Volume 115, Issue 5-6
October 2003, pages 319-805
pp 319-319 October 2003
pp 321-332 October 2003
By using phenylacetylene based rigid-rod linkers (PhA), we have successfully synthesized two fullerene derivatives, C60-PhA and C60-PhA-C60.The absorption spectral features of C60, as well as that of the phenylacetylene moiety are retained in the monomeric forms of these fullerene derivatives, ruling out the possibility of any strong interaction between the two chromophores in the ground state. Both the fullerene derivatives form optically transparent clusters, absorbing in the UV-Vis region; this clustering leads to a significant increase in their molar extinction coefficients. TEM characterization of the C60-PhA showed large spherical clusters, with sizes ranging from 150–350 nm, while an elongated wire-type structure was observed for the bisfullerene derivative (C60-PhA-C60).AFM section analysis studies of isolated nanoclusters of C60-PhA-C60, deposited on mica, indicate that smaller clusters associate to form larger nanostructures.
pp 333-339 October 2003
The title transformations are oxygen atom transfer, twin isomerization and regiospecific imine oxidation.Bispyridyldiazole ligands have furnished new oxygen atom transfer reagents of coordination type ReVOCl3(NN) which undergo a slower transfer to PPh3 than the corresponding azole reagents. The rate of twin isomerization (linkage and geometrical) of meridional azole complexes of coordination type ReIII(OPnP)Cl3(NN) to facial ReIII(PnPO)Cl3(NN) decreases rapidly asn increases in the interval 1–4 (PnP is Ph2P(CH2)nPPh2). An α-diimine chelate of type ReV(NPh)Cl3(NN) is shown to undergo facile oxidation to the corresponding iminoamide complex ReVI(NPh)Cl3(NN) upon treating with dilute nitric acid. The reaction proceeds via regiospecific nucleophilic addition of waterto the more polarized imine function.
pp 341-348 October 2003
Composites of nanometre-sized copper core-copper oxide shell with diameters in the range 6.1 to 7.3 nm dispersed in a silica gel were synthesised by a technique comprising reduction followed by oxidation of a suitably chosen precursor gel. The hot pressed gel powders mixed with nanometre-sized copper particles dispersed in silica gel showed electrical resistivities several orders of magnitude lower than that of the precursor gel. Electrical resistivities of the different specimens were measured over the temperature range 30 to 300°C. Activation energies for the coreshell nanostructured composites were found to be a fraction of that of the precursor gel. Such dramatic changes are ascribed to the presence of an interfacial amorphous phase. The resistivity variation as a function of temperature was analysed on the basis of Mott’s small polaron hopping conduction model. The effective dielectric constant of the interfacial phase as extracted from the data analysis was found to be much higher than that of the precursor glass. This has been explained as arising from the generation of very high pressure at the interface due to the oxidation step to which the copper nanoparticles are subjected.
pp 349-364 October 2003
For the NO molecule, modelled as a Morse oscillator, time-dependent (TD) nuclear Schrödinger equation has been numerically solved for the multiphoton vibrational dynamics of the molecule under a far-infrared laser of wavelength 10503 nm, and four different intensities,I = 1 × 108, 1 × 1013, 5 × 1016, and 5 × 1018 W cm−2 respectively. Starting from the vibrational ground state at zero time, various TD quantities such as the norm, dissociation probability, potential energy curve and dipole moment are examined. Rich high-harmonics generation (HHG) spectra and above-threshold dissociation (ATD) spectra, due to the multiphoton interaction of vibrational motions with the laser field, and consequent elevation to the vibrational continuum, have been obtained and analysed.
pp 365-372 October 2003
New organic-inorganic hybrid catalysts were synthesized by covalent grafting of proline and benzylpenicillin derivatives into mesoporous MCM-41. These catalysts were extensively characterized using FT-IR,13C CP MAS solid stateNMR, XRD and TEM techniques. These were used as catalysts for direct, asymmetric aldol reaction between acetone and activated aromatic aldehydes. In the reaction of 4-nitro and 4-fluoro benzaldehyde, the aldol products were obtained in 36% and 59%ee respectively. The catalysts were reusable with neither significant drop in enantioselectivity nor loss of mesostructure. An attempt was made to substantiate the proposed ’enamine’ mechanism for direct aldol reaction by trapping the intermediate between proline-MCM-41 and acetone.
pp 373-400 October 2003
An overview of the use of non-protein amino acids in the design of conformationally well-defined peptides, based on work from the author’s laboratory, is discussed. The crystal structures of several designed oligopeptides illustrate the useα-aminoisobutyric acid (Aib) in the construction of helices, D-amino acids in the design of helix termination segments andDPro-Xxx segments for nucleating ofβ-hairpin structures.β- andγ-amino acid residues have been used to expand the range of designed polypeptide structures.
pp 401-410 October 2003
Initial growth stages of the ultra thin films of germanium (Ge) prepared by ion beam sputter deposition have been studied using atomic force microscope (AFM) and interference enhanced Raman scattering. The growth of the films follows Volmer-Weber growth mechanism. Analysis of the AFM images shows that Ostwald ripening of the grains occurs as the thickness of the film increases. Raman spectra of the Ge films reveal phonon confinement along the growth direction and show that the misfit strain is relieved for film thickness greater than 4 nm.
pp 411-417 October 2003
Here we report direct hydrothermal synthesis of a few hexagonal molybdates with composition, Mx/+Mo6−xEmphasis>/3/6+O18−x(OH)x.yH2O (M = Li, Rb, Cs, NH4). The molybdates crystallize in the space groupP63/m with a r∼ 10.5 andc r∼ 3.7 Å . Unlike previous studies, our work suggests that hexagonal molybdates could be stabilized in the presence of monovalent cations with varying ionic size (smaller lithium to larger cesium) under hydrothermal condition. The phases showed exceptional thermal stability till 550°C.
pp 419-429 October 2003
Three new, isostructural methylenediphosphonates of molybdenum, A[MoO2(O3PCH2PO3H)] (A = Rb (1), NH4 (2) and Tl (3)) have been synthesized by hydrothermal method and structurally characterised by X-ray diffraction and spectroscopic techniques. These compounds crystallize in monoclinic space group,P21/c with Z = 4 and consist of [MoO2(O3PCH2PO3H)]-anionic layers interleaved with A+ ions.
pp 431-438 October 2003
Two new oxides, La2Ba2CaZn2Ti3O14 and Pr2Ba2CaZn2Ti3O14, have been synthesized by the ceramic route at 1100°C. These oxides crystallize in the disordered cubic structure with an ‘a’ lattice parameter of 3.9728 (2) and 3.9448 (5) respectively. These oxides show high dielectric constant (70 and 57) and low loss (0.003 and 0.013 at 100 kHz) for La2Ba2CaZn2Ti3O14 and Pr2Ba2CaZn2Ti3O14 respectively. The dielectric constant is highly stable with frequency and temperature.
pp 439-446 October 2003
We study structure and magnetic properties of La0.5Ba0.5CoO3(LBGO) usingab initio density functional theory (DFT) method based on pseudopotentials and a plane-wave basis. We find the cubic structure of LBCO is ferromagnetic and lower in energy than the nonmagnetic rhombohedral structure. Through the calculation of Γ-point phonons of LBCO in the cubic structure, we determine its structural instabilities and find that they correspond to the structural phase transition accompanying a para-ferromagnetic transition observed recently.
pp 447-457 October 2003
We report a low temperature synthesis of layered Na0.20Co02 and K0.44CoO2 phases from NaOH and KOH fluxes at 400°C. These layered oxides are employed to prepare hexagonal HCoO2, LixCoO2and Delafossite AgCoO2 phases by ion exchange method. The resulting oxides were characterised by powder X-ray diffraction, X-ray photoelectron spectroscopy, SEM and EDX analysis. Final compositions of all these oxides are obtained from chemical analysis of elements present. Na0.20Co02 oxide exhibits insulating to metal like behaviour, whereas AgCoO2 is semiconducting.
pp 459-463 October 2003
We investigate the properties of GaN semiconducting nanoparticles as a potential candidate for photodetection in the solar-blind region. The photocurrent spectral response is studied spanning the range 1.6-5.5 eV. A significant fraction of the response is in the range 4–5.5 eV. The results are compared to other optical properties and the origins of the features observed in the spectra are speculated upon.
pp 465-472 October 2003
A reliable diagnostics of lead-acid batteries would become mandatory with the induction of an improved power net and the increase of electrically assisted features in future automobiles. Sparse-impedance spectroscopic technique described in this paper estimates the internal resistance of sealed automotive lead-acid batteries in the frequency range 10 Hz-10 kHz, usually produced by the alternators fitted in the automobiles. The state-of-health of the battery could be monitored from its internal resistance.
pp 473-490 October 2003
The crystal structure of ferroelastic Rb4Li(HSO4)3(SO4) has been determined at two temperatures, which indicates a structural phase transition, tetragonal P43 witha = 7.629(1) Å,c = 29.497(2) Å at 293 K and monoclinic P21 witha = 7.583(3) Å,b = 29.230(19) Å,c = 7.536(5) Å,β = 90.14(1)° at 90 K. The crystal structure of K4Li(HSO4)3(SO4)4 has also been determined at two temperatures, tetragonalP41 witha = 7.405(1) Å,c = 28.712(6) Å at 293 K and tetragonalP41 witha = 7.371(5) Å,c = 28.522(5) Å at 100 K. The overall coordination features in both the structures have been analysed in terms of bond valence sum calculations.
pp 491-498 October 2003
We have carried out detailed electron spectroscopic investigation of Ce1−x SrxTiO3 exhibiting insulator-metal transition withx. Core level X-ray photoelectron spectra of Ce3d as well as resonant photoemission spectra obtained at the Ce4d→4f resonant absorption threshold establish Ce as being in the trivalent state throughout the series. Using the ’off-resonance’ condition for Ce 4f states, we obtain the Ti3d dominated spectral features close toEF, exhibiting clear signatures of coherent and incoherent peaks. We discuss the implications of our findings in relation to the metalinsulator transition observed in this series of compounds.
pp 499-508 October 2003
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.
pp 509-518 October 2003
A novel method of purification for single-walled carbon nanotubes, prepared by an arc-discharge method, is described. The method involves a combination of acid washing followed by high temperature hydrogen treatment to remove the metal nanoparticles and amorphous carbon present in the as-synthesized singlewalled carbon nanotubes. The purified single-walled carbon nanotubes have been characterised by low-angle X-ray diffraction, electron microscopy, thermo-gravi-metric analysis and Raman spectroscopy.
pp 519-524 October 2003
Fine powders of lanthanum iron oxide, LaFeO3, have been prepared by solid state reaction as well as sol-gel synthesis and nebulized spray pyrolysis. Structures, morphologies and magnetic susceptibility measurements of these powders have been examined. The powders prepared by all the three low-temperature routes contain nearly spherical particles with an average diameter of 40 nm. These samples show a lower Neel temperature than the powder prepared by solid state reaction besides showing much lower magnetic susceptibility at low temperatures.
pp 525-532 October 2003
The formation ofβ-phase Bi2Mo2O9 catalyst from a precursor precipitate has been studied using thein situ combined XRD/QuEXAFS technique and DSC during calcination. Accordingly the precursor was observed to undergo a number of changes in both the molybdenum (VI) coordination and long-range ordering during this heating. Initially the two other forms of bismuth molybdate (α-andγ-phases) were observed to form from the poorly crystalline precursor at about 230°C, however, theβ-phase eventually crystallised after prolonged heating at 560°C.
pp 533-542 October 2003
We consider an interacting one-dimensional molecular wire attached to two metal electrodes on either side of it. The electrostatic potential profile across the wire-electrode interface has been deduced solving the Schrodinger and Poisson equations self-consistently. Since the Poisson distribution crucially depends on charge densities, we have considered different Hamiltonian parameters to model the nano-scale wire. We find that for very weak electron correlations, the potential gradient is almost zero in the middle of the wire but are large near the chain ends. However, for strong correlations, the potential is essentially a ramp function. The nonlinear current, obtained from the scattering formalism, is found to be less with the ramp potential than for weak correlations. Some of the interesting features in current-voltage characteristics have been explained using one-electron formalism and instabilities in the system.
pp 543-552 October 2003
In zeolites, diffusion is often accompanied by a reaction or sorption which in turn can induce temperature inhomogeneities. Monte Carlo simulations of Lennard-Jones atoms in zeolite NaCaA are reported for the presence of a hot zone presumed to be created by a reaction or chemi- or physi-sorption site. These simulations show that the presence of localized hot regions can alter both kinetic and transport properties such as diffusion. Further, we show that enhancement of diffusion constant is greater for systems with larger barrier height, a surprising result that may be of considerable significance in many chemical and biological processes. We find an unanticipated coupling between reaction and diffusion due to the presence of a hot zone in addition to that which normally exists via concentration. Implications of this coupling for the product profile of a reaction are discussed. We also propose a mechanism by which mobility of ions or diffusion of molecular species within biomembranes may take place.
pp 553-560 October 2003
The results of X-ray diffraction, andac anddc magnetisation as a function of temperature are reported for a new class of spin-chain oxides, Ca3Co1+x Ir1−xO6. While thex = 0.0, 0.3, 0.5 and 1.0 are found to form in the K4CdCl6-derived rhombhohedral (space group$$R\bar 3c$$) structure, thex = 0.7 composition is found to undergo a monoclinic distortion in contrast to a literature report. Apparently, the change in the crystal symmetry withx manifests itself as a change in the sign of paramagnetic Curie temperature for this composition as though magnetic coupling sensitively depends on such crystallographic distortions. All the compositions exhibit spin-glass anomalies with an unusuallylarge frequency dependence of the peak temperature inac susceptibility in a temperature range below 50 K, interestingly obeying Vogel-Fulcher relationship even for the stoichiometric compounds.
pp 561-571 October 2003
Ce-Zn-O mixed oxides were prepared by amorphous citrate process and decomposition of the corresponding acetate precursors. The resulting materials were characterised by TGA, XRD, UV-Vis-DRS, EPR, SEM and surface area measurements. XRD and DRS results indicated fine dispersion of the ceria component in the ZnO matrix. EPR results clearly indicate the presence of oxygen vacancy and defect centres in the composite oxide. Addition of CeO2 to ZnO produced mixed oxides of high surface area compared to the pure ZnO. Hydrogen transfer reaction was carried out on these catalytic materials to investigate the effect of rare earth oxide on the activity of ZnO. Addition of ceria into zinc oxide was found to increase the catalytic activity for hydrogen transfer reaction. The catalytic activity also depended on the method of preparation. Citrate process results in uniformly dispersed mixed oxide with higher catalytic activity.
pp 573-586 October 2003
A hydrothermal reaction of a mixture of Y(NO3)3, 1,2-benzenedicarboxylic acid (1,2-BDC) and NaOH gives rise to a new yttrium phthalate coordination polymer, [Y4H2O2C8H4O4)6]∞,I. The Y ions inI are present in four different coordination environments with respect to the oxygen atoms (CN6 = octahedral, CN7 = pentagonal bipyramid, CN8 = dodecahedron and CN9 = capped square anti-prism). The oxygen atoms of the 1,2-BDC are fully deprotonated, and show variations in their connectivity with Y atoms. The Y atoms themselves are connected through their vertices forming infinite Y-O-Y one-dimensional chains. The Y-O-Ychains are cross-linked by the 1,2-BDC anions forming a corrugated layer structure. The layers are supported by favourableπ…π interactions between the benzene rings of the 1,2-BDC anions. The variations in the coordination environment of the Y atoms and the presence of Y-O-Y interactions along with the favourableπ…π interactions between the benzene rings from different layers are noteworthy structural features. Crystal data: triclinic, space group =P−1 (no. 2),a = 12.6669 (2),b = 13.8538 (2),c = 16.0289 Å,α = 75.20 (1),β = 69.012 (1),γ= 65.529 (1)°,V = 2371.28 (7) Å3,Dcalc = 1.922 g cm−1, μ(MoKα) = 4.943 mm−1. A total of 9745 reflections collected and merged to give 6566 unique reflections (Rint = 0.0292) of which 5252 withI>2σ(I) were considered to be observed. FinalR2 = 0.0339,wR2 = 0.0724 andS = 1.036 were obtained for 704 parameters.
pp 587-606 October 2003
Colossal magnetoresistance (CMR) composites form an interesting field of study. The intrinsic CMR, governed by the intragrain transport of the conduction electrons through the double exchange, limits its application owing to the high field requirement and generally low transition temperatures. Extrinsic CMR, a function of the intergrain transport between ferromagnetic (FM) particles, plays a crucial role in enhancing low field magnetoresistance (LFMR) or increasing room temperature magnetoresistance. Since extrinsic CMR is a grain boundary controlled phenomena, magnetically dirty grain boundaries in the virgin state of the sample help in achieving a high LFMR or increase the field sensitivity. In this article, we give the properties of different composites of magnetoresistive LCMO or LSMO where the second component is (a) an insulating phase, (b) a conducting phase, (c) a nonmagnetic phase and (d) a magnetic phase. We present here some of our recent work on CMR composites where the CMR phase is chosen as LCMO and SiO2, ZnO, ZrO2 and SiCN have been used as the different second phase of the composites. We summarise some of the salient features of the results.
pp 607-612 October 2003
A newchola lariat ether (1, a 21-crown-6) was constructed fromreadily available precursors. The association constant of compound1 with alkali metal picrates was measured using Cram’s extraction protocol. Evidence is presented for the involvement of the 3-methoxy group for the complexation. Energy minimised structures show that the A-ring gets slightly distorted upon metal ion binding.
pp 613-619 October 2003
Nanoparticles have properties that can be fine-tuned by their size as well as shape. Hence, there is significant current interest in preparing nano-materials of small size dispersity and to arrange them in close-packed aggregates. This manuscript describes ways of synthesising gold nanoparticles using a metal-chelator derivative1, as stabiliser. Controlled synthesis conditions lead to formation of nanoparticles thereby indicating the ability of1 to act as efficient stabiliser. The nanoparticles formed were characterised by transmission electron microscopy and UV-Vis spectroscopy. TEM analysis showed the formation of dense aggregates of nanoparticles. This can be ascribed to the inter-particle hydrogen bonding possible by the carboxylic acid moiety of1 that leads to aggregation. The aggregation can be controlled by the pH of the solution employed for dispersing the particles.
pp 621-636 October 2003
Folding dynamics and energy landscape picture of protein conformations of HP-36 andβ-amyloid (Aβ) are investigated by extensive Brownian dynamics simulations, where the inter amino acid interactions are given by a minimalistic model (MM) we recently introduced [J. Chem. Phys.118 4733 (2003)]. In this model, a protein is constructed by taking two atoms for each amino acid. One atom represents the backbone Cαs atom, while the other mimics the whole side chain residue. Sizes and interactions of the side residues are all different and specific to a particular amino acid. The effect of water-mediated folding is mapped into the MM by suitable choice of interaction parameters of the side residues obtained from the amino acid hydropathy scale. A new non-local helix potential is incorporated to generate helices at the appropriate positions in a protein. Simulations have been done by equilibrating the protein at high temperature followed by a sudden quench. The subsequent folding is monitored to observe the dynamics of topological contacts (Ntopo), relative contact order parameter (RCO), and the root mean square deviation (RMSD) from the real-protein native structure. The folded structures of different model proteins (HP-36 and Aβ) resemble their respective real native state rather well. The dynamics of folding showsmultistage decay, with an initial hydrophobic collapse followed by a long plateau. Analysis ofNtopo and RCO correlates the late stage folding with rearrangement of the side chain residues, particularly those far apart in the sequence. The long plateau also signifies large entropic free energy barrier near the native state, as predicted from theories of protein folding.
pp 637-647 October 2003
An experimental charge density study has been carried out on perylene based on X-ray diffraction measurements at 130 K. The electron density and its associated properties have been evaluated at the bond and the ring critical points for the naphthalene residues as well as for the central ring. The variation of the Laplacian along the axis, above and below the ring plane, is found to be symmetric for the central ring while for the naphthalene rings, the Laplacian values are enhanced under the bow-shaped region. A plot of the Laplacian versus density evaluated at various points along the axis above the ring plane, shows a steep variation in the case of the central ring implying that theπ-density is smeared out compared to that over the naphthalene rings. Similar data extracted from a quinoid ring and a regular phenyl ring (both based on earlier reports from this laboratory) exhibit increasingly shallower trends and indicate, by contrast, that the central ring of perylene is much less aromatic.
pp 649-661 October 2003
Formation of solid solution has been explored in the valence compensated perovskite oxide system Sr1−xLaxTi1−xFexO3 (x ≤ 0.50). XRD studies indicate the formation of solid solution for the whole range investigated. All the compositions synthesised have cubic structure similar to undoped SrTiO3. Study of dielectric behaviour of these materials show that orientational polarisation and space charge polarisation contribute significantly to it.
pp 663-677 October 2003
We present all-atom molecular dynamics simulations ofn-hexane on the basal plane of graphite at monolayer and multilayer coverages. In keeping with experimental data, we find the presence of ordered adsorbed layers both at single monolayer coverage and when the adsorbed layer coexists with excess liquid adsorbate. Using a simulation method that does not impose any particular periodicity on the adsorbed layer, we quantitatively compare our results to the results of neutron diffraction experiments and find a structural transition from a uniaxially incommensurate lattice to a fully commensurate structure on increasing the coverage from a monolayer to a multilayer. The zig-zag backbone planes of all the alkane molecules lie parallel to the graphite surface at the multilayer coverage, while a few molecules are observed to attain the perpendicular orientation at monolayer coverage.
pp 679-687 October 2003
This paper describes the formation of water-dispersible gold nano-particles capped with a bilayer of sodium dodecylsulphate (SDS) and octadecylamine (ODA) molecules. Vigorous shaking of abiphasic mixture consisting of ODA-capped gold nanoparticles in chloroform and SDS in water results in the rapid phase transfer of ODA-capped gold nanoparticles from the organic to the aqueous phase, the latter acquiring a pink, foam-like appearance in the process. Drying of the coloured aqueous phase results in the formation of a highly stable, reddish powder of gold nanoparticles that may be readily redispersed in water. The water-dispersible gold nanoparticles have been investigated by UV-Vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). These studies indicate the presence of interdigitated bilayers consisting of an ODA primary monolayer directly coordinated to the gold nanoparticle surface and a secondary monolayer of SDS, this secondary monolayer providing sufficient hydrophilicity to facilitate gold nanoparticle transfer into water and rendering them water-dispersible.
pp 689-694 October 2003
The paper discusses the design of a low cost Raman spectrometer. Single-walled nanotubes (SWNT) have been studied to demonstrate the reach of such a system. We observe both the radial-breathing mode (RBM) and the tangential mode from the SWNT. The tube diameters of the SWNT used in these experiments have been determined using RBM to be predominantly 1.4 and 1.6 nm. These are consistent with the TEM images taken of the same sample. The new method of producing SWNT using Ni-Y catalyst in electric-arc discharge method produces nanotubes with very small dispersion in diameter and high yields. The chirality of the SWNT can be deduced from their radial breathing modes, and it suggests that they are metallic in nature.
pp 695-701 October 2003
Y2W3O12 exhibits negative thermal expansion along the three crystallographic directions due to the transverse thermal vibrations perpendicular to the Y-O-W linkage. It is highly hygroscopic and forms a trihydrate structure at room temperature. Dilatometric studies of Y2W3O12 show large thermal expansion hysteresis due to large grain size and a large initial positive thermal expansion due to the removal of water molecules. Al2O3 has been added to Y2W3O12 upto 10 wt% in an attempt to overcome the hygroscopicity and reduce the particle size and thereby the thermal expansion hysteresis. Thermo gravimetric, dilatometric and electron microscopic studies are presented to support these observations.
pp 703-709 October 2003
We report a new nanomaterial in which ciprofloxacin molecules are incorporated inside silica nanobubbles, denoted as ciprofloxacin@SiO2. The material has been characterised using UV/Vis absorption spectroscopy, transmission electron microscopy, cyclic voltammetry, and emission spectroscopy. The material is stable and the freestanding particles can be precipitated and redispersed in several solvents. Confinement of the molecule is complete as leaching through the shell is minimal. The material behaves like free ciprofloxacin in solution; however, effects of confinement are manifested. Energy transfer reaction between ciprofloxacin@SiO2 and Tb3+ was monitored by emission spectroscopy. The emission intensity decreased with metal ion exposure indicating selective electronic interaction.
pp 711-720 October 2003
Expanded porphyrins containing seven pyrrole/heterocyclic rings linked in a cyclic fashion are termed heptaphyrins. The number ofπ-electrons in heptaphyrins depends on the number ofmeso carbon bridges used to link the heterocyclic rings, accordingly heptaphyrins with 28π-electrons and 30π-electrons are reported to date. Both condensation reactions of the appropriate precursors and acid-catalysed oxidative coupling reactions have been utilized to synthesise the heptaphyrins. The 30π heptaphyrins exhibit rich structural diversity where some of the heterocyclic rings in the macrocycle undergo a 180° ring flipping. An overview of the synthetic methods employed for the synthesis of heptaphyrins, their spectroscopic properties, structural behaviour and aromatic properties are highlighted in this paper.
pp 721-726 October 2003
We use the finite-size, density-matrix-renormalization-group(DMRG) method to obtain the zero-temperature phase diagram of the one-dimensional, extended Bose-Hubbard model, for mean boson densityρ = 1, in theU-V plane (U andV are respectively, onsite and nearest-neighbour repulsive interactions between bosons). The phase diagram includes superfluid (SF), bosonic-Mott-insulator(MI), and mass-density-wave (MDW) phases. We determine the natures of the quantum phase transitions between these phases.
pp 727-740 October 2003
The effect of mechanical milling on materials has been studied using simple model systems. The results show that milling leads to enhancement in both thermodynamic driving force and transport kinetics. A study of some characteristic physical properties of the milled samples in comparison to the bulk shows how milling affects the properties.
pp 741-749 October 2003
Rhodium(I) complexes of the hybrid ylide-phosphine ligands, Ph2P(CH2)nPPh2(CHC(O)C6H5) (n = 1: dppm-yl, or 2: dppe-yl) have been synthesised from [Rh(μ-C1)(COD)]2 (COD = 1,5-cyclooctadiene) and characterized by NMR spectroscopic and X-ray structural methods. The dppe-yl behaves as an ambidentate ligand; it functions as a monodentate P-donor ligand with a dangling ylidic carbon in the neutral chloro complex, [(COD)Rh(Cl)(dppe-yl)] (1), whereas replacement of the chloride by a non-coordinating counter anion results in the formation of the complexes, [(COD)Rh(L-L’)]+ (L-L’ = dppe-yl (2) or dppm-yl (3)) respectively in which the ligands are bonded to the metal via the phosphorus and the ylidic carbon atoms. The 1,5-cyclooctadiene (COD), present in the Rh(I) precursor, remains intact in the products. The structures of1,2 and3 have been confirmed by X-ray crystallography.
pp 751-766 October 2003
Binding energies of the interaction of collagen like triple helical peptides with a series of polyphenols, viz. gallic acid, catechin, epigallocatechingallate and pentagalloylglucose have been computed using molecular modelling approaches. A correlation of calculated binding energies with the interfacial molecular volumes involved in the interaction is observed. Calculated interface surface areas for the binding of polyphenols with collagen-like triple helical peptides vary in the range of 60–210 Å2 and hydrogen bond lengths vary in the range of 2.7–3.4 Å. Interfacial molecular volumes can be calculated from the solvent inaccessible surface areas and hydrogen bond lengths involved in the binding of polyphenols to collagen. Molecular aggregation of collagen in the presence of some polyphenols and chromium (III) salts has been probed experimentally in monolayer systems. The monolayer arrangement of collagen seems to be influenced by the presence of small molecules like formaldehyde, gluteraldehyde, tannic acid and chromium (III) salts. A fractal structure is observed on account of two-dimensional aggregation of collagen induced by tanning species. Atomic force microscopy has been employed to probe the topographic images of two-dimensional aggregation of collagen induced by chromium (III) salts. A case is made that long-range ordering of collagen by molecular species involved in its stabilisation is influenced by molecular geometries involved in its interaction with small molecules.
pp 767-774 October 2003
Manganites of the LA1−xCaxMnO3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach recently proposed by us, where we argue that due to strong Jahn-Teller (JT) coupling with phonons the twofold degenerateeg states at the Mn sites dynamically reorganize themselves into localised, JT polaronsl with exponentially small inter-site hopping, and band-like, nonpolaronic statesb, leading to anew 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment of the model which explains quantitatively a wide variety of experimental results, including insulator-metal transitions and CMR, in terms of the influence of physical conditions on the relative energies and occupation of thel andb states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and spatial correlation is the key to manganite physics.
pp 775-788 October 2003
Relaxor properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) and non-lead perovskite thin films have been analysed in terms of large frequency dispersion of dielectric response at low temperatures. A wide spectrum of dielectric relaxation was observed in the frequency-dependent response of the imaginary part of the dielectric permittivity. Transformation from normal ferroelectric to relaxor behaviour has been observed in the case of the Ca substituting the BaTiO3 thin films. A number of techniques were exploited to investigate the wide spectrum of relaxation times in pulsed laser ablated thin films.ac anddc electric field induced complex dielectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) thin films were studied as function of frequencies at different temperatures. Nonlinear behaviour of dielectric susceptibility with respect to the amplitude of theac drive was observed at lower temperatures. The frequency dependence of transition temperatureTm (temperature of the maximum of dielectric constant) was studied using the Vogel-Fulcher relation.
pp 789-799 October 2003 Emerging Directions in Chemical Sciences
pp 800-805 October 2003 Emerging Directions in Chemical Sciences
Volume 132, 2020
Continuous Article Publishing mode
Click here for Editorial Note on CAP Mode