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G U Kulkarni

Publications and source records attributed to G U Kulkarni.

15 recordsLinked to original sources
PubMed2006

Role of triple bond in 1,2-diphenylacetylene crystal: A combined experimental and theoretical study.

We have performed a combined experimental and theoretical study of the molecular system of 1,2-diphenylacetylene. The occurrence of two different geometries of the molecule in the crystal structure, one being planar and the other tilted by approximately 6 degrees , has been investigated in relation to the nature of the acetylenic linker. The experimental charge density analysis shows that the acetylenic linker exhibits a noncylindrical density reminiscent of the strong conjugation present in the molecule. The pi-orbitals of the acetylenic linker derived from density functional theory (DFT) calculations are found to sustain a variety of conjugation lengths between the phenyl rings, thereby giving flexibility to the molecule to arrange itself in various packing conformations in the crystal. It is interesting that the energy involved for such distortions is only kBT, allowing several polymorphic forms of the crystal structure as reported in the literature. The distortions entertained by the molecule and the corresponding changes in the charge density distribution and energy are all relevant to molecular electronics.

Journal Article↗
PubMed2006

A facile method of producing femtoliter metal cups by pulsed laser ablation.

Cuplike structures of Au, Ag, Cu, Zn, Nb, Cd, Al, In, and Sn in the size range of 300 nm to a few micrometers with an internal volume of a few femtoliters have been produced by the laser ablation of metal targets in a vacuum, by optimizing, in each case, the laser fluence and the substrate temperature. The metal droplets impinging on the substrate seem to undergo a hydraulic jump driven by the surface tension forces before solidifying into cups. The cups are robust and can be functionalized with biomarkers, filled with nanoparticle sols, oxidized to crucibles, or detached from the substrate without causing any deformation. We envisage their potential applications as femtoliter metal containers.

Journal Article↗
PubMed2006

Nanocrystalline films of Au-Ag, Au-Cu, and Au-Ag-Cu alloys formed at the organic-aqueous interface.

By the reaction of the appropriate organometallic precursors and tetrakishydroxymethyl phosphonium chloride (THPC) at the toluene-water interface, we have prepared nanocrystalline films of Au-Ag and Au-Cu alloys with a range of compositions. The films have been characterized by X-ray diffraction, electron microscopy, optical spectroscopy, and other techniques, besides compositional analysis. The particle size of the nanocrystals in the films varies with the composition. The surface plasmon band of the alloy films as well as of the organosols obtained by the disintegration of the films by the addition of an alkanethiol show the expected composition dependence. It has been possible to prepare the nanocrystalline films of a ternary Au-Ag-Cu alloy.

Journal Article↗
PubMed2006

Pyramidal nanostructures of zinc oxide.

Zinc oxide (ZnO) nanostructures have been prepared by pulsed laser deposition of the oxide onto Si(100) substrate at 600 degrees C. An examination of the morphology using atomic force microscopy and scanning electron microscopy reveals well formed pyramidal structures consistent with the growth habit of ZnO. A domain matched epitaxy across the interface makes the ZnO pyramids orient along the axes of Si(100) surface. The pyramidal nanostructures signify an intermediate state in the growth of hexagonal nanorods of ZnO. The hardness of the nanostructures as well as their response to oxygen gas have been investigated using nanoindentation and conducting probe methods respectively. ZnO nanostructures are much harder than their bulk. The hardness of ZnO pyramids obtained by nanoindentation is 70 +/- 10 GPa which is about one order more that of bulk ZnO. Besides, the nanostructures exhibit high sensitivity towards oxygen. A 70% increase in the resistance of ZnO nanostructures is observed when exposed to oxygen atmosphere.

Hot Temperature↗
PubMed2005

Aromaticity in stable tiara nickel thiolates: computational and structural analysis.

Quantum chemical calculations as well as crystallographic analyses show that the Ni(n) rings in the tiara Ni thiolates, [NiS2]n and [Ni(SR)2]n (n = 3-6), have highly symmetric polygonal structures. We find that such structural features primarily arise from the effective delocalization of the d-orbital electrons across the Ni(n) rings leading to bond length equalization and thereby aromaticity. We introduce the d-orbital aromaticity for the first time to explain the experimentally observed polygonal structures of these cyclic metal rings bridged by thiol linkages.

Journal Article↗
PubMed2005

Hydrogen bonding patterns in the cocrystals of 5-nitrouracil with several donor and acceptor molecules.

: Cocrystals of 5-nitrouracil with solvent molecules, dioxane, pyridine, DMSO, formamide and ethanol as well as with piperazine, N, N'-dimethylpiperazine, 3-aminopyridine and diazabicyclo [2.2.2]octane obtained by deliberate inclusion, have been examined by X-ray crystallography. The tape structure found in the parent centric form of nitrouracil is retained with some modifications in the cocrystals with dioxane, piperazine, diazabicyclo [2.2.2]octane, N,N'-dimethylpiperazine, pyridine and DMSO, with the guest molecules forming alternate tapes. In cocrystals involving formamide, ethanol and 3-aminopyridine, the molecular tapes exhibit mixed compositions. The observed bonding patterns have been classified into six schemes. Interestingly, quadruple type hydrogen bonding patterns are seen in cocrystals containing 3-aminopyridine or ethanol and water, while a network of acyclic tetrahedral pentamers of water is found in the cocrystal containing diazabicyclo [2.2.2]octane and water.

Journal Article↗
PubMed2005

Use of the liquid-liquid interface for generating ultrathin nanocrystalline films of metals, chalcogenides, and oxides.

The air-water interface has traditionally been employed to prepare particle assemblies and films of metals and semiconductors. The interface between water and an organic liquid, however, has not been investigated sufficiently for possible use in preparing nanocrystals and thin films of materials. In this article, we demonstrate the use of the liquid-liquid interface as a medium for preparing ultrathin films of metals, chalcogenides and oxides. The method involves the reaction at the interface between a metal-organic compound in the organic layer and an appropriate reagent for reduction, sulfidation, etc. in the aqueous layer. Some of the materials discussed are nanocrystalline films of gold, CuS, CuSe, CuO, and Cu(OH)2 formed at the liquid-liquid interface. The results reported in this article should demonstrate the versatility and potential of the liquid-liquid interface for preparing nanomaterials and ultrathin films and encourage further research in this area.

Chalcogens↗
PubMed2005

Nature and properties of ultrathin nanocrystalline gold films formed at the organic-aqueous interface.

Ultrathin nanocrystalline films of gold formed at different temperatures at the organic-aqueous interface have been investigated by X-ray diffraction, electron microscopy, atomic force microscopy, and electronic spectroscopy. The films are smooth and continuous over relatively large length scales and are generally approximately 100 nm thick. The size of the nanocrystals is sensitive to the reaction temperature, which also determines whether the film is metallic or an activated conductor. The surface plasmon band of gold is highly red-shifted in the films. Alkanethiols perturb the structure of the films, with the magnitude of the effect depending on the chain length. Accordingly, the position of the plasmon band and the electrical resistance of the films are affected by interaction with alkanethiols; the plasmon band approaches that of isolated nanocrystals in the presence of long-chain thiols.

Journal Article↗
PubMed2005

Gold-coated conducting-atomic force microscopy probes.

Some aspects of the performance of gold-coated conductive probes used in conducting atomic force microscopy (C-AFM) technique are discussed. The resistance of the nanocontact between the gold-coated AFM tip and the graphite substrate has been monitored at various applied forces. For small forces (<50 nN), resistance on the order of a few kiloohms was observed. Minimal contact resistance was observed for forces in the range 100-150 nN, beyond which the tip seems to undergo plastic deformation. The resistance of the nanocontact increased when current on the order of 100 microA was allowed to pass through, finally resulting in melting of the gold coating.

Chemical Phenomena↗
PubMed2005

Perpendicular magnetization in self-assembled films of citrate-capped gamma-Fe2O3 nanocrystals on Si(100) surfaces.

gamma-Fe2O3 nanocrystals capped with citrate and octylamine have been chemically prepared. The octylamine-capped nanocrystals exhibit a tendency to form ordered lattices. Films of nanocrystals of varying thickness (454, 720, and 1400 microg/cm2 in the case of citrate-capped nanocrystals and 300 microg/cm2 in the case of octylamine-capped nanocrystals) have been prepared on Si(100) substrates by drop casting and have been characterized by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Magnetic measurements have been carried out on the films as well as on nanocrystal powders. The films of citrate-capped gamma-Fe2O3 nanocrystals exhibit enhanced perpendicular magnetization, with the anisotropy depending on the film thickness.

Amines↗
PubMed2003

Novel iron(III) complexes of tripodal and linear tetradentate bis(phenolate) ligands: close relevance to intradiol-cleaving catechol dioxygenases.

Four new iron(III) complexes of the bis(phenolate) ligands N,N-dimethyl-N',N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine [H2(L1)], N,N-dimethyl-N',N'-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine [H2(L2)], N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine [H2(L3)], and N,N'-dimethyl-N,N'-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine [H2(L4)] have been isolated and studied as structural and functional models for the intradiol-cleaving catechol 1,2-dioxygenases (CTD). The complexes [Fe(L1)Cl] (1), [Fe(L2)(H2O)Cl] (2), [Fe(L3)Cl] (3), and [Fe(L4)(H2O)Cl] (4) have been characterized using absorption spectral and electrochemical techniques. The single-crystal X-ray structures of the ligand H2(L1) and the complexes 1 and 2 have been successfully determined. The tripodal ligand H2(L1) containing a N2O2 donor set represents the metal-binding region of the iron proteins. Complex 1 contains an FeN2O2Cl chromophore with a novel trigonal bipyramidal coordination geometry. While two phenolate oxygens and an amine nitrogen constitute the trigonal plane, the other amine nitrogen and chloride ion are located in the axial positions. In contrast, 2 exhibits a rhombically distorted octahedral coordination geometry for the FeN2O3Cl chromophore. Two phenolate oxygen atoms, an amine nitrogen atom, and a water molecule are located on the corners of a square plane with the axial positions being occupied by the other nitrogen atom and chloride ion. The interaction of the complexes with a few monodentate bases and phenolates and differently substituted catechols have been investigated using absorption spectral and electrochemical methods. The effect of substituents on the phenolate rings on the electronic spectral features and FeIII/FeII redox potentials of the complexes are discussed. The interaction of the complexes with catecholate anions reveals changes in the phenolate to iron(III) charge-transfer band and also the appearance of a low-energy catecholate to iron(III) charge-transfer band similar to catechol dioxygenase-substrate complexes. The redox behavior of the 1:1 adducts of the complexes with 3,5-di-tert-butylcatechol (H2DBC) has been also studied. The reactivities of the present complexes with H2DBC have been studied and illustrated. Interestingly, only 2 and 4 catalyze the intradiol-cleavage of H2DBC, the rate of oxygenation being much faster for 4. Also 2, but not 4, yields an extradiol cleavage product. The reactivity of the complexes could be illustrated not on the basis of the Lewis acidity of the complexes alone but by assuming that the product release is the rate-determining phase of the catalytic reaction.

Catechol 1,2-Dioxygenase↗
PubMed2002

Size-dependent chemistry: properties of nanocrystals.

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.

Journal Article↗
PubMed2001

Mesoscopic assembly of magnetic Pd-Ni nanocrystals into ordered arrays by using alkane thiols.

Two-dimensional arrays of core-shell Pd561 Nin (n = 38, 280, 561, 800, 1500, 3000, 4500) nanocrystals have been prepared by using alkanethiols. The arrays of Pd561 Nin nanocrystals are particularly well ordered; those with high Ni coverage show a tendency to form multilayers or crystallites. Two-dimensional arrays also have been obtained of Pd561 Ni3000 nanocrystals further covered by a Pd layer with a Pd561 Ni3000 Pd1500 composition.

Alkanes↗
PubMed1997

Transition-metal saccharide chemistry and biology: saccharide complexes of Cu(II) and their effect on in vivo metallothionein synthesis in mice.

Monosaccharide (D-Glc, D-Fru, D-Gal, D-Xyl, D-Rib) and disaccharide (Mal) complexes of Cu(II) were synthesized from two different precursors, viz. [NEt4]2 [CuCl2Br2] and CuCl2.2H2O, in nonaqueous media, and isolated in the solid state. The complexes were found to be primarily dimeric and trimeric and water soluble. These were characterized by diffuse reflectance, aqueous solution absorption, CD, FTIR, magnetic susceptibility, EPR, EXAFS, XANES, and elemental analysis. Aqueous solution stability in the pH range 4-8 was studied by cyclic voltammetry and absorption spectroscopy. The effect of subcutaneously injected Cu-Fru, Cu-Xyl, and Cu-Rib complexes on the in vivo metallothionein synthesis in mice was found to be significant in liver, but not in brain, in accordance with the observed copper accumulation in these tissues.

Animals↗