bluuebunny/crossref_metadata_2025 huggingface dataset:
Article
Authors: Höpfl Herbert , Farfán Norberto
Published: April 2011

Boron chelates obtained from salicylaldehyde and 2prime-hydroxyacetophenone azines are colored compounds with potential applications in analytical chemistry. Up to now these complexes have not been studied by X-ray crystallography, although two structures with a six- or a seven-membered chelate ring are possible. This contribution presents the X-ray analysis of 2prime-hydroxyacetophenone azine and its corresponding new mono(diphenylboron) chelate with a six-membered boron heterocyclic ring. With these data structural changes of the ligand on chelate formation and structural differences in comparison to salicylaldehyde azomethine boron chelates are discussed.Key words: boron complex, borinate, azine, azomethine.
Article
Authors: Ahrika A , Anouti M , Robert J E , Paris J L
Published: April 2011

S-Aryl thiol esters RC(O)SAr and ArS2- ions are the end products resulting from the reactions between bis(2-nitrophenyl) disulfide Ar2S2 and thiocarboxylate ions RC(O)S- (R = Me, Ph) at 20°C. The apparent SNAr process in fact occurs in two steps as shown by UV-vis absorption spectrophotometry coupled with voltammetry: (i) formation of diacyl disulfides [RC(O)]2S2 and ArS- ions by redox exchange; (ii) subsequent nucleophilic substitution of 2-nitrophenyl thiolate ions at the carbonyl carbon of diacyl disulfides.Key words: diacyl disulfides, 2-nitrophenyldisulfide ions, N,N-dimethylacetamide, nucleophilic substitution, spectroelectrochemistry.
Article
Authors: El-Banna Mohamed M , El-Batouti Mervette M
Published: April 2011

Excess molar volumes, VE, for three binary liquid mixtures of cyclohexane + n-dodecane, n-tetradecane, and n-hexadecane were determined from density measurements at seven different temperatures in the range from 283.15 to 313.15 K and atmospheric pressure, over the entire range of mole fractions. The VE values were derived and the results show a regular negative pattern of behaviour for each of the three sets of the binary system. For cyclohexane, the magnitude of VE increases with the alkane chain length. The Prigogine-Flory-Patterson theory was applied to analyze the experimental behaviour by calculating three different contributions to VE, i.e., interactional, free volume, and internal pressure. The theory gives a representation of the main feature of the systems studied by using a fitted parameter to describe the behaviour throughout the composition range.Key words: excess molar volumes, binary mixtures, cyclohexane, n-dodecane, n-tetradecane, n-hexadecane, Prigogine-Flory-Patterson theory.
An ether-linked tetrafunctional acylating reagent and its cross-linking reactions with hemoglobin
Authors: Kluger Ronald , Paal Krisztina , Adamson J Gordon
Published: April 2011

A new type of tetrafunctional reagent for cross-linking proteins has been prepared and used to modify human hemoglobin A. DPEE (1,2-bis{2-[3,5-bis(3,5-dibromosalicyloxycarbonyl) phenoxy]ethoxy}ethane)) has two separate pairs of reacting sites connected by a flexible tetraether chain. DPEE is capable of connecting a cross-link within a protein to another cross-link, either within the same protein molecule or between molecules. DPEE was readily prepared by esterification of a tetraether-linked bisphthalate (prepared by coupling of 1,2-bis(2-iodoethoxy)ethane and 5-hydroxyisophthalic acid). DPEE reacts with deoxy hemoglobin to produce a mixture of modified proteins. Ion-exchange HPLC was used to separate the modified proteins in the mixture. The most abundant products were selected for structural analysis, which used data from reverse-phase chromatography and tryptic peptide mapping. To prevent dissociation of the modified proteins during analysis, the products were further reacted with the bifunctional reagent, bis(3,5-dibromosalicyl) fumarate, which produces fumaryl cross-links between α-subunits. From peptide analysis of the separated products, the major modified protein from DPEE was identified as a novel species with four links within the same α2β2 tetramer. In addition, a minor product that involves cross-links in two different proteins was observed. These results imply that the reagent reacts primarily in a folded state within the protein.Key words: acylation: cross-linking, multifunctional, hemoglobin, reaction pattern.
Photochemical reaction of β-ionone derivatives in the presence of oxygen. A convenient synthesis of substituted 1,2,4-trioxanes
Authors: Mischne Mirta P , Huber Silvia N , Zinczuk Juan
Published: April 2011

This paper describes a simple and convenient method for the preparation of substituted 1,2,4-trioxanes. The scope of the one-pot photooxidation of β-ionones has been extended to include derivatives bearing additional functionalities in the beta-ionone ring. The relative stereochemistry of the products has been established by analysis of 1H and 13C NMR spectra.Key words: 1,2,4-trioxanes, photooxidation, cyclic peroxides, tetrahydrobenzopyranes, antimalarials.
α-Halogenohydrazides: useful starting material for the synthesis of hydrazinoazapeptides and azauracils
Authors: Barré Catherine , Carret Sophie , Guerro Michel , Grel Philippe Le , Baudy-Floc'h Michèle
Published: April 2011

An efficient method for the synthesis of pseudodipeptides, which combines an aza glycine residue with various hydrazinoacid moieties by simple substitution of the halogen of α-halohydrazides, is described. Some of these hydrazinoazapeptides lead to hydrazones by oxidation or to azaaminouracils by cyclization.Key words: hydrazinohydrazides, hydrazones, hydrazinoazapeptides, azaaminouracils, pseudopeptides.
Cyclic voltammetry of ferrocene-containing β-diketones as a tool to obtain group electronegativities. The structure of 3-ferrocenoyl-1,1,1-trifluoro-2-hydroxyprop-2-ene
Authors: du Plessis WC (Ina) , Erasmus Johannes JC , Lamprecht Gert J , Conradie Jeanet , Cameron T Stanley , Aquino Manuel AS , Swarts Jannie C
Published: April 2011

Formal reduction potentials (E°' values vs. Ag/Ag+ given in parentheses) in acetonitrile of 1-ferrocenyl-4,4,4-trifluorobutane-1,3-dione (ferrocenoyltrifluoroacetone, Hfctfa, 0.394 V), 1-ferrocenyl-4,4,4-trichlorobutane-1,3-dione (ferrocenoyltrichloroacetone, Hfctca, 0.370 V), 1-ferrocenylbutane-1,3-dione (ferrocenoylacetone, Hfca, 0.313 V), 1-ferrocenyl-3-phenylpropane-1,3-dione (benzoylferrocenoylmethane, Hbfcm, 0.306 V), and 1,3-diferrocenylpropane-1,3-dione (diferrocenoylmethane, Hdfcm, 0.265 and 0.374 V for the two electrochemically nonequivalent ferrocenyl groups) were determined. A linear relationship between observed group electronegativity, χ, and E°' as well as observed infrared carbonyl stretching frequencies of methyl esters of the type RCOOMe resulted in average χ-values, on the Gordy scale, of 1.87 for the ferrocenyl group and 2.82 for the ferrocenium group. A single crystal X-ray structure determination of Hfctfa (monoclinic, P21/n, Z = 4, R = 0.029) showed that this β-diketone crystallizes in the enol form. Asymmetric enolization in a direction furthest from the aromatic ferrocenyl group was observed.Key words: ferrocene, β-diketone, group electronegativity, cyclic voltammetry.
Preparation, isolation, and characterization of permethylated galactosides and fucosides
Authors: Asres Daniel D , Perreault Hélène
Published: April 2011

Sugar analysis involving permethylation followed by methanolysis can lead to significant results, given that permethylated monosaccharide standards are available for comparison of spectral and chromatographic data. Such standards are not readily commercially available, especially the furanoside forms. This note describes the isolation and characterization of permethylated pyranoside and furanoside species of D(+)-galactose and L(-)-fucose. Separation of the isomers was optimized using a combination of column chromatography and continuous elution thin-layer chromatography (TLC). TLC, gas chromatography - mass spectrometry, and 1H nuclear magnetic resonance (NMR) spectroscopy were used as the characterization methods. The isolation of furanosides is emphasized, since no specific NMR data have been reported on those to date, while several reports have already discussed the structural aspects of pyranosides.Key words: permethylation, monosaccharide, GC-MS, TLC, NMR.
Energetics of the encapsulation of o-, m-, and p-hydroxybenzoic acids by β-cyclodextrin and its methylated and hydroxypropylated derivatives in aqueous solution
Authors: Junquera Elena , Baonza Valentin G , Aicart Emilio
Published: April 2011

A fully computerized potentiometric technique has been used to analyze the energetics of the encapsulation of o-, m-, and p-hydroxybenzoic acids by β-cyclodextrin (β-CD) and (or) two of its most used derivatives: 2,6-di-O-methyl-β-cyclodextrin (DIMEB) and hydroxypropyl-β-cyclodextrin (HPBCD). The study has been carried out by measuring the pH of aqueous solutions of the three hydroxyacids in the absence of cyclodextrin at 25°C, and in the presence of cyclodextrin at several temperatures ranging from 15 to 40°C, keeping constant the concentration of the guest. The dissociation constant of the acids and the binding constants of the inclusion complexes formed have been simultaneously determined at all the temperatures by using a model previously derived by us. The carboxylic forms have been found to bind the CD with higher affinities than the carboxylate partners do, irrespective of the temperature, the cyclodextrin used, and the position of the substituents on the guest molecules. The formation of the inclusion complexes studied in this work has been found to be enthalpy driven, with a favorable enthalpic term dominant over an unfavorable entropic one. The effect of the substituents of both guest and host molecules on the thermodynamics of the inclusion processes has been analyzed. Key words: binding constant, β-cyclodextrin, hydroxybenzoic acid, inclusion complex, substituent effect.
Free radicals formed by H(Mu) addition to pyrene
Authors: Percival Paul W , Addison-Jones Brenda , Brodovitch Jean-Claude , Ghandi Khashayar , Schüth Joachim
Published: April 2011

Muonium addition to pyrene has been investigated by means of transverse field muon spin rotation and muon level-crossing spectroscopy. Two hydropyrenyl radicals are produced and easily detected while a third is formed in low abundance. Muon and proton hyperfine constants were experimentally determined and compared with semiempirical and ab initio calculations. It is concluded that the radicals are of cyclohexadienyl type, i.e., the Mu adds only to carbon atoms that can assume a tetrahedral -CHMu- configuration without distorting the planar carbon skeleton of pyrene.Key words: muonium, pyrene, radical, hyperfine.
Geometric isotope effects in crystalline sodium hydrogen bis(4-nitrophenoxide) dihydrate
Authors: Kreevoy Maurice M , Young, Jr. Victor G
Published: April 2011

The crystal structure of sodium hydrogen bis(4-nitrophenoxide) dihydrate, 1, with deuterium replacing hydrogen in the bridge and the structural water molecules, has been determined crystallographically at 113, 173, and 295 K. The structure of 1 had previously been determined at similar temperatures (Kreevoy et al.). The O,O distances are 1.5-1.7 pm greater in the deuterated compound than in the undeuterated, at all three temperatures, providing another example of an Ubbelohde effect in a Speakman-Hadzi compound. The temperature invariance of the Ubbelohde effect at temperatures up to room temperature is evidence against centralization of the hydron within this temperature range. It has previously been suggested (Kreevoy et al.) that simplification of the IR spectrum of 1 with increasing temperature is due to an increase in the rate of the hydron shift between the two basic oxygens. This suggestion is strengthened by the elimination of hydron centralization as an alternative. The O,O distance in 1 also increases with temperature, and the dihedral angle between the mean planes of the two aromatic rings decreases. Similarly, the increase in the O,O distance with isotopic substitution is accompanied by a small decrease in the dihedral angle; another geometric isotope effect. Ubbelohde effects in Speakman-Hadzi compounds make the geometric isotope effects found computationally in the critical complexes for hydron, hydrogen atom, or hydride transfer more credible.Key words: low-barrier hydrogen bond, Speakman-Hadzi compound, geometric isotope effect, Ubbelohde effect.
Regioselective protonation of ferrocene in superacid and formation of a CHFe bond. An experimental and theoretical study of the structure and dynamics of the ferrocenonium ion
Authors: Karlsson Annika , Broo Anders , Ahlberg Per
Published: April 2011

Protonation of ferrocene has been suggested to take place on carbon (exo-protonation) or iron (endo-protonation). However, experiments have not been conclusive because of interfering exchange reactions. Now low-temperature protonation of ferrocene and [2H10]-ferrocene in superacid and direct observation of the carbocation by 1H NMR at low temperature shows only primary protonation and that it exclusively takes place in an endo-fashion. Studies by DFT calculations using B3LYP hybrid functional indicate the presence of an intramolecular nonlinear CHFe bond and that the proton might be delocalized between carbon and iron. Potential energy barriers for degenerate rearrangements of the hydride bridged carbocation are low, suggesting that the proton might be delocalized between all 10 carbons and iron. The NMR results are consistent with such an interpretation.Key words: regioselective, protonation, superacid, ferrocenonium ion.
Iron(II) 2-methylimidazolate and copper(II) 1,2,4-triazolate complexes: systems exhibiting long-range ferromagnetic ordering at low temperatures
Authors: Rettig Steven J , Storr Alan , Summers David A , Thompson Robert C , Trotter James
Published: April 2011

Crystals of [Fe (2methylimidazolate)2·0.13(FeCp2)]x,1, are orthorhombic, a = 8.0654(11), b = 15.3504(5), c = 19.3388(9) Å, Z = 8, space group Pnnm. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R (F, I [Formula: see text] 3σ(I)) = 0.057 (Rw (F2, all data) = 0.142). The structure involves tetrahedral iron(II) centers linked in chains by single imidazolate ligand bridges. The chains are cross-linked by additional imidazolates to generate a complex 3-D network of linear channels in which ferrocene molecules are trapped. [Cu(1,2,4-triazolate)2]x, 2, is shown by indirect evidence to have a polymeric structure in which copper(II) ions are bridged by triazolate ligands. Magnetic susceptibilities were measured on powdered samples over the temperature range 2-300 K at applied fields of 0-55 000 G. Both materials exhibit antiferromagnetic exchange at temperatures above a magnetic phase transition: 27 K for 1 and 35 K for 2. At temperatures below these transitions the compounds exhibit weak ferromagnetism, likely resulting from canted spin structures. Cycling the applied magnetic field between +55 000 G and -55 000 G at 4.8 K generates hysteresis loops for both materials. Remnant magnetizations of 200 and 55 cm3 G mol-1 and coercive fields of 5000 and 2500 G are obtained for 1 and 2, respectively.Key words: iron, 2-methylimidazolate, copper, 1,2,4-triazolate, canted spins, ferromagnets.
Quantitative investigations of the aggregation behaviour of hydrophobic anilino squaraine dyes through UV/vis spectroscopy and dynamic light scattering
Authors: Wojtyk James , McKerrow Andrew , Kazmaier Peter , Buncel Erwin
Published: April 2011

In continuing studies of the aggregation behaviour of squaraine dyes in DMSO-water mixtures, we have examined a series of symmetrical anilino-based squaraines with increasing N-alkyl chain length (n-butyl, n-octyl, and n-dodecyl). The aggregation behaviour was assessed through UV/vis spectroscopic and Dynamic Light Scattering (DLS) studies with quantitative interpretation based on exciton theory. This class of N-alkyl squaraines forms two distinct solution aggregates, Type J and Type H, depending on the composition of the DMSO-water medium. DLS measurement of the aggregates in the Type J and Type H morphologies showed that (i) the particle diameter increased with increasing hydrophobicity; (ii) DMSO has a large effect on the aggregate size; (iii) Type J aggregates are of equal dimensions with Type H aggregates. Moreover, even though as the DMSO-water solvent composition is varied the UV/vis spectra of Type H and Type J aggregates remained unchanged, the number of molecules comprising each aggregate varies widely. It is proposed that the dynamic conversion (J →> H) results from intramolecular reorganization of individual molecules within the aggregate. Quantification of aggregation through DLS has further developed the "kinetic" versus "thermodynamic" control model of dye aggregation.Key words: squaraine, dynamic light scattering, DMSO-water mixtures, self-assembly, kinetic/thermodynamic control.
Acid–base chemistry in high-performance lubricating oils
Authors: Hone Duncan C , Robinson Brian H , Steytler David C , Glyde Roger W , Cleverley John A
Published: April 2011

We have used stopped-flow kinetic studies to conduct an investigation into the acid neutralization reaction between overbased detergent additives and aqueous acid dispersed as w/o microemulsion droplets. Water-soluble pH indicators have been used to monitor the change in pH during the reaction. The acid neutralization rate was found to be dependent on the concentration of overbased additive, the temperature of the system, and the hydrocarbon chain length of the solvent. The systems are of importance in relation to the mechanism of acid neutralization by engine lubricants.Key words: overbased detergent additive, acid neutralization, pH indicator, stopped flow.
Correlation of the rates of solvolysis of phenyl chlorothionoformate and phenyl chlorodithioformate
Authors: Kevill Dennis N , D’Souza Malcolm J
Published: April 2011

The specific rates of solvolysis of phenyl chlorothionoformate (PhOCSCl) are remarkably similar to those previously reported for phenyl chlorothioformate (PhSCOCl). When analyzed using the extended Grunwald-Winstein equation over the usual range of solvent types, these solvolyses show essentially identical divisions into the solvents favoring the addition-elimination channel and those favoring the ionization channel. The introduction of one sulfur caused a partial shift away from the addition-elimination pathway, which was dominant over the full range of solvents for phenyl chloroformate (PhOCOCl). Consistent with these results, introduction of the second sulfur within phenyl chlorodithioformate (PhSCSCl) leads to a completion of this shift, such that an extended Grunwald-Winstein treatment of the specific rates of solvolysis now shows the ionization pathway to be dominant over the full range of solvents.Key words: Grunwald-Winstein equation, solvent nucleophilicity, solvolysis, phenyl chlorothionoformate, phenyl chlorodithioformate.
Exploring zero-point energies and hydrogen bond geometries along proton transfer pathways by low-temperature NMR
Authors: Smirnov Sergei N , Benedict Hans , Golubev Nikolai S , Denisov Gleb S , Kreevoy Maurice M , Schowen Richard L , Limbach Hans-Heinrich
Published: April 2011

We have followed by NMR the zero-point energy changes of the hydrogen bond proton in 1:1 acid-base complexes AHB triple bond {AH···B <-–> Aδ-···H···Bδ+ <-–> A-···HB+} as a function of the proton position between A and B. For this purpose, the isotopic fractionation factors K between the acid-base complexes AHB + Ph3COD···B –><- ADB + Ph3COH···B, where AH represents a variety of acids and B represents pyridine-15N, were measured around 110 K, using a 2:1 mixture of liquefied CDClF2-CDF3 as solvent. As under these conditions the slow hydrogen bond exchange regime is reached, the values of K could be obtained directly by integration of appropriate proton NMR signals. Using the valence-bond order concept established previously by crystallography, the fractionation factors and corresponding zero-point energy changes (ΔZPE) are related in a quantitative way to the hydrogen bond geometries, the 1H chemical shift of the hydrogen bond proton, and the pyridine-15N chemical shift. The K values are related in a quasi-linear way to the chemical shifts of the hydrogen bond proton, where the slope depends on whether the proton is closer to oxygen or nitrogen. In the region of the strongly hydrogen-bonded quasi-symmetric complexes, which are characterized by a strong hydrogen bond contraction, the variation of K is very small in spite of substantial proton displacements.Key words: NMR, isotopic fractionation, hydrogen bonding, acid-base complexes, proton transfer, geometric isotope effects.
Spectral and cyclic voltammetric characterization of self-assembled monolayers on gold of pyrene end-labeled oligonucleotide duplexes
Authors: Reese R Scott , Fox Marye Anne
Published: April 2011

Self-assembled monolayers of sulfur-terminated oligonucleotide duplexes were formed on flat gold surfaces, either by exposure of a self-assembled monolayer bearing one oligonucleotide strand to the complementary strand or by preformation of a oligonucleotide duplex that was then deposited on a fresh gold surface. Virtually identical spectral behavior was observed whether the duplex was produced before deposition or by in situ complementary association. With a duplex bearing an appropriate pyrene end-label, the resulting thin film was photoresponsive. Surface emission measurements show no evidence for pyrene aggregation on the modified surfaces. The polarity of the photocurrent, reflecting photoinduced electron transfer initiated by photoexcitation of pyrene, is opposite that expected from the oligonucleotide-mediated reduction of the appended pyrene excited state.Key words: oligonucleotide, self-assembled monolayer, duplex formation, photoelectrochemistry, surface emission.
About environmental isomers and their existence in liquid water
Authors: Grunwald Ernest , Steel Colin
Published: April 2011

Environmental isomers in a liquid are molecules of a single species that become distinguishable because they exist in distinct solvent cages. Concepts involved in environmental isomerism are defined and criteria for distinguishability derived, first by neglecting Brownian potential energy noise, then by including it. Two models are used in the simulations of a molecule jumping from a cage of one type to another. (i) On every Brownian "collision," the associated Schroedinger state wave is continuous, i.e., collisions and transits between cages are accompanied by zero phase change. (ii) On every collision and transit, there is a random phase change in the state wave. The properties of liquid water are reviewed in terms of a two-state model in which the states are environmental isomers. It is shown that (i) the difference in enthalpy and entropy between the isomers, (ii) the positions and width of their IR stretching frequencies, as well as (iii) dielectric constant, (iv) molar volume, and (v) O···O radial distribution data are in agreement with a model in which the more stable isomer is H2O\4w (a water molecule in a cage of four water molecules) and the less stable isomer is H2O\5w. The widths of the IR bands indicate that the water-water interactions are through "linkage pairs" in which the motions of the individual molecules comprising a cage are essentially independent.Key words: isomers, isomerism, water, liquid cages, Brownian noise, state broadening, exchange.
Synthesis, characterization, and biological relevance of hydroxypyrone and hydroxypyridinone complexes of molybdenum
Authors: Lord Sarah J , Epstein Noah A , Paddock Robert L , Vogels Christopher M , Hennigar Tracy L , Zaworotko Michael J , Taylor Nicholas J , Driedzic William R , Broderick Tom L , Westcott Stephen A
Published: April 2011

We have prepared a number of complexes of the type cis-MoO2L2 where L represents a hydroxypyronato or hydroxypyridinonato ligand. Both the maltol (3-hydroxy-2-methyl-4-pyrone, Hma) and kojic acid (5-hydroxy-2-hydroxymethyl-4-pyrone, Hka) complexes, cis-MoO2(ma)2 (1) and cis-MoO2(ka)2 (2), have been characterized by X-ray diffraction studies. The pyrone ligands are bound to molybdenum in a cis bidentate fashion via the deprotonated hydroxyl groups and the ketone moieties. Crystals of 1 are orthorhombic, a = 12.107 (1), b = 8.6169 (8), c = 16.472 (1) Å, Z = 4, space group Pca21, and those of 2 are monoclinic, a = 8.4591 (5), b = 16.3453 (10), c = 10.2954 (7) Å, β = 103.0320 (10)°, Z = 4, space group P21/c. Hydroxypyridinone molybdenum complexes have been prepared for both maltol and kojic acid derivatives with the substituents Me, n-Pr, CH2Ph, Ph at the ring nitrogen. Crystals of the 3-hydroxy-2-methyl-1-phenyl-4-pyridinone (Hppp) derivative, MoO2(ppp)2 (9), are monoclinic, a = 10.9476 (6), b = 13.5353 (9), c = 17.4877 (10) Å, β = 93.465 (4)°, Z = 4, space group P21/n. Initial investigations into the effects molybdenum compounds have on diabetic hearts are presented. Both Na2MoO4 (used as a control) and 1 were effective in lowering blood glucose and free fatty acid levels. Diabetic rats treated with molybdate showed significant improvements in postischemic cardiac function.Key words: molybdenum, hydroxypyrones, hydroxypyridinones, heart function.
Cage peroxides having planar bridgehead nitrogen atoms
Authors: Edward John T , Chubb Francis L , Gilson Denis FR , Hynes Rosemary C , Sauriol Françoise , Wiesenthal Alain
Published: April 2011

Three new cage peroxides, 1,6-diaza-3,4,8,9-tetraoxabicyclo[4.4.2]dodecane (3a),1,6-diaza-3,4,8,9-tetraoxa-11-methylbicyclo[4.4.2]dodecane (3b), and 1,6-diaza-3,4,8,9-tetraoxatricyclo[4.4.2.411,12]hexadecane (4), have been prepared by reaction of 1,2-diaminoethane, 1,2-diaminopropane, and trans-1,2-diaminocyclohexane, respectively, with formaldehyde and hydrogen peroxide in aqueous acidic solution. Their structures have been established by X-ray diffraction, and show the bridgehead nitrogen atoms to be predominantly sp2 hybridized. The structures accord with 1H and 13C NMR spectra. Variable temperature NMR studies show that the diperoxide 3a begins to undergo rapid inversion (on the NMR time scale) at about 303 K; up to 370 K the diperoxides 3b and 4 show no conformational change.Key words: cage compounds, formaldehyde, peroxides, amine nitrogen, hybridization.
The structure and some molecular properties of acetone-picrylhydrazone
Authors: Brown K C , Nelson B R , Quail J W , Robertson B E , Weil J A , Zimpel Z
Published: April 2011

Various physical measurements and quantum-mechanical computations to characterize molecular 2-propanone(2,4,6-trinitrophenyl)hydrazone, alias acetone-picrylhydrazine (AH), are reported, including an X-ray diffraction structural determination, an 1H and 13C NMR study of its internal hindered reorientation, and a theoretical (SCF-MO) interpretation of these observations. The structure of AH was determined by X-ray crystallography. The space group is Pbar over 1, with a = 10.1768(9) Å, b = 7.7968(18) Å, c = 8.0018(5) Å, α = 92.102(6)°, β = 99.919(7)°, γ = 105.926(6)°, Z = 2, wR2(F2) = 0.1995 based on all 2748 unique reflections. The (picryl) proton NMR thermal work yielded a Gibbs activation energy ΔG = 46.9 ± 0.4 kJ mol-1 in acetone-d6 and 48.1 ± 0.2 kJ mol-1 in chloroform-d, whereas 13C NMR (two pairs in the picryl ring) yielded 46.6 ± 1.0 and 46.4 ± 1.0 kJ mol-1 in acetone-d6. The SCF-MO computations yielded a detailed model of the conformerization path. Various model conformations and tautomers of AH have been considered, as has removal of H+ or of H0 from its hydrazinic linkage.Key words: dynamic NMR, picrylhydrazone, hindered rotation, activation parameters, SCF-MO model.
1998 Alcan Award Lecture Surface electrochemistry - surface science with a joy stick
Authors: Lipkowski Jacek
Published: April 2011

This lecture gives a review of thermodynamic, spectroscopic, STM and AFM imaging, and X-ray diffraction studies of molecular and ionic adsorption at Au(111) electrodes. In the first part, thermodynamics of adsorption of simple ions such as sulfate, chloride, bromide, and iodide will be discussed. At high coverages, anions adsorbed at a single crystal surface form ordered 2D adlayers. We show that the structure of these adlayers can be studied by STM and surface X-ray diffraction techniques. Next, the information concerning adsorption of simple ions is used to describe mixed adlayers formed by coadsorption of anions and metal adatoms. We demonstrate how to combine electrochemical experiments with in situ polarization-dependent Cu K-edge X-ray absorption spectroscopy to determine the composition and the structure of mixed films formed by deposition of Cu on Au(111) in the presence of SO42-. In the last part we review our efforts to describe coordination of organic molecules to gold electrodes. First, we will discuss adsorption of benzonitrile at the Au(111) electrode surface. We combine electrochemical methods with in situ infrared spectroscopy to describe (i) the energetics of molecular adsorption at the gold electrode surface, (ii) the character of the interaction of the adsorbed molecule with the metal substrate, and (iii) the influence of the electric field on the orientation of the adsorbed molecule. In the last section we describe surface aggregation phenomena. We apply AFM and STM to determine the structure of hemimicelles formed at the Au(111) electrode surface by adsorbed molecules of sodium dodecyl sulfate and we discuss the potential-controlled transformation of these hemimicelles into a condensed monolayer.Key words: surface electrochemistry, electrosorption, surface aggregation, gold electrodes, molecular adsorption, ionic adsorption.
Preferred conformation and barriers to internal rotation of ortho-disubstituted cyclopropylbenzenes
Authors: Ernst Ludger , Rieck Thomas , Soliven Mark
Published: April 2011

Schaefer's "J method" was employed to show that 2-cyclopropyl-1,3-dimethylbenzene (5) in solution prefers the perpendicular conformation in which the torsional angle Θ between the C(1)-H bond of the cyclopropyl group and the plane of the benzene ring is 90°. This is opposed to the situation in cyclopropylbenzene (3) where the bisected conformer (Θ = 0°) prevails. From the value of -0.85 ± 0.01 Hz for 6J(H-alpha,H-para) in 5 (for solutions in CS2 and in acetone) a barrier to rotation about the cyclopropyl-aryl bond of 6.4 kJ/mol can be derived if a predominantly two-fold potential and a vanishing 6J(H,H) for Θ = 0° are assumed. The introduction of the two ortho-methyl groups into 3 thus effectively interchanges the ground and transition state conformations of the internal rotation. This effect is well reproduced by ab initio (STO-3G and 6-31G*) and semiempirical (AM1) molecular orbital computations. The preference for the perpendicular conformation of an ortho-disubstituted cyclopropyl substituent was also demonstrated by a dynamic NMR study of 2-cyclopropyl-4-isopropyl-1,3,5-trimethylbenzene (10). Exchange-broadened 1H spectra due to slow rotation of the cyclopropyl group were only obtained near the low-temperature limit of the spectrometer (-140°C), and the barrier to rotation is estimated to lie near 28 kJ/mol (deltaG). MM3 molecular mechanics computations suggest that the rather large increase of the rotational barrier in 10 relative to 5 is caused by the combined buttressing effect of the isopropyl and the 5-methyl groups. The present findings explain why an earlier attempt (in 1970) to determine the rotational barrier in 2-cyclopropyl-1,3,5-trimethylbenzene by dynamic NMR was bound to fail.Key words: conformation, cyclopropylbenzenes, ortho-disubstituted, NMR spectroscopy, rotational barrier.
Électrosynthèse de films de polyparaphénylène déposés sur carbones
Authors: Sadki S , Fauvarque J F , Descarsin M T , Simon P
Published: April 2011

This present work deals with the formation of polyparaphenylene (PPP) electrodes in which the polymer is associated with conductive carbons. Regular and reproducible coatings of PPP on different types of carbon electrodes were obtained by catalytic electrosynthesis (Fauvarque method) and were examined by electron microscopy. Electrochemical properties of PPP, coated on conductive carbons, were studied in sulfolane-NBu4BF4 electrolyte. Coatings on carbon lead to a good cyclability and a high apparent current density (as high as 2 mA/cm2). Key words : conductive polymers, polyparaphenylene, electrolytic coating.
Geometrical isomerism and 19F NMR spectroscopy of octahedral perfluoroethyl- and perfluoropropyl-substituted β-diketonates and monothio-β-diketonates of rhodium(III)
Authors: Stern Gary A , Westmore John B
Published: April 2011

In solution in acetone at room temperature, tris(perfluoropropanoyl and perfluorobutanoyl)benzoylmethanate complexes of rhodium(III) are shown by 19F NMR spectroscopy to exist as nearly statistical 1:3 mixtures of their respective fac and mer octahedral isomers, whereas the analogous perfluoroalkanoylthiobenzoylmethanate complexes have a fac-octahedral structure. For all four complexes studied, vicinal F-F coupling constants are too small (0-3 Hz) to lead to resolved multiplet peaks, so that perfluoroethyl groups show singlet resonances for their CF3 groups. Since four-bond coupling constants are significantly larger, ~9 Hz, perfluoropropyl groups show the expected multiplet resonances for their CF3 groups. In acetone, AB patterns, with geminal coupling constants equal to approx. 270 Hz, are observed for the CF2 groups next to the chelate ring (except for the perfluoropropanoylthiobenzoylmethanate complex). (These observations are modified by solvent. In toluene, one of the AB patterns of a mer-octahedral complex collapses to a single peak.) The AB spectra, which are subject to substituent and solvent effects, can be interpreted in terms of perfluoroalkyl groups rotating rapidly about the Cring-Calkyl bond, with an unsymmetric double potential well, giving unequal rotamer populations. Changes in the spectra were not noticeable over the temperature range accessible in the acetone and toluene solvents used.Key words: rotation, perfluoroalkyl groups, fluorine NMR spectra, fluorine-fluorine coupling constants, fluorinatedβ-diketonate complexes, monothio-beta-diketonates, rhodium complexes.
Dendritic aryl imides containing the tetrahydro[5]helicene unit: synthesis, characterization, electrochemical behavior, and comparison with a linear oligoimide analogue
Authors: Bender Timothy P , Qi Yu , Desjardins Pierre , Wang Zhi Yuan
Published: April 2011

Herein, we describe the synthesis, characterization, and electrochemical behavior of dendritic aryl imides derived from a core compound 3,12-dinitro-5,6,9,10-tetrahydro[5]helicene-7,8-dicarboxylic anhydride (3). Compound 3 is an AX2 type system (where X is a masked or protected group), possesses a well-defined spatial orientation, is conformationally rigid, and contains a conjugated o-terphenyl moiety as part of its molecular structure. This compound is used as a building block in the construction of dendritic aryl imides using a divergent synthetic approach. Dendrons from generations one through three were synthesized, and their identities were confirmed. A comparison of the properties of the dendrons, several model compounds, and a linear oligoimide, all containing either the tetrahydro[5]helicene or o-terphenyl units, has been made. There are differences in their hydrodynamic volume, glass transition temperature, and electrochemical behaviour. All dendrons are electrochemically active and exhibit well-defined and reversible redox cycles. Furthermore, all dendrons are fluorescent, and the intensity of the fluorescent emission is dependent on the generation of the dendron.Key words: dendrimer, dendron, electrochemistry, helicene, tetrahydro[5]helicene.
Long chain ß-aminoethanols as micellar catalysts for the hydrolysis of diphenyl-p-nitrophenyl phosphate. The observation of an unusual enhancement of catalysis in the presence of added polyols
Authors: Oumar-Mahamat H , Slebocka-Tilk H , Brown R S
Published: April 2011

The hydrolysis of p-nitrophenyldiphenyl phosphate (PNPDPP, 7) catalyzed by N-dodecylaminoethanol (6a), N-tetradecylaminoethanol (6b), N-methyl-N-tetradecylaminoethanol (6c), and N-hexadecylaminoethanol (6d) was investigated at 25°C at various pH values in aqueous media containing additives. Added ethanol or dimethoxyethane produces very little activity of 6a, b, c toward 7. The highest activities of 6a, b were found at pH 8.0 and 8.5 in solvents containing 20% glycerol or ethylene glycol. Under no conditions was N-hexadecylaminoethanol (6d) found to be active. The kinetics of the reaction of 6b with 7 were also investigated in a medium containing various amounts of glycerol, polyethylene glycol 400 (PEG 400), and water at pH 7.6-8.5. The best catalytic system comprised 3 mM 6b in 20% aqueous ethylene glycol, pH 8.5, which accelerated the hydrolysis of 7 by 1700-fold over the blank rate. Kinetic experiments conducted using [7]/[6b] = 1 and 2 demonstrate that 6b exhibits true turnover catalysis. A mechanism for the catalyzed reaction is proposed.Key words: ß-aminoethanols, hydrolysis, p-nitrophenyldiphenyl phosphate, micelles, polyhydroxy alcohols.
La radiolyse de l'eau et des solutions aqueuses : historique et actualité
Authors: Ferradini Christiane , Jay-Gerin Jean-Paul
Published: April 2011

Experiments showing that water is decomposed by the action of high-energy radiations date back to the first days of the discovery of radioactivity, a century ago. On the occasion of this anniversary, we have attempted to give a comprehensive account of the radiation chemistry of water and its solutions since its origin, with special emphasis on the various physical and chemical stages that led to the present state of this science. To this aim, we describe the effect of different intervening factors on the molecular and radical yields, including dissolved solute concentration, pH, radiation intensity (or dose rate), type and energy of the radiation, presence of oxygen, temperature, phase, and pressure. We also discuss briefly the chemical behavior of the free radicals produced in radiolyzed aqueous solutions. A good, albeit incomplete, description of the phenomena is obtained that leads to various perspectives concerning, on the one hand, the development of this science and, on the other hand, its potential for applications.Key words : radical chemistry, dilution curve, water, hydrated electron, hydroxyl and superoxide radicals, free radicals, radiolysis, chain reactions, molecular and radical yields, cell survival, linear energy transfer.
New aspects on the reactivity of MoO2(S2CNEt2)2 with Me3SiX. A peculiar trans effect control
Authors: Teruel H , Sierraalta A
Published: April 2011

The reaction of MoO2(S2CNEt2)2 with Me3SiI and Me3SiNCS yielded MoOI2(S2CNEt2)2 and MoO(NCS)2(S2CNEt2)2, respectively, similar to results reported for Me3SiCl and Me3SiBr. On the contrary, MoO(OSiMe3)CN(S2CNEt2)2 was the only compound isolated from the reaction of MoO2(S2CNEt2)2 with Me3SiCN. To study the effects of ligands on the reactivity of siloxy compounds, ab initio calculations were undertaken for Mo(OSiH3)L(S2CNH2)2 molecules, for L = Cl, Br, I, NCS, and CN. The electronic density, ρ(r), was analysed by means of the topological theory of Atoms in Molecules (AIM). The analysis of -del2ρ(r) critical points (CPs) at the molybdenum valence shell showed, only in the case of L = CN, a local charge concentration region (N-type CPs) susceptible to electrophilic attacks. However, for L = Cl, Br, I, and NCS, local charge depletion sites (E-type CPs) were found in a bond-free region. The absence of any accessible E-type CPs and the presence of an N-type CP in the same region, opposite to the Mo=O group, justified the stability of Mo(OSiMe3)L(S2CNEt2)2 for L = CN, and the formation of MoOL2(S2CNEt2)2 for L = Cl, Br, I, and NCS by further reaction with the corresponding Me3SiL.Key words: ab initio calculation, electronic density, Mo-siloxy compound, reactivity, theoretical.
Temperature dependence of the creatine kinase reaction measured in rat brain in vivo by31P NMR saturation transfer
Authors: Buist Richard , Kroeker Scott , Peeling James
Published: April 2011

Creatine kinase (CK) catalyzes the reversible phosphorylation of MgADP by phosphocreatine and thus regulates cellular concentrations of ADP and ATP. The temperature dependence of this reaction has been determined in rat brain in vivo between 30 and 40°C using31P NMR saturation transfer measurements. The pseudo-first-order rate constant for the forward CK reaction, kf, varies little with temperature over this range, with an apparent activation energy Ea= 14.2 ± 4.9 kJ/mol. This is considerably lower than the values of Eafor isolated CK enzymes. However, when changes in [MgADP] and [H+] with temperature are considered, a substrate concentration-independent value of Ea= 65.3 ± 9.7 kJ/mol is obtained for the maximum forward reaction velocity Vmax. This agrees well with literature values for the isolated brain-type isoform of CK.Key words: creatine kinase, activation energy, temperature, brain, rat.
Assignment of 1H and 13C spectra for polyprenol-12, a molecule with severe 1H and 13C spectral crowding, with the aid of highresolution, 13C-detected, 13C–1H shift correlation spectra
Authors: Reynolds William F , McLean Stewart , Jacobs Helen , Harding Wayne W
Published: April 2011

Polyprenol-12 contains a linear sequence of 12 isoprene units. As a result, there is severe crowding in several regions of the 1H and 13C spectra. Nevertheless, it is possible to assign a substantial majority of individual resonances as well as to confirm the number and sequence of E and Z isoprene units with the aid of high-resolution (0.001 ppm along each axis) 13C-detected 13C-1H shift correlation spectra. By contrast, the corresponding 1H-detected sequences (e.g., HSQC and HMBC) do not have sufficient resolution to allow these assignments, in spite of their higher sensitivity.Key words: 13C-detected 2D shift correlation, polyprenol 12, spectral assignments.
Electron transfer reactions of bis(1,4,7-trithiacyclononane)palladium(III/II), -platinum(III/II), and -gold(III/II) complexes in aqueous solution and in acetonitrile
Authors: Matsumoto Mitsuru , Itoh Makoto , Funahashi Shigenobu , Takagi Hideo D
Published: April 2011

The oxidation reactions of Pd(ttcn)22+, Pt(ttcn)22+, and Au(ttcn)22+ (ttcn = 1,4,7-trithiacyclononane) complexes by a typical outer-sphere oxidizing reagent, Ni(tacn)23+ (tacn = 1,4,7-triazacyclononane), were studied in aqueous acidic solution and in acetonitrile. The rate constants corresponding to the oxidation and reduction reactions were isolated for Pd(ttcn)23+/2+ and Au(ttcn)23+/2+ couples in both media. The electron exchange rate constants (kex) for Pd(ttcn)23+/2+, Pt(ttcn)23+/2+, and Au(ttcn)23+/2+ couples at 25°C were estimated as 1.7 × 103, 59, 3.9 × 104 mol-1 dm3 s-1 in aqueous solution and 6.7 × 103, 1.3 × 102, and 2.7 × 105 mol-1 kg s-1 in acetonitrile, respectively. An attempt to observe the reduction reaction of Pt(ttcn)23+ was not successful because of the instability of Pt(ttcn)23+ in aqueous solution. A comparison of the electron exchange rate constants of Pd(tacn)23+/2+, Pd(ttcn)23+/2+, Pt(ttcn)23+/2+, and Au(ttcn)23+/2+ couples in aqueous solution and in acetonitrile revealed that the inner-sphere contribution to the activation free energy for each reaction is identical in both solvents and that the self-exchange rate constants of these complexes directly reflect the structural differences of these complexes.Key words: electron transfer, kinetics, Marcus theory, inner-sphere contribution.
Spectroscopic conformational study of PMS 952, a specific imidazoline ligand. Experimental results
Authors: Baron Marie-Hélène , Halut-Desportes Sabine , Ye Hai-Fen , Huet Jack , Favre Edith
Published: April 2011

X-ray analysis of PMS 952 displays two kinds of very strong hydrogen bonds, an intramolecular one stabilizing the conformer A and an intermolecular one that associates conformers A to form two infinite chains. The 1H NMR study of vicinal coupling constants based on dihedral angle values, evidences that PMS 952 takes up an eclipsed conformation, in CCl4 and CDCl3 solutions, closely related to the conformation in solid state. Deuterium exchange in methanol-d4 points out the presence of a tautomeric form. However, as there is spectral evidence neither from NMR nor from FTIR studies for such a tautomer, this specie cannot be taken into account to explain the complexity of the FTIR spectra. NMR data also reveal that the imidazoline ring freely rotates while FTIR analysis allows characterizing two conformers (A and B), with lifetime shorter than NMR time scale. These two conformers are stabilized by two different intramolecular H-bonds. Taking into account that conformer A could exist in two states (monomer A and (A)n-chains), an equilibrium between A, (A)n, and B entities allowed the assignment of all main bands of the IR spectra, assigned to vNH, vOH, vCH, and vCN modes. The strength of hydrogen bonds is solvent dependent. Polar solvents, either proton donor or acceptor stabilized the conformer A, while CCl4 favors the conformer B.Key words: hydrogen bonds, X-ray, NMR, FTIR, imidazoline derivative.
Cationic iron(III) complex with a hexadentateN2,N'2',O2-aminopyridylphenolate ligand
Authors: Setyawati Ika A , Rettig Steven J , Orvig Chris
Published: April 2011

Iron(III) complexation with potentially hexadentate H2bbpen (N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethylenediamine) was studied. The resulting monocationic complex, [Fe(bbpen)]+as its NO3-and PF6-salts, was characterized by infrared spectroscopy, mass spectrometry, elemental analyses, cyclic voltammetry, and X-ray crystallographic analysis. Crystals of [Fe(bbpen)]NO3·CH3OH are monoclinic, space group P21/c, a = 10.2640(13), b = 14.7526(10), c = 18.3172(5) Å, β = 97.904(1)°, and Z = 4. The structure was solved by heavy-atom Patterson methods and refined to an R factor of 0.039 for 3526 reflections with I > 3σ(I). The structure of the [Fe(bbpen)]+cation showed that the ligand is bound in a hexadentate fashion to the central Fe(III) ion, resulting in a distorted octahedral geometry. Magnetic susceptibility measurements indicated the presence of a high-spin Fe(III) ion, and the UV-vis spectrum of [Fe(bbpen)]NO3exhibits absorption maxima, λmax, at 575 nm (ε = 5400 M-1cm-1), 323 (8900), and 275 (13 500). The cyclic voltammogram of [Fe(bbpen)]NO3showed a quasi-reversible, one-electron process corresponding to [FeIII(bbpen)]++ e-<_> [FeII(bbpen)] at -0.47 V vs. SCE.Key words: cationic, iron(III), phenolate, pyridyl, hexadentate.
Temperature and pressure dependence of the volumetric properties of binary mixtures containing polyhaloalkanes
Authors: Jarne Carmen , Artal Manuela , Embid José Muñoz , Velasco Inmaculada , Otín Santos
Published: April 2011

Densities of binary mixtures of 1,1,2-trichlorotrifluoroethane + dibromomethane, + bromochloromethane, or + bromotrichloromethane were measured over their entire composition ranges at 288.15 and 308.15 K. Thermal expansion coefficients (α) and excess molar volumes (VEm) were calculated. Moreover, densities at 298.15 K and pressures up to 80 bar (1 bar = 100 kPa) were determined for these same mixtures. Isothermal compressibilities (KT) of the pure liquids and their mixtures were obtained.Key words: density, excess volume, thermal expansion coefficient, isothermal compressibility.
Photoyellowing of peroxide-bleached lignin-rich pulps: a photochemical study on stilbene-hydroquinone chromophores issued from β-5 units of lignin during refining and (or) bleaching
Authors: Ruffin Brigitte , Castellan Alain
Published: April 2011

The contribution of stilbene-parahydroquinone structures, formed from β-5 units during refining and bleaching, in photoreversion of peroxide-bleached high-yield pulps, was studied using 4'-benzyloxy-2,5-dihydroxy-3,3'-dimethoxystilbene (I), as a model compound. The various photoproducts formed during irradiation of I in solution or adsorbed on cellulose paper were identified. Isomerization of the stilbene double bond and oxidation of the hydroquinone into quinone were observed in solution, whereas in the solid state only the formation of the quinone was found to operate. Our attempts to detect and quantify stilbene-hydroquinones forming from β-5 units created on peroxide bleached mechanical pulp were not met with success.Key words: lignin, stilbene, hydroquinone, lignin model compound, pulp, photoyellowing.