• Vandana

      Articles written in Journal of Chemical Sciences

    • Functional properties and electroanalytical selectivity of some anion exchange membranes

      S K Srivastava Vandana Sahgal C K Jain

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      Heterogeneous membranes of Fe(III)-Zr(IV), Cr(III)-Zr(IV) mixed hydrous oxides and one doped with Sn(II) ion have been prepared using polystyrene as a binding material. Functional properties like water content, porosity, swelling, electrolyte absorption and conductance of these membranes have been determined in various anionic forms and correlated with their electroanalytical selectivity.

    • A semiempirical MO study of tautomerism and the electronic structure of barbituric acid

      Rita Kakkar Vandana Katoch

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      The electronic structure of barbituric acid has been investigated, keeping in mind the possibility of tautomerism. It has been found that the triketo form is the most stable, followed by the 4-hydroxy tautomer. The difference in their stabilities decreases on substitution at C5. Substituents that allow a greater degree of delocalization with the ring system (nitro, bromo, thiol) stabilize the 4-hydroxy tautomer to a greater extent since it is planar. The AM1 method is found to be the best suited for studying the electronic structure of barbituric acid, as it gives the best agreement with the experimental geometries. Of the other two methods investigated, MNDO gives erroneous results for relative energies, while PM3 gives unsatisfactory geometries. The stabilities of the charged species resulting from deprotonation, and the radicals resulting from the removal of a hydrogen atom by heat treatment or irradiation with gamma rays, have also been investigated. Their electronic structures are also discussed.

    • Theoretical study of the mechanism of proton transfer in tautomeric systems: Alloxan

      Rita Kakkar Bhupendra K Sarma Vandana Katoch

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      Semiempirical SCF-MO studies of tautomerism in alloxan preclude the possibility of direct proton transfer in the gas phase due to the strain in the four-centred transition state, in which the proton being transferred is forced to come close to the positively charged carbon atom at the opposite corner of the four-membered ring. However, in aqueous solution, the activation barrier reduces appreciably, not only due to reduction in strain, but also due to charge separation in the transition state, which is stabilized due to ionic resonance. The N-H bond is almost broken, while the O-H bond is only partially formed in the transition state. The other stabilizing effect in aqueous solution is due to bulk solvent dielectric effects, which stabilize the transition state to a greater extent due to its higher dipole moment. Although the transition states for proton transfer to the neighbouring oxygen atoms on either side have comparable energies, as the mechanisms of proton transfer leading to the formation of the 2-hydroxy and 4-hydroxy tautomers are similar, bulk solvent effects are larger in the latter due to the higher dipole moment of the transition state. The reason is the almost complete separation of the two entities, i.e. the alloxan anion and the hydronium ion in the latter case, indicating that in this case a dissociative mechanism of the kind encountered in acid-base equilibria is operating.

    • Preparation of Cu, Ag, Fe and Al nanoparticles by the exploding wire technique

      P Sen Joyee Ghosh Alqudami Abdullah Prashant Kumar Vandana

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      We describe a novel process for the production of nanoparticles of Cu, Ag, Fe and Al which involves exploding their respective wires, triggered by large current densities in the wires. The particles are characterised by X-ray diffraction (XRD) and atomic force microscopy (AFM). Particle sizes in the range 20–100nm were obtained employing this technique. The XRD results reveal that the nanoparticles continue to retain lattice periodicity at reduced particle sizes, displaying in some cases evidence of lattice strain and preferential orientation. In the case of Fe, Mossbauer spectroscopy reveals loss of ferromagnetism as a result of the reduced size of the particles.

    • Linear, branched and network polysilanes with thienyl/furyl substituted sila-alkyl side chains and their applications for the synthesis of fluorescent silver nanoparticles/clusters

      Ravi Shankar Usharani Sahoo Vandana Shahi Manchal Chaudhary

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      The scope of Wurtz coupling and catalytic dehydrocoupling methods for the synthesis of functional polysilanes of compositions, [RR'Si]$_n$ (linear), [(PhMeSi)$_x$-co-(RSi)$_{1−x}$]$_n$ (branched) and [RSi]$_n$ (network) [R = Et3SiCH2CH2, 2-Fu/2-ThMe2SiCH2CH2 (Fu = Furyl, Th = Thienyl), 𝑛-Hex or Ph; R' = H or Me] is presented. By virtue of 𝜎-delocalized silicon backbone and variable HOMO-LUMO band gap energies, these polymers are found to be promising candidates as reducing agents for Ag(I) ions in toluene/cyclo-hexane and provide a simple approach for ‘size-controlled’ synthesis of silver nanoparticles (AgNPs) as well as fluorescent polymer-silver nanocomposites.

    • Thiophene-based donor-acceptor conjugated polymer as potential optoelectronic and photonic material

      Maluvadi G Murali Udayakumar Dalimba Vandana Yadav Ritu Srivastava K Safakath

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      In this paper, we report the synthesis, characterization and optical properties of a donor-acceptor conjugated polymer, PTh-CN, containing 3,4-didodecyloxythiophene and cyanovinylene units. The polymer possesses a low band gap of 1.75 eV as calculated from the onset absorption edge. From the electrochemical study, the HOMO and LUMO energy levels of the polymer are figured out to be −5.52 eV and −3.52 eV, respectively. Polymer light-emitting diodes are fabricated using PTh-CN as the emissive layer with a device configuration of ITO/PEDOT:PSS/PTh-CN/Al. The device showed stable saturated red electroluminescence with CIE coordinate values (0.65, 0.32) at 12 V, which are very close to the values for standard red demanded by the NTSC. In addition, the device showed good colour stability under different bias voltages and the threshold voltage of the PLED device is found to be as low as 3.1 V. Further, a nanocomposite of the polymer and TiO2 nanoparticles is prepared by the dispersion method. The nonlinear optical properties of PTh-CN and PTh-CN/TiO2 nanocomposite are studied using z-scan technique. The polymer solution, polymer film and polymer/TiO2 nanocomposite film show a strong saturable absorption behaviour. The value of saturation intensity (Is) is found to be of the order 1011-1012 W/m2, indicating that the materials are useful candidates for photonic applications.

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