Showing papers by "Indian Association for the Cultivation of Science published in 1998"

Indium(III) Chloride-Promoted Rearrangement of Epoxides: A Selective Synthesis of Substituted Benzylic Aldehydes and Ketones

Abstract: A simple and efficient procedure for the rearrangement of substituted epoxides catalyzed by InCl3 has been developed. Aryl-substituted epoxides isomerize with complete regioselectivity to form a single carbonyl compound via cleavage of the benzylic C−O bond. The reactions are simple, fast, and high yielding. This procedure is very mild compared to those catalyzed with BF3 and other Lewis acids and compatible with several acid-sensitive functionalities. This protocol provides a highly selective synthesis of substituted benzylic aldehydes and ketones. However, rearrangement of alkyl-substituted epoxides is not very selective.

Chemistry of Azoimidazoles: Synthesis, Spectral Characterization, Electrochemical Studies, and X-ray Crystal Structures of Isomeric Dichloro Bis[1-alkyl-2-(arylazo)imidazole] Complexes of Ruthenium(II)

Abstract: Several new ligands, azoimidazoles belonging to the class 1-methyl-2-(arylazo)imidazoles (L1 (3)) and 1-benzyl-2-(arylazo)imidazoles (L2 (4)) (R = H (a), Me (b), OMe (c), Cl (d), NO2 (e)) have been synthesized and reacted with RuCl3 in ethanol under refluxing conditions. Two isomers of the composition RuL2Cl2, green (i) and blue (iii), are chromatographically separated. The green isomer is quantitatively transformed to the blue isomer on refluxing in a high boiling solvent. The isomeric structures have been confirmed by X-ray crystallography. Crystal data are as follows. Green complex C38H34Cl2N8Ru (6a): crystal system monoclinic; space group C2/c; a = 15.680(8) A; b = 22.766(14) A; c = 11.473(5) A; β = 119.27(4)°; V = 3573(3) A3 Z = 4; R = 3.59%; Rw = 4.38%. Blue complex C22H24Cl2N8Ru (7b): crystal system monoclinic; space group P21/n, a = 9.547(6) A; b = 22.554(14) A; c = 11.748(8) A; β = 99.07(5)°; V = 2498(3) A3; Z = 4; R = 3.15%; Rw = 4.51%. With reference to the pairs of Cl, N(imidazole), and N(az...

State-Specific Multi-Reference Coupled Cluster Formulations: Two Paradigms

Abstract: The traditional multi-reference coupled cluster (MRCC) methods are based on effective hamiltonian formalism and often suffer from the problem of intruders. A state-specific MRCC approach, focusing on only one state, offers the attractive possibility of avoiding intruders while at the same time incorporating the nondynamical correlation in a size-extensive manner. In this paper we discuss two alternative paradigms which allow us to achieve this goal. The first, to be called the decontracted description, we deliberately retain certain linearly dependent cluster amplitudes and allow the combining coefficients of the reference determinants to be updated to their values for the exact function. The presence of the linearly dependent cluster amplitudes requires imposition of suitable sufficiency conditions, which are invoked in a manner which naturally ensures size-extensivity. In the second approach, to be called the contracted description, we would generate a cluster expansion with respect to the entire reference function consisting of a combination of reference determinants and retain only the linearly independent cluster-amplitudes in the cluster expansion. For an efficient implementation of the formalism, we shall introduce the notion of extended normal ordering and an analogue of Wick's theorem which uses the entire reference function as the multi-determinantal analogue of the vacuum. This necessarily imposes the restriction that the combining coefficients appearing in the reference function have to remain frozen in the equations for the cluster amplitudes. Relaxation of the coefficients can be acheived only after the cluster-amplitudes with the current coefficients are solved.

Solvation Dynamics of Coumarin 480 in Vesicles

Abstract: Solvation dynamics of Coumarin 480 (C-480) in dimyristoylphosphatidylcholine (DMPC) vesicles in water is studied using picosecond time-resolved Stokes shift. In sonicated unilamellar DMPC vesicles C-480 exhibits wavelength dependent fluorescence decays. At short wavelengths a fast decay is observed while at the long wavelengths a growth in the nanosecond time scale precedes the decay. The solvation dynamics of C-480 in DMPC vesicles is found to be bimodal with two components of 0.6 and 11 ns, which is similar to the solvation dynamics of C-480 in the large water pools of AOT/n-heptane/water (AOT = aerosol OT) microemulsions.

Morphology of thin silver film grown by dc sputtering on Si(001)

Abstract: The morphology and growth mechanism of silver films approximately 150 A in thickness on Si(001) substrates have been studied by atomic force microscopy and x-ray reflectivity. The thin films prepared by dc sputtering at room temperature are composed of islands of silver. The shape and size distribution of these islands are studied using these two complementary measurement techniques.

Solvation dynamics of 4-aminophthalimide in water-in-oil microemulsion of triton X-100 in mixed solvents

Abstract: Solvation dynamics of 4-aminophthalimide (4-AP) in water-in-oil microemulsion of Triton X-100 (TX) in mixed solvents containing 30% benzene and 70% n-hexane is studied using picosecond time-resolved Stokes shift. The steady-state emission spectrum of 4-AP in this microemulsion is very different from that in TX reverse micelle or in water. This indicates that in this microemulsion the 4-AP molecule stays in the water pool and the micropolarity of the water pool is very much different from that of ordinary bulk water. In TX microemulsions 4-AP exhibits wavelength-dependent fluorescence decays and a growth in the nanosecond time scale at long wavelengths. The solvation dynamics of 4-AP in TX microemulsion is bimodal with a short component of 0.74 ns and a very long component of 29.73 ns. This is similar to the solvation dynamics of coumarin 480 in the large water pools of sodium dioctyl sulfosuccinate (AOT)/n-heptane/water microemulsions.

Stereoselective Synthesis of Polysubstituted Tetrahydrofurans by Radical Cyclization of Epoxides Using a Transition-Metal Radical Source. Application to the Total Synthesis of (±)-Methylenolactocin and (±)-Protolichesterinic Acid

Abstract: Radical cyclization reactions of substituted α-(prop-2-ynyloxy) epoxides using bis(cyclopentadienyl)titanium(III) chloride as the radical source resulted in polysubstituted tetrahydrofurans. Titanium(III) species were prepared in situ from commercially available titanocene dichloride and zinc dust in tetrahydrofuran. Upon radical cyclization, 2-aryl epoxides 3a−e afforded only trans products 4a−e whereas the 2-alkyl epoxides 3f−h formed a mixture of isomeric products 4f−h in a ratio of 5:1. Some of the aryl tetrahydrofuran derivatives have already been used for the synthesis of bioactive furofuran lignans. 2-Pentyl-3-(hydroxymethyl)-4-methylenetetrahydrofuran (4f) and 2-tridecyl-3-(hydroxymethyl)-4-methylenetetrahydrofuran (4g) have been transformed to two antitumor antibiotics (±)-methylenolactocin (1f) and (±)-protolichesterinic acid (1g), respectively, in good overall yield.

Isolation and Structure of the First Azo Anion Radical Complexes of Ruthenium.

Abstract: The anaerobic reaction of 1 (X = H, Cl) with 2 in heptane affords the green paramagnetic (S = 1/2) and EPR-active (g = 2.00) title complexes 3a (X = H) and 3b (X = Cl), which furnish the corresponding nonradical diamagnetic salts 3a+ PF6- and 3b+ PF6- upon oxidation in solution. In contrast, the reaction of 2 with N-(p-tolyl)pyridine-2-aldimine (pal) affords [Ru(pal)(H)(Cl)(CO)(PPh3)]. The X-ray structures of 3a and 3b+ have been determined.

Excited-state proton transfer of 1-naphthol in micelles

Abstract: The fast deprotonation of 1-naphthol, which occurs in 35 ps in aqueous solution, is studied in neutral (triton X 100, reduced, TX-100R), cationic (cetyl trimethylammonium bromide, CTAB), and anioni...

Spin Exchange Coupling in Heterobimetallic MIIVIVO (M = Cu, Ni, Co, Fe, Mn) Macrocyclic Complexes. Synthesis, Structure, and Properties

Abstract: A series of heterobimetallic complexes [(VIVO)L1CuII(μ-OAC)(MeOH)](ClO4) (1) and [(VIVO)L1MII(μ-OAc)(H2O)](ClO4)·H2O (M = Ni (2), Co (3), Fe (4), Mn (5)) derived from a symmetric tetraaminodiphenol...

Photophysical Processes of Ethidium Bromide in Micelles and Reverse Micelles

Abstract: Photophysical processes of ethidium bromide (EB) in homogeneous solutions, micelles, and reverse micelles have been investigated. In acetonitrile, the fluorescence intensity and lifetime of EB are 6.3 ± 0.3 times those in water and 1.25 ± 0.1 times those in acetone. This is attributed to the weaker hydrogen-bond acceptor property of acetonitrile, compared to water and acetone. Addition of water to acetonitrile leads to a marked quenching of the EB emission, with a quenching constant of (1.7 ± 0.3) × 107 M-1 s-1. In aqueous solution, hydroxyl ion quenches EB emission more dramatically with a quenching constant of (4.4 ± 0.4) × 1010 M-1 s-1. This is ascribed to the abstraction of the amino proton of the excited ethidium ion by the hydroxyl ion. Emission properties of EB are found to be good monitors for the micellization of an anionic micelle, sodium dodecyl sulfate (SDS). In AOT microemulsion, EB exhibits nearly (1.8 ± 0.1)-fold emission enhancement relative to water. The emission properties of EB are foun...

Palladium-Catalyzed Heteroannulation Leading to Heterocyclic Structures with Two Heteroatoms: A Highly Convenient and Facile Method for a Totally Regio- and Stereoselective Synthesis of (Z)-2,3-Dihydro-2-(ylidene)-1,4-benzo- and -naphtho[2,3-b]dioxins†

Abstract: A facile method for the synthesis of (Z)-2,3-dihydro-2-(ylidene)-1,4-benzo- and naphthodioxins (3) has been developed using palladium−copper catalysis. Aryl halides 2 were found to react with mono-prop-2-ynylated catechol (1a) or 2-hydroxy-3-(prop-2-ynyloxy)naphthalene (1b) in the presence of (PPh3)2PdCl2 (3.5 mol %) and CuI (7 mol %) in triethylamine by stirring at room temperature for 20 h followed by heating at 100 °C for 16 h to give products 3 in good yields. The method is regio- and stereoselective and also amenable to bisheteroannulation. The Z-stereochemistry of products 3 was established firmly from 1H NMR, 3JCH values (between vinylic proton and methylenic carbon of the heterocyclic ring), proton NOE measurements, and finally from X-ray analysis. Based on experimental observations and known palladium chemistry, a mechanism has been proposed to explain the regio- and stereoselective product formation. Some of the products 3 were also converted to 1,4-benzodioxan derivatives 6 using hydrogenation ...

Ethyl(hydroxyethyl)cellulose stabilized polyaniline dispersions and destabilized nanoparticles therefrom

Abstract: Ethyl(hydroxyethyl)cellulose (EHEC) has been used as a stabilizer for the dispersion polymerization of aniline in water and in aqueous ethanols. An increasingly greater amount of aniline could be used to prepare the dispersion as the ethanol concentration increased in the medium over the range 50−70 vol %. EHEC incorporation in the particles increases and particle size decreases with an increase in the ethanol concentration under otherwise identical experimental conditions. Particles prepared in aqueous ethanols and containing less than ca. 8 wt % EHEC are easily disintegrated to nanoparticles (∼20 nm) using ultrasound. These nanoparticles are unstabilized and undergo fractal aggregation even in a medium that contains a substantial amount of EHEC. This fact suggests that in the dispersion polymerization the real stabilizer is not the homopolymer EHEC but could be its graft copolymer. The fractal dimension (D) of the aggregates lies in the range 1.69−1.89. These values agree well with the theoretically det...

Fock space multireference coupled cluster theory: noniterative inclusion of triples for excitation energies

Abstract: In this paper we propose and numerically implement a specific scheme for calculating the excitation energies (EEs) within the Fock space multireference coupled cluster framework, which includes the contributions from noniterative triples cluster amplitudes. These contribute to the EEs at the third order. We present results for CH+ and N2, and study the effects of these noniterative triples on EEs.

Thermoreversible gelation of poly(vinylidene flouride) in diethyl adipate: a concerted mechanism

Abstract: Poly(vinylidene flouride) (PVF 2 ) gels in diethyl adipate (DEA) with fibrillar morphology. The gels are transparent. The Wide-angle X-ray scattering pattern and FT-IR spectra of the gel indicate the presence of solvated a-phase PVF 2 crystallites in the gel. The intensities of the X-ray diffraction peaks of the gel are mostly different than those of the melt-crystallized sample. The plots of enthalpy of gel fusion and enthalpy of gel formation with a weight fraction of PVF 2 (W PVF 2 ) indicate a positive deviation from linearity. Analysis of these results indicates polymer-solvent complex formation in the molar ratio of 1:2 for the DEA and PVF 2 repeating unit, respectively. The phase diagram also supports polymer-solvent complex formation with an incongruent melting point. The gelation kinetics is studied by the test tube tilting method. Analysis of the concentration function of the gelation rate supports the theory that this gelation process obeys the three-dimensional percolation mechanism. The temperature coefficient analysis is done both by the Flory and Weaver theory of coil-to-helix transtion and by the growth rate theory of fibrillar crystallization extended to the polymer-diluent system. A comparison of energy barrier values of both the processes indicates that the gelation is a concerted process of conformational ordering and crystallization.

A Family of (N-Salicylidene-α-amino acidato)vanadate Esters Incorporating Chelated Propane-1,3-diol and Glycerol: Synthesis, Structure, and Reaction

Abstract: The brown colored title complexes, VVO(Asal)(Hpd) and VVO(Asal)(H2pt), have been synthesized in excellent yields by reacting VIVO(Asal)(H2O) with propane-1,3-diol (H2pd) and glycerol (H3pt), respectively, in methanol. Here Asal2- is the deprotonated salicylaldimine of glycine (A = g), l-alanine (A = a), l-valine (A = v), or l-phenylalanine (A = p). The complexes have relatively low oxovanadium(V)−oxovanadium(IV) reduction potentials (−0.2 V vs SCE in dimethyl sulfoxide). The X-ray structures of VO(gsal)(Hpd) and VO(gsal)(H2pt) have revealed distorted octahedral VO5N coordination. Six-membered and five-membered O,O chelation occur for Hpd- and H2pt-, respectively, an undissociated alcohol function lying trans to the oxo oxygen atom. The tridentate salicylaldimine ligand spans meridionally and has a folded structure consisting of two planar parts intersecting along a C−N bond. The V−O(alkoxide) bond in the complexes is ∼0.6 A shorter than the V−O(alcohol) bond. The alkoxidic chelate ring partially hydrolyze...

Ring-Closing Metathesis in Carbohydrate Annulation.

Abstract: Even eight-membered rings (such as in 2) can be formed by ring-closing metathesis of glucose derivatives such as 1. Enantiomerically pure tricyclic spiro compounds can also be prepared.

Development of an amperometric enzyme electrode biosensor for fish freshness detection

Abstract: A biosensor system has been developed for quantitative measurement of fish freshness by determining the ATP degradation products, hypoxanthine (HX), inosine (INO) and inosine monophosphate (IMP) metabolites in fish- tissue. Freshness is indicated by an index K1 or H which depends on the amount of these ATP degradation products present in fish meat. These degradation metabolites have been detected efficiently by amperometric method using three new ferrocene carboxylic acid (FCA) mediator incorporated conducting polypyrrole enzyme electrodes with immobilized xanthine oxidase, nucleoside phosphorylase and nucleotidase enzymes for quantitative measurement of HX, INO and IMP respectively. This simple, fast and ready to use biosensor has been used to measure the H values of fresh water fish Catla-Catla over days of storage. Results show that the fish degrades very rapidly after 7 days even when stored at −12°C.

A fluorescence spectroscopic and molecular dynamics study of bis-ANS/protein interaction.

Abstract: Despite emergence of bis-ANS as a major fluorescence probe of proteins structure, conformational and spectroscopic properties of protein/bis-ANS complexes remains largely unexplored. We have shown that fluorescence polarization of both ANS and bis-ANS is excitation wavelength dependent and this is a property of all protein-ANS/bis-ANS complexes studied. Bis-ANS excitation maximum is always more red shifted than the corresponding ANS complex. Even when corrected for the red shift, the bis-ANS complexes in some, but not all, cases show only a little lowering of polarization, suggesting modest additional depolarization in bis-ANS compared to ANS. Calculation of energy migration rate between the two rings suggests that energy migration rate should be high at all values of the naphthyl-naphthyl dihedral angle. Although, Molecular mechanics and dynamics calculations show that the lowest energy conformation of bis-ANS is when the two naphthalene rings are roughly perpendicular to each other, due to rapi...

Intramolecular Proton Transfer in Inclusion Complexes of Cyclodextrins: Role of Water and Highly Polar Nonaqueous Media

Abstract: The molecule 4-methyl-2,6-dicarbomethoxyphenol (CMOH) which undergoes ultrafast excited-state intramolecular proton transfer (ESIPT) has been employed as a guest to probe ESIPT within the interior of the host α- and β-cyclodextrins (CDx) in water and highly polar nonaqueous solvents. The ESIPT reaction of CMOH is favored in its microencapsulated form in both aqueous and nonaqueous media. As a consequence, resultant enhancement of tautomer emission, following ESIPT, relative to that in homogeneous bulk media is observed. CMOH forms 1:2 and 1:1 complexes with α- and β- CDx's (host), respectively, in both water and highly polar nonaqueous media. However, tautomer emission due to ESIPT is significantly enhanced in water compared to nonaqueous solvents, despite formation of stronger guest−host (inclusion) complexes with much higher binding constant in the nonaqueous media. The results have been interpreted as due to the formation of ternary complexes comprising CMOH, second guest, and cyclodextrin in nonaqueou...