Showing papers by "Indian Association for the Cultivation of Science published in 2004"

Effect of substrate-induced strain on the structural, electrical, and optical properties of polycrystalline ZnO thin films

Abstract: The effect of substrate-induced strain in polycrystalline ZnO thin films on different substrate, e.g., GaN epilayer, sapphire (0001), quartz glass, Si(111)∕SiO2, and glass deposited by sol-gel process, has been investigated by x-ray diffraction, scanning electron microscope, electrical resistivity, and photoluminescence measurements. A strong dependence of orientation, crystallite size, and electrical resistivity upon the substrate-induced strain along the c axis has been found. The results of structural and morphological studies indicate that relatively larger tensile strain exists in ZnO deposited on sapphire and glass, while a smaller compressive strain appears in film deposited on GaN and the strain is relaxed in larger crystallite size. The electrical resistivity of the films increases exponentially with increasing strain. The excitonic peak positions are found to shift slightly towards lower energy side with increasing strain. The analysis shows that GaN being a closely lattice-matched substrate produces ZnO films of better crystallinity with a lower resistivity.

Size-Controlled Synthesis of Magnetite Nanoparticles in the Presence of Polyelectrolytes

Abstract: Magnetite nanoparticles of nearly uniform size have been prepared by precipitating ferrous ions in the presence of two different polyelectrolytes, viz., poly(acrylic acid) and the sodium salt of carboxymethyl cellulose at high pH (∼13). The size of the magnetite nanoparticles can be controlled easily by varying the concentration of the polyelectrolyte in the medium. Transmission electron microscopy study indicates that the average particle size varies from 5 to 15 nm, depending on the concentration and the nature of the polyelectrolyte. X-ray diffraction study shows the presence of only magnetite phase. FTIR spectroscopy and thermogravimetric analysis confirmed the presence of polyelectrolyte on the magnetite surface. The magnetization and Mossbauer studies were performed on two samples with mean diameters 7.0 and 14.7 nm. Magnetization measurements suggest that both of these particles are of single magnetic domain. The measurements also estimate the superparamagnetic blocking temperature, TB = 145 K for ...

Characterization and dielectric properties of polyaniline?TiO2 nanocomposites

Abstract: Nanocomposites of polyaniline (PANI)–titanium dioxide (PANI–TiO2) are prepared from a colloidal sol of TiO2 nanoparticles. The dc and ac conductivities of samples with different concentrations of PANI have been investigated as a function of frequency and temperature. The dc conductivity follows three-dimensional variable range hopping. The ac conductivity has been interpreted as a power law of frequency. The temperature variation of the frequency exponent suggests a correlated barrier hopping conduction process in the nanocomposites. A very large dielectric constant of about 3700 at room temperature has been observed. An electric modulus presentation is used to interpret the dielectric spectra. The interface between polyaniline and TiO2 plays an important role in yielding a large dielectric constant in the nanocomposite.

Indium(I) iodide-promoted cleavage of diaryl diselenides and disulfides and subsequent condensation with alkyl or acyl halides. One-pot efficient synthesis of diorganyl selenides, sulfides, selenoesters, and thioesters

Abstract: Diphenyl diselenides and disulfides undergo facile cleavages by indium(I) iodide and the corresponding generated selenate and thiolate anions condense in situ with alkyl or acyl halides present in the reaction mixture. Thus, a simple, efficient, and general procedure has been developed for the synthesis of unsymmetrical diorganyl selenides, sulfides (thioethers), selenoesters, and thioesters by this one-pot reaction at room temperature.

Physical, Mechanical, and Conductivity Properties of Poly(3-hexylthiophene)−Montmorillonite Clay Nanocomposites Produced by the Solvent Casting Method

Abstract: Polymer nanocomposites (PNCs) of poly(3-hexylthiophene) (P3HT) with organically modified montmorillonite (om-MMT) clay have been prepared by the solvent casting method. WAXS and TEM studies indicate exfoliated clay structure for lower clay content, but at higher clay content (5%, w/w) intercalated structures appear. The interchain lamella of P3HT exists in the nanocomposite, and the P3HT crystals become more ordered, showing better X-ray diffraction peaks. The thermal stability of PNCs increases significantly, and 1% clay content PNC exhibits the maximum thermal stability. The glass transition temperature (Tg), β-transition temperature (Tβ), the melting point (Tm), and the enthalpy of fusion (ΔH) of the PNCs are increased as compared to those of pure P3HT. The storage modulus (G‘) of PNCs showed a dramatic increase from that of pure P3HT, and the increase is larger in the temperature range 20−50 °C. The FTIR study indicates a decrease in Si−O−Si and Si−O stretching frequency for the exfoliated clay struct...

A state-specific approach to multireference coupled electron-pair approximation like methods: development and applications.

Abstract: The traditional multireference (MR) coupled-cluster (CC) methods based on the effective Hamiltonian are often beset by the problem of intruder states, and are not suitable for studying potential energy surface (PES) involving real or avoided curve crossing. State-specific MR-based approaches obviate this limitation. The state-specific MRCC (SS-MRCC) method developed some years ago [Mahapatra et al., J. Chem. Phys. 110, 6171 (1999)] can handle quasidegeneracy of varying degrees over a wide range of PES, including regions of real or avoided curve-crossing. Motivated by its success, we have suggested and explored in this paper a suite of physically motivated coupled electron-pair approximations (SS-MRCEPA) like methods, which are designed to capture the essential strength of the parent SS-MRCC method without significant sacrificing its accuracy. These SS-MRCEPA theories, like their CC counterparts, are based on complete active space, treat all the reference functions on the same footing and provide a descrip...

Synthesis, crystal structure and magnetic behavior of three polynuclear complexes: [Co(pyo)2(dca)2]n, [Co3(ac)4(bpe)3(dca)2]n and [{Co(male)(H2O)2}(H2O)]n [pyo, pyridine-N-oxide; dca, dicyanamide; ac, acetate; bpe, 1,2-bis-(4-pyridyl)ethane and male, maleate]

Abstract: Three polymeric cobalt(II) complexes of formulae [Co(pyo)2(dca)2]n (1), [Co3(ac)4(bpe)3 (dca)2]n (2) and [{Co(male)(H2O)2(H2O)]n (3) [pyo, pyridine-N-oxide; dca, dicyanamide; ac, acetate; bpe, 1,2-bis-(4-pyridyl)ethane and male, maleate] have been synthesized and characterized structurally as well as magnetically. The structure determination of complex 1 shows that each octahedral Co(II) in the 1D coordination chain is attached with four μ-1,5-dicyanamide and two pendant pyridine-N-oxide ligands, which form mutual relationships with other 1D chains through non-covalent π-π interactions, giving rise to a 2D infinite sheet-like structure. The molecular structure reveals that complex 2 adopts an infinite three-leg ladder-like structure in which three parallel 1D Co(bpe) chains are connected by syn-syn and oxo bridging acetate ligands. The dicyanamide ligands are pendant to the terminal Co(II) centers. Complex 3 is an infinite 3D network in which carboxylate groups of the maleate ligand are linked to Co(II) centers in syn-anti fashion. The structure of complex 3 has already been reported. The variable temperature (300–2 K) magnetic measurements have been performed for all three complexes. In the case of 2, the full structure can be rationalized as quasi-isolated trimers; the exchange Hamiltonian that describes magnetic interactions between the effective S′ = 1/2 spins, at low temperature is . Fixing g⊥ = 6.01 and g∣∣ = 2.25, according to the EPR measurements at 4 K, 2J = −3.3 cm−1 is the best-fit parameter. For 1 and 3, complete fits are not possible for calculating the corresponding J parameters for the one- and three-dimensional structures, respectively. Only an approximate J value has been calculated for 1.

Polymeric networks of copper(II) using succinate and aromatic N-N donor ligands: synthesis, crystal structure, magnetic behaviour and the effect of weak interactions on their crystal packing.

Abstract: Four succinato-bridged complexes of copper(II) have been synthesized. Complex 1, [Cu2(μ-OH2)2L(bpy)2(NO3)2]n and 2, [Cu2(μ-OH2)2L(phen)2(NO3)2]n (bpy = 2,2′-bipyridine; phen = 1,10-phenanthroline and LH2 = succinic acid) exhibit 1D coordination polymer structures where both the nitrate ions are directly linked to the copper(II) producing synthons in a 2D sheet. A novel 2D grid-like network, {[Cu4L2(bpy)4(H2O)2](ClO4)4(H2O)}n3, is obtained upon changing the nitrate by perchlorate anion in complex 1, where the channels are occupied by the anions. On changing the nitrate by tetrafluoroborate anion in complex 2, a novel octanuclear complex, [Cu8L4(phen)12](BF4)8·8H2O 4, is isolated. The coligand bpy and phen in these complexes show face-to-face (in 1, 2, 3, 4) or edge-to-face (in 4) π–π interactions forming the multidimensional supramolecular architectures. Interestingly, the appearance of edge-to-face π–π interactions in complex 4 facilitates the formation of discrete octanuclear entities. Variable-temperature (300–2 K) magnetic measurements of complexes have been done. Complexes 1 and 2 show very weak antiferromagnetic (OOC–CH2–CH2–COO) and ferromagnetic coupling (μ-H2O). Complex 3 also shows antiferromagnetic (syn–syn μ-OCO), and ferromagnetic coupling (μ-O of the -COO group). Complex 4 with two types (syn–syn and syn–anti) of binding modes of the carboxylate group shows strong antiferromagnetic interaction.

Efficient Proton-Templated Synthesis of 18- to 38-Membered Tetraimino(amino)diphenol Macrocyclic Ligands: Structural Features and Spectroscopic Properties

Abstract: A whole range of Robson-type tetraiminodiphenol macrocyclic ligands have been prepared as their perchlorate salts [H 4 L](ClO 4 ) 2 in high yield (ca. 90%) by a single-step [2 + 2] condensation reaction between 4-methyl(or tert-butyl)-2,6-diformyl(or diacyl)phenols and α,ω-diaminoalkanes (C 2 -C 12 ) in the presence of acetic acid and NaClO 4 . The reduction of these 18- to 38-membered macrocyclic salts with NaBH 4 have afforded corresponding tetraaminodiphenol macrocycles H 2 L'. The X-ray crystal structures of two of the tetraiminodiphenol macrocycles with the C 2 and C 4 lateral chains have been determined, and the optimized configurations for all of the macrocycles have been obtained by molecular mechanics calculations. The macrocycles have been characterized by elemental analysis and by IR, absorption, emission, and NMR spectroscopic study. The protonated tetraiminodiphenol macrocycles exhibit strong fluoroscence in methanol, acetonitrile, and nitromethane and undergo quenching when treated with triethylamine. The neutral macrocycles H 2 L, isolated by treating [H 4 L](ClO 4 ) 2 with excess of triethylamine, lack luminescence, as do the reduced tetraaminodiphenol macrocycles H 2 L'. The hydrolytic cleavage of [H 4 L](ClO 4 ) 2 has been studied.

Synthesis, structural analysis, and magnetic behaviour of three fumarate bridged coordination polymers: five-fold interpenetrated diamond-like net of NiII, sheets of NiII and CoII

Abstract: The hydrothermal reactions of Ni(NO3)2·6H2O, disodium fumarate (fum) and 1,2-bis(4-pyridyl)ethane (bpe)/1,3-bis(4-pyridyl)propane (bpp) in aqueous–methanol medium yield one 3-D and one 2-D metal–organic hybrid material, [Ni(fum)(bpe)] (1) and [Ni(fum)(bpp)(H2O)] (2), respectively. Complex 1 possesses a novel unprecedented structure, the first example of an “unusual mode” of a five-fold distorted interpenetrated network with metal–ligand linkages where the four six-membered windows in each adamantane-type cage are different. The structural characterization of complex 2 evidences a buckled sheet where nickel ions are in a distorted octahedral geometry, with two carboxylic groups, one acting as a bis-chelate, the other as a bis-monodentate ligand. The metal ion completes the coordination sphere through one water molecule and two bpp nitrogens in cis position. Variable-temperature magnetic measurements of complexes 1 and 2 reveal the existence of very weak antiferromagnetic intramolecular interactions and/or the presence of single-ion zero field splitting (D) of isolated NiII ions in both the compounds. Experimentally, both the J parameters are close, comparable and very small. Considering zero-field splitting of NiII, the calculated D values are in agreement with values reported in the literature for NiII ions. Complex 3, [{Co(phen)}2(fum)2] (phen = 1,10-phenanthroline) is obtained by diffusing methanolic solution of 1,10-phenanthroline on an aqueous layer of disodium fumarate and Co(NO3)2·6H2O. It consists of dimeric CoII(phen) units, doubly bridged by carboxylate groups in a distorted syn–syn fashion. These fumarate anions act as bis-chelates to form corrugated sheets. The 2D layer has a (4,4) topology, with the nodes represented by the centres of the dimers. The magnetic data were fitted ignoring the very weak coupling through the fumarate pathway and using a dimer model.

Honeycomb nets with interpenetrating frameworks involving iminodiacetato-copper(II) blocks and bipyridine spacers: syntheses, characterization, and magnetic studies.

Abstract: Three coordination polymers of copper(II), viz. {[Cu(ida)(4,4‘-bipyH)]ClO4}∝ (1), {[Cu2(ida)2(μ-4,4‘-bipy)]·2H2O}∝ (2), and [Cu2(ida)2(bpa)]∝ (3) have been synthesized by the process of self-assembly using Cu(ida) [ida = iminodiacetate(2-)] as the building block and 4,4‘-bipyridyl and 1,2-bis(4-pyridyl)ethane (bpa) as linkers. Crystals of 1 are orthorhombic, of space group Pna21, with a = 13.8956(12) A, b = 16.3362(16) A, c = 7.3340(12), and Z = 4. Both compounds 2 and 3 crystallize in monoclinic space group P21/a with a = 10.1887(8) A (9.6779(10) A for 3), b = 8.0008(11) A (9.1718(10) A), c = 11.6684(9) A (12.9144(12) A), β = 98.307(11)° (102.796(18)°), and Z = 2 (2). Compound 1 has a zigzag chain structure with an extensive hydrogen-bonded network while compounds 2 and 3 are honeycomb (6,3) nets with interpenetrating structures. Variable temperature (2−300 K) magnetic study indicates the presence of weak antiferromagnetic interactions (J = 0.82 ± 0.01 cm-1) in 1 and ferromagnetic in 2 (J = −0.45 ± 0.05 ...

Novel solid-state molecular self-assemblies of manganese (II) constructed with flexible-ligands: Influences of π-π and C-H...π interactions on their crystal packing

Abstract: Three supramolecular networks are synthesized by the combination of Mn(II) and flexible ligands [1,2-bis (4-pyridyl) ethane (bpe); 1,3-bis (4-pyridyl) propane (bpp); 4,4‘-bipyridyl disulfide (bpds)], characterized by single-crystal X-ray analysis All three complexes have extended solid-state structure formed by π−π interactions, and there is an excellent opportunity to elucidate the difference in the formation of extended networks in these three complexes Complex 1, which is reported in a paper earlier by our group, is an unprecedented three-dimensional (3D) “honeycomb”-like architecture constructed from a novel sinusoidal one-dimensional (1D) chain [Mn(bpe)(H2O)4]2+ through π−π interactions in which bpe is used as a bidentate bridging ligand Complex [{Mn(bpp)3Cl2}(H2O)2]n 2 generates a “standing wave”-like architecture by π−π interactions from a neutral zigzag 1D chain of [Mn(bpp)3Cl2] Complex [{Mn(bpds)2(dca)2(H2O)2}(H2O)4(bpds)]n 3 is a neutral monomer [Mn(bpds)2(dca)2(H2O)2], which is self-assembl

Electronic structure and magneto-optical properties of magnetic semiconductors: Europium monochalcogenides

Abstract: Electronic structure and magnetic properties of Europium monochalcogenides ($\mathrm{Eu}X$, $X=\mathrm{O}$, S, Se, and Te) have been investigated by the full potential linear muffin-tin orbital method. The generalized gradient approximation (GGA) including on site Coulomb interaction of $5d$ and $4f$ states $(\mathrm{GGA}+{U}_{d}+{U}_{f})$ has been used to describe the complicated properties of $\mathrm{Eu}X$. The $\mathrm{GGA}+{U}_{f}$ band structure underestimates the band gap. $\mathrm{GGA}+{U}_{d}+{U}_{f}$ gives better agreement in terms of band gap and the positions of $4f$ bands of $\mathrm{Eu}X$. $\mathrm{GGA}+{U}_{d}+{U}_{f}$ gives much improved Kerr spectra for all the $\mathrm{Eu}X$ over the standard LSDA and $\mathrm{GGA}+{U}_{f}$ spectra although the magnitude of the first peak is less satisfactory in the dielectric function spectra. The Coulomb correlation effects among $4f$ and $5d$ electrons of Eu play important role to determine the optical spectra of $\mathrm{Eu}X$.

Morphology, crystal structure, and thermal and spectral studies of semiorganic nonlinear optical crystal LAHClBr

Abstract: Solubility and specific heat properties of the semiorganic nonlinear optical crystal, L-arginine hydrocholorobromide (LAHClBr), have been measured as a function of temperature. The compound crystallizes in a rectangular bipyramidal shape with well-developed {100} and {001} pinacoids. X-ray diffraction analysis shows that it belongs to the space group P2 1 with two independent molecules in asymmetric units and lattice parameters a = 11.158(2) A, b = 8.579(3) A, c = 11.235(3) A, β = 91.55°(4). The arginine molecules occur as zwitterions, with the amino groups and the guanidyl groups each accepting a proton from the acid groups present in the molecule. The molecules are held together by a three-dimensional network of hydrogen bonds.

Structural and Magnetic Properties of Two Carboxylato‐Bridged Manganese(II) Complexes with N‐Donor Coligands

Abstract: Two polymeric MnII complexes, [(m-phth)(Mn)(bpp)2]·0.5H2O (1) and [(mal)2(Mn)2(H2O)2(hmt)] (2) [mal = malonate; hmt = hexamethylenetetramine; bpp = 1,3-bis(4-pyridyl)propane; m-phth = 1,3-benzenedicarboxylate] have been synthesized. Complex 1 consists of 1D chains (with monocoordinated bpp) that are held together to form a 2D extended network of hydrogen bonding between lattice water molecules and uncoordinated bpp donors. Variable-temperature magnetic susceptibility data of the complex have been fitted applying the formula used for dinuclear complexes of MnII ions, considering interdimer interactions (J′). The following parameters were obtained from the best fit: J = −0.88 ± 0.02 cm−1, J′ = −0.1 cm−1 and g = 1.99 ± 0.01. The agreement factor R = Σ(χmTobsd. − χmTcalcd.)2/Σ(χmTcalcd.)2 is 1.110−5. Complex 2 is a 3D polymeric network where syn/anti-bridging malonate ligands and MnII centers are assembled in the form of [Mn(H2O)(mal)]n layers, which are further connected by hexamethylenetetramine. The magnetic data have been fitted considering the complex as a pseudo-two-dimensional sheet of MnII cations and malonate anions since the coupling through the hexamethylenetetramine ligand is almost zero. We have followed two approaches for fitting the data, (a) through the expansion series of Line’s equation for the S = 5/2 antiferromagnetic quadratic layer and (b) by using the equation based on an infinite 2D square lattice composed of isotropically coupled classical spin S = 5/2. Both approaches result in excellent theoretical fits and similar J values of −0.24 cm−1 and −0.21 cm−1 for the Line’s and Curely equations, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes: first experimental evidence of valence tautomerism in an iron complex.

Abstract: The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)]·DMF (1), and catecholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen = N,N‘-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N‘-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV−vis−NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon kno...

Thermal behavior of vinyl ester resin matrix composites reinforced with alkali‐treated jute fibers

Abstract: The thermal behavior of vinyl ester resin matrix composites reinforced with jute fibers treated for 2, 4, 6, and 8 h with 5% NaOH was studied with Thermo-gravimetric analysis and differential scanning calorimetry. The moisture desorption peak shifted to a higher temperature, from 37 to 58.3°C, for all the treated-fiber composites because of improved wetting of the fibers by the resin and stronger bonding at the interface. The degradation temperature of the vinyl ester resin in the composites was lowered to 410.3°C from that of the neat resin, 418.8°C. The X-ray diffraction studies showed increased crystallinity of the treated fibers, which affected the enthalpy of the α-cellulose and hemicellulose degradation. The hemicellulose degradation temperature remained the same (299.7°C) in all the treated-fiber composites, but the enthalpy associated with the hemicellulose degradation showed an increasing trend in the treated composites with a small increase in the weight loss. This could be attributed to the increased hydrogen bonding between the more accessible OH groups of the hemicellulose in the noncrystalline region of the jute fiber and the resin. The degradation temperature of α-cellulose was lowered from 364.2 to 356.8°C in the treated composites. The enthalpy of α-cellulose degradation showed a decreasing trend with a lowering of the weight loss. The crystalline regions of the fiber, consisting of closely packed α-cellulose chains, were bonded with the resin mainly on the surface through hydrogen bonds and became more resistant to thermal degradation; this reduced the weight loss. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 123–129, 2004

Bi- and trinuclear copper(II) complexes of a sterically constrained phenol-based tetradentate ligand: syntheses, structures, and magnetic studies.

Abstract: Copper(II) complexes (1−3) of a sterically constrained phenol-based tetradentate N2O2 ligand 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)piperazine (H2L) have been reported. The associated anions of the c...

Solution of quantum Langevin equation: Approximations, theoretical and numerical aspects

Abstract: Based on a coherent state representation of noise operator and an ensemble averaging procedure using Wigner canonical thermal distribution for harmonic oscillators, a generalized quantum Langevin equation has been recently developed [Phys. Rev. E 65, 021109 (2002); 66, 051106 (2002)] to derive the equations of motion for probability distribution functions in c-number phase-space. We extend the treatment to explore several systematic approximation schemes for the solutions of the Langevin equation for nonlinear potentials for a wide range of noise correlation, strength and temperature down to the vacuum limit. The method is exemplified by an analytic application to harmonic oscillator for arbitrary memory kernel and with the help of a numerical calculation of barrier crossing, in a cubic potential to demonstrate the quantum Kramers’ turnover and the quantum Arrhenius plot.

A Simple, Efficient and General Procedure for Acetalization of Carbonyl Compounds and Deprotection of Acetals under the Catalysis of Indium(III) Chloride

Abstract: Indium(III) chloride efficiently catalyzes the protection of a variety of aldehydes and ketones to their corresponding 1,3-dioxolanes and dialkyl acetals in refluxing cyclohexane. On the other hand, deprotection of acetals is also achieved in refluxing aqueous methanol under the catalysis of indium(III) chloride.