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Various Solvents (various + solvent)
Selected AbstractsChemInform Abstract: Synthesis of Substituted 2,2, and 4,4,-Bithiazoles in Various Solvents.CHEMINFORM, Issue 39 2001R. Mostaghim Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Force Field Evaluation for Biomolecular Simulation: Free Enthalpies of Solvation of Polar and Apolar Compounds in Various SolventsCHEMPHYSCHEM, Issue 3 2006Daan P. Geerke Abstract Recently, the GROMOS biomolecular force field parameter set 53A6,which has been parametrized to reproduce experimentally determined free enthalpies of hydration and solvation in cyclohexane of amino acid side-chain analogs,was presented. To investigate the transferability of the new parameter set, we calculated free enthalpies of solvation of a range of polar and apolar compounds in different solvents (methanol, dimethyl sulfoxide (DMSO), acetonitrile, and acetone) from molecular dynamics simulations using the GROMOS 53A6 force field. For methanol and DMSO, parameters were used that are available in the 53A6 parameter set. For acetonitrile, a recently developed model was taken and for acetone, two models available in literature were used. We found that trends in and values for the solvation free enthalpies are in satisfactory agreement with experiment, except for the solvation in acetone for which deviations from experiment can be explained in terms of the properties of the models used. [source] Fluorescence and photoisomerization studies of p -nitrophenyl-substituted ethenylindolesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2006Anil K. Singh Abstract The synthesis, electronic absorption, fluorescence (,f, ,ex, ,f, ,f) and photoisomerization (,t,c, photostationary state composition) properties of 3-(4-nitrophenylethenyl- E)-NH-indole (1), 3-(4-nitrophenylethenyl- E)- N -ethylindole (2) and 3-(4-nitrophenyl ethenyl- E)- N -benzenesulfonylindole (3) in organic solvents of varying polarity are reported. The absorption maximum of these compounds undergoes a moderate red shift with increasing solvent polarity. However, the fluorescence maximum becomes highly red shifted with increasing solvent polarity. Whereas 1 and 2 show broad fluorescence bands, 3 exhibits dual fluorescence. Further, 1 and 2 fluoresce much more efficiently than 3. Correlation of the Stokes shift with solvent polarity parameters such as ,f and ET(30) and excited-state dipole moment indicate a highly polar excited state for 1,3. Time-resolved fluorescence studies show that the fluorescence decays are single- and multi-exponential type, depending on the solvent polarity. Further, 1 and 2 do not show photoisomerization on irradiation. However, 3 is photoactive and shows efficient photoisomerization in non-polar heptane. The sensitivity (,) of the photoreaction is determined in various solvent in terms of the Hammett plot, which showed that the excited states involved are electron deficient in nature and consequently stabilized more by an electron sufficient polar solvent and electron donating substituent. These results led us to suggest the existence of three types of excited states, namely the locally excited state, the intramolecular charge-transfer excited state and the conformationally relaxed intramolecular charge-transfer excited state in the photoprocesses of these compounds. Copyright © 2005 John Wiley & Sons, Ltd. [source] Chlorophyll a Self-assembly in Polar Solvent,Water Mixtures ,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2000Radka Vladkova ABSTRACT The conversion of chlorophyll a (Chl a) monomers into large aggregates in six polar solvents upon addition of water has been studied by means of absorption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chl a self-assembly and determining the type of its organization. Two empirical solvent parameter scales were used for quantitative characterization of the different solvation properties of the solvents and their mixtures with water. The mole fractions of water f1/2 giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solvent, and then various solvent,water mixture parameters for the f1/2 values were compared. On the basis of their comparison, it is concluded that the hydrogen-bonding ability and the dipole,dipole interactions (function of the dielectric constant) of the solvent,water mixtures are those that promote Chl a self-assembly. The influence of the different nature of the nonaqueous solvents on the Chl aggregation is manifested by both the different water contents required to induce Chl monomer , aggregate transition and the formation of two types of aggregates at the completion of the transition: species absorbing at 740,760 nm (in methanol, ethanol, acetonitrile, acetone) and at 667,670 nm (in pyridine and tetrahydrofuran). It is concluded that the type of Chl organization depends on the coordination ability and the polarizability (function of the index of refraction) of the organic solvent. The ordering of the solvents with respect to the f1/2 values,methanol < ethanol < acetonitrile < acetone < pyridine < tetrahydrofuran,yielded a typical lyotropic (Hofmeister) series. On the basis of this solvent ordering and the disparate effects of the two groups of solvents on the Chl a aggregate organization, it is pointed out that the mechanism of Chl a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effect, as displayed in the lipid-phase behavior (Koynova et al., Eur. J. Biophys. 25, 261,274, 1997). It relates to the solvent ability to modify the bulk structure and to distribute unevenly between the Chl,water interface and bulk liquid. [source] Kinetic Studies of the Oxidative Addition and Transmetallation Steps Involved in the Cross-Coupling of Alkynyl Stannanes with Aryl Iodides Catalysed by ,2 -(Dimethyl fumarate)(iminophosphane)palladium(0) ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 4 2004Bruno Crociani Abstract The complexes [Pd(,2 -dmfu)(P,N)] {dmfu = dimethyl fumarate; P,N = 2-(PPh2)C6H4,1-CH=NR, R = C6H4OMe-4 (1a), CHMe2 (2a), C6H3Me2 -2,6 (3a), C6H3(CHMe2)2 -2,6 (4a)} undergo dynamic processes in solution which consist of a P,N ligand site exchange through initial rupture of the Pd,N bond at lower energy and an olefin dissociation-association at higher energy. According to equilibrium constant values for olefin replacement, the complex [Pd(,2 -fn)(P,N)] (fn = fumaronitrile, 1b) has a greater thermodynamic stability than its dmfu analogue 1a. The kinetics of the oxidative addition of ArI (Ar = C6H4CF3 -4) to 1a and 2a lead to the products [PdI(Ar)(P,N)] (1c, 2c) and obey the rate law, kobs = k1A + k2A[ArI]. The k1A step involves oxidative addition to a reactive species [Pd(solvent)(P,N)] formed from dmfu dissociation. The k2A step is better interpreted in terms of oxidative addition to a species [Pd(,2 -dmfu)(solvent)(,1 -P,N)] formed in a pre-equilibrium step from Pd,N bond breaking. The complexes 1c and 2c react with PhC,CSnBu3 in the presence of an activated olefin (ol = dmfu, fn) to yield the palladium(0) derivatives [Pd(,2 -ol)(P,N)] along with ISnBu3 and PhC,CAr. The kinetics of the transmetallation step, which is rate-determining for the overall reaction, obey the rate law: kobs = k2T[PhC,CSnBu3]. The k2T values are markedly enhanced in more polar solvents such as CH3CN and DMF. The solvent effect and the activation parameters suggest an associative SE2 mechanism with substantial charge separation in the transition state. The kinetic data of the above reactions in various solvents indicate that, for the cross-coupling of PhC,CSnBu3 with ArI catalysed by 1a or 2a, the rate-determining step is represented by the oxidative addition and that CH3CN is the solvent in which the highest rates are observed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Extended X-ray Absorption Fine Structure Study of Copper(I) and Copper(II) Complexes in Atom Transfer Radical PolymerizationEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2003Tomislav Pintauer Abstract Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy has been used to investigate structural features of CuIBr and CuIIBr2 complexes with dNbpy, PMDETA, Me6TREN, tNtpy, and Me4CYCLAM in various solvents {dNbpy = 4,4,-bis(5-nonyl)-2,2,-bipyridine, PMDETA = N,N,N,,N,,,N,, -pentamethyldiethylenetriamine, Me6TREN = tris[2-(dimethylamino)ethyl]amine, tNtpy = 4,4,,4,,-tris(5-nonyl)-2,2,:6,,2,,-terpyridine, Me4CYCLAM = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane}. The structures of the CuI and CuII complexes were found to depend on the solvent polarity and the number of nitrogen atoms in the ligand. Generally, in non-polar media and with monomers typically used in ATRP, CuI complexes preferred a tetracoordinate geometry, and were either ionic as observed in [CuI(dNbpy)2]+[CuIBr2], (CuI,NAV = 2.00 Å, CuI,BrAV = 2.25 Å) and [CuI(Me4CYCLAM)]+[CuIBr2], (CuI,NAV = 2.06 Å, CuI,BrAV = 2.23 Å), or neutral as in [CuI(PMDETA)Br] (CuI,NAV = 2.12 Å, CuI,BrAV = 2.33 Å), and [CuI(tNtpy)Br] (CuI,NAV = 2.03 Å, CuI,BrAV = 2.29 Å). The EXAFS analysis of CuIIBr2 complexes indicated a preference for a coordination number of five, such as in [CuII(dNbpy)2Br]+[Br], (CuII,NAV = 2.03 Å, CuII,BrAV = 2.43 Å), [CuII(PMDETA)Br2] (CuII,NAV = 2.03 Å, CuII,Br1,AV = 2.44 Å, CuII,Br2,AV = 2.64 Å) and [CuII(Me6TREN)Br]+[Br], (CuII,NAV = 2.09 Å, CuII -BrAV = 2.39 Å), with the exception of the neutral tetracoordinate [CuII(dNbpy)Br2] (CuII,NAV = 2.02 Å, CuII,BrAV = 2.36 Å), which has been observed in non-polar media. Additionally, polar media were found to favor bromide dissociation in [CuII(Me6TREN)Br]+[Br], and [CuII(PMDETA)Br2], as indicated by a decrease in the Br and Cu coordination numbers at the Cu- and Br- K -edges, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Kinetics of the Solvolyses of Fluoro-Substituted Benzhydryl Derivatives: Reference Electrofuges for the Development of a Comprehensive Nucleofugality ScaleEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2010Christoph Nolte Abstract A series of m -fluoro-substituted benzhydryl chlorides, bromides, mesylates, and tosylates 1 -X to 5 -X were prepared and subjected to solvolysis reactions in various solvents. The observed first-order rate constants ks(25 °C) were found to follow the correlation equation log,ks(25 °C) = sf(Nf + Ef), which allowed us to determine the electrofugality parameters Ef for these destabilized benzhydrylium cations and the nucleofugality parameters Nf, sf for a series of leaving group/solvent combinations. [source] Highly Enantioselective Synthesis of No-Carrier-Added 6-[18F]Fluoro- L -dopa by Chiral Phase-Transfer AlkylationEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 13 2004Christian Lemaire Abstract [18F]Fluoro- L -dopa, an important radiopharmaceutical for positron emission tomography (PET), has been synthesized using a phase-transfer alkylation reaction. A chiral quaternary ammonium salt derived from a Cinchona alkaloid served as phase-transfer catalyst for the enantioselective alkylation of a glycine derivative. The active methylene group of this Schiff-base substrate was deprotonated with cesium hydroxide and rapidly alkylated by the 2-[18F]fluoro-4,5-dimethoxybenzyl halide (X = Br, I). The reaction proceeded with high yield (> 90%) at 0 °C or room temperature in various solvents such as toluene or dichloromethane. Preparation of the [18F]alkylating agent on a solid support was developed. After labelling, the labeled [18F]fluoroveratraldehyde was trapped on a tC18 cartridge and then converted on the cartridge into the corresponding benzyl halide derivatives by addition of aqueous sodium borohydride and gaseous hydrobromic or -iodic acid. Hydrolysis and purification by preparative HPLC made 6-[18F]fluoro- L -dopa ready for human injection in a 25,30% decay-corrected radiochemical yield in a synthesis time of 100 min. The product was found to be chemically, radiochemically and enantiomerically pure (ee > 95%). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Restoration of Conductivity with TTF-TCNQ Charge-Transfer SaltsADVANCED FUNCTIONAL MATERIALS, Issue 11 2010Susan A. Odom Abstract The formation of the conductive TTF-TCNQ (tetrathiafulvalene,tetracyanoquinodimethane) charge-transfer salt via rupture of microencapsulated solutions of its individual components is reported. Solutions of TTF and TCNQ in various solvents are separately incorporated into poly(urea-formaldehyde) core,shell microcapsules. Rupture of a mixture of TTF-containing microcapsules and TCNQ-containing microcapsules results in the formation of the crystalline salt, as verified by FTIR spectroscopy and powder X-ray diffraction. Preliminary measurements demonstrate the partial restoration of conductivity of severed gold electrodes in the presence of TTF-TCNQ derived in situ. This is the first microcapsule system for the restoration of conductivity in mechanically damaged electronic devices in which the repairing agent is not conductive until its release. [source] Solvent-Resistant PDMS Microfluidic Devices with Hybrid Inorganic/Organic Polymer CoatingsADVANCED FUNCTIONAL MATERIALS, Issue 23 2009Bo-Yeol Kim Abstract This study presents a method for the fabrication of solvent-resistant poly(dimethylsiloxane) (PDMS) microfluidic devices by coating the microfluidic channel with a hybrid inorganic/organic polymer (HR4). This modification dramatically increases the resistance of PDMS microfluidic channels to various solvents, because it leads to a significant reduction in the rate of solvent absorption and consequent swelling. The compatibility of modified PDMS with a wide range of solvents is investigated by evaluating the swelling ratio measured through weight changes in a standard block. The HR4-modified PDMS microfluidic device can be applied to the formation of water-in-oil (W/O) and oil-in-water (O/W) emulsions. The generation of organic solvent droplets with high monodispersity in the microfluidic device without swelling problems is demonstrated. The advantage of this proposed method is that it can be used to rapidly fabricate microfluidic devices using the bulk properties of PDMS, while also increasing their resistance to various organic solvents. This high compatibility with a variety of solvents of HR4-modified PDMS can expand the application of microfluidic systems to many research fields. [source] The Synthesis, Photophysical Characterization, and X-Ray Structure Analysis of Two Polymorphs of 4,4,-DiacetylstilbeneHELVETICA CHIMICA ACTA, Issue 6 2010Cameron Pye Abstract A palladium(II) acetate-catalyzed synthesis of 1 that utilizes the novel triazene 1-{4-[(E)-morpholin-4-yldiazenyl]phenyl}ethanone as a synthon is described. The room temperature absorption spectra of 1 in various solvents exhibited a ,,,* transition in the range of 330,350,nm. Compound 1 was observed to be luminescent, with room-temperature solution and solid-state emission spectra that exhibited maxima in the range 400,500,nm. All room-temperature absorption and emission spectra exhibited some degree of vibrational structure. The emission spectrum of 1 at 77,K in propanenitrile glass was broad and featureless with a maximum at 447,nm. Compound 1 crystallized as a yellow and colorless polymorph. X-Ray structure analyses of both of these polymorphs and 1-{4-[(E)-morpholin-4-yldiazenyl]phenyl}ethanone are presented. [source] Solvent and structural effects on the kinetics of the reactions of 2-substituted cyclohex-1-enylcarboxylic and 2-substituted benzoic acids with diazodiphenylmethaneINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2007J. B. Nikoli The rate constants for the reaction of 2-methyl-cyclohex-1-enylcarboxylic, 2-phenylcyclohex-1-enylcarboxylic, and 2-methylbenzoic and 2-phenylbenzoic acids with diazodiphenyl-methane were determined in 14 various solvents at 30°C. To explain the kinetic results through solvent effects, the second-order rate constants of the examined acids were correlated using the Kamlet,Taft solvatochromic equation. The correlations of the kinetic data were carried out by means of multiple linear regression analysis, and the solvent effects on the reaction rates were analyzed in terms of initial and transition state contributions. The quantitative relationship between the molecular structure and the chemical reactivity has been discussed, as well as the effect of geometry on the reactivity of the examined molecules. The geometric data of all the examined compounds corresponding to the energy minima in solvent, simulated as dielectric continuum, obtained using semiempirical MNDO-PM3 energy calculations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 664,671, 2007 [source] Radiochemical stability of 14C-compounds on storage: benefits of thioethersJOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 3 2003Andreas Fredenhagen Abstract Storage of radiochemicals is a significant practical problem. Storage as a solution in various solvents was compared to the storage as a neat oil or solid over an extended period of time. Dichloromethane, a solvent previously not recommended for storage, was found to be a good choice in certain solvent mixtures. Addition of methylsulfide or 2-methyl-2-butene was shown to reduce the radiochemical decomposition by a factor of 1.7,3.2 in ethanol-free solvents. General points to consider for storage of radiochemicals are discussed. Radiochemical purity was determined by HPLC. Copyright © 2002 John Wiley & Sons, Ltd. [source] Distributed midpoint chain scission in ultrasonic degradation of polymersAICHE JOURNAL, Issue 9 2004G. Sivalingam Abstract The ultrasonic degradation of poly(bisphenol A carbonate), poly(,-caprolactone), and poly(vinyl acetate) was investigated with various solvents such as benzene, monochlorobenzene, and dichlorobenzene. The time evolution of molecular weight was determined using gel permeation chromatography. A limiting molecular weight was observed for all the systems and was a function of solvent properties. The degradation rates increased with increase in viscosity and decrease in vapor pressure. The polydispersity reached a maximum before reaching a constant value at longer times. The experimental data indicate that the breakage of the polymer is around midpoint, with a distribution rather than an ideal midpoint scission. A continuous distribution model with a stoichiometric kernel based on Lorentzian probability distribution function was developed to satisfactorily model the experimental data. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2258,2265, 2004 [source] Quantitative analysis of the major constituents of St John's wort with HPLC-ESI-MSJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 12 2005Dhammitha H. Chandrasekera A method was developed to profile the major constituents of St John's wort extracts using highperformance liquid chromatography-electrospray mass spectrometry (HPLC-ESI-MS). The objective was to simultaneously separate, identify and quantify hyperforin, hypericin, pseudohypericin, rutin, hyperoside, isoquercetrin, quercitrin and chlorogenic acid using HPLC-MS. Quantification was performed using an external standardisation method with reference standards. The method consisted of two protocols: one for the analysis of flavonoids and glycosides and the other for the analysis of the more lipophilic hypericins and hyperforin. Both protocols used a reverse phase Luna phenyl hexyl column. The separation of the flavonoids and glycosides was achieved within 35 min and that of the hypericins and hyperforin within 9 min. The linear response range in ESI-MS was established for each compound and all had linear regression coefficient values greater than 0.97. Both protocols proved to be very specific for the constituents analysed. MS analysis showed no other signals within the analyte peaks. The method was robust and applicable to alcoholic tinctures, tablet/capsule extracts in various solvents and herb extracts. The method was applied to evaluate the phytopharmaceutical quality of St John's wort preparations available in the UK in order to test the method and investigate if they contain at least the main constituents and at what concentrations. [source] Effect of hydrophobic side-chains on the solvation of imidazolium saltsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2005Allan D. Headley Abstract The chemical shifts of the aromatic hydrogens of 12 symmetrical imidazolium salts were determined in different deuterated solvents. Based on the magnitude of the chemical shift change for the hydrogens of the imidazolium ion in the various solvents, relationships were developed to determine the relative solute/solvent interactions for these compounds. Owing to different degrees of interactions involving the aromatic hydrogens of the imidazolium cations and anions, there is a variation in the interaction of the hydrogens with the solvent molecules. The intimate interaction that exists between the hydrogens of the imidazolium cation and the BF anion results in the BF salts being less solvated compared with salts containing BF and SbF anions. For imidazolium salts that contain C2H5, C4H9 and C8H17 side-chains bonded in the 1 and 3 positions, the interaction between H2 and the solvents was observed to be greater than for imidazolium salts with C16H33 substituents. On the other hand, for imidazolium salts that have C16H33 substituents the interaction between H2 and the solvents is similar to that for H4 and H5. Copyright © 2005 John Wiley & Sons, Ltd. [source] NMR, solvation and theoretical investigations of conformational isomerism in 2-X-cyclohexanones (X=NMe2, OMe, SMe and SeMe)JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2003Matheus P. Freitas Abstract The conformational equilibria of 2- N,N -dimethylamino- (1), 2-methoxy- (2), 2-methylthio- (3) and 2-methylselenocyclohexanone (4) were determined in various solvents by measurement of the 3JH-2,H-3 couplings. The observed couplings were analyzed using theoretical and solvation calculations to give both the conformer energies in the solvents studied plus the vapor-phase energies and the coupling constants for the distinct conformers. These gave the conformer energies and couplings of 2,4. The intrinsic couplings for the 2- N,N -dimethylamino compound were determined by the molecular mechanics PCMODEL program. The axial conformation in 1 is the most polar and also more stable in DMSO solution (Eeq,Eax=0.05,kcal,mol,1) and the pure liquid, while the equatorial conformer predominates in the remaining solvents studied (except in CCl4, where self-association is observed). In the methoxy ketone (2) the equatorial conformation is more stable in the vapor (Eeq,Eax=,0.30,kcal,mol,1) and in all solvents. The opposite behavior is shown by 3 and 4, where the axial conformation is the more stable one in the vapor phase (Eeq,Eax=1.60 and 2.95,kcal,mol,1 for 3 and 4, respectively) and is still the prevailing conformer in solution. The axial predominance for 3 and 4 is attributed to hyperconjugation between the electron lone pair of the hetero-substituent and the ,*CO orbital. This interaction is stronger for 3 and 4 than in the case of 1 and 2, where the ,gauche effect' in the equatorial conformation should be more effective in stabilizing this conformation. Copyright © 2003 John Wiley & Sons, Ltd. [source] Solvatochromism of carbenium,arene EDA (electrondonor,acceptor) complexes and their behaviour on silicaJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2001Stefan Spange Abstract Spectral characteristics of two different types of electron donor,acceptor (EDA) complexes with charged components (arene,carbenium and anion,, acceptor) are presented. The UV,Vis absorption maxima of the charge-transfer (CT) band (,max,CT) of the EDA complexes were measured in various solvents and after adsorption on silica. Tropylium, triarylmethylium and diarylmethylium ions in combination with aromatic , donors, e.g. pyrene, acenaphthene and methoxy-substituted benzene derivatives and also the tetraethylammonium iodide,1,3,5-trinitrobenzene complex have been investigated by means of a special UV,Vis technique in transparent slurries of silica nanoparticles in 1,2-dichloroethane. Multiple linear regression analyses of the ,max,CT values of the EDA complexes in various solvents and on silica with the Kamlet,Taft solvent parameters and structure,reactivity terms of the complex components, e.g. the oxidation potential of the arene component or the Hammett substituent constant, ,p+, of the arylmethyl component, show their related properties to the solution complexes. The influence of the electron-pair donating capacity of a solvent on ,max,CT of carbenium/arene complexes was compared with the property of the silica surface in order to link EDA complexes. For the diarylmethyl carbenium ion,pyrene complexes on silica, the presence of a ,-like structure is discussed. Copyright © 2001 John Wiley & Sons, Ltd. [source] Cyclic poly(pyridine ether)s by the polycondensation of 2,6-difluoropyridine with various diphenolsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 20 2005Hans R. Kricheldorf Abstract The bistrimethylsilyl derivatives of six different diphenols were polycondensed with 2,6-difluoropyridine in N -methylpyrrolidone in the presence of K2CO3. On the basis of previous studies, the reaction conditions were optimized for almost quantitative conversions. The feed ratio was systematically varied to optimize the molecular weight. A 2 mol % excess of 2,6-difluoropyridine was needed to obtain maximum molecular weights. In the matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra of the optimized polyethers, only cycles were found (detectable up to 5000 Da). Obviously, the relatively low molecular weights obtained under optimized conditions resulted from a limitation of the chain growth by cyclization, indicating a high cyclization tendency for poly(pyridine ether)s. The size exclusion chromatography measurements not only proved low molecular weights but also demonstrated the existence of bimodal mass distributions and high polydispersities. Protonation of the poly(pyridine ether)s required strong acids such as methane or trifluoromethane sulfonic acid. The solubilities of the neutral and protonated polyethers derived from bisphenol A were studied in various solvents. The MALDI-TOF mass spectra proved that protonation at 20,25 °C did not cause cleavage of ether bonds. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4781,4789, 2005 [source] Controlled synthesis of amphiphilic biodegradable polylactide-grafted dextran copolymersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 11 2004Cécile Nouvel Abstract The whole controlled synthesis of novel amphiphilic polylactide (PLA)-grafted dextran copolymers was achieved. The control of the architecture of such biodegradable and potentially biocompatible copolymers has required a three-step synthesis based on the "grafting from" concept. The first step consisted of the partial silylation of the dextran hydroxyl groups. This protection step was followed by the ring-opening polymerization of D,L -lactide initiated from the remaining OH functions of the partially silylated polysaccharide. The third step involved the silylether group deprotection under very mild conditions. Based on previous studies, in which the control of the first step was achieved, this study is focused on the last two steps. Experimental conditions were investigated to ensure a controlled polymerization of D,L -lactide, in terms of grafting efficiency, graft length, and transesterification limitation. After polymerization, the final step was studied in order to avoid degradation of both polysaccharide backbone and polyester grafts. The chemical stability of dextran backbone was checked throughout each step of the synthesis. PLA-grafted dextrans and PLA-grafted (silylated dextrans) were proved to adopt a core-shell conformation in various solvents. Furthermore, preliminary experiments on the potential use of these amphiphilic grafted copolymers as liquid/liquid interface stabilizers were performed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2577,2588, 2004 [source] Preferential extractability of ,-oryzanol from dried soapstock using different solventsJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 2 2009Raj R Kumar Abstract BACKGROUND: ,-Oryzanol from rice bran has lately gained potential importance because of its proven health benefits. Thus the extractability of ,-oryzanol from the soapstock of crude rice bran oil is important from the perspective of future large-scale production, which would give value addition to this by-product obtained from the rice bran oil industry. The aim of the present study was to investigate the extraction of ,-oryzanol from the drum-dried soapstock of rice bran oil using various solvents. RESULTS: It was found that ,-oryzanol could be extracted most effectively using ethyl acetate, followed by dichloromethane and ethyl methyl ketone. All components of ,-oryzanol have an alcohol group in the ferulate portion giving rise to relatively high polarity, thereby increasing the extraction in more polar solvents efficiently. Ethyl acetate showed maximum extractability of ,-oryzanol by the Soxhlet method. To quantify ,-oryzanol, reverse phase high-performance liquid chromatography (RP-HPLC) was used for fingerprinting the ,-oryzanol analogues with respect to standard ,-oryzanol. CONCLUSION: A new RP-HPLC method for determining the individual components of ,-oryzanol has been reported that can be used for performing an online characterisation of ,-oryzanol analogues by liquid chromatography/mass spectrometry. Copyright © 2008 Society of Chemical Industry [source] Coil-Globule Collapse in Flexible MacromoleculesMACROMOLECULAR THEORY AND SIMULATIONS, Issue 9 2003Bahattin M. Baysal Abstract The transition of a solvated flexible macromolecular chain from random coil behavior in the , -state to a globular compact form in the collapsed state has been the subject of extensive theoretical and experimental studies. Most of the coil-globule transition studies of macromolecules have concentrated on the prototypical polystyrene-cyclohexane system. However, chain contractions reported in this system have been around 75% of those in the unperturbed , -state. This relatively small decrease in size does not satisfy the criterion for a densely packed, collapsed globule. Experimentally, the collapse from a coil to a true compact globular state has now been established for two flexible macromolecules: poly(N -isopropylacrylamide) in water and poly(methyl methacrylate) in various solvents. In this contribution, we review recent theoretical studies covering phenomenological and Langevin models as well as computer simulations. In addition, we outline recent experimental studies of the coil-globule transition of various flexible polymers, copolymers, and polyelectrolytes. Expansion factor, ,, versus temperature for NaPSS in 4.17 M aqueous NaCl solution. (,): NaPSS-1, (,): NaPSS-2. [source] Identification of taxanes in extracts from leaves of Taxus baccata L. using 13C-NMR spectroscopyPHYTOCHEMICAL ANALYSIS, Issue 3 2009Emilie Duquesnoy Abstract Introduction The study of taxanes is a hot topic worldwide because of their potential activity against cancer. Analysis of taxanes in extracts of various Taxus species is generally achieved using HPLC. Objective The aim of the current work was to identify taxanes in different fractions of extracts from Taxus baccata L. using the computer-aided analysis of their 13C-NMR spectra. Methodology A computerised procedure, based on 13C-NMR spectroscopy using literature data, has been developed in order to identify taxoids in natural mixtures. Leaves and twigs of T. baccata were extracted with various solvents and the extracts were fractionated according to rapid fractionation schemes described in the literature. Results The computer-assisted analysis of the 13C-NMR spectra of the fractions of chromatography led to the identification of 16 taxanes belonging to the taxine, taxicine, taxinine and taxuspine families. Conclusion It has been shown that 13C-NMR is a powerful alternative tool for the identification of taxanes in extracts of leaves and twigs of various Taxus species after a rapid fractionation step. Copyright © 2009 John Wiley & Sons, Ltd. [source] Batch solvent extraction of flavanolignans from milk thistle (Silybum marianum L. Gaertner)PHYTOCHEMICAL ANALYSIS, Issue 1 2005Sunny N. Wallace Abstract Seeds of milk thistle (Silybum marianum L. Gaertner) contain silymarins and ca. 25% (w/w) of oil. A pre-treatment step involving re,uxing with petroleum ether is usually performed before extraction of the silymarins using organic solvents. This paper compares the extraction of whole and defatted milk thistle seeds in various solvents as a function of temperature. The extraction of whole seeds of milk thistle with water at 50, 70 and 85°C was also examined: the yield of silymarin increased with increasing water temperature. In most cases, ethanol at 60°C recovered the largest quantities of silymarins. However, boiling water proved to be an ef,cient extraction solvent for the more polar silymarins such as taxifolin and silychristin, even when using whole seeds. Extractions of defatted seed meal with boiling ethanol returned maximum yields of 0.62, 3.89, 4.04, and 6.86 mg/g defatted seed of taxifolin, silychristin, silybinin A and silybinin B, respectively. When extracting defatted seed meal with ethanol, yields of taxifolin, silybinin A and silybinin B were, respectively, 6.8-, 0.95-, 1.7- and 1.6-fold higher than when extracting whole seeds. When extracting with boiling water, the yields of silychristin, silybinin A, and silybinin B were 380, 47 and 50% higher for whole seeds compared with defatted seeds. Copyright © 2005 John Wiley & Sons, Ltd. [source] Structure-property relationship in polyurethane elastomers containing starch as a crosslinkerPOLYMER ENGINEERING & SCIENCE, Issue 5 2000Sonal Desai Polyurethane elastomers were synthesized using polypropylene glycol (PPG 2000) as the polyol and starch as the multifunctional crosslinker in varying concentrations. Thermal and mechanical properties were measured by DSC, DMA and tensile tests. The morphology was examined by SEM. The swelling behavior of the polyurethanes in various solvents was investigated and the solubility parameter was determined. All these properties were compared with those of polyurethanes containing 1,1,1 -trimethylol propane (TMP) as the crosslinker. Starch-based polyurethanes exhibited better mechanical properties. The effect of varying the starch:TMP ratio on the mechanical strength was also studied. With increasing starch content, the tensile strength and elongation increased. The starch-based PUs exhibited two glass transitions, whereas TMP-based PUs exhibited one Tg. No significant difference in the Tgs of the two PUs was observed. The activation energy of St-PU calculated from DMA was 69 kcal/mol. Soil degradation tests indicated greater biodegradability in polyurethanes containing starch than in those containing TMP. [source] Mortality among unionized construction plasterers and cement masons,AMERICAN JOURNAL OF INDUSTRIAL MEDICINE, Issue 4 2001Frank Stern Abstract Background Plasterers perform a variety of duties including interior and exterior plastering of drywall, cement, stucco, and stone imitation; the preparation, installation, and repair of all interior and exterior insulation systems; and the fireproofing of steel beams and columns. Some of the current potential toxic exposures among plasterers include plaster of Paris, silica, fiberglass, talc, and 1,1,1-trichloroethylene; asbestos had been used by the plasterers in the past. Cement masons, on the other hand, are involved in concrete construction of buildings, bridges, curbs and gutters, sidewalks, highways, streets and roads, floors and pavements and the finishing of same, when necessary, by sandblasting or any other method. Exposures include cement dust, silica, asphalt, and various solvents. Methods Proportionate mortality ratios (PMRs) and proportionate cancer mortality ratios (PCMRs) were calculated for 99 causes of death among 12,873 members of the Operative Plasterers' and Cement Masons' International Association who died between 1972 and 1996 using United States age-, race-, and calender-specific death rates. Statistical significance (P value) of results was based upon the Poisson distribution. Results Among plasterers, statistically significant elevated mortality was observed for asbestosis, where the PMR reached 1,657 (P,<,0.01) with eleven observed deaths and less than one death expected, for lung cancer (PCMR,=,124, P,<,0.01), and for benign neoplasms (PMR,=,210, P,<,0.05). Among cement masons, statistically significant elevated mortality was observed for cancer of the stomach (PCMR,=,133, P,<,0.01), benign neoplasms (PMR,=,132, P,<,0.01), and poisonings (PMR,=,159, P,<,0.05). Except for poisonings, which were not thought to be occupationally related, all of the statistically significant results occurred among those members who entered the union prior to 1950. However, the risk for lung cancer among plasterers was still elevated among those entering the union after 1970 as was the risk for stomach cancer among cement masons who entered the union after 1950. Conclusions The present study suggests that plasterers and cement masons still have elevated risks for certain diseases, especially lung and stomach cancer. Therefore, union members currently living should be screened for asbestos-related diseases and educated about the future risks for these diseases. Am. J. Ind. Med. 39:373,388, 2001. Published 2001 Wiley-Liss, Inc. [source] Studies on the disproportionation of trichloromethyltinAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 8 2003Christopher A. Bertelo Abstract The rate of disproportionation of trichloromethyltin in various solvents was studied by two methods, derivatization with sodium tetraethylborate and 13C NMR measurements, which gave similar results. The reaction followed second-order kinetics and was more rapid in coordinating solvents than in uncoordinating ones. Moreover, it was catalyzed by nucleophiles, such as amines or alcohols, which is in favor of a nucleophile-assisted electrophilic reaction. Copyright © 2003 John Wiley & Sons, Ltd. [source] Efficient Extraction of Lycopene from Rhodopseudomonas palustris with n -Hexane and Methanol after Alkaline WashCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 10 2010Y. Bao Abstract Extraction of lycopene from Rhodopseudomonas palustris with various solvents and alkaline wash was investigated. Dichloromethane or benzene as single polar or nonpolar solvent were the most effective solvents. The maximum extraction efficiency was achieved with a combination of n -hexane and methanol (1:1 v/v). which was approximately one time higher than that obtained with a single solvent. The partitioning behavior of lycopene in n -hexane/methanol indicated that almost all extracted lycopene from R. palustris cells was dissolved in the n -hexane phase. Further studies showed that lycopene extraction was much improved after an alkaline wash of R. palustris cells. The measured lycopene content was much higher than that in tomatoes which indicates that R. palustris will become an important biological resource of lycopene. [source] Lewis Base Catalyzed Mannich-Type Reactions between Trimethylsilyl Enol Ethers and AldiminesCHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2006Hidehiko Fujisawa Abstract Lewis base catalyzed Mannich-type reaction between trimethylsilyl enol ethers and N -tosylaldimines is described. Nitrogen anions generated from amides or imides such as lithium benzamide or potassium phthalimide are found to be effective Lewis base catalysts in DMF at room temperature to afford the corresponding ,-amino carbonyl compounds in good to high yields; the oxygen anion generated from carboxylic acids such as lithium acetate was also found to be effective in dry DMF. The above-mentioned lithium acetate-catalyzed Mannich-type reaction between aldimines and various trimethylsilyl (TMS) enol ethers such as silyl ketene acetal proceeded smoothly even in water-containing DMF. Then, Lewis base catalyzed three-component Mannich-type reactions of TMS enol ether, tosylamide, and aromatic aldehyde having electron-withdrawing group such as p -nitrobenzaldehyde were investigated. The reaction proceeded smoothly to afford the corresponding ,-amino ester in good yield. Further, ammonium carboxylates such as tetrabutyl ammonium acetate or tetrabutyl ammonium benzoate were found to be more effective Lewis base catalysts in the above-mentioned Mannich-type reaction. The synthesis proceeded in various solvents at lower temperatures. The reaction between aldimines and TMS enol ethers generated from thioester and various ketones such as propiophenone or cyclohexanone also proceeded smoothly to afford the corresponding ,-amino carbonyl compounds in high yields with good to high anti -selectivities. [source] How Does the Trans,Cis Photoisomerization of Azobenzene Take Place in Organic Solvents?CHEMPHYSCHEM, Issue 5 2010Giustiniano Tiberio Dr. Abstract The trans,cis photoisomerization of azobenzene-containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the isomerization in a vacuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion,inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans,cis photoisomerization quantum yield and decay times in various solvents (n -hexane, anisole, toluene, ethanol, and ethylene glycol), and obtain results comparable with experimental ones where available. A profound difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional,inversion pathway being the most important in solvents. [source] | |||||||||||||||||||||||||