasol catalytic system for ethylene tetramerization is composed of a Cr source, a PNP ligand, and MAO (methylaluminoxane). The use of expensive MAO in excess has been a critical concern in commercial operation. Many efforts have been made to replace MAO with non‐coordinating anions (e.g., [B(C₆F₅)₄]^?); however, most of such attempts were unsuccessful. Herein, an extremely active catalytic system that avoids the use of MAO is presented. The successive addition of two equivalent [H(OEt₂)₂]⁺[B(C₆F₅)₄]^? and one equivalent CrCl₃(THF)₃ to (acac)AlEt₂ and subsequent treatment with a PNP ligand [CH₃(CH₂)₁₆]₂C(H)N(PPh₂)₂ ( 1 ) yielded a complex presumably formulated as [ 1 ‐CrAl (acac)Cl₃(THF)]²⁺[B(C₆F₅)₄]^?₂, which exhibited high activity when combined with iBu₃Al (1120 kg/g‐Cr/h; ~4 times that of the original Sasol system composed of Cr (acac)₃, iPrN(PPh₂)₂, and MAO). Via the introduction of bulky trialkylsilyl substituents such as –SiMe₃, –Si(nBu)₃, or –SiMe₂(CH₂)₇CH₃ at the para‐position of phenyl groups in 1 (i.e., by using [CH₃(CH₂)₁₆]₂C(H)N[P(C₆H₄‐p‐SiR₃)₂]₂ instead of 1 ), the activities were dramatically improved, i.e., tripled (2960–3340 kg/g‐Cr/h; more than 10 times that of the original Sasol system). The generation of significantly less PE (<0.2 wt%) even at a high temperature is another advantage achieved by the introduction of bulky trialkylsilyl substituents. NMR studies and DFT calculations suggest that increase of the steric bulkiness on the alkyl‐N and P‐aryl moieties restrict the free rotation around (alkyl)N–P (aryl) bonds, which may cause the generation of more robust active species in higher proportion, leading to extremely high activity along with the generation of a smaller amount of PE.     

Experimental and Computational Studies of a Multi‐Electron Donor–Acceptor Ligand Containing the Thiazolo[5,4‐d]thiazole Core and its Incorporation into a Metal–Organic Framework

A ligand containing the thiazolo[5,4‐d]thiazole (TzTz) core (acceptor) with terminal triarylamine moieties (donors), N,N′‐(thiazolo[5,4‐d]thiazole‐2,5‐diylbis(4,1‐phenylene))bis(N‐(pyridine‐4‐yl)pyridin‐4‐amine ( 1 ), was designed as a donor–acceptor system for incorporation into electronically active metal–organic frameworks (MOFs). The capacity for the ligand to undergo multiple sequential oxidation and reduction processes was examined using UV/Vis‐near‐infrared spectroelectrochemistry (UV/Vis‐NIR SEC) in combination with DFT calculations. The delocalized nature of the highest occupied molecular orbital (HOMO) was found to inhibit charge‐transfer interactions between the terminal triarylamine moieties upon oxidation, whereas radical species localized on the TzTz core were formed upon reduction. Conversion of 1 to diamagnetic 2+ and 4+ species resulted in marked changes in the emission spectra. Incorporation of this highly delocalized multi‐electron donor–acceptor ligand into a new two‐dimensional MOF, [Zn(NO₃)₂( 1 )] ( 2 ), resulted in an inhibition of the oxidation processes, but retention of the reduction capability of 1 . Changes in the electrochemistry of 1 upon integration into 2 are broadly consistent with the geometric and electronic constraints enforced by ligation.     

Preparation of cycloolefin copolymers of a bulky tricyclopentadiene

Copolymerizations of ethylene and tricyclopentadiene (TCPD) are realized without formation of a gel with a high activity (3–18 × 10⁶ g/molZr·h) using a catalytic system of [Ph₂C(Flu)(Cp)]ZrCl₂/MMAO. The monomer reactivity ratios, r_ethylene and r_TCPD, determined through the Fineman‐Ross plot, are 6.4 and 0.044, respectively, indicating that TCPD insertion is unfavorable, negligibly allowing the successive TCPD insertions. A significant higher glass transition temperature (T_g) is attained than those observed for other reported cycloolefin copolymers at the same cycloolefin content. A T_g as high as 214 °C is attainable at 41 mol % TCPD content. The remaining double bond can be hydrogenated to saturated hydrocarbon or converted to an epoxide group. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

The influence of a transverse magnetic field on a subnormal glow discharge in air

In subnormal glow discharge under d.c. excitation at different pressure in a varying transverse magnetic field (0 to 30 G) some measurements have been carried out for various initial average tube currents. The voltage across the discharge increases and average tube current and residual current decreases in the magnetic field. With the help of Beckman’s expression [4] for the axial field and the electron density distribution in a transverse magnetic field the observed variation of current and voltage can be satisfactorily explained. The variation of axial electric field with transverse magnetic field can be represented to a fair degree of accuracy by the derived equation. The behaviour of residual current with magnetic field has been observed in these oscillations.     

Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.