• Volume 114, Issue 1

      February 2002,   pages  1-91

    • Coenzyme B12 model studies: Equilibrium constants for the pH-dependent axial ligation of benzyl(aquo)cobaloxime by various N- and S-donor ligands

      D Sudarshan Reddy N Ravi Kumar Reddy V Sridhar S Satyanarayana

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      Equilibria of the axial ligation of benzyl(aquo)cobaloximes by imidazole, 1-methyl imidazole, histidine, histamine, glycine, ethyl glycine ester, thiourea and urea have been spectrophotometrically measured in aqueous solutions of ionic strength 1.0M (KCl) at 25°C as a function of pH. The equilibrium constants are in the order CN> 1-methyl imidazole > imidazole > histidine > histamine>glycine>ethyl glycine ester > thiourea > urea. The order of stability of benzyl(ligand)cobaloxime is explained based on the basicity of the ligand, Co(III) →>L dπ- pπback bonding and soft-soft and soft-hard interaction. Imidazole, substituted imidazoles, histidine and histamine form more stable complexes than glycine, ethyl glycine ester in contrast to the basicity of the ligands. Benzyl(ligand)cobaloximes were isolated and characterized by elemental analysis, IR and1H NMR spectra.

    • Equilibria and kinetics forpH-dependent axial ligation of alkyl(aquo) cobaloximes with aromatic and aliphatic N-donor ligands

      V Sridhar D Sudarshan Reddy N Ravikumar Reddy S Satyanarayana

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      Equilibria and kinetics of the reaction of bromomethyl(aquo) cobaloxime with histamine, histidine, glycine and ethyl glycine ester and iodomethyl(aquo) cobaloxime with cyanide, imidazole and substituted imidazoles were studied as a function ofpH at 25°C, 10 M ionic strength (KCl) by spectrophotometry technique. The rate of substitution of H2O varies with thepKa of the incoming ligand, thus establishing the existence of nucleophilic participation of the ligand in the transition state. Dissociation kinetic reactions were also studied as a function ofpH. Binding and kinetic data were interpreted based on the basicity, steric crowd of the entering ligand and HSAB principle. To compare the rate constants of the entering ligandspH independent second-order rate constants were calculated.

    • Dipicolinate complexes of main group metals with hydrazinium cation

      K Saravanan S Govindarajan

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      Some new coordination complexes of hydrazinium main group metal dipicolinate hydrates of formulae (N2H5)2M(dip)2.nH2O (where, M = Ca,Sr,BaorPb andn = 0, 2, 4 and 3 respectively and dip = dipicolinate), N2H5Bi(dip)2.3H2O and (N2H5)3Bi(dip)3.4H2O have been prepared and characterized by physico-chemical techniques. The infrared spectra of the complexes reveal the presence of tridentate dipicolinate dianions and non-coordinating hydrazinium cations. Conductance measurements show that the mono, di and trihydrazinium complexes behave as 1:1, 2:1 and 3 :1 electrolytes respectively, in aqueous solution. Thermal decomposition studies show that these compounds lose water followed by endothermic decomposition of hydrazine to give respective metal hydrogendipicolinate intermediates, which further decompose exothermically to the final product of either metal carbonates (Ca, Sr, Ba and Pb) or metal oxycarbonates (Bi). The coordination numbers around the metal ions differ from compound to compound. The various coordination numbers exhibited by these metals are six (Ca), seven (Ba), eight (Sr) and nine (Pb and Bi). In all the complexes the above coordination number is attained by tridentate dipicolinate dianions and water molecules. The X-ray diffraction patterns of these compounds differ from one another suggesting that they are not isomorphous.

    • Interactions between oxovanadium (IV), glycylvaline and imidazoles: An aqueous potentiometric and spectroscopic study

      N Patel V K Soni K K Shukla S Sharma K B Pandeya

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      Speciation has been determined in aqueous oxovanadium, glycylvaline and imidazoles at 25 ± 1°C and μ = 0.1M NaClO4 using a combination of potentiometry, and visible and EPR spectroscopy. Results of potentiometric and spectroscopic methods are consistent. Calculations of stability constants have been made using the SCOGS computer program.

    • Lifetimes ofα-halo andα-azidobenzyl carbocations in aqueous solution

      R Sanjeev V Jagannadham

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      The title cations were produced in aqueous solution by chemical initiation (solvolysis) of benzyl-gem-dihalides and benzyl-gem-diazides. The solvolysis reactions of benzyl-gem-dihalides and benzyl-gem-diazides in water proceed by a stepwise mechanism through α-halobenzyl carbocation and α-azidobenzyl carbocation intermediates, which are captured by water to give the corresponding carbonyl compounds as the sole detectable products. Rate constant ratiokx/ks(M−1) for partitioning of the carbocation between reaction with halide/azide ion and reaction with water is determined by analysis of halide/azide common ion inhibition of the solvolysis reaction. The rate constantsks(s-1) for the reaction of the cation with solvent water were determined from the experimental values ofkx/ks andksolv, for the solvolysis of the benzyl-gem-dihalides and benzyl-gem-diazides respectively, usingkx = 5 × 109M−1 s−1 for diffusion-limited reaction of halide/azide ion with α-substituted benzyl carbocations. The values of 1/ks are thus the lifetimes of the α-halobenzyl carbocations and α-azidobenzyl carbocations respectively.

    • An analysis on the transmission of electron density through sulphur atoms in the quaternization reactions of benzothiazoles

      B K Mishra Sabita Patel

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      The kinetics of quaternization of a number of 2-substituted benzothiazoles with alkyl iodides and phenacyl bromide have been studied in nitrobenzene. The order of reactivity of different substituted benzothiazoles has been found to be -H > NO2 > Cl. The relative reactivity of 2-amino and 2-methyl derivatives has been ascribed to a solvation phenomenon. The analysis of rho values from the Hammett equation suggests two possible routes for the transmission of electron density.

    • Partitioning of methyl internal rotational barrier energy of thioacetaldehyde

      Nikhil Guchhait

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      The nature of methyl internal rotational barrier in thioacetaldehyde has been investigated by relaxation effect, natural bond orbital (NBO) analysis and Pauling exchange interactions. The true experimental barrier can be obtained by considering fully relaxed rotation. Nuclear-electron attraction term is a barrier forming term in the fully relaxed rotation, but it appears as an antibarrier for rigid rotation. It is seen that during methyl rotation, the torsional mode is coupled with the aldehydic hydrogen out-of-plane wagging motion. Natural bond orbital analysis shows that the principal barrier forming term originates from the C-C bond. The lengthening of the C-C bond is explained by considering charge transfer interaction between several bonding and antibonding orbitals in the C-C bond region, which leads to higher bonding overlap for the eclipsed conformer compared to the staggered conformer. S-C(σ)/Cme-Hp and C-Hald/Cme-Hop interactions appear to be the main barrier-forming Pauling exchange terms but have less contribution to make to the barrier compared to the C-C bond interaction.

    • EPR investigations of electron transfer in one-electron reduced α-1,4 K5[PV2W10O40].3H2O

      C M Varghese A Shunmugasundaram R Murugesan T Jey Abalan

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      Heteropoly blues of α-1,2 and α-1,4 isomers of [PV2W10O40]5−have been prepared by using the electrochemical technique. EPR spectra, measured as a function of temperature over a wide range (20-300 K), are explicable in terms of electron-hopping processes in heteropoly blues. Temperature dependence of A∥of the isomers suggest that the activation energy for electron hopping is greater for the α-1,4 isomer than the α-1,2 isomer. Other parameters like stability of the blues and intra-molecular electron transfer rate constants are also evaluated using EPR as the tool.

    • Photoisomerization of merocyanine 540 in polymer-surfactant aggregate

      Sobhan Sen Dipankar Sukul Partha Dutta Kankan Bhattacharyya

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      Photoisomerization of merocyanine 540 (MC540) in a polymer-surfactant aggregate is studied using picosecond time resolved emission spectroscopy. The aggregate consists of the polymer, poly(vinylpyrrolidone) (PVP) and the surfactant, sodium dodecyl sulphate (SDS). With increase in the concentration of SDS in an aqueous solution of MC540 containing PVP, the emission quantum yield and lifetime of MC540 increase markedly. This indicates marked retardation in the nonradiative photoisomerization process of MC540, when it binds to the polymer-surfactant aggregate. The critical association concentration of SDS for binding to PVP has been found to be 0.5 mM. This is about 16 times lower than the CMC of SDS in pure water (8 mM).

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