Characterization and thermal investigations of ammonium heptamolybdate

Thermal decomposition of ammonium heptamolybdate was studied by means of differential scanning calorimetry and thermogravimetry. DSC showed that the material decomposes in the temperature region 463–608 K in static air atmosphere, in three steps. The products of decomposition were characterized by IR spectroscopy, elemental analysis and magnetic susceptibility measurements. All the phases are paramagnetic in nature. Kinetic analysis suggested that the first two steps are controlled by a diffusion mechanism, while the third step is controlled by nuclear-growth.

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Zusammenfassung Mittels Differential-Scanning-Kalorimetrie und Thermogravimetrie wurde die thermische Zersetzung von Ammoniumheptamolybdat untersucht. Laut DSC zersetzt sich die Substanz in statischer Luftatmosphäre im Temperaturbereich von 463 bis 608 K in drei Schritten. Die Produkte wurden durch IR-Spektroskopie, Elementaranalyse und Messungen der magnetischen Suszeptibilität charakterisiert. Alle Phasen sind paramagnetischer Natur. Kinetische Untersuchungen besagen für die ersten zwei Schritte einen diffusionskontrollierten, für den dritten Schritt einen wachstumskontrollierten Mechanismus.

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The authors are grateful to the UGC, New Delhi, for financial assistance.     

Design and synthesis of site directed maleimide bifunctional chelators for technetium and rhenium

A new family of heterobifunctional linkers (L1-L9) containing a terminus consisting of a tridentate donor set for coordination of the {M(CO)(3)}(+) core (M = Tc, Re), and a thiol reactive maleimide group has been prepared conveniently and in high yield under Mitsunobu reaction conditions by the coupling of an appropriate alcohol derivative with maleimide. The rhenium complexes [Re(CO)(3)(Lx)]Br (x= 1-9) were prepared in good yields from the reactions of the ligands and (NEt(4))(2)[Re(CO)(3)Br(3)] in refluxing methanol. The ligands and their Re complexes were characterized by (1)H and (13)C NMR, IR, and ESI-MS. Ligand L4 and [Re(CO)(3)(L5)]Br have been structurally characterized by X-ray crystallography. Photoexcitation of solutions of the complexes [Re(CO)(3)(Lx)]Br (x= 4-6) gives rise to intense and prolonged luminescence at room temperature (fluorescence lifetimes of ca. 16 micros). The ligands and their Re complexes react smoothly at the maleimide linker with sulfhydryl groups of peptides and proteins at room temperature in phosphate-buffered saline (PBS, pH 7.4) to form stable thioether bioconjugates. The photoluminescence properties of the labeled conjugates are similar to those of the parent complexes, but with even longer lifetimes. The ligands can also be labeled at room temperature with (99m)Tc to give chemically robust complexes. The corresponding hydrazinonicotinamide derivative N-[5-(6'-hydrazinopyridine-3'-carbonyl)aminopentyl]maleimide (L10) was also prepared. While coupling of L10 to cysteine ethylester and synthesis of the rhenium derivative [ReCl(3)(HYNIC-maleimide)(2)] were successfully accomplished, attempts to couple [ReCl(3)(HYNIC-maleimide)(2)] to glutathione or BSA yielded intractable mixtures.     

Fluorescence resonance energy transfer (FRET) in chemistry and biology: Non-F?rster distance dependence of the FRET rate

Fluorescence resonance energy transfer (FRET) is a popular tool to study equilibrium and dynamical properties of polymers and biopolymers in condensed phases and is now widely used in conjunction with single molecule spectroscopy. In the data analysis, one usually employs the F?rster expression which predicts (l/R ⁶) distance dependence of the energy transfer rate. However, critical analysis shows that this e_xpression can be of rather limited validity in many cases. We demonstrate this by explicitly considering a donor-acceptor system, polyfluorene (PF₆)-tetraphenylporphyrin (TPP), where the size of both donor and acceptor is comparable to the distance separating them. In such cases, one may expect much weaker distance (as l/R ² or even weaker) dependence. We have also considered the case of energy transfer from a dye to a nanoparticle. Here we find l/R ⁴ distance dependence at large separations, completely different from F?rster. We also discuss recent application of FRET to study polymer conformational dynamics. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Cloaking and imaging effects in plasmonic checkerboards of negative and and dielectric photonic crystal checkerboards

Negative refractive index materials are known to be able to support a host of surface plasmon states for both polarizations of light. This makes possible unique effects such as a perfect lens. Checkerboards consisting of alternating cells of positive and negative refractive index represent a very singular situation in which the density of modes diverges at the corners. This raises the question as to whether such effects will still be observed in a real dissipative system of finite size. We have considered several aspects of such structures including these and symmetry aspects (rectangular against triangular checkerboards). We have also studied silver checkerboards whose transverse extent is finite. Negative refractive index checkerboards bring new electromagnetic paradigms both through the intriguing possibilities they appear to offer, and the challenges they present to our understanding of the diffraction process. Most intriguing of all is the possibility of a triangular checkerboard lens whose resolution is limited not by wavelength, but only by the losses in the constituent materials, while a ray picture suggests it behaves as a perfect mirror. The resolution of this lens increases without limit as the losses tend to zero as shown by the generalized lens theorem. We finally show that light confinement can be achieved to certain extent using dielectric triangular photonic crystal (PC) checkerboards displaying the all-angle-negative-refraction (AANR) within the Bragg regime in p polarization. Effectively even a single rectangular or triangular PC can act as an open resonator that confines light in its neighborhood. This cloaking effect has been previously observed in PC slabs displaying the AANR effect. We show that the cloaking is enhanced for three triangular PC wedges sharing a vertex and further improved for 12 triangular PC wedges arranged in a checkerboard fashion.     

Effective properties of a piezocomposite containing shape memory alloy and inert inclusions

Received July 2, 2001 / Published online February 4, 2002     

Self-assembled, mesoporous polymeric networks for patterned protein arrays

A facile, self-assembly approach to the fabrication of a robust, mesoporous, biocompatible polymeric network for the spatial organization of proteins is described. Surface-deposited poly(styrene) (PS) beads that assemble into a two-dimensional (2-D) hexagonal array are used to template cross-linked poly(vinyl alcohol) (PVA), yielding an inverse opal structure. The porous, water insoluble network is used to entrain a model, soluble protein, green fluorescent protein (GFP). The polymeric network is characterized by atomic force microscopy (AFM) and optical microscopy, and the spatial localization of the incorporated GFP is determined by fluorescence microscopy. The results demonstrate that this system may constitute a versatile platform for the lateral organization of biomolecules.     

Investigation of lateral interactions in chemisorption

It is believed that lateral interactions play an important role in chemisorption site selection. With the advent of angular resolved and polarization dependent photoemission spectroscopies, one is able to compare computed band structure with angular resolved photoemission data. This advance coupled with theoretical calculations provides an excellent opportunity for investigating direct (adsorbate-adsorbate) and indirect (through-substrate) lateral interactions at surfaces. We have modelled the chemisorption of oxygen on Al(111) using direct lateral interactions only (isolated monolayer). The calculated results provide a good framework for interpreting the data. For O₂ monolayer the direct lateral interactions are also appreciable and lead to significant dispersion in the calculated energy bands. We have also computed the electronic structure for two dimensional isolated layers of CO in ordered configurations observed on a Pd(100) surface. The shape and width of the 4σ-derived band is satisfactorily accounted for by this direct lateral interaction only model. Having demonstrated that direct lateral interactions lead to observable band dispersions, it is suggested that they might influence normal vibrational frequencies and ought to be included.     

A study of the high-temperature transport anomaly in AgBr,by diffusion of sodium tracer

A detailed measurement of the temperature dependence of the tracer diffusivity of substitutional sodium ion in AgBr indicates the activation energy for the formation of a Frenkel pair to be temperature-dependent. The approach used earlier by Müller and by Aboagye and Friauf in interpreting the “extra” ionic conductivity at high temperatures as due to an anomalous increase in the defect concentrations thus appears to be essentially correct. However, the decreases in defect formation enthalpy at high temperatures, as deduced from these diffusion experiments, are somewhat smaller than those obtained earlier from conductivity data. Also, the activation enthalpy for the jumping of a sodium ion into a neighboring vacancy is found to be nearly twice that for motion of a free vacancy, the increased migration barrier presumably resulting from the absence of covalent interactions in the case of the sodium ion.     

Cluster model electronic structure calculations for the ideal and hydrogen chemisorbed Si (100) surfaces

The electronic structure of the ideal and hydrogen chemisorbed Si(100) surfaces is calculated using the self-consistent scattered-wave cluster model. The results are presented for total and local density of states. These are compared with experiments and other calculations, where available.