bluuebunny/crossref_metadata_2025 huggingface dataset:
Phomalone, an antifungal metabolite of Phoma etheridgei
Authors: Ayer William A. , Jimenez Luis Diego
Published: May 2006

The metabolites produced by liquid cultures of Phoma etheridgei, a new fungus isolated from black galls of aspen, have been examined. This fungus is antagonistic to the aspen decay fungus Phellinus tremulae. The compound responsible for this antagonism has been isolated and its structure elucidated. Phomalone (1) is a previously undescribed natural product that possesses the same carbon skeleton as the Phoma pigmentivora metabolite LL-D253α. The structure was determined by physical methods, mainly 1H and 13C NMR. INAPT experiments played an important role. Some chemical transformations of phomalone (1) are described and it is shown by 13C-labelling experiments to be derived from six acetate units. Phomalone is active against Phellinus tremulae. Compounds 4, 6, 7, and 8 are also new natural products produced by P. etheridgei.
Enhanced basicity of 1,3-dialkoxy-substituted benzenes: cyclophane derivatives
Authors: Zhang Guangzhong , Shi Yijian , Mosi Renée , Ho Thao , Wan Peter
Published: May 2006

The photoprotonation of four dialkoxy-substituted benzenes in their excited singlet states has been studied. The parent systems 4 and 5 are regioselectively photoprotonated at the 2-position, with significant quantum efficiencies for deuterium incorporation at acidities greater than pH 2. The structurally related cyclophane derivatives 6 and 7 did not show any deuterium exchange over the same acidity range but fluorescence quenching by proton (in aqueous solution) and laser flash photolysis studies (in 1,1,1,3,3,3-hexafluoro-2-propanol) indicate that photoprotonation does take place with these compounds. Due to structural constraints imposed by the most stable conformation adopted by these molecules (6 and 7), the attacking proton (deuteron) is also the proton (deuteron) that is deprotonated from the cyclohexadienyl cation (2,6-dialkoxybenzenonium ion) intermediate, resulting in the absence of incorporation of deuterium.
An AM1 calculational study of the Diels–Alder addition of maleic anhydride to C5-substituted pentamethylcyclopentadienes and 2,5-dimethylthiophene oxide. An attempt to ascertain the factors controlling the π-facial selectivities and relative reactivities
Authors: Werstiuk Nick Henry , Ma Jiangong
Published: May 2006

The Diels–Alder reactions of maleic anhydride (1) with a group of C5-substituted cyclopentadienes and 2,5-dimethylthiophene oxide (5) have been studied with the semi-empirical method AM1. We find that the facial selectivities and relative reactivities found experimentally for six dienes (2a–2f) are accurately reproduced at this level of theory. The results of the calculational study provide no concrete support for the proposal that σ–σ* hyperconjugation in the transition state – an interaction between a σ bond at C5 of the cyclopentadiene and the incipient σ bonds anti to it (the Cieplak effect) – is the major source of the syn facial selectivity observed for 2a–2d. In addition, the relative reactivities and facial selectivities are predicted for a number of cyclopentadienes which will be synthesized and studied experimentally.
A study of the vacuum pyrolysis of 6,6-dihalobicyclo[3.1.0]hexanes with Hel ultraviolet photoelectron spectroscopy
Authors: Werstiuk Nick Henry , Roy Chandra Deo , Ma Jiangong
Published: May 2006

To establish the utility of using a high-power CW (continuous wave) CO2 laser as a directed heat source for preparing organic transients targeted for gas-phase ultraviolet photoelectron spectroscopic studies, we studied the vacuum pyrolysis of 6,6-dibromobicyclo[3.1.0]hexane (1a), 6,6-dichlorobicyclo[3.1.0]hexane (1b), and exo-6-bromo-endo-6-chlorobicyclo[3.1.0]hexane (1c). While it has already been established that 1a and 1b rearrange to the 2,3-dihalocyclohexenes 3a and 3b, respectively, when heated in the condensed phase, upon vacuum pyrolysis 1a and 1b readily eliminate HX — not the dihalogens — and give the 2-halo-1,3-cyclohexadienes 2a and 2b in high yield. That 2,3-dibromocyclohexene (3a) does not eliminate HBr at the laser power used to pyrolyze 1a and 1b and that bromochloro compound 1c selectively loses HCl to form 2a suggests that the 1,3-elimination of HX occurs in a concerted fashion without isomerization of the cyclopropane.
On the purported axial preference in 2-methylthio and 2-methoxycyclohex-anones: steric effects versus orbital interactions
Authors: Fraser Robert R. , Faibish Neil C.
Published: May 2006

The value for the equilibrium constant representing the ratio of equatorial to axial conformers for 2-methylthio- and 2-methoxycyclohexanone has been measured in five solvents using 500 MHz 1H nuclear magnetic resonance. In cyclohexane solvent, the methylthio group is found to have a large axial preference while that for the methoxy is negligible. For both compounds the equilibrium shows a strong solvent dependence. Comparison of these results with those known for the halo derivatives shows the amount of axial to increase in the order: fluoro, methoxy, chloro, bromo, and methylthio. With the aid of molecular mechanics calculations it is concluded that the order of conformational preference can be attributed to a variation in the van der Waals interaction between the substituent and the carbonyl group in the equatorial conformer. Keywords: α-substituted cyclohexanones, conformation, MMX calculations, steric and dipolar effects.
Novel O-C-C-O and C-O-C-C stereochemistry in crown ether analogs. X-ray crystal structure and NMR studies of dibenzo-20-crown-6
Authors: Buchanan G.W. , Moghimi A. , Bensimon C.
Published: May 2006

The X-ray crystal structure of the title material indicates that the molecule possesses a pseudo-centre of inversion. A pair of O-C-C-O bonds have a trans conformation in contrast to normal gauche stereochemistry for such units in crown ethers. For the C-O-C-C networks, which are normally transoid in crown ethers, all four such units involving methylene carbons of the 20-membered ring exhibit unusual geometries. Two units possess gauche conformations and the other two have torsion angles near 120°. Solution 1H and 13C NMR spectra have been recorded as a function of temperature and 13C solid state spectra are included. Keywords: crown ether, solid state conformation.
Étude par radiolyse pulsée des propriétés spectrales et cinétiques de l'électron solvaté dans l'hexane-1,2,6-triol liquide
Authors: Jay-Gerin J.-P. , Chevrel J. , Ferradini C. , Ray E. , Klapper M.H. , Faraggi M.
Published: May 2006

The optical absorption spectrum of the solvated electron (es−) in liquid hexane-1,2,6-triol has been measured by nanosecond pulse radiolysis at different temperatures (10–40 °C) to investigate the influence of high solvent viscosity values on the spectral and kinetic properties of es−. The wavelength at the absorption maximum, λmax, is equal to 560 nm, and its variation with temperature, if it exists in the considered zone, is less than the experimental error. At 20 °C and 150 ns, the value of the product [Formula: see text] of the yield of es− and the molar extinction coefficient at λmax is 2.55 × 104 molecule/(M cm 100 eV). In the context of this work, we have compared results obtained with both a linear accelerator and a Febetron, a comparison that has allowed us to evaluate the influence of variations of the dose per pulse and to extend measurements to short times. In the case of experiments performed with the linear accelerator, es− is found to decay at all wavelengths by a first-order reaction (or by a pseudo-first-order reaction) with an activation energy of ~45 kJ mol−1. By contrast, kinetic curves obtained with the Febetron seem to show a competition in which a second-order law is followed at short times. The fact that the shape of the spectra seems to vary as a function of the dose per pulse indicates the possible intervention of another species whose formation is favored by the use of high radiation doses. In other respects, the kinetics of electron solvation does not seem to be controlled by the viscosity of the solvent in our experimental conditions. Keywords: liquid hexane-1,2,6-triol, pulse radiolysis, linear accelerator and Febetron, solvated electron, optical absorption spectrum, kinetic properties, solvent viscosity, dose and temperature effects.
Design and synthesis of near-infrared absorbing pigments. II. Structure determination of aceanthrene green and derivatives
Authors: Désilets Denis , Kazmaier Peter M. , Burt Richard A. , Hamer Gordon K.
Published: May 2006

The reported structure of aceanthrene green, a pigment prepared by potassium hydroxide fusion of 1,9-anthracenedicarboxylic imide, was found to be incorrect. The structure of the pigment is reassigned to 7,8,15,16-dibenzo[a,j]perylenetetracarboxylic diimide on the basis of COSY, NOESY, and inversion–recovery 1H NMR experiments. N-Alkyl- or N-phenyl-1,9-anthracenedicarboxylic imides, aceanthryleno[1,2-b]quinoxaline, and a benzimidazole derivative of 1,9-anthracenedicarboxylic anhydride were found to give the same dibenzo[a,j]perylene structure when reacted in potassium hydroxide. The electronic spectra of these derivatives is reported and it is shown that, as predicted by Pariser–Parr–Pople calculations, they absorb in the near-infrared. Finally, a mechanistic outline for the fusion is proposed on the basis of AM1 and frontier molecular orbital calculations. Keywords: photoconductor, near-infrared, aceanthrene green, alkalifusion, polycyclic aromatic hydrocarbon.
Solvated electron mobility in liquid tert-butanol
Authors: Zhao Yixing , Freeman Gordon R.
Published: May 2006

The mobility μe of solvated electrons in liquid tert-butanol was estimated from the measured rate constant of the diffusion-controlled reaction [Formula: see text] combined with the measured molar conductivities of perchloric acid and ammonium perchlorate solutions; T = 300–330 K. The values μe = 4.1 × 10−8 m2 V−1 s−1 at 300 K and Eμ = 41 kJ mol−1 indicate that electrons migrate by a hopping mechanism in liquid tert-butanol. This contrasts with the ion-like diffusion mechanism of [Formula: see text] in methanol and ethanol. Keywords: tert-butanol, solvated electron, activation energy, mobility.
Thermochemistry of the liquid bismuth(III) oxide – boron oxide: a new solvent for oxide melt solution calorimetry
Authors: Rogez Jacques , Mathieu Jean Claude
Published: May 2006

In solution calorimetry, the choice of solvent is important. Some criteria for this choice for oxide melt solution calorimetry are reviewed. A new solvent is proposed and the corresponding physicochemical study is presented. Some structural interpretations are suggested for these Bi2O3–B2O3 melts. Keywords: thermochemistry, calorimetry, high temperature, oxide melt.
Isolation and structures of the uprolides. I. Thirteen new cytotoxic cembranolides from the Caribbean gorgonian Euniceamammosa
Authors: Rodríguez Abimael D. , Piña Ivette C. , Soto Javier J. , Rojas Dalila R. , Barnes Charles L.
Published: May 2006

Thirteen new cembranolide diterpenoids, uprolides 3–15, have been isolated from the Caribbean gorgonian Euniceamammosa collected off the West coast of Puerto Rico. Several known metabolites, such as eupalmerin acetate (1) and eupalmerin (2), were also isolated from the same organism. The structures of the new compounds, which also showed modest in vitro cytotoxic activity, were assigned on the basis of extensive nuclear magnetic resonance analysis, chemical correlation studies, and by comparison with analogous spectral data from known cembranolide diterpenoids. One structure (3) was confirmed by X-ray diffraction analysis. Keywords: uprolides, Euniceamammosa, cytotoxicity, Caribbean, gorgonian.
The 1994 Alcan Award Lecture Density functional theory as a practical tool in studies of organometallic energetics and kinetics. Beating the heavy metal blues with DFT
Authors: Ziegler Tom
Published: May 2006

An evaluation is given of approximate density functional theory as a practical tool in studies on organometallic energetics and kinetics. The evaluation covers electronic excitations and ionizations, electron capture, conformational changes, molecular vibrations, and bond energies, as well as reaction profiles. Keywords: density functional theory, energetics, kinetics, organometallic, catalysis.
Molecular mechanics calculations on the complexes between analogues of Hoechst 33258 and d(CGCGAAT-TCGCG)2: influence of bulky group substitution on base pair preference of DNA minor groove binders
Authors: Chang Ding-Kwo , Cheng Shu-Fang , Chien Ting-Lin
Published: May 2006

Molecular mechanics calculations were performed on the three structures of the complexes formed by the derivatives of Hoechst 33258 and dodecameric DNA duplex d(CGCGAATTCGCG)2. Formation and docking energies of these complexes were compared. It was found that the CG site that is 3′ to the central AATT region can be tolerated by the drugs. This is probably due to the presence of the bulky piperazine ring and, more pronouncedly, by alkylated analogues of the drug that prefer the wider minor groove formed by the GC base pair region of B-DNA. The argument of bulkiness of the piperazine moiety as the origin of enhancement of GC affinity is supported by detailed structural analysis of the intermolecular interface and widening of the DNA minor groove at the binding site. Implications of the results are discussed. Keywords: minor groove binder, docking energy, sequence specificity.
Dinuclear dihydride complexes of iridium: a study of structure and dynamics
Authors: Heinekey D. Michael , Fine David A. , Harper T. Gregory P. , Michel Suzanne T.
Published: May 2006

Reaction of the neutral dihydride complexes (η-C5R5)Ir(L)H2(L = P(OPh)3, CO; R = H, Me) with triflic acid (HO3SCF3) at ambient temperature or above affords hydrogen and the dimeric hydride bridged species {[(η-C5R5)Ir(L)H]2(µ-H)}O3SCF3. Deprotonation of the cationic complexes gives the neutral dimers of the form [(η-C5R5)Ir(L)H]2. Spectroscopic data are consistent with the presence of only terminal hydride ligands in these complexes. Variable temperature 1H and 31P NMR studies indicate that a rapid dynamic process exchanges the two terminal hydride ligands and that the complexes exist as unequal mixtures of the racemic and meso diastereomers. Synthesis of a lower symmetry derivative, [(η-C5Me4Et)Ir(CO)H]2, reveals that a rapid epimerization process occurs that interconverts the two diastereomers. Thermolysis of the carbonyl complex [(η-C5Me5)Ir(CO)H]2 affords [(η-C5Me5)Ir(CO)]2 and dihydrogen. Keywords: iridium, hydride, bimetallic, dynamics, epimerization.
The synthesis of tellurometalates by the reduction of binary telluride melts with potassium in liquid ammonia
Authors: Park Chang-Woo , Salm Robert J. , Ibers James A.
Published: May 2006

The reduction of binary telluride melts with potassium in liquid ammonia in the presence of 18-Crown-6 or 2,2,2-Crypt has been employed to synthesize and precipitate tellurometalates. The method is more rapid than conventional extraction techniques and, since lower temperatures are involved, potentially unstable fragments are conserved. A melt of nominal composition CdTe3 dissolved with K and 18-Crown-6 in liquid NH3 affords [K(18-Crown-6)]4[Cd4Te12] (1) after recrystallization from CH3CN–Et2O. Compound 1 crystallizes in the triclinic space group [Formula: see text] with one formula unit in a cell of dimensions a = 12.673(1) Å, b = 13.694(3) Å, c = 14.483(3) Å, α = 117.54(2)°, β = 96.43(1)°, and γ = 100.00(1)° at 113 K. Full anisotropic refinement of the structure on Fo2 led to a value of R1 = 0.026 for those 6654 reflections having Fo2 > 2σ(Fo2). The [Cd4Te12]4− anion of 1, which may be formulated as [Cd4(Te)2(Te2)2(Te3)2]4−, consists of a rectangular array of four Cd2+ centers linked by Te2−, Te2−, and Te32− ligands. As expected from the solid-state structure, the 125Te NMR spectrum of 1 in CD3CN shows four resonances at −11, −259, −548, and −1682 ppm. A melt of nominal composition PbTe2 was dissolved with K and 2,2,2-Crypt in liquid NH3 to afford [K(2,2,2-Crypt)]2[Pb2Te3]•CH3CN (2) after recrystallization from CH3CN. Compound 2 crystallizes in the triclinic space group CI1−P1 with one formula unit in a cell of dimensions a = 11.105(3) Å, b = 11.748(4) Å, c = 11.922(3) Å, α = 118.78(2)°, β = 96.52(2)°, and γ = 91.03(3)° at 113 K. Full anisotropic refinement of the structure on Fo2 led to a value of R1 of 0.074 for those 11 135 reflections having Fo2 > 2σ(Fo2). The anion of 2 consists of a triangle of Te2− ligands capped above and below the plane by two Pb2+ centers to define a trigonal bipyramid. The 125Te NMR spectrum of 2 in CD3CN shows one resonance at −928 ppm (JPb–Te: 1090 Hz). Keywords: tellurometalates, liquid ammonia, synthesis, crystal structure.
The acidity and tautomerism of β-diketones in aqueous solution
Authors: Bunting John W. , Kanter James P. , Nelander Raymond , Wu Zhennan
Published: May 2006

The acidity and keto–enol tautomerism of a series of symmetrical β-diketones (RCOCH2COR (1): R = methyl (a), phenyl (b), 3-pyridinyl (c), 4-pyridinyl (d), 3-(N-methyl)pyridinio (e), and 4-(N-methyl)pyridinio (f)) and two series of unsymmetrical β-diketones (RCOCH2COCH3(7a–7f) and RCOCH2COC6H5(8a–8f)) have been investigated in aqueous solution at 25 °C and ionic strength 0.1. Values of [Formula: see text] were measured spectrophotometrically, and the acidities of the enols [Formula: see text] were obtained from the analysis of the pH dependence of the buffer catalysis for the general acid-catalyzed protonation of the enolateconjugate bases. These data in turn allowed the calculation of the acidities of the keto tautomers [Formula: see text] and the equilibrium constants for enolization (KE = [Enol]/[Keto]). In general, KEis greater for the symmetrical ketones(1) than for the corresponding R-substituted unsymmetrical ketones (7 and 8). KEis much more sensitive to the nature of the R substituent in these three series of β-diketones than in the corresponding series of β-keto esters and amides. Correlations between [Formula: see text] for 8 and 7 combined with the known acidities of the corresponding β-keto esters and amides provide the first accurate estimates of the acidities of dimethyl malonate (pKa = 13.0)and malonamide (pKa = 12.5) in aqueous solution. Keywords: acidity, tautomerism, β-diketones.
Kinetic resolution of sulfoxides with pendant acetoxy groups using cholesterol esterase: substrate mapping and an empirical rule for chiral phenols
Authors: Serreqi Alessio N. , Kazlauskas Romas J.
Published: May 2006

Sulfoxides are valuable chiral auxiliaries because they direct the formation of carbon–carbon bonds. As a route to enantiomerically pure sulfoxides, we examined hydrolase-catalyzed kinetic resolution by hydrolysis of a pendant acetoxy group. Screening hydrolases for the enantioselective hydrolysis of the acetate in 2-(methylsulfinyl)phenyl acetate, 1b, identified cholesterol esterase (CE) as the most enantioselective enzyme. The enantiomeric ratio, E, ranged from 10 to 25, favoring the (R) configuration at sulfur. Competing chemical hydrolysis of 1b caused the large range in the measured values of E. A small-scale (250 mg) resolution of (±)-1b yielded (S)-1b with >99% ee (11% yield) after recrystallization. Changing the methyl substituent to phenyl or n-butyl did not significantly change the enantioselectivity (E = 10 and 15, respectively), but changing it to a chloromethyl substituent lowered the enantioselectivity slightly (E = 5). Changing the phenyl acetate to a naphthyl acetate (2-(phenylsulfinyl)phenyl acetate vs. 1-(phenylsulfinyl)-2-naphthyl acetate) increased the enantioselectivity from 10 to 19. In all cases CE favored the (R)-sulfoxide. To aid the design of new resolutions with CE, we propose an empirical rule that accounts for the observed enantiopreference of CE toward these 5 sulfoxides and 15 other chiral aryl acetates. This empirical rule uses both the size of the substituents and their conformational preferences to predict which enantiomer reacts faster. Keywords: kinetic resolution, lipase, cholesterol esterase, sulfoxides, empirical rule.
Application pratique des réseaux neuro mimétiques aux données spectroscopiques (infrarouge et masse) en vue de l'élucidation structurale
Authors: Cabrol-Bass D. , Cachet C. , Cleva C. , Eghbaldar A. , Forrest T.P.
Published: May 2006

In the last few years, intensive research by several groups has shown that neural networks can be used to analyse spectral data for structural elucidation, and that their performance approaches that of an expert in the field. The construction of such networks, their training and evaluation, requires large structural and spectral databases and significant computational resources and time. However, once the network has been completed it can be used very effectively for practical applications on an ordinary desktop computer. In this article we describe the methodology for creating such a network for infrared and mass spectra, and present a program for use on a personal computer, either connected to a spectrometer or independently. The program accepts data in ASCII format, both for the network description and for the spectral information. This approach permits the use of neural networks in an analytical laboratory with limited computational resources. Keywords: neural networks, infrared spectroscopy, mass spectroscopy, structure determination.
Thermodynamic properties of the NiCl2–CoCl2–KCl system
Authors: Tumidajski Peter J. , Pickles Christopher A.
Published: May 2006

The thermodynamic behavior of NiCl2 in NiCl2–CoCl2–KCl ternary solutions was investigated using formation cells of the type, (−) Ni/NiCl2–CoCl2–KCl/C, Cl2(gas, 1 atm)(+). The values of the NiCl2 activity coefficients indicate a non-ideal system consonant with the formation of complex species. The NiCl2 activity coefficients at 1000 K were fitted to the polynomial equation,[Formula: see text]where[Formula: see text]The experimental data were limited to the ranges 0 ≤ t ≤ 0.80 and 0.10 ≤ y ≤ 0.5. Keywords: molten salt, thermodynamic properties, nickel chloride, non-ideal solution, complex species.
Structural characterization of the O-chain polysaccharide from Proteusmirabilis strain 7570
Authors: Uhrín Dušan , Chandan Vandana , Altman Eleonora
Published: May 2006

The O-chain polysaccharide produced by Proteusmirabilis strain 7570 was shown by chemical analysis and 1D and 2D NMR spectroscopy to be a high molecular weight acidic linear polymer of tetrasaccharide repeating units, composed of D-galacturonic acid, 2-acetamido-2-deoxy-D-galactose, and 2-acetamido-2-deoxy-D-glucose (2:1:1). In addition, native O-chain was randomly substituted by O-acetyl groups. Keywords: Proteus, lipopolysaccharide, O-chain, structure, NMR.
The role of polymer flexibility on the interaction with surfactant micelles: poly(vinyl alcohol) and sodium dodecyl sulphate aqueous micelle interactions studied by dynamic fluorescence quenching
Authors: Stam Jan. Van , Wittouck Nadine. , De Schryver Frans C. , Almgren Mats , Miguel Maria da Graça
Published: May 2006

The interactions between poly(vinyl alcohol) and the surfactant sodium dodecyl sulphate in aqueous solutions have been investigated by means of static and dynamic fluorescence measurements. By static fluorescence measurements the onset of interaction, the critical aggregation concentration, is determined to be 6.5 mM at 20 °C and 7.0 mM at 40 °C. By dynamic fluorescence measurements without added quencher the influence on the lifetime of the fluorescent probe, pyrene, by the presence of the polymer is investigated. From measurements with an added quencher, dimethylbenzophenone, the surfactant aggregation numbers of the micellar aggregates formed upon interaction between the surfactant and the polymer and the quenching rate constant are obtained. At the two polymer concentrations, i.e., 0.2% and 2.0%, the surfactant aggregation numbers at low surfactant concentration are very small, but increase rapidly with increasing surfactant concentration. Comparisons are made with results from a system where poly(ethylene oxide) instead of poly(vinyl alcohol) is the polymer. Key words: dynamic fluorescence quenching, polymer–surfactant interactions, aggregation numbers.
Studies of the inhibition of autooxidation of polypropylene with 4,4′-bis(2,6-di-tert-butylphenol)
Authors: Shanina E.L. , Zaikov G.E. , Mukmeneva N.A.
Published: May 2006

The kinetics of 4,4′-bis(2,6-di-tert-butylphenol) (BP) consumption in the oxidation of polypropylene (PP) at 130 °C in air has been studied. It is shown that 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ) is the main product of conversion of the starting BP. An effect of synergism has been revealed on studying stabilization of PP with BP–DPQ mixtures. The antioxidative activity of BP was compared with that of some other stabilizers. Keywords: polypropylene oxidation, antioxidative activity, 4,4′-bis(2,6-di-tert-butylphenol), 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone.
Luminescence measurements of oxygen permeation and oxygen diffusion in thin polymer films
Authors: Yekta Ahmad , Masoumi Zahra , Winnik Mitchell A.
Published: May 2006

A new method is developed for analyzing the diffusion of oxygen in thin polymer films via fluorescence quenching measurements. We begin by reviewing previous methods, all of which involve approximations, for the measurement of permeability and diffusion coefficient by luminescence quenching; their shortcomings are clarified. An exact analytic theory is developed that successfully couples Fick's laws of diffusion to the Stern–Volmer equation of intensity quenching. Various modes of experimentation with polymeric films are considered. The equations we derive make the unexpected prediction that the rate of emission intensity decay when O2 diffuses into a polymer film is much faster than the rate of emission intensity enhancement when O2 diffuses out of the same film, even when the molecular diffusivity remains unchanged. Experiments show that this is indeed the observed behaviour. Keywords: diffusion coefficient, oxygen permeability, polymers, quenching, luminescence quenching, Fick's laws. Stern–Volmer equation.
Pyrene-labeled amphiphilic poly-(N-isopropylacrylamides) prepared by using a lipophilic radical initiator: synthesis, solution properties in water, and interactions with liposomes
Authors: Winnik Françoise M. , Adronov Alexander , Kitano Hiromi
Published: May 2006

Fluorescently labeled amphiphilic poly-(N-isopropylacrylamides) (PNIPAM) substituted with a N-[4-(1-pyrenyl)butyl]-N-n-octadecyl group at the chain end were prepared by free-radical polymerization in dioxane of N-isopropylacrylamide (NIPAM) using 4,4′-azobis{4-cyano-N,N-[4-(1-pyrenyl)butyl]-n-octadecyl}pentanamide as the initiator. The solution properties of the polymers in water were studied as a function of polymer concentration and temperature. Quasi-elastic light-scattering measurements and fluorescence experiments monitoring the pyrene excimer and pyrene monomer emissions revealed the presence of multimolecular polymeric micelles below the lower critical solution temperature (LCST) of PNIPAM. These underwent partial, reversible reorganization as they were heated above the LCST. The interactions of the pyrene-labeled amphiphilic PNIPAM with dimyristoylphosphatidylcholine (DMPC) liposomes have been examined in water at 25 °C. From fluorescence experiments it was established that the polymeric micelles are disrupted irreversibly upon contact with the liposomes. The anchoring of the polymer chains occurs by insertion of their hydrophobic tail within the phospholipidic bilayer, as evidenced from a large decrease of the pyrene excimer emission relative to pyrene monomer emission. The copolymers remained anchored within the bilayer as the temperature of the copolymer–liposome suspension was raised above the LCST of PNIPAM. Keywords: liposome, poly-(N-isopropylacrylamide), fluorescence, micelles.
Optical absorption spectra of solvated electrons in 1-butylamine–water mixed solvents
Authors: Zhao Yixing , Freeman Gordon R.
Published: May 2006

The optical absorption spectra of es− in 1-butylamine–water mixed solvents increase smoothly in energy and intensity as the water content is increased, with the exception of a small decrease in intensity on going from 95 to 100 mol% water. At 298 K the value of Gεmax increases from 1.42 × 10−21 m2/16 aJ (8.6 × 103 es−L/100 eV mol cm) in pure 1-butylamine to 8.3 × 10−21 m2/16 aJ (50 × 103 es−L/100 eV mol cm) in pure water, and the value of EAmax increases from 115 zJ (0.72 eV) to 278 zJ (1.74 eV). In the pure amine, if G(es−) = 0.27, then εmax = 5.3 × 10−21 m2/es− (3200 m2/mol). The solvent composition dependences of Gεmax and EAmax indicate little selective solvation of es− by water; this might be due to relatively "ideal" mixing of water and amine in the binary solvents. The temperature coefficient −dEAmax/dT = 0.43 zJ/K in pure 1-butylamine, 0.47 in pure water, and has a minimum of 0.27 in the 50:50 mixture. Keywords: 1-butylamine–water mixed solvents, optical absorption spectra, solvated electron, temperature dependence.
Iron porphyrin catalyzed oxidation of lignin model compounds: oxidation of phenylpropane and phenylpropene model compounds
Authors: Cui Futong , Dolphin David
Published: May 2006

The oxidation of 1-(4-ethoxy-3-methoxyphenyl)propane (2) and 1-(4-ethoxy-3-methoxyphenyl)propene (3) by meso-tetra(2,6-dichloro-3-sulphonatophenyl)porphyrin iron chloride (TDCSPPFeCl, 1) and tert-butylhydroperoxide (t-BuOOH) are discussed. In addition to a Cα-hydroxylation product, demethoxylation and direct aromatic ring cleavage products were found in the oxidation of 2. When 3 was oxidized by 1 and t-BuOOH in aqueous acetonitrile, an acetonitrile-incorporated product was found. A mechanism for the oxidation of 3 is proposed. Keywords: lignin, ligninase, iron porphyrin, model, degradation.
Attachment of C5Me4HSi(Me2)NH-t-C4H9 to yttrium via amine elimination. X-ray structure of [(η5-C5Me4)Si(Me2)-η1-N-t-C4H9]YN(SiMe3)2
Authors: Mu Ying , Piers Warren E. , MacDonald Mary-Anne , Zaworotko Michael J.
Published: May 2006

Reaction of the homoleptic yttrium amido complex Y[N(SiMe3)2]3 with C5Me4HSiMe2N(H)-t-C4H9, Cp*HSiNHR, at 100 °C resulted in elimination of 2 equivalents of HN(SiMe3)2 and attachment of the Cp-amido donor to yttrium. The product (Cp*SiNR)YN(SiMe3)2, 1, was isolated in 40% yield and was characterized crystallographically (triclinic, space group [Formula: see text] a = 8.9759(23) Å, b = 9.504(7) Å, c = 18.294(6) Å, V = 1423.4(12) Å3, Z = 2, R = 0.061, Rw = 0.053). Metrical parameters associated with the N(SiMe3)2 ligand were suggestive of weak β-SiC agostic interactions that stabilize the electron-deficient yttrium center. Alkylation of 1 resulted in unstable alkyl derivatives that could not be isolated. Keywords: cyclopentadienyl-amido, amine elimination, yttrium complex.
Near-infrared and visible luminescence and absorption spectroscopy of low-coordinate organometallic rhenium compounds
Authors: Savoie Carole , Reber Christian
Published: May 2006

Low-temperature absorption and luminescence spectra of four organorhenium compounds were measured in the visible and near-infrared spectral regions. Effects of the temperature, formal oxidation state, and the oxo-, bromo-, and organic ligands on the metal center are reported. The emission maxima of the four compounds show a large variation between 9000 and 14 200 cm−1. Stokes shifts show a smaller variation from 3700 to 4200 cm−1. Luminescence intensities are used to qualitatively characterize nonradiative relaxation processes. Key words: luminescence spectra, absorption spectra, organorhenium compounds.
The photochemistry of 3,3′,4,4′-tetramethoxy-and 4-hydroxy-3,3′,4′-trimethoxystilbene — models for stilbene chromophores in peroxide-bleached, high-yield wood pulps
Authors: Leigh William J. , Lewis T. Johnathan , Lin Vincent , Postigo J. Alberto
Published: May 2006

The photochemistry of the title compounds has been investigated in ethanol and tetrahydrofuran solution under aerobic and anaerobic conditions. Direct irradiation of trans-3,3′,4,4′-tetramethoxystilbene (trans-1) in deoxygenated ethanol leads to the rapid establishment of a photostationary state with the cis isomer, and the slower formation of the ethyl ether corresponding to addition of ethanol across the olefinic C=C bond and cyclobutane dimers. The same products are formed upon photolysis in the presence of oxygen under the same conditions but, in addition, two isomeric tetramethoxyphenanthrenes and 3, 4-dimethoxybenzaldehyde are formed. Photolysis of trans-1 in oxygenated tetrahydrofuran leads to the same products in different relative yields. Quantum yields for cis, trans photoisomerization, phenanthrene formation, and addition of ethanol have been determined by ferrioxalate actinometry. Direct irradiation of tran-4-hydroxy-3,3′,4′-trimethoxystilbene (trans-2) in ethanol solution also results in rapid cis–trans isomerization and the formation of (three) isomeric phenanthrene derivatives in photolyses carried out in the presence of oxygen, although the material balance is low. The various products of photolysis of trans-2 have been independently synthesized by desilylation of the products isolated from photolysis of trans-4-tert-butyldimethylsiloxy-3,3′,4′-trimethoxystilbene (trans-3) under similar conditions. Fluorescence-quenching experiments have been carried out to determine the relative rates of quenching of the excited singlet states of trans-1 and trans-2 by alcohols and oxygen. The formation of aldehydes is proposed to arise via reaction of superoxide ion with stilbene radical cations, which are formed by electron-transfer quenching of the stilbene excited singlet state by oxygen. Key words: stilbenes, lignin, photochemistry, photooxidation.
Hydrogen evolution reaction at nickel boride electrodes in aqueous and in aqueous methanolic and ethanolic solutions
Authors: Mahdavi Behzad , Miousse Danielle , Fournier Joël , Ménard Hugues , Lessard Jean
Published: May 2006

The hydrogen evolution reaction (HER) was investigated at nickel boride pressed powder electrodes in aqueous and in aqueous methanolic (ethanolic) alkaline (1 M NaOH) solutions in the presence and the absence of NaCl (0.1 M) and H3BO3 (0.1 M) at 25 °C by the steady-state polarization and ac impedance techniques. The ac impedance data were analyzed using the constant-phase element (CPE) model. It was found that the electrocatalytic behaviour of a nickel boride electrode for the HER was the same in all the above media and that the HER proceeds via the Volmer–Heyrovsky mechanism. The kinetic parameters of the HER were determined. Key words: hydrogen evolution reaction, nickel boride electrodes, Tafel curves, ac impedance, kinetic parameters.
Electron transfer kinetics of cobaloxime complexes
Authors: Wang Kefei , Jordan R.B.
Published: May 2006

The rates of oxidation of CoII(dmgBF2)2(OH2)2by CoIII(NH3)5X2+(X = Br−, Cl−, and N3−) have been studied at 25 °C in 0.10 M LiClO4. The rate constants are 50 ± 9, 2.6 ± 0.2, and 5.9 ± 1.0 M−1s−1for X = Br−, Cl−, and N3−, respectively, in 0.01 M acetate buffer at pH 4.7. The relative rates are consistent with the inner-sphere bridging mechanism established earlier by Adin and Espenson for the analogous reactions of CoII(dmgH)2(OH2)2. The rate constants with CoII(dmgBF2)2(OH2)2typically are ~103times smaller and this is attributed largely to the smaller driving force for the CoII(dmgBF2)2(OH2)2complex. The outer-sphere oxidations of cobalt(II) sepulchrate by CoIII(dmgH)2(OH2)2+(pH 4.76–7.35, acetate, MES, and PIPES buffers) and CoIII(dmgBF2)2(OH2)2+(pH 3.3–7.42, chloroacetate, acetate, MES, and PIPES buffers) have been studied. The pH dependence gives the following rate constants (M−1s−1) for the species indicated: (1.55 ± 0.09) × 105(CoIII(dmgBF2)2(OH2)2+); (5.5 ± 0.3) × 103(CoII(dmgH)2(OH2)2+); (3.1 ± 0.5) × 102(CoIII(dmgH)2(OH2)(OH)); (2.5 ± 0.3) × 102(CoIII(dmgBF2)2(OH2)(OH)). The known reduction potentials for cobalt(III) sepulchrate and the diaqua complexes, and the self-exchange rate for cobalt(II/III) sepulchrate, are used to estimate the self-exchange rate constants for the dioximate complexes. Comparisons to other reactions with cobalt sepulchrate indicates best estimates of the self-exchange rate constants are ~2.4 × 10−2 M−1s−1for CoII/III(dmgH)2(OH2)2and ~5.7 × 10−3 M−1s−1for CoII/III(dmgBF2)2(OH2)2. Key words: electron transfer, cobaloxime, inner sphere, outer sphere, self-exchange.
An unusual fatty acid and its glyceride from the marine fungus Microsphaeropis olivacea
Authors: Yu Chao-Mei , Fathi-Afshar Ziba R. , Curtis Jonathan M. , Wright Jeffrey L.C. , Ayer Stephen W.
Published: May 2006

Two new metabolites (1 and 2) were isolated from the marine fungus Microsphaeropsis olivacea. The structures were elucidated, through analysis of the spectroscopic data, as an unusual methyl-branched unsaturated fatty acid, 10-methyl-9Z-octadecenoic acid (1), and the glyceride (2). The locations of the double bond and the methyl branching were determined from the electron impact (EI) mass spectrum of the picolinyl derivative of 1 and from MS/MS data on the hydrogenation products of 1 and 2. Key words: Microsphaeropsis olivacea, marine fungus, methyl-branched unsaturated fatty acid, picolinyl ester.
Mixed alkali effect in sodium thiocyanate–potassium thiocyanate – acetamide melt systems
Authors: Mahiuddin Sekh
Published: May 2006

Electrical conductivity and molar volume of the 0.25[xNaSCN + (1 −x)KSCN] + 0.75CH3CONH2 systems were measured as functions of temperature [Formula: see text] and composition (x = 0.0 – 1.0 mol fraction). Temperature dependence of the electrical conductivity was non-Arrhenius in nature and has been analysed by using the Vogel–Tammann–Fulcher (VTF) equation. Molar volume data were fitted to an equation similar to the VTF equation based on the free volume model. Molar volumes and intrinsic volumes were found to be additive in nature. Electrical conductivity isotherms deviate from linearity in different fashion for different temperature regions. The onset of the mixed alkali effect is governed by the anion polarization effect, by a contribution of the auto-dissociated molten acetamide, and by polymeric-type solvated ions. Key words: electrical conductivity, sodium thiocyanate, potassium thiocyanate, acetamide, mixed alkali effect.
Density-functional calculation of core-electron binding energies of glycine conformers
Authors: Chong Delano P.
Published: May 2006

Our recent procedure of computing accurate core-electron binding energies (CEBEs) with density-functional theory is applied to glycine conformers in this work. The procedure uses the unrestricted generalized transition-state model and a combined functional of Becke's 1988 exchange with Perdew's 1986 correlation. When a large basis set such as Dunning's correlation-consistent polarized valence quadruple zeta set is used, the average absolute deviation from experiment for the CEBEs of the most stable conformer of glycine is only 0.2 eV, compared with 18 eV for Koopmans' theorem. Key words: core-electron binding energies, density-functional theory, glycine.
A comparative study of the energetics, structures, and mechanisms of the HCN ↔ HNC and LiCN ↔ LiNC isomerizations
Authors: Rao V. Sreedhara , Vijay Amrendra , Chandra A.K.
Published: May 2006

The potential energy surfaces of the HCN ↔ HNC and LiCN ↔ LiNC isomerization processes were determined by ab initio theory using fully optimized triple-zeta double polarization types of basis sets. Both the MP2 corrections and the QCISD level of calculations were performed to correct for the electron correlation. Results show that electron correlation has a considerable influence on the energetics and structures. Analysis of the intramolecular bond rearrangement processes reveals that, in both cases, H (or Li+) migrates in an almost elliptic path in the plane of the molecule. In HCN ↔ HNC, the migrating hydrogen interacts with the in-plane π,π* orbitals of CN, leading to a decrease in the C—N bond order. In LiCN ↔ LiNC, Li+ does not interact with the corresponding π,π* orbitals of CN. Key words: potential energy surfaces, intra-molecular bond rearrangement, bond orders, elliptic path, migration of Li+.
Charge density in crystalline citrinin from X-ray diffraction at 19 K
Authors: Roversi Pietro , Merati Felicita , Destro Riccardo , Barzaghi Mario
Published: May 2006

For the fungal metabolite citrinin, C13H14O5, the total experimental electron distribution ρ(r) and its Laplacian [Formula: see text] have been obtained from an extensive set (36 564 measurements) of single-crystal X-ray diffracted intensities at a temperature of 19 ± 2 K. Relevant steps in data collection and processing are reported. The resulting 7698 independent intensity data have been analysed with a multipole (pseudoatoms) formalism. The topological properties of ρ(r) have been determined according to the quantum theory of atoms in molecules. CC and CO bond path lengths have been obtained by numerical integration; their values are found to be well correlated with those of the electron density at the bond critical points. Topological features have been used to characterize the extension of the conjugated system of the molecule, and to confirm the stability of its rings, particularly the two formed by intramolecular H bonds. Maps of [Formula: see text] are presented, showing details in the valence charge distribution and providing a very sensitive tool for analysing dependence of the density on the model adopted to interpret X-ray data. The known chemical reactivity of the molecule towards nucleophiles at a Csp2 atom is confirmed by the shape of the molecular reactive surface (the zero envelope of [Formula: see text]). Key words: experimental electron density, low-temperature X-ray diffraction, topological analysis, Laplacian of ρ.
Variability of shapes and properties of atoms in molecules: a case study of the carbonyl oxygen
Authors: Stefanov Boris B. , Cioslowski Jerzy
Published: May 2006

A comparative study of carbonyl oxygen atoms in diverse molecular environments is presented. The variability of shapes of oxygen atoms is quantified with a newly developed similarity measure that confirms the qualitative conclusions of visual assessment. Electronic properties of these atoms, such as charges, energies, and dipole moments, are computed and their possible correlations with the atomic shapes are studied. Factors that affect atomic shapes are investigated and found to be distinct from those influencing electronic properties of atoms in molecules. The kinetic energies of the atoms under study correlate poorly with the atomic charges. The second-neighbor effects on the atomic energies and charges are approximately additive. Both the theoretical considerations and the numerical results definitively rule out the possibility of the shapes of atomic basins unambiguously determining the properties of atoms in molecules. The consequences of this observation for the recently contemplated approaches to the prediction of electronic properties of large molecules are discussed. Key words: atoms in molecules, properties of —; similarity of —; transferability of —.
1995 McBryde Medal Award Lecture Solid phase microextraction – a unique tool for chemical measurements
Authors: Górecki Tadeusz , Boyd-Boland Anna , Zhang Zhouyao , Pawliszyn Janusz
Published: May 2006

The paper presents recent advances in solid phase microextraction (SPME). The method utilizes a small fused silica fibre coated with a suitable polymeric stationary phase for analyte extraction from various matrices. The fibre is mounted in a syringe-like holder for protection. SPME has been used for the characterization of polymeric coating properties. Infinite-dilution weight-fraction activity coefficients have been determined for five solutes in two different stationary phases. Their values correlate well with the structure of the analytes and the coating. Other SPME uses include simultaneous determination of 60 pesticides in liquid matrices. A comparison between SPME, liquid extraction, and Soxhlet extraction for the analysis of Atrazine, Simazine, and Metolachlor in well water, as well as Metolachlor in soil samples, produced similar results for all three methods. The solvent-free character of SPME enables its use as sample introduction technique for fast GC analysis. Separations of BTEX in less than 9 s, and of volatile compounds described in EPA method 624 in 2.5 min, are described. SPME has also been used for the analysis of tetraethyllead and Pb(II) in water. Before the analysis, lead is derivatized with sodium tetraethylborate, and the tetraethyllead formed is extracted from sample headspace. Sub-ppb detection limits have been achieved with FID. Finally, SPME has been coupled to HPLC with the use of a specially designed interface. The combination has been tested on samples of polyaromatic hydrocarbons and surfactants in water. Very good precision of retention times and peak areas has been achieved. Key words: solid phase microextraction, coating characterization, pesticide analysis, fast GC, lead and organolead compounds, SPME/HPLC.
The acid-catalyzed demetalation of 1-(tri-n-butylstannyl)-2-phenylethyne. A surprisingly small β-stannyl effect
Authors: Egle I. , Gabelica V. , Kresge A.J. , Tidwell T.T.
Published: May 2006

Rates of conversion of 1-(tri-n-butylstannyl)-2-phenylethyne to phenylethyne in H2O and D2O solutions of perchloric acid were found to be proportional to acid concentration, giving the hydronium ion rate constant [Formula: see text] and the isotope effect [Formula: see text]. The magnitude of this isotope effect suggests that the reaction occurs by rate-determining hydron transfer to the substrate, producing a vinyl carbocation; this carbocation then loses its tributylstannyl group, giving phenylacetylene as the only detectable aromatic product. The hydronium ion rate constant, when compared to the rates of protonation of phenylethyne and 1-(trimethylsilyl)-2-phenylethyne, gives a β-stannyl stabilizing effect of δΔG≠ = 6.6 kcal mol−1 and a differential β-stannyl/β-silyl effect of δΔG≠ = 3.2 kcal mol−1. These stabilizations are very much smaller than previously reported β-stannyl effects. Possible reasons for the difference are suggested. Key words: β-stannyl effect, β-silyl effect, carbocation stabilization, protodemetalation.
Études des réactions d'oxydation du n-heptane et de l'isooctane
Authors: Simon Yves , Scacchi Gérard , Baronnet François
Published: May 2006

The oxidation mechanisms of two alkanes that have quite different octane numbers: n-heptane (0) and isooctane (100), were investigated to obtain a better understanding of the chemical mechanism of autoignition phenomena and to improve the compatibility of the available fuels with the engines. The experimental study was performed at 923 K in a setup equipped with a jet-stirred reactor. The oxidation mechanisms of n-heptane and isooctane were simplified first by a purely kinetic analysis based on the product formation and then by using a software of simulation of reaction mechanisms. The very different behaviour of these two hydrocarbons was explained by the presence, in the products of isooctane oxidation, of alkenes, which would have an antiknock effect due to the formation of resonance-stabilized radicals. Key words: oxidation reaction, n-heptane, isooctane, autoignition, modelling.
Synthesis of 5-(1-azidovinyl) and 5-[2-(1-azirinyl)] analogs of 2′-deoxyuridine
Authors: Kumar Rakesh , Wiebe Leonard I. , Knaus Edward E.
Published: May 2006

The regiospecific addition of bromine azide to the vinyl substituent of 5-vinyl-3′,5′-di-O-acetyl- (or tert-butyldimethylsilyl)-2′-deoxyuridines (2) yielded the corresponding 5-(1-azido-2-bromoethyl)-3′,5′-di-O-protected-2′-deoxyuridines (3). Treatment of the 5-(1-azido-2-bromoethyl) compounds 3 with t-BuOK, to effect the base-catalyzed elimination of HBr, afforded the corresponding 5-(1-azidovinyl)-2′-deoxyuridines (4, 7). Thermal decomposition of 5-(1-azidovinyl)-2′-deoxyuridine (7) at 110 °C in dioxane yielded 5-[2-(1 -azirinyl)]-2′-deoxyuridine (9). 5-(1 -Azidovinyl)-2′-deoxyuridine (7) exhibited appreciable in vitro antiviral activities againist herpes simplex virus type 1 (HSV-1) and varizella zoster virus (VZV). Athough 7 increased the length of survival of HSV-1 brain-infected mice, it did not decrease the mortality rate relative to placebo. 5-[2-(1-Azirinyl)]-2′-deoxyuridine (9) was an inactive antiviral agent. Key words: azidovinyl, azirinyl, 2′-deoxyuridine, antiviral activity.