Spin-transition behaviour of transition metal complexes with 2,6-bis-(benzimidazol-2′-yl)-pyridine induced by deprotonation of the complex

Summary 2,6-bis-(Benzimidazol-2330/xxlarge8242.gif" alt="prime" align="BASELINE" BORDER="0">-yl)-pyridine (bzimpy = H₂ L) acts as a bidentate ligand when combining with transition metal ions. The complexes [M(bzimpy)₂](ClO₄)₂ (M = Fe²⁺, Mn²⁺, Zn²⁺, Co²⁺, and Ni²⁺) were obtained as solids. The protonation constants (logK) for the ligand and the complexes were evaluated in 30:70 (v/v) H₂O:EtOH at 293 K and at constant ionic strength of 0.12M KCl. Coordination of the ligand to the metal ions leads to an increase of acidity of the imino-hydrogen of the benzimidazole group of the ligand as a function of the complex stability. Deprotonation leads to a spin-state transition (intermediate spin-state 330/xxlarge8594.gif" alt="rarr" align="BASELINE" BORDER="0"> low-spin) of the iron(II)-complex, followed by a shift of the metal-to-ligandcharge transfer band (MLCT) to lower energies (330/xxlarge955.gif" alt="lambda" align="BASELINE" BORDER="0">_max=563 to 580 nm). The d-d absorption bands are found to shift to higher energies and the low-spin isomer is favoured at room temperature. An opposite shift of theMLCT band (330/xxlarge955.gif" alt="lambda" align="BASELINE" BORDER="0">_max=563 to 557 nm) is observed when HClO₄ is added to the complex solution, rendering the high-spin state of the complex more favourable.On leave from the Chemistry Department, Jahangirnagar University, Dhaka, Bangladesh     

Conformational studies of 1,2,3-trichloro- and 1,2,3-tribromopropane by nuclear magnetic resonance spectroscopy: attractive parallel (1:3) halogen-hydr

Analysis of the NMR spectra of 1,2,3-trichloropropane and 1,2,3-tribromopropane in various media shows the most stable conformer to be AG_-. The populations of several conformera have been estimated by using pure trans and gauche coupling constants obtained from closely similar molecules. The calculated populations found in non-polar solvents agree well with those obtained by electron diffraction studies in the gas phase. It is suggested that the AG_- form is stabilised relative to AG₊ by the former having two parallel (1:3) halogen-hydrogen attractions against one in the AG₊ form. Comparison is made to related molecules where the most stable conformers also have the greatest number of parallel (1:3) halogen-hydrogen interactions.     

Photochemically induced fluorescence investigation of aβ-cyclodextrin: Azure A inclusion complex and determination of analytical parameters

The photooxidation of Azure A and fluorescence properties of Azure A and its photoproduct have been investigated in aqueous media and in the presence of330h5325n6/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-cyclodextrin (330h5325n6/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-CD). The fluorescence intensity of the complex formed between the photoproduct and 330h5325n6/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-CD was found to be three times higher than that of the uncomplexed Azure A photoproduct. A complex formation constant of 110±40 M^–1 was calculated using the Benesi-Hildebrand treatment of the fluorescence emission data. Although the stoichiometry of the Azure A photoproduct: 330h5325n6/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-CD complex was found to be 1: 1, it seems that the Azure A structure is only partially included. Calibration graphs were plotted for the free Azure A photoproduct and the photogenerated product included in 330h5325n6/xxlarge946.gif" alt="beta" align="MIDDLE" BORDER="0">-CD. The analytical parameters and quantification limits were determined.     

Synthesis of H (type 4) trisaccharide,key structural fragment of globo-H and fucosyl-GM1 cancer-associated antigens

A convenient synthesis of Fucα1-2Galβ1-3GalNAcβ-OCH₂CH₂CH₂NH₂ (trisaccharide H type 4) is described. This glycan is the terminal part of glycosphingolipids globo-H and fucosyl-GM1 known as cancer-associated carbohydrate antigens.     

Syntheses of chiral fused 4,5-diazafluorene–bis(nopinane) derivatives

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Nonlinear dynamics of the heartbeat: I. The AV junction: Passive conduit or active oscillator?

Under physiologic conditions, the AV junction is traditionally regarded as a passive conduit for the conduction of impulses from the atria to the ventricles. An alternative view, namely that subsidiary pacemakers play an active role in normal electrophysiologic dynamics during sinus rhythm, has been suggested based on nonlinear models of cardiac oscillators. A central problem has been the development of a simple but explicit mathematical model for coupled nonlinear oscillators relevant both to stable and perturbed cardiac dynamics. We use equations describing an analog electrical circuit with an external d.c. voltage source (V₀) and two nonlinear oscillators with intrinsic frequencies in the ratio of 3:2, comparable to the SA node and AV junction rates. The oscillators are coupled by means of a resistor. 1:1 (SA:AV) phase-locking of the oscillators occurs over a critical range of V₀. Externally driving the SA oscillator at increasing rates results in 3:2 AV Wenckebach periodicity and a 2:1 AV block. These findings appear with no assumptions about conduction time or refractoriness. This dynamical model is consistent with the new interpretation that normal sinus rhythm may represent 1:1 coupling of two or more active nonlinear oscillators and also accounts for the appearance of an AV block with critical changes in a single parameter such as the pacing rate.     

Infinite transition solutions for a class of Allen–Cahn model equations

This paper constructs a family of solutions that undergoes an infinite number of spatial transitions for an Allen–Cahn model equation.      

Chemoselective hydrogenation of 3-nitrostyrene over Ag/TiO2-SiO2 catalyst in a flow reactor

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Mechanistic view of the relaxation dynamics of a simple glass-former. A bridge between the topographic and the dynamic approaches

We provide a link between the two main approaches to the relaxation dynamics of glassy systems: The `real dynamics' scheme and the inherent dynamics or topographic formalism. The first approach is based on molecular dynamics (MD) simulations, whilst the second one reflects the underlying influence of the energy `landscape' (within a timescale separation and activated dynamics scenario) and constitutes a widespread picture within the realm of complex systems ranging from glasses to biopolymers. For a model glass-former (a binary Lennard-Jones system), MD studies which characterized in detail the movements of the different particles led to the discovery of dynamical heterogeneities. On the other hand, the topographic approach identified activated events on the potential energy surface of this system corresponding to transitions between different basins of attraction or inherent structures. In this work we demonstrate that at low temperature the relevant events identified by both methods conform to a basic mechanistic phenomenology with elementary steps involving ballistic string-like particle movements. We also show that as temperature increases and the timescales characterizing events of different range become comparable, these elemental steps loose their nature of rare activated events. Concurrently, the system looses diversity and complexity, signatures of glassy behavior. This fact enables us to furnish for the first time the microscopic structural and dynamical basis and conditions for the prevalence of the `landscape paradigm' for this class of systems.     

Unusual examples of methane hydrate nucleation in bilayer water–oil systems

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