Showing papers by "Indian Association for the Cultivation of Science published in 2003"

Solvation dynamics and proton transfer in supramolecular assemblies.

Abstract: Water molecules confined in a supramolecular assembly control reactivity and dynamics of biological systems in a unique way. In a confined system, water molecules display an ultraslow component of solvation which is slower than that in bulk water by 2−4 orders of magnitude. The ultraslow component arises mainly from the disruption of the hydrogen-bond network of water and the binding of water molecules to a macromolecule. The ultraslow component of solvation markedly retards polar reactions. Many examples of slow dynamics in complex systems, and their implications in biological and natural processes are discussed.

WMAP constraints, supersymmetric dark matter, and implications for the direct detection of supersymmetry

Abstract: Recently WMAP has measured the cosmological parameters to a much greater accuracy We analyze the implications of this more precise measurement for supersymmetric dark matter and for the direct detection of supersymmetry at accelerators We consider MSUGRA including also the hyperbolic branch (HB) in the radiative breaking of the electroweak symmetry On the part of the hyperbolic branch where the lightest neutralino is dominantly a Higgsino rather than being mostly a B-ino, the relic density constraints are satisfied by coannihilation with the next lightest neutralino and the light chargino Including this branch the lightest neutralino mass satisfies ${m}_{{\ensuremath{\chi}}_{1}^{0}}l~1200\mathrm{GeV}$ for $\mathrm{tan}\ensuremath{\beta}l~50$ Constraints of $\stackrel{\ensuremath{\rightarrow}}{b}s+\ensuremath{\gamma},$ of ${g}_{\ensuremath{\mu}}\ensuremath{-}2,$ and of ${B}_{s}^{0}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ are also analyzed It is shown that the neutralino-proton cross section in each case will fall within the reach of dark matter experiments The possibility for the direct detection of supersymmetry is discussed in the allowed regions of the parameter space consistent with WMAP constraints A brief discussion of the hyperbolic branch and focus point region (HB/FP) is also given

Magneto-structural xorrelations in 2D and 3D extended structures of manganese(II)-malonate systems.

Abstract: Two polymeric malonato-bridged manganese(II) complexes of formula [Mn(mal)(H(2)O)(2)](n)() (1) and [Mn(2)(mal)(2)(4,4'-bipy)(H(2)O)(2)](n)() (2) have been synthesized and characterized (mal = malonate dianion; 4,4'-bipy = 4,4'-bipyridine). The crystal structure of complex 1 was already known. Complex 2 crystallizes in monoclinic space group P2(1)/n, Z = 2, with unit cell parameters of a = 7.2974(10) A, b = 18.7715(10) A, c = 7.514(3) A, and beta = 91.743(12) degrees. The structure determination reveals that the complex [Mn(2)(mal)(2)(4,4'-bipy)(H(2)O)(2)](n)() (2) is a 3D network being composed of Mn-malonate sheets which are pillared by bidentate 4,4'-bipy spacer forming small voids. The Mn-Mn distances through Mn-mu-(O3-C8-O4)-Mn, Mn-mu(O1-C6-O2)-Mn, and Mn-mu-4,4'-bipy-Mn bridges are 5.561, 5.410, and 11.723 A, respectively. The magnetic behaviors of complexes 1 and 2 in the temperature range 300-2 K are very close, corresponding to a weak antiferromagnetic coupling. The magnetic pathways of complex 1 are through two Mn-O-C-O-Mn with anti-anti conformation and two Mn-O-C-O-Mn with syn-anti conformations and in complex 2 through all Mn-O-C-O-Mn with syn-anti conformations. Both syn-anti and anti-anti conformations create weak antiferromagnetic coupling, and the susceptibility data are fitted by the expansion series of Lines and the Curely formula for an S = 5/2 antiferromagnetic quadratic layer, based on the exchange Hamiltonian H = -Sigma(nn)()JS(i)()S(j)(). The best fit is given by the superexchange parameters J = -0.32 cm(-)(1) and g = 2.00 for complex 1 and J = -0.14 cm(-)(1), J(inter) = -0.031 cm(-)(1), and g = 2.00 for complex 2. Finally, in both the complexes there is a magnetic pathway Mn-O-C-C-C-O-Mn, and this pathway through the three carbon atoms of the malonato-bridging ligand could be considered negligible.

Structural analyses and magnetic properties of 3D coordination polymeric networks of nickel(II) maleate and manganese(II) adipate with the flexible 1,2-bis(4-pyridyl)ethane ligand.

Abstract: Two novel inorganic-organic hybrid 3D extended networks of Ni(II) and Mn(II) having molecular formulas [(maleate)(2)Ni(3)(bpe)(4)(H(2)O)(4)](NO(3))(2).H(2)O (1) and [(adipate)Mn(bpe)] (2) (bpe = 1, 2-bis(4-pyridyl)ethane), respectively, have been synthesized and characterized by single-crystal X-ray diffraction studies and low-temperature (300-2 K) magnetic measurements. Compound 1 crystallizes in the monoclinic system, space group C2/c (No. 15), with chemical formula C(56)H(62)N(10)Ni(3)O(19), a = 30.955(4) A, b = 12.705(3) A, c = 17.058(5) A, beta = 117.26(2) degrees, and Z = 4. Compound 2 crystallizes in the triclinic system, space group Ponemacr; (No. 2), with chemical formula C(18)H(20)MnN(2)O(4), a = 8.492(2) A, b = 9.444(2) A, c = 11.533(3) A, alpha = 97.19(1) degrees, beta = 94.64(1) degrees, gamma = 105.02(1) degrees, and Z = 2. The structure determination reveals for both a 3D network. Compound 1 contains two crystallographically independent Ni(II) ions in different octahedral environments. Ni(1) lies on an inversion center, and its coordination environment comprises two chelating maleate anions and two bpe nitrogen donors, while the Ni(2) ion is surrounded by meridionally disposed three bpe N atoms, two water molecules, and one oxygen donor from the dicarboxylate anion. Of the three crystallographic independent bpe ligand, one presents an anti and the others a gauche conformation. The corresponding N-to-N distances are 9.344, 6.543, and 6.187 A. Variable-temperature magnetic susceptibility measurement of the complex reveals the existence of a dominant ferromagnetic interaction within the molecule. Compound 2 is composed of Mn(2) dimer units linked by adipate anions to form corrugated 2D sheets which, on interconnection through bpe (anti conformation, N-to-N distance of 9.391 A), produces an interpenetrated 3D alpha-polonium-related type net. Complex 2 reveals to be antiferromagnetic fitting data using a dimeric Mn(II) model that considers negligible magnetic transmission through the carbon skeleton of adipate and the bpe pathway.

Syntheses of Two New 1D and 3D Networks of Cu(II) and Co(II) Using Malonate and Urotropine as Bridging Ligands: Crystal Structures and Magnetic Studies

Abstract: Two new metal-organic based polymeric complexes, [Cu(4)(O(2)CCH(2)CO(2))(4)(L)].7H(2)O (1) and [Co(2)(O(2)CCH(2)CO(2))(2)(L)].2H(2)O (2) [L = hexamethylenetetramine (urotropine)], have been synthesized and characterized by X-ray crystal structure determination and magnetic studies. Complex 1 is a 1D coordination polymer comprising a carboxylato bridged Cu(4) moiety linked by a tetradentate bridging urotropine. Complex 2 is a 3D coordination polymer made of pseudo-two-dimensional layers of Co(II) ions linked by malonate anions in syn-anti conformation which are bridged by bidentate urotropine in trans fashion. Complex 1 crystallizes in the orthorhombic system, space group Pmmn, with a = 14.80(2) A, b = 14.54(2) A, c = 7.325(10) A, beta = 90 degrees, and Z = 4. Complex 2 crystallizes in the orthorhombic system, space group Imm2, a = 7.584(11) A, b = 15.80(2) A, c = 6.939(13) A, beta = 90.10 degrees (1), and Z = 4. Variable temperature (300-2 K) magnetic behavior reveals the existence of ferro- and antiferromagnetic interactions in 1 and only antiferromagnetic interactions in 2. The best fitted parameters for complex 1 are J = 13.5 cm(-)(1), J' = -18.1 cm(-)(1), and g = 2.14 considering only intra-Cu(4) interactions through carboxylate and urotropine pathways. In case of complex 2, the fit of the magnetic data considering intralayer interaction through carboxylate pathway as well as interlayer interaction via urotropine pathway gave no satisfactory result at this moment using any model known due to considerable orbital contribution of Co(II) ions to the magnetic moment and its complicated structure. Assuming isolated Co(II) ions (without any coupling, J = 0) the shape of the chi(M)T curve fits well with experimental data except at very low temperatures.

Synthesis, Growth, and Characterization of l-Arginine Acetate Crystal: A Potential NLO Material

Abstract: Single crystals of nonlinear optical material l-arginine acetate (LAA) (C6H14N4O2·CH3COOH), space group P21, were successfully grown for the first time by the temperature-lowering method and also by the slow evaporation method at constant temperature (30 °C) from its aqueous solution with pH at 6 and dimension of 21 × 15 × 3 mm3. Initially, solubility tests were carried out for four solvents such as water, water and methanol, water and ethanol, and water and acetone. Among the four solvents, the solubility of LAA was found to be the highest in water, so crystallization of LAA was done from its aqueous solution. Morphological analysis reveals that the crystal is a polyhedron with 16 developed faces with major face forms {100}, {001}, and {102} (pinacoids) parallel to the polar axis. As grown crystals were characterized by chemical analysis, density measurement, and X-ray powder diffraction studies. Infrared spectroscopy, thermogravimetric analysis, and differential thermal analysis measurements were perfor...

Synthesis, Structure, and Reactivity of a New Mononuclear Molybdenum(VI) Complex Resembling the Active Center of Molybdenum Oxotransferases

Abstract: The design, synthesis, and structure determination of a pentacoordinate square-pyramidal molybdenum(VI) complex MoO2L (where LH2 is a diacidic tridentate ONO donor ligand) is reported. The substrate binding capacity of MoO2L has been demonstrated by the formation and isolation of six-coordinate octahedral complexes [MoO2L(B)] (where B = imidazole, 1-methylimidazole, pyridine, or γ-picoline) and the structures of [MoO2L(Imz)] and [MoO2L(Py)] have been solved by X-ray crystallography. Oxo transfer of MoO2L to the substrate PPh3 has been demonstrated by the formation of MoOL and [MoOL(bipy)]. Thus, the complex MoO2L behaves as a model for the active center of oxotransfer molybdoenzymes and is of interest in bioinorganic chemistry. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Large conductance switching and binary operation in organic devices: Role of functional groups

Abstract: We have realized tuning of electronic memory-switching property via functional group modification in solid-state devices. Apart from their large ON/OFF ratio and long memory retention time, solid-state devices sustain repetitive switching between the two ON/OFF states at high frequencies. We have chosen several molecular systems with same backbone structure and tuned ON/OFF ratio from 4 to 300 000 simply by increasing the number of deactivating groups. A key to the large ON/OFF ratio in these devices has been the presence of acceptor groups in the molecules and consequently low OFF state current. We analyzed the appearance of ON state in terms of conjugation restoration of the molecules. A redox active group has been found to be necessary in the molecules for continuous flip-flop between “1” and “0” states for random access memory (RAM) applications.

Higgsino dark matter in a supergravity model with nonuniversal gaugino masses

Abstract: We study a specific SUGRA model with nonuniversal gaugino masses as an alternative to the minimal SUGRA model in the context of supersymmetric dark matter. The lightest supersymmetric particle in this model turns out to be a Higgsino-dominated instead of a B-ino-dominated lightest neutralino. The thermal relic density of this Higgsino dark matter is somewhat lower than the cosmologically favored range, which means that it may be only a subdominant component of cold dark matter. Nonetheless, it predicts favorable rates of indirect detection, which can be seen in square-km-size neutrino telescopes.

Synthesis and Characterization of Iron-Rich Highly Ordered Mesoporous Fe-MCM-41

Abstract: Iron-rich and highly ordered 2D-hexagonal mesoporous ferrisilicates containing predominantly tetrahedrally coordinated Fe3+ in the silica network have been prepared using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent (SDA) under mild alkaline hydrothermal conditions (initial pH = 8−8.5 in the synthesis gel). The optimum limit of iron loading for the ordered mesophase in the present study was 8.2 wt %; beyond this limit, disordered iron oxide/silica phase was observed in the same pH range. These mesoporous ferrisilicate samples were characterized using XRD, N2 sorption, EPR, SEM-EDX, TEM, FT-IR and UV−visible spectroscopies, and Mossbauer measurements. The average pore diameters were 2.2−2.8 nm, and the mesopore volumes were 0.39−0.5 cm3g-1. Moderately high Bronsted acidity was observed in the temperature-programmed desorption (TPD) pattern of ammonia over these mesoporous ferrisilicate materials. Wet chemical analysis, EPR, and Mossbauer spectroscopic data suggested that most of t...

Syntheses, Structural Analyses, and Magneto-Structural Correlations of Three Polymeric Fe(II) Complexes with Azide Ligand

Abstract: Three new metal-organic polymeric complexes, [Fe(N(3))(2)(bpp)(2)] (1), [Fe(N(3))(2)(bpe)] (2), and [Fe(N(3))(2)(phen)] (3) [bpp = (1,3-bis(4-pyridyl)-propane), bpe = (1,2-bis(4-pyridyl)-ethane), phen = 1,10-phenanthroline], have been synthesized and characterized by single-crystal X-ray diffraction studies and low-temperature magnetic measurements in the range 300-2 K. Complexes 1 and 2 crystallize in the monoclinic system, space group C2/c, with the following cell parameters: a = 19.355(4) A, b = 7.076(2) A, c = 22.549(4) A, beta = 119.50(3) degrees, Z = 4, and a = 10.007(14) A, b = 13.789(18) A, c = 10.377(14) A, beta = 103.50(1) degrees, Z = 4, respectively. Complex 3 crystallizes in the triclinic system, space group P(-)1, with a = 7.155(12) A, b = 10.066(14) A, c = 10.508(14) A, alpha = 109.57(1) degrees, beta = 104.57(1) degrees, gamma = 105.10(1) degrees, and Z = 2. All coordination polymers exhibit octahedral Fe(II) nodes. The structural determination of 1 reveals a parallel interpenetrated structure of 2D layers of (4,4) topology, formed by Fe(II) nodes linked through bpp ligands, while mono-coordinated azide anions are pendant from the corrugated sheet. Complex 2 has a 2D arrangement constructed through 1D double end-to-end azide bridged iron(II) chains interconnected through bpe ligands. Complex 3 shows a polymeric arrangement where the metal ions are interlinked through pairs of end-on and end-to-end azide ligands exhibiting a zigzag arrangement of metals (Fe-Fe-Fe angle of 111.18 degrees) and an intermetallic separation of 3.347 A (through the EO azide) and of 5.229 A (EE azide). Variable-temperature magnetic susceptibility data suggest that there is no magnetic interaction between the metal centers in 1, whereas in 2 there is an antiferromagnetic interaction through the end-to-end azide bridge. Complex 3 shows ferro- as well as anti-ferromagnetic interactions between the metal centers generated through the alternating end-on and end-to-end azide bridges. Complex 1 has been modeled using the D parameter (considering distorted octahedral Fe(II) geometry and with any possible J value equal to zero) and complex 2 has been modeled as a one-dimensional system with classical and/or quantum spin where we have used two possible full diagonalization processes: without and with the D parameter, considering the important distortions of the Fe(II) ions. For complex 3, the alternating coupling model impedes a mathematical solution for the modeling as classical spins. With quantum spin, the modeling has been made as in 2.

A novel layered and pillared topology in a 3D open framework: synthesis, crystal structure and magnetic properties

Abstract: A one pot reaction of Cu(NO3)2, malonate dianion (mal) and 1,2-bis(4-pyridyl)ethane (bpe) leads to a novel 3D open framework, [{Cu3(mal)2(bpe)3(H2O)2}(NO3)2(H2O)2]n (1), composed of Cu–malonate layers pillared by bidentate bpe ligands encapsulating NO3− and water molecules in bidirectional intersecting channels; the framework's secondary building unit comprises unprecedented two hexa- and four penta-coordinate copper(II) nodes which are 6- and 4-connecting, respectively, and the magnetic properties of the complex show the highest overall ferromagnetic interaction (J = 22 cm−1) of the previously reported syn–anti carboxylate bridged complexes.

Equilibrium Studies in Solution Involving Nickel(II) Complexes of Flexidentate Schiff Base Ligands: Isolation and Structural Characterization of the Planar Red and Octahedral Green Species Involved in the Equilibrium

Abstract: Three new flexidentate 5-substituted salicylaldimino Schiff base ligands (L1-OH-L3-OH) based on 1-(2-aminoethyl)piperazine (X=H, L1-OH; X=NO2, L2-OH; and X=Br, L3-OH) and their nickel(II) complexes (1a, 1b, 2, and 3) have been reported. The piperazinyl arm of these ligands can in principle have both boat and chair conformations that allow the ligands to bind the Ni(II) center in an ambidentate manner, forming square-planar and/or octahedral complexes. The nature of substitution in the salicylaldehyde aromatic ring and the type of associated anion in the complexes have profound influences on the coordination geometry of the isolated products. With the parent ligand L1-OH, the product obtained is either a planar red compound [Ni(L1-O)]+, isolated as tetraphenylborate salt (1a), or an octahedral green compound [Ni(L1-NH)(H2O)3](2+), isolated with sulfate anion (1b); both have been crystallographically characterized. In aqueous solution, both these planar (S=0) and octahedral (S=1) forms are in equilibrium that has been followed in the temperature range 298-338 K by 1H NMR technique using the protocol of Evans's method. The large exothermicity of the equilibrium process [Ni(L1-O)]+ + 3H2O + H+ [Ni(L1-NH)(H2O)3](2+) (DeltaH degrees=-46 +/- 0.2 kJ mol(-1) and DeltaS degrees=-133 +/- 5 J K(-1) mol(-1)) reflects formation of three new Ni-OH2 bonds in going from planar to the octahedral species. With the 5-nitro derivative ligand L2-OH, the sole product is an octahedral compound 2, isolated as a sulfate salt while with the bromo derivative ligand L3-OH, the exclusive product is a planar molecule 3 with associated tetraphenylborate anion. Both 2 and 3 have been structurally characterized by X-ray diffraction analysis.

Living Radical Polymerization. 1. The Case of Atom Transfer Radical Polymerization of Acrylamide in Aqueous-Based Medium

Abstract: Atom transfer radical polymerization (ATRP) of acrylamide has been carried out in water or in glycerol−water (1:1 v/v) medium at 130 °C using water-soluble initiators, viz. 2-chloropropionamide (2-Cl-PA) or 2-bromopropionamide (2-Br-PA) and CuX (X = Cl, Br) bipyridine complex as catalyst. Extraneous addition of CuX2 (20 mol % of CuX) and/or excess X- ions (1 M alkali halide) in the reaction mixture helps to reduce molecular weight polydispersity (PDI). However, even under the best conditions (using both CuX2 and X- ion additives), the PDI is high, ca. 1.6−1.7. Also, the GPC traces show shoulders. The chain extension experiment, however, confirms the living nature of the polymers. Replacement of glycerol−water with water as the medium results in sluggish polymerization. The ln M0/M vs t plots are curved to start with but become linear after polymerization proceeds to variable extents depending on the additives used. The molecular weights tend to agree with the theoretical values as conversion increases. A ...

Excited State Prototropic Activities in 2-Hydroxy 1-Naphthaldehyde

Abstract: Excited state proton transfer in 2-hydroxy 1-naphthaldehyde (HNL) has been reported on the basis of steady state absorption and emission, time-resolved emission, and semiempirical quantum chemical calculations. The existence of open and closed ring conformers in the ground state gives rise to different emissions upon excitation in hydroxylic and hydrocarbon solvents, respectively. Formation of the ground state HNL ion in open conformer was observed with addition of base. Excitation of the closed conformer leads to intramolecular proton transfer across the preexisting intramolecular hydrogen bond to give excited zwitterion in hydrocarbon solvents. On excitation of the open ring conformer in hydroxylic solvents, emission from the neutral excited state could be observed for the first time apart from the emission of ionic intermolecular proton transfer species. An increase or decrease in pH results in enhancement or a decrease of emission from the ionic conformer with parallel dwindling or intensification of ...

Tuning of Organic Reversible Switching via Self‐Assembled Supramolecular Structures

Abstract: Single-layer devices based on Rose Bengal show large conductance switching between the on and off states (on/off ratio =10 4 ) in the pristine form. Rose Bengal molecules have been incorporated into different water soluble "host" polymer-based supramolecular matrices by layer-by-layer electrostatic self-assembly. Different ionic groups in the supramolecular structures tune the leakage current and the on/off ratio by several orders of magnitude. The switching devices may be used in random-access and read-only memory applications.

Cocrystallization Behavior of Poly(3-alkylthiophenes): Influence of Alkyl Chain Length and Head to Tail Regioregularity

Abstract: The cocrystallization behavior of poly(3-alkylthiophene)s (P3ATs) of varying regioregularity and alkyl chain length was explored via differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) studies. For the regioregular samples both DSC and WAXS studies revealed that cocrystallization was limited to the difference of the alkyl chain length by two carbon atoms. On the other hand, P3AT samples with the same alkyl chain length but different regioregularities cocrystallized with a difference of 17 mol % head to tail (H−T) regularity studied here. P3AT samples with varying regioregularity and alkyl chain length by two carbon atoms showed cocrystallization for a regioregularity difference of 7 mol %. The WAXS study indicated that cocrystals had crystallized within the same interchain lamella and noncocrystals had crystallized within the independent interchain lamella of the components. It also indicated the formation of type 1 P3AT crystals with a noninterdigited side chain during the cocr...

A Three-Dimensional Honeycomb-Like Network Constructed with Novel “Sinusoidal” One-Dimensional Chains via Hydrogen Bonding and π−π Interactions

Abstract: A one pot reaction of MnCl2, acetylene dicarboxylate (C4O42-), and 1,2-bis(4-pyridyl)ethane (bpe) leads to a compound formulated as [{Mn(bpe)(H2O)4}(C4O4)·4.5H2O] and has been shown by single crystal structure analysis to be a three-dimensional network with hexagonal and rhombohedral channels encapsulating C4O42- and water molecules, constructed by the one-dimensional “sinusoidal” [Mn(bpe)(H2O)4]n2+ cationic chains through extensive hydrogen-bonding and π−π interactions.

Remarkably selective reduction of the alpha,beta-carbon-carbon double bond in highly activated alpha,beta,gamma,delta-unsaturated alkenes by the InCl3-NaBH4 reagent system.

Abstract: A combination of sodium borohydride and a catalytic amount of indium(III) chloride in acetonitrile reduces exclusively the alpha,beta-carbon-carbon double bond in alpha,beta,gamma,delta-unsaturated diaryl ketones, dicarboxylic ester, cyanoester, and dicyano compounds.

Improving the Lipase Activity Profile in Cationic Water-in-Oil Microemulsions of Hydroxylated Surfactants

Abstract: The activity of Chromobacterium viscosum lipase (CV-lipase) in cationic water-in-oil (w/o) microemulsions is significantly lower compared to that in bis(2-ethylhexyl)sulfosuccinate sodium salt (AOT)-based (anionic) systems.26,36,38,39 In the present study, we estimated the second-order rate constants k2 in lipase-catalyzed hydrolysis of p-nitrophenylcaproate, in newly developed cationic w/o microemulsions of synthesized surfactants (2−6, Chart 1) containing hydroxyethyl moieties at the polar headgroup. The kinetic studies at pH = 6.0 (pH refers to the pH of the aqueous buffer solutions used in preparing the w/o microemulsions) show that the catalytic efficiency of CV-lipase was systematically increased with the sequential increment of hydroxyethyl groups at the polar heads of surfactants (1−4, Chart 1), possibly due to the increase in the interfacial concentration of water [H2O]i in consequence of the added hydrogen bonding ability of hydroxyl groups. To this end, we found that in a 0.05 M 4/water/32.3:1 ...