bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal expansion and Grüneisen functions of polymer crystal models: 1. Central forces
Authors: Barron Ruth Margaret , Barron Thomas Hugh Kenneth , Mummery Paul Malcolm , Sharkey Michael
Published: May 2006

With typical pair potentials, a bond is stretched by vibrations along the bond directions, while contraction is caused by the tension set up by vibrations normal to the bond direction. The rôle of these two effects is studied in calculations for parallel linear-chain and zigzag-chain models. For linear chains and rigid zigzag chains, expansion is positive in the plane normal to the chain direction, although it may be strongly anisotropic in this plane. Expansion along the chain direction is negative, and typically an order of magnitude less than that normal to the chain. For a flexible zigzag chain with bond angle 90°, expansion is positive in all directions; the Grüneisen tensor is approximately isotropic, but elastic anisotropy leads to smaller expansion coefficients in the molecular planes. The Reuss-averaged Grüneisen function derived from (6–12) pair potentials is about 3.5 for most of the models; this is higher than observed experimentally in polycrystalline polyethylene, indicating the need for more realistic models of organic polymers.
Lattice and molecular dynamics
Authors: Janzen Alec R. , Leech John W.
Published: May 2006

A frequency analysis is made of a molecular dynamical (MD) simulation of crystalline CS2. At 20 K the peak MD frequencies agree with those calculated using lattice dynamics (LD). However, they are not as dominant as might be expected. At 150 K the peak frequencies are generally not the LD ones. Experience with the Fermi–Pasta–Ulam linear chain problem suggests the reason may be obscuring of fundamental by combination frequencies in highly anharmonic situations.
Infrared multiphoton absorption and decomposition of ethanol vapour
Authors: Back R. A. , Evans D. K. , McAlpine Robert D. , Verpoorte E. M. , Ivanco Mike , Goodale J. W. , Adams H. M.
Published: May 2006

Multiphoton absorption (MPA) by ethanol has been studied using pulsed HF and CO2 lasers. For the same laser fluence, a CO2 laser excites the ethanol molecule to a much higher energy than an HF laser. The multiphoton decomposition (MPD) of methanol has been studied using the 9P(16) CO2 laser line for pulse widths of 10 and 60 ns. The major decomposition channel produces C2H4 and H2O. However, other decomposition channels, resulting in the products CH3CHO, C2H6, CH4, and C2H2, are also studied and the channel ratios are found to vary with laser pulse width and with pressure. The mechanistic implications of these results are discussed.
A light scattering and calorimetric study of micelle formation by a polystyrene -b-hydrogenated polyisoprene block copolymer in a binary solvent (pentane–cyclopentane)
Authors: Procházka Karel , Delcros Hélène , Delmas Geneviève
Published: May 2006

A light scattering and calorimetric study was made of micelle formation by a polystyrene-b-hydrogenated polyisoprene in a binary mixture. The polymer 0.42 weight percent PS, is unassociated in cyclopentane (c-C5) but forms micelles with a PS core in n-pentane, a non-solvent for PS. Light scattering measurements and heats of mixing, obtained over the binary composition range at a polymer concentration higher than the c.m.c., show that the solution changes from the unassociated to the micellar state for solutions richer in pentane than 0.4 volume fraction. Using heats of mixing of the corresponding homopolymers, the heat of micellization is found to be −5.1 + 0.3 J g−1, a value which supports the model of an enthalpy driven process of micellization.
Structure and conformation of 5′-chloro-5′-deoxyarabinosylcytosine: X-ray, 1H and 13C nmr analyses
Authors: Birnbaum George I. , Buděšínský Miloš , Beránek Jiří
Published: May 2006

Crystals of 5′-chloroarabinosylcytosine, an inhibitor of nucleic acid synthesis, belong to the orthorhombic space group P212121, and the cell dimensions are a = 6.801(1), b = 9.698(2) and c = 16.497(3) Å. X-ray intensity data were measured on a diffractometer and the structure was determined by direct methods. Least-squares refinement, which included all hydrogen atoms, converged at R = 0.032 for 1251 observed reflections. The conformation about the glycosyl bond is anti, the furanose ring adopts a C3′ endo/C4′ exo pucker and the conformation of the side chain is gauche+, stabilized by an intramolecular [Formula: see text] hydrogen bond. 1H and 13C nmr spectra confirm the anti conformation about the glycosyl bond. In D2O solution there is an approximately equal population of N- and S-type conformers of the furanose ring and a trans > gauche+ > gauche− distribution of the 5′-CH2Cl side chain rotamers.
The photochemistry of the rhodizonate dianion in aqueous solution
Authors: Iraci G. , Back M. H.
Published: May 2006

The photochemistry of the rhodizonate dianion at 483 nm has been studied in aqueous solution at pH 8.3. In the absence of oxygen no reaction was observed. In the presence of oxygen, 6.2 × 10−5 M, the dianion was consumed with a quantum yield of 0.044. The oxidation apparently did not involve the formation of O2(1Δg). Electron transfer from the excited dianion was observed with methyl viologen.
The prototropic equilibria in pentenylphosphonic systems
Authors: Modro Agnes M. , Modro Tomasz A.
Published: May 2006

Dialkyl 2-chloropentylphosphonates, C3H7CHClCH2P(O)(OR)2, and monoalkyl 2-chloropentylphosphonic acids, C3H7CHClCH2PO2H(OR), have been dehydrohalogenated by strong bases to the corresponding pentenylphosphonic compounds. In all cases the kinetically controlled product was shown to be the E-1-pentenylphosphonic derivative. The α,β-unsaturated products underwent base-catalysed isomerization to the thermodynamically controlled E-2-pentenylphosphonic systems, thus indicating that the PO3R2 and PO3R− groups destabilize (relative to hydrogen) the adjacent olefinic bond. The hydrogen–deuterium isotope exchange was studied for the α,β- and β,γ-unsaturated products and the results obtained were in full agreement with the isomerization results.
Biosynthesis of the lupine alkaloids. II. Sparteine and lupanine
Authors: Golebiewski W. Marek , Spenser Ian D.
Published: May 2006

Contrary to predictions based on earlier observations, it is now found that label from DL-[6-14C]lysine and from [6-14C]-Δ1-piperideine does not enter the three C5 segments of lupanine with equal efficiency. One of the C5 segments is labelled at a significantly higher level than the other two C5 segments, which show identical levels of labelling. A new biogenetic model is proposed, which is consistent with these new findings as well as with all other regio- and stereochemical evidence on the mode of incorporation of precursors that is available.
A comparative electron spin resonance study of the selenothiadiazolyl radicals cyclo-Se3–nSnN2+ (n = 0–3)
Authors: Awere Ed , Passmore Jack , Preston Keith F. , Sutcliffe Leslie H.
Published: May 2006

Electron spin resonance spectra observed in dilute solutions of the hexafluoroarsenate salts of selenothiadiazolylium cations in polar solvents are ascribed to the parent cation radicals [Formula: see text], [Formula: see text], [Formula: see text], and[Formula: see text]. The g- and nitrogen hyperfine tensors for the three selenium-containing species are entirely in line with the planar π-radical description deduced in earlier studies of trithiadiazolylium. Substantial increases in the g-shifts on substitution of sulphur by selenium result in broadening of the solution spectra and concomitant loss of nitrogen hyperfine structure. From measurements made in solid SO2, the following principal tensor components were deduced for the new, Se-containing radicals:[Formula: see text]: g = (1.9930, 2.0108, 2.1379), aN(2) = (7.4, ≈0, ≈0) gauss, aSe(2) = (180, 70, 43) gauss;[Formula: see text]: g = (1.9941, 2.0108, 2.1355), aN(2) = (6.9, ≈0, ≈0) gauss, aSe(2) = (180, 70, 45) gauss;[Formula: see text]: g = (1.9957, 2.0086, 2.0736), aN(2) = (8.3, ≈0, ≈0) gauss, aSe = (198, 78, 75) gauss, where the values are given in the order x, y, z, x being the direction perpendicular to the radical plane.
Electron spin resonance spin-trapping study of peroxidation of human low density lipoprotein
Authors: Krilov Dubravka , Pifat Greta , Herak Janko N.
Published: May 2006

The electron spin resonance spin-trapping method has been used for detection of radicals in human low density lipoprotein (LDL) solution saturated with oxygen. The trapped radicals could be detected after about a day in an antioxidant-free solution. Two types of radicals are formed and stabilized in the lipid domain of LDL. The nature of the trapped radicals remains unclear:kinetic considerations suggest the trapped radicals to be of the LO- type, while the spectroscopic parameters are in favour of L- type radicals.
Étude thermodynamique du mélange chlore – acide isocyanurique en milieu aqueux
Authors: Solastiouk Bernard , Deglise Xavier
Published: May 2006

A method of calculation of the equilibrium concentrations in aqueous solution of cyanuric acid and its chlorinated derivatives as a function of pH, molar ratio of chlorine/cyanuric acid, and chloride ion concentration is presented. The influence of each of these parameters is studied in conditions close to those used for these compounds in swimming pools.
Voltammetric stabilization of electrofaceted (100)-type platinum electrodes in acidic solutions
Authors: Bilmes Sara Alfonsina Dora Aldabe De , Giordano María Cristina , Arvia Alejandro Jorge
Published: May 2006

The voltammetric stabilization of electrofaceted (100)-type Pt electrodes prepared from polycrystalline Pt by fast repetitive triangular potential cycling are followed by conventional voltammetry in the H-electroadsorption/electrodesorption potential range in either 1 M H2SO4 (0 to 35 °C) or 0.5 M HClO4 (−2 to 35 °C). The influence of CO adsorption on the stabilized voltammetric profile is also considered. Depending on the potential cycling used in conventional voltammetry, two well-defined stabilized profiles at 0.1 V/s in the H-electroadsorption/electrodesorption range can be obtained, namely, that corresponding to the H-adsorption stabilized electrofaceted (100)-type Pt, and that related to the O-adsorption stabilized electrofaceted (100)-type Pt.The changes in the conventional voltammetric profiles can be interpreted in terms of surface modifications basically associated with either adsorption or electroadsorption processes. The influence of the anion (electrolyte composition) prevailing in solution is accounted for in terms of a competitive adsorption between anions and H-adatoms. The influence of temperature in the range covered by this work is smaller than that of the structure of the solution at the electrode/solution interface.
The structures and stabilities of singlet and triplet dithiatriazines: a computational study
Authors: Hoffmeyer R. E. , Chan W.-T. , Goddard J. D. , Oakley R. T.
Published: May 2006

Ab initio molecular orbital and Møller–Plesset perturbation theory calculations have been carried out on two model dithiatriazines RCN3S2 (R = H, NH2). With geometry optimization and the inclusion of electron correlation both of these dithiatriazines are predicted to be ground state singlets. Both molecules have low-lying triplet excited states, with energy gaps of 6.6 (R = H) and 13.0 (R = NH2) kcal mol−1. The singlet dithiatriazines distort from high (C2v) to low (Cs) symmetry, and these changes are important in determining the relative energies of the singlet and triplet molecules. The structural distortions experienced by these molecules are related to Hartree–Fock and Jahn–Teller instabilities in other thiazene heterocycles.
Cyclopalladation of trimesitylarsine. The X-ray crystal structure for
Authors: Alyea Elmer C. , Ferguson George , Malito John , Ruhl Barbara L.
Published: May 2006

The bulky trimesitylarsine ligand, As(mes)3, undergoes facile cyclopalladation to yield the dimeric complex, [Formula: see text], characterized by microanalysis, IR and 1H NMR spectroscopy. This complex is very stable but readily undergoes bridge-cleavage reactions with tertiary phosphine ligands having ligand cone angles less than 170°. The crystal structure for the PPh3 bridge-cleavage product is reported. This complex, [Formula: see text] is monoclinic, space group P21/c with a = 20.469(2), b = 12.702(2), c = 15.401(4) Å, β = 98.46(1)°, V = 3961 Å3Z = 4, R = 0.0284 and Rw = 0.0305. The Pd geometry is distorted square-planar with principal dimensions, Pd—Cl 2.395(1), Pd—P 2.318(1), Pd—C 2.056(3), and Pd—As 2.437(1) Å; As—Pd—Cl 96.5(1), Cl—Pd—P 90.9(1), P—Pd—C 93.7(1), As—Pd—C 78.9(1), As—Pd—P 172.6(1), and Cl—Pd—C 171.7(1)°. The average C—P—C angle (104.3(1)°) is smaller than expected and is rationalized on the basis of steric effects operative within the complex.
Apparent molar volume and adiabatic compressibility studies of aqueous solutions of some drug compounds at 25 °C
Authors: Iqbal Mohammad , Verrall R. E.
Published: May 2006

Apparent molar adiabatic compressibilities and volumes of aqueous solutions of the following drugs (sodium salicylate, methyl orange, L-tryptophan, phenol, and the hydrochloride salts of propranalol, procaine, pilocarpene, and ephedrine) have been calculated from high precision density and sound velocity measurements at 25 °C. These drugs are considered important in several biochemical phenomena such as anomalous protein binding, anesthesia, or unique structure–activity relationship and this study is part of a comprehensive investigation of drug–protein interactions in biological systems. The data are qualitatively interpreted in terms of solute–solvent and solute–solute interactions. Some plausible modes of drug hydration are discussed with particular reference to the structure of the drug molecules. The adiabatic compressibility shows a strong correlation with the hydrational behaviour of the solute molecule and appears to be sensitive to the structural features of the solute, such as shape, size, branching, and presence of aromatic rings. It is argued that these aspects are important in terms of considering the geometrical fit of the solute into the ordered form of the aqueous solvent surrounding these solutes. Keywords: apparent molar adiabatic compressibilities, apparent molar volumes, hydration of drugs.
Novel sesquiterpenoids from the fairy ring fungus, Marasmiusoreades
Authors: Ayer William A. , Craw Peter A. , Stout Thomas J. , Clardy Jon
Published: May 2006

The structures of four new drimane-type sesquiterpenes 1–4 produced in liquid cultures of the fairy ring fungus, Marasmiusoreades, have been determined by a combination of chemical and spectroscopic methods. The structure assigned to one of these sesquiterpenes (1) has been confirmed by an X-ray crystallographic study. An unprecedented feature of all four sesquiterpenes is the biosynthetic elaboration of C8–C12 and C15 of the drimane skeleton to a dioxabicyclooctane moiety. In addition, two of the sesquiterpenes possess an uncommon α orientation of the hydroxyl groups at C3. Keywords: sesquiterpenes, fungal metabolites, drimanes, fairy ring mushroom, crystal structure.
The chemistry of the chromotropic acid method for the analysis of formaldehyde
Authors: Georghiou Paris E. , Ho Chi Keung (Jimmy)
Published: May 2006

Formaldehyde is present in the indoor air of many industrial and non-industrial environments. The procedure for its analysis which is commonly employed in North America is a NIOSH recommended one that uses chromotropic acid and concentrated sulphuric acid. The nature of the purple chromogen that is produced in the analytical procedure has not been fully understood until now. Using 1H and 13C nuclear magnetic resonance spectroscopic techniques and calibration line studies, evidence has been obtained to support the hypothesis that the chromogen has a mono-cationic dibenzoxanthylium structure and not a para,para-quinoidal one that is commonly cited. Keywords: Formaldehyde, formaldehyde analysis, chromotropic acid, chromogen, dibenzoxanthylium cation.
Structure of the capsular polysaccharide antigen of type IV group B Streptococcus
Authors: Fabio Jose L. Di , Michon Francis , Brisson Jean-Robert , Jennings Harold J. , Wessels Michael R. , Benedí Vicente-Javier , Kasper Dennis L.
Published: May 2006

The native polysaccharide antigen isolated from type IV group B Streptococcus contains D-galactose, D-glucose, 2-acetamido-2-deoxy-D-glucose, and sialic acid in the molar ratio 2:2:1:1 and is composed of the following repeating unit:[Formula: see text]The structural analysis of this antigen was based on results obtained from methylation analysis, partial hydrolysis, nitrous acid deamination, and nuclear magnetic resonance spectroscopic techniques. Keywords: capsular polysaccharide, high resolution NMR spectroscopy, group B Streptococcus, polysaccharide structure, polysaccharide degradation.
Conformational properties of benzyl alcohol in dilute solution
Authors: Schaefer Ted , Sebastian Rudy , Peeling James , Penner Glenn H. , Koh Kevin
Published: May 2006

STO-3G MO computations, with substantial geometry optimization, are reported for benzyl alcohol. They are performed at 15° intervals for the angle by which the C—O bond twists out of the plane of the phenyl group, as well as for 30° intervals for the HCOH dihedral angle. The ensuing conformational energies are used to estimate expectation values of these two angles as well as of functions of these angles. The latter are compared with those obtained from long-range proton–proton coupling constants for dilute solutions in CS2 and acetone-d6. At infinite dilution in CS2 these coupling constants give a distribution for motion about the [Formula: see text] bond that happens to agree with the theoretical results for the free molecule. The latter are used to derive the dihedral angle dependence of 3J(HCOH), the coupling constant between the hydroxyl and methylene protons. This dependence is compared with the observed coupling constants in benzyl alcohol in the gas phase at 422 K and with those measured for the 2,6-difluoro and 2,6-dichloro derivatives of benzyl alcohol. Infinite dilution shifts are reported for the latter two compounds and for α,α-dimethylbenzyl alcohol. The first self-association complex of any significance for benzyl alcohol in CS2 at 300 K is most likely a trimer. Between zero and 2.5 mol% in CS2, all five proton chemical shifts of this molecule are linearly proportional to one another, four of them becoming less shielded at decreasing concentrations. Some nuclear magnetic resonance data are also reported for 3,5-dichlorobenzyl alcohol. Keywords: conformations of benzyl alcohol, 1H NMR of benzyl alcohol, STO-3G MO calculations of benzyl alcohol.
Drop calorimetry of the compounds NbCl5, TaCl5, RbNbCl6, CsNbCl6, RbTaCl6, and CsTaCl6
Authors: Shawki E. E. , Flengas S. N. , Sadoway D. R.
Published: May 2006

Enthalpy contents for NbCl5, TaCl5, RbNbCl6, CsNbCl6, RbTaCl6, and CsTaCl6 were measured as functions of temperature using a high temperature aluminum block drop calorimeter. It was found that the solid compounds RbNbCl6, CsNbCl6, RbTaCl6, and CsTaCl6 undergo allotropie solid–solid transformations and the enthalpies and entropies associated with these phase changes, as well as from fusion, have been evaluated.Molar heat capacities for the systems investigated are reported as linear functions of temperature.The molar heat capacities for solid and molten NbCl5 or TaCl5 were used together with available vapour pressure data to express enthalpies and free energies of vaporization for these compounds as functions of temperature through the third law calculation method. Keywords: calorimetry, heat capacities, transition enthalpies, niobium compounds, tantalum compounds.
Chemoselective reductions with sodium borohydride
Authors: Ward Dale E. , Rhee Chung K.
Published: May 2006

Under the appropriate reaction conditions, sodium borohydride is a highly chemoselective reducing agent. The order of reactivity among carbonyl groups is conjugated enones < ketones < conjugated aldehydes < aldehydes. In general, a carbonyl group of one type can be selectively reduced in the presence of a carbonyl group of a less reactive type. Reactions are conducted with excess sodium borohydride at −78 °C in solvent mixtures of methanol or ethanol in dichloromethane. Keywords: sodium borohydride, chemoselective reductions.
Étude structurale de la liaison thioamide: Synthèse et conformation de dérivés de la thioalanine et de la thioglycine
Authors: Michel Andre G. , Ameziane-Hassani Chakib , Boulay Gaston , Lajoie Gilles
Published: May 2006

The present study reports the synthesis, crystal structure determinations, and the conformational analysis of N-tertiobutyloxycarbonyl N′-methylthioalanine (Boc-AlaS-NHCH3, C9H18N2O2S) and of N-tertiobutyloxycarbonyl N′-methylthioglycine (Boc-GlyS-NHCH3, C8H16N2O2S). The particular feature of these compounds is the replacement of the classical oxopeptide linkage by a thioamide bond. Crystals of Boc-AlaS-NHCH3 are tetragonal, space group P43212. Those of Boc-GlyS-NHCH3 are monoclinic, space group P21/c. Both structures were solved by direct methods and refined by full-matrix least-squares methods to Rw = 0.045 and 0.035 for 827 and 1335 reflections respectively, with intensities greater than 2.5σ(I). The conformations of both compounds correspond to conformational energy minima, calculated for classical amino acids. The C=S bond lengths of 1.665(9) and 1.650(3) Å constitute the major difference compared to oxopeptides; the crystal structures reveal that the presence of the sulfur atom does not change the electronic properties of the peptide bond. Using a classical method for the study of peptides (ECEPP/2), conformational energy maps were computed for derivatives of dithioalanine and dithioglycine and are compared to the oxo residues. We conclude that the synthesis and conformational analysis of thionated amino acids allow us to introduce the thioamide linkage into more complex peptide structures and to predict the conformational behaviour. Keywords: molecular conformations, peptidic structure, crystallography.
Adsorption of dicarboxylic ions on polycrystalline gold electrode from solutions at constant cationic strength
Authors: Dutkiewicz Edward , Skoluda Piotr
Published: May 2006

Studies on the specific adsorption of succinate and maleate anions at polycrystalline gold electrode from solutions at constant cationic strength are reported. The adsorption has been studied by differential capacity – potential measurements. The adsorption can be described by one-constant virial isotherm. The basic parameters of this isotherm are determined. Our experimental results are partly interpreted with the help of the results of the adsorption of dicarboxylic anions on the (210) and (311) faces of gold. Keywords: electrochemistry, adsorption, double layer, gold electrode, dicarboxylic ions.
New investigations of the reaction of epichlorohydrin with hindered amines: X-ray and NMR analyses
Authors: Laguerre Michel , Boyer Chantal , Leger Jean-Michel , Carpy Alain
Published: May 2006

During the synthesis of a series of new antiarhythmic drugs, the reaction between epichlorohydrin and hindered amines was reinvestigated. With primary amines, 3-azetidinols were formed in fair yields. The X-ray structure elucidation of one of these compounds (1: P21/n, a = 17.210, b = 6.665, c = 23.585, β = 102.29, and R = 0.064 for 2005 observed reflections) and NMR studies led to the conclusion that both the hydroxy group and the bulky nitrogen substituent are on the same side of the azetidine ring. With secondary amines, the reaction was very complex, leading to diamino dioxane, diamino alcohols, amino alcohol ethers, and one glycol. X-ray analysis and 1H and 13C NMR studies allowed complete assignments for all compounds. All but one exhibited a slow interconversion of the nitrogen visible in the 13C NMR spectra. Keywords: epichlorohydrin, hindered amines, azetidinol, adamantane, alkanol amines.
The preparation of thiazyltrifluoromethane, CF3SN, the hexachloro-3-cyclopentenylidenaminosulfides C5Cl6NSX (X = CF3, Cl, NSO, SCF3, C5Cl6N) and the crystal structures of C5Cl6NSX (X = CF3, Cl)
Authors: L Alois Haas , Mischo Thomas
Published: May 2006

CF3S(Cl) = NSi(CH3)3 is found to undergo intermolecular decomposition to give oligomeric, possibly cyclic (CF3SN)x rather than monomeric CF3SN, formed by intramolecular decomposition. Monomeric thiazyltrifluoromethane, CF3SN, is formed in the thermal decomposition of CF3SN3 and may be trapped before polymerizing by formation of a 1:1 adduct with hexachlorocyclopentadiene. The structure of this adduct is determined by single crystal X-ray diffraction. From the molecular structure it is evident that a (4 + 2) cycloaddition has not occurred but rather the formation of a thioketimine (I). The same material is formed when a suspension of CF3SNH2 and lead(IV) acetate in CH2Cl2 is refluxed and the intermediate CF3SN is again trapped by C5Cl6.By comparison ClSN and C5Cl6 interact in an analogous manner to give C5Cl6NSCl (II), whose structure was determined by single crystal X-ray diffraction. FSN, on the other hand, when reacted with CsCl6 yields the corresponding fluoro derivative only as an intermediate. At 20 °C using SO2 as solvent C5Cl6NSCl (II) is formed. At 75 °C in SO2 the new compound C5Cl6N—S—NSO forms. The latter compound may also be formed from (II) and Hg(NSO)2. In the reaction of FSN with CsF6 only decomposition products were observed.Some of the reactions of C5Cl6NSCl (II) were investigated. With Hg(SCF3)2 C5Cl6N—SSCF3 is obtained; with SbCl5 in SO2 C5Cl6N—S—NC5Cl6 forms as the main product. Keywords: synthesis of CF3SN, preparation of C5Cl6NSX (X = CF3, Cl, NSO, SCF3, C5Cl6N), crystal structures of C5Cl6NSX (X = CF3, Cl), chemical properties of C5Cl6NSCl, nmr-, ir-, ms-data.
Photodissolution of iron oxides II: The lack of efficiency of thiocyanate
Authors: Litter Marta I. , Blesa Miguel A.
Published: May 2006

Thiocyanate does not enhance, and even inhibits, the photochemical dissolution of iron oxides at 254 nm. At low SCN− concentrations, surface complexes [Formula: see text] are formed only to a small extent due to the low affinity for the surface by the monovalent anion, and electron-hole pairs essentially recombine. At higher ligand concentrations, only the homogeneous photolysis of SCN− is observed. At 450 nm, there is no appreciable light absorption by the oxide but homogeneous photolysis of FeIII—SCN− complexes produces FeII, which dissolves the oxide by a reductive mechanism. Keywords: iron oxides photodissolution, thiocyanate heterogeneous photochemistry, metal oxides surface complexation.
Raman spectroscopy and electrochemistry of water-soluble porphyrins at a silver electrode
Authors: Orita Hideo , Shimizu Masao , Nishihara Chizuko , Hayakawa Takashi , Takehira Katsuomi
Published: May 2006

Cyclic voltammetry and laser Raman spectroscopy were applied to investigate insitu the properties and structures of metal meso-tetrakis (p-sulfonatophenyl)porphyrins (MeTPPS, Me = Co, Fe, Mn, Cu, Ni) adsorbed on a silver electrode in 0.05 M H2SO4 aqueous solution. The cyclic voltammograms for O2 reduction were investigated by changing the metal cations in the porphyrin ring. Among the porphyrins examined, only CoTPPS shows catalytic behavior for O2 electrochemical reduction. The Raman spectra of MeTPPS in aqueous solution are very similar to those of metal tetraphenylporphyrins (MeTPP), and the band assignments were carried out on the basis of the results for [FeTPP]2O. The Raman spectra of MeTPPS on a silver electrode were measured under a N2 or O2 atmosphere, changing the applied potentials and the metal cations in the porphyrin ring. Their spectra are very sensitive to metal cations, but not so to applied potential. Keywords: porphyrin, SERS, electrochemistry, dioxygen, silver electrode.
The mechanism of the infrared laser induced multiphoton decomposition of ethyl alcohol
Authors: Goodale J. W. , Evans D. K. , Ivanco M. , McAlpine Robert D.
Published: May 2006

The infrared laser induced multiphoton decomposition of ethyl alcohol has been studied in experiments with iodine as a radical scavenger and the deuterium substituted form CH3CD2OH of the alcohol. These experiments show that the principal product, C2H4, arises from a concerted molecular elimination, as do most of the CH4 and CH3CHO. The C2H6 product results from recombination of the nascent methyl radicals formed in the minor bond breaking channel. Keywords: ethyl alcohol, multiphoton decomposition.
1H NMR studies of proton transfer in Schiff base and carboxylic acid systems
Authors: Le-Thanh Hoa , Vocelle D.
Published: May 2006

When the unconjugated Schiff base, isobutylidene isopropylamine (2), reacts with acids such as the mineral acids HCl, HBr, and HI, or the carboxylic acids trichloroacetic, monochloroacetic, and propionic acid, in CDCl3, several complexes are formed. In this study, 1H NMR allows us to differentiate between several possibilities. With the mineral acids, only 1:1 structures are seen and the chemical shift of the [Formula: see text] proton varies according to the strength of the hydrogen bond between the protonated Schiff base and its counterion. With the carboxylic acids, 1:1 and 1:2 structures are present in rapid equilibrium. In the presence of an excess of acid, protonation of the Schiff base increases considerably. Extrapolation of the results is given in terms of the molecular mechanism of vision. Keywords: protonation, proton nuclear magnetic resonance, proton transfer, rhodopsins, Schiff base.
Synthesis, characterization, and dynamic behaviour of pseudoferrocene complexes
Authors: Clerk Michael D. , Zaworotko Michael J. , Borecka Bozena , Cameron T. Stanley , Hooper Donald L. , Linden Anthony
Published: May 2006

Carbanion sources (organolithium and Grignard reagents) react with [(mesitylene)2Fe]2+ and [(mesitylene)(Ch)Fe]+ cations (Ch = cyclohexadienyl) to afford neutral Ch2Fe and ChCh′Fe ("pseudoferrocene") complexes, respectively. Addition of R− (R− = methyl, phenyl, benzyl, and t-butyl) occurs stereospecifically via exo addition at an unsubstituted carbon atom of the arene ring. The synthesis and characterization of these complexes is detailed and the structure of bis(η5-exo-6-phenyl-1,3,5-trimethylcyclohexadienyl)iron(II), 3b, has been determined by single crystal X-ray diffraction analysis. 3b crystallizes in the triclinic space group [Formula: see text], with a = 11.947(2), b = 12.784(1), c = 8.865(2), α = 108.59(1)°, β = 108.19(1)°, γ = 84.91(1)°, and Dcalcd 1.227 g cm−3 for Z = 2. Least-squares refinement gave a conventional R value of 0.0452 for 2080 independent observed reflections. The structure reveals that the complex adopts a gauche-eclipsed conformation. The variable temperature (vt) 1H NMR spectra of pseudoferrocenes are interpreted in terms of restricted ring rotation around the ligand—metal—ligand axis. Keywords: arene, cyclohexadienyl, pseudoferrocene, variable temperature NMR, X-ray diffraction.
A study of the Ca2+ – Arsenazo III system and its application to the spectrophotometric determination of free calcium in solution
Authors: Kratochvil Byron , He Xi-Wen
Published: May 2006

A two-wavelength method was applied to the determination of free, ionic calcium with Arsenazo III in solutions containing calcium-binding ligands. By this procedure impurities in the indicator can be corrected for, thereby allowing the use of commercial indicator preparations with purities as low as 80%. Only a 1:1 complex with a conditional log stability constant of 4.28 ± 0.13 at pH 4.6 and ionic strength 0.1 was found under the conditions studied. Key words: free metal ion determination, calcium ion speciation, spectrophotometry, arsenazo III, ion increment method for speciation.
Synthesis and properties of DNA oligomers containing an O4-ethylated thymine. d(e4TpA), d(Ape4T), d(Ape4TpA), and d(Tpe4TpT)
Authors: Buchko Garry W. , Hruska Frank E. , Sadana Krishan L.
Published: May 2006

Syntheses via the phosphotriester method are described for DNA oligonucleotides containing O4-ethylated thymine (e4Thy) and adenine (Ade) bases, d(e4TpA), d(Ape4T), and d(Ape4TpA), for their unmodified analogs, d(TpA), d(ApT), and d(ApTpA), and for d(Tpe4TpT). 1H NMR chemical shift data obtained from 10 to 70 °C at 300 and 500 MHz show that these molecules form right-handed minihelices at low temperature in aqueous solution; the presence of the O4-ethyl group does not seem to have a drastic effect on the stacking geometry of the thymine base. The presence of right-handed stacking is confirmed by circular dichroic data obtained over a similar temperature range. Coupling constants for the sugar ring indicate that the e4Tp unit of d(e4TpA) does not show the shift towards the 3′-endo (N) pucker noted for the corresponding unit of d(e4TpT). A prominent quasimolecular ion peak [M – H]− is observed for the modified and unmodified molecules in the spectra obtained by Fast Atom Bombardment mass spectrometry (FAB-MS) operating in the negative ion mode, indicating that the labile O4-ethyl group is sufficiently stable to withstand the ionization process used in this method. In addition, a number of fragment ion peaks such as [O4-ethylthymine – H]− are observed that reveal the potential of FAB-MS for characterizing DNA oligomers modified by alkylating agents. Keywords: oligonucleotides, O-alkylation, stereochemistry, NMR, mass spectrometry.
Étude structurale d'une imidazolylcétone l'isopropyl-1-phényl-2-diaza-1,3-one-6-bicyclo[3,3,0]octène-2
Authors: Reboul Jean-Pierre , Brouant Pierre , Barbe Jacques , Caranoni Claude , Soula Catherine , Marsura Alain , Thomasson François , Luu-Du Cuong , PEPE GÉRARD
Published: May 2006

The solid state configuration of 1-isopropyl-2-phenyl-1,3-diaza-6-one-bicyclo[3.3.0]oct-2-ene was determined from X-ray diffraction data (a = 10.039(3) Å; b = 8.683(3) Å; c = 15.505(6) Å; β = 90.72(9)°; P21/c;R = 0.047). This structure is compared to those of three other related (Δ-2)-imidazolines. Molecular structure in solution was investigated by 1H nuclear magnetic resonance spectroscopy. Results clearly indicate that the title compound has the same structure in the solid state as in solution. On the contrary, this configuration differs appreciably from those of other (Δ-2)-imidazolines considered in this report. Keywords: (Δ-2)-imidazolines, crystal structure, NMR, conformations.
The synthesis and X-ray structures of η1-, η3-, and η5-dienolate complexes of manganese carbonyl
Authors: Masters Andrew P. , Richardson John F. , Sorensen Ted S.
Published: May 2006

The η1-, η3-, and η5-ester dienolate complexes of Mn(CO)n have been prepared and the crystal structures of the phenyl ester member have been obtained for all three bonding modes. The allyl (anti- η3) member shows some bonding distortions compared to a simple allyl ligand. The structure of the η5-1-oxapentadienyl ligand complex is substantially different from a rhenium analog in that the organic ligand is non-planar with significantly different bond distances. The η5-complex reacts rapidly with triphenylphosphine to generate the syn-η3-complex. Keywords:
A new palladium-catalyzed coupling reaction of vinylic and allylic triflates with pyrimidine nucleosides
Authors: Hassan Mohamed Ezeldin
Published: May 2006

Vinylic trifluoromethane sulfonates (triflates) form palladium intermediates that react with pyrimidine nucleosides to produce C-5 alkyl substituted nucleosides after hydrogenation of the vinylic coupling products with hydrogen and Pd/C in methanol. The reaction, which is run under mild conditions, appears to be a general one since both mono- and disubstituted vinylic triflates, as well as the cyclic ones, gave high yields of the corresponding C-5 alkyl-substituted nucleoside. The reaction with allyl triflate requires the use of the 5-chloromercurinucleoside. Key words: nucleosides, triflates, palladium catalysis
Obtention de cyclopropanes gem-dicarboxylate par cyclocondensation de carbanions bromo- et chloromalonate sur des accepteurs de Michaël
Authors: Menn Jean-Christophe Le , Tallec André , Sarrazin Jean
Published: May 2006

Cyclopropane gem-dicarboxylates are prepared through cyclocondensation of chloro or bromomalonate carbanion and electrophilic alkenes. The carbanion can be formed in THF by deprotonation of the monohalomalonates with powdered potassium carbonate; the chloromalonate always leads to better yields. The influence of the nature and the number of substituents on the alkene reactivity has been investigated. If side reactions (for instance protonation of anionic intermediates by halomalonate) can take place, better yields are obtained when the carbanion is generated by electroreduction of dihalomalonate in the presence of the Michael acceptor. Key words: halomalonates, cyclopropane gem-dicarboxylates, electroreduction.
Chromodorane diterpenes from the tropical dorid nudibranch Chromodoris cavae
Authors: Morris Sandra A. , Silva E. Dilip de , Andersen Raymond J.
Published: May 2006

Two novel diterpenoids, chromodorolides A (1) and B (2), have been isolated from skin extracts of the tropical dorid nudibranch Chromodoris cavae. Complete NMR assignments are presented for chromodorolide A (1). The structure of chromodorolide B (2) has been solved by spectroscopic analysis. Chromodorolides A (1) and B (2) are the first examples of diterpenoids with the highly rearranged chromodorane carbon skeleton. Key words: nudibranch, diterpene, chromodorane, Chromodoris.
Thermodynamic treatment of multicomponent fused salt solutions: common anion reactive systems approximation
Authors: Flengas S. N.
Published: May 2006

From a modified F.F.G. (1) thermodynamic cycle, equations have been developed from which the molar and partial molar excess thermodynamic properties of j-component molten salt solutions may be predicted from binary data.The theory is applicable to reactive molten salt systems and particularly to charge asymmetric fused salt solutions with common anions.It is shown that for a system having j + 1 components, such as [Formula: see text], any molar or partial molar property of mixing for MXq, may be calculated from available data on corresponding j-binary systems, such as MXq–AX, MXq–BX, and [Formula: see text].The equations are of the general form,[Formula: see text]The latter is applicable only when the multicomponent and the binary solutions have the same MXq content and same temperature.The theoretical expressions have been found to predict quite well available data on ternary and quarternary systems. New composition parameters for expressing the compositions in quarternary and quinary systems have been established. Key words: molten salt solutions, multicomponent systems, ternary systems, quarternary systems, quinary systems.
Molecular orbital and 1H nuclear magnetic resonance studies of the inversion potentials of thianthrene and thioxanthene
Authors: Schaefer Ted , Sebastian Rudy , Beaulieu Christian
Published: May 2006

The inversion potentials, obtained from STO-3G, STO-3G(*), 3-21G, 3-21G(*), and 4-31G basis sets, are reported for thianthrene and thioxanthene, molecules in which both or only one of the methylene groups have been replaced by sulfur in 9,10-dihydroanthracene. Comparison with the available experimental data suggests that the split-valence bases lead to an overestimate, possibly by about 10 kJ/mol, of the inversion barrier in the crystal, whereas the STO-3G and STO-3G* basis sets underestimate this barrier. It appears that the inversion barrier for thianthrene is much lower in solution than in the crystal. The long-range coupling constants between the methylene and ring protons for thioxanthene in solution are consistent with an inversion barrier somewhat smaller than those obtained with the split-valence bases but rather larger than those predicted with the STO-3G basis set. The bond lengths and angles in the equilibrium structures of the two molecules, as computed with the 3-21G(*) basis, agree reasonably well with those in their crystals, except that the theoretical folding angles are smaller than measured. These discrepancies become less marked when expectation values are calculated from the theoretical inversion potentials at finite temperatures. Key words: MO calculations, inversion potentials of thianthrene and thioxanthene; 1H NMR, thioxanthene; spin–spin coupling constants, long range, in thioxanthene.
Forbeside D (C62H101O31SNa), a new saponin from Asterias forbesi. Complete structure by nuclear magnetic resonance (300 MHz) methods
Authors: Findlay John A. , He Zheng-Quan , Sauriol Françoise
Published: May 2006

The structure of forbeside D (C62H101O31SNa), a saponin from the starfish Asterias forbesi, has been deduced entirely by nuclear magnetic resonance methods using a 300 MHz instrument. Based on 2D-COSY, HETCOR, single and multiple RCT, NOESY and HETCORSL experiments, the structure of forebeside D has been shown to be the monosodium salt of 6α-O-{β-D-galactopyranosyl-(1→3)-β-D-fucopyranosyl-(1→2)-β-D-fucopyranosyl-(1→4)-[β-D-quinovopyranosyl-(1→2)]-α-L-arabinopyranosyl-(1→3)-β-D-quinovopyranosyl}-20S-hydroxy-3β-sulfo-oxy-5α-cholesta-9(11),24-diene. This is the first example of a starfish saponin featuring an arabinopyranosyl unit. Key words: marine saponin, Asterias forbesi saponin, forbeside D structure, structure elucidation by NMR.
Photoelectrochemical transduction between ionic and electronic conduction mediated by bis(maleonitriledithiolato)nickelate(II) in a polyviologen film
Authors: Lefebvre Mark C. , Biro David A. , Langford Cooper H.
Published: May 2006

The photoelectrochemistry of a polymer coated electrode exploiting the chromophore bis(maleonitriledithiolato)nickelate(II) to link the redox self-exchange conductor poly(xylylviologen) to the solution couple Fe(CN)63−/4− is described. The diffusion coefficient for the Fe(CN)63−/4− redox couple in the film is 4.2 × 10−9 cm2/s. The system was developed in an attempt to improve on a system where a quaternary ammonium substituted polystyrene/polyvinylpyridine blend films functioned as a good ionic conductor but poor electronic conductor in consequence of the trap states. The expected improvement in electronic conductivity is seen but at the expense of lower ionic mobility. Key words: polyviologen, bis(maleonitriledithiolato)nickelate(II), polymer conduction, photoelectrode.
Effets des sels de terres rares sur quelques phosphates dérivés des metabolites de la glycolyse
Authors: Rabiller Claude , Eymard Philippe , Mesbahi Mahmoud
Published: May 2006

Phosphate derivatives of glucose and fructose are well-known intermediates in sugar fermentation. Proton and deuterium NMR spectroscopy studies of the fermentation process provide information about proton (deuteron) transfers that are likely to occur in such biotransformations. In this paper we describe the influence of complexation of those metabolites with praseodymium trichloride and of pH on their proton NMR chemical shifts. We determine the conditions under which all the protons of both the α. and β forms of these cyclic sugars are separated. Glucose-1 phosphate was found to be the most suitable derivative for this type of study. Furthermore, comparing the behaviour of these compounds with 2-phosphoglyceric acid allows us to specify the exact conformation of the latter. Key words: phosphate sugar derivatives, NMR, LIS effects.
X-ray crystal structure determination and NMR solution studies of 3′,3″5′,5″-tetrabromophenolphthalein ethyl ester
Authors: Chiverton Antony C. , Fortier Suzanne , Bovenkamp John W. , Thoraval D. , Buchanan G. W. , Dawson B. A.
Published: May 2006

The crystal structure of 3′,3″,5′,5″-tetrabromophenolphthalein ethyl ester (TBPE) has been determined by X-ray diffraction at room temperature. This compound crystallizes in space group [Formula: see text], with a = 12.361(4), b = 12.533(4), c = 9.986(2) Å, α = 99.57(3), β = 112.76(2), γ = 100.73(3) °, and Z = 2. The structure was solved by direct methods and refined by full-matrix least-squares calculations to a residual of 0.034 for 1862 observed reflections. TBPE is propeller shaped about the central carbon and crstallizes with one molecule of benzene solvent per asymmetric unit. In 1:1 CD2Cl2:CDCl3 solution complete 1H and 13C NMR signal assignments have been made via 1H1H COSY and 1H13C HETCOR experiments. The quinoidal and phenolic integrities are retained on the NMR timescale. By contrast for an acetone-d6 solution only 14 13C resonances are found and the colour of the solution changed from green to dark blue. These changes in acetone solution are attributed to the presence of a trace of base which induces the loss of the phenolic proton of TBPE and the existence of resonance forms which render the phenolic and quinoidal rings equivalent. Key words: crystal structure, NMR, 3′3″5′5″-tetrabromophenolphthalein ethyl ester.
A facile preparation of 4-aryl-3-[(tert-butyldimethylsilyl)oxymethyl]furans
Authors: Keay Brian A. , Bontront Jean-Louis J.
Published: May 2006

A series of aryl bromides undergo a palladium-catalyzed coupling reaction with 3-[(tert-butyldimethylsilyl)oxymethyl]-4-(tri-n-butylstannyl)furan to provide 4-aryl-3-[(tert-butyldimethylsilyl)oxymethyl]furans in moderate to good yields. Key words: palladium, cross-couplings, stannanes, 3,4-disubstituted furans.
Thermodynamical detection of hydrophobic hydration of tris(2,4-pentanedionato)-cobalt(III) and tris(3,5-heptanedionato)cobalt(III) in water
Authors: Yoshimura Yasuki
Published: May 2006

The solubilities of tris(2,4-pentanedionato)cobalt(III) and tris(3,5-heptanedionato)cobalt(III) in water, heptane, and 1,2-ethanediol were determined over the temperature range 5–50 °C and from these data the thermodynamic quantities of solution at 25 °C were estimated. The free energy [Formula: see text], enthalpy [Formula: see text], and entropy [Formula: see text] of transfer of these chelates from heptane to some solvents were calculated from the corresponding thermodynamic quantities of solution. When [Formula: see text] and [Formula: see text] were separately plotted against [Formula: see text], the data of transfer from heptane to water deviated markedly from a correlation obtained for the data of transfer to the solvents other than water. This finding indicates that these chelates are subject to hydrophobic hydration in their aqueous solutions. The solubility of tris(glycinato)cobalt(III) in water was also determined over the temperature range 5–60 °C and its temperature dependence of the solubility is compared with that for the cobalt(III) chelates of the β-diketones. Key words: tris cobalt(III) chelates of β-diketones and glycine, temperature dependence of solubility, thermodynamic parameters of solution, thermodynamic parameters of transfer, hydrophobic hydration.
Synthesis and electrochemistry of linear and cofacial conjugated binuclear phthalocyanines covalently linked by alkyne and alkene bridges
Authors: Vigh Steven , Lam Herman , Janda Pavel , Lever A. B. P. , Leznoff Clifford C. , Cerny Ronald L.
Published: May 2006

Semihydrogenation of 1,2-bis(3,4-dicyanophenyl)ethyne gave cis-1,2-bis(3,4-dicyanophenyl)ethene. From these precursors, binuclear phthalocyanines were produced containing linear binuclear phthalocyanines having an ethyne bridge and a mixture of cis and trans isomers of binuclear phthalocyanines linked by an ethene bridge. The cis and trans isomers could be separated and differentiated by gel permeation chromatography as the very extended trans isomer elutes faster than the cofacial globular cis isomer. Strong coupling between the conjugated binuclear phthalocyanine was not observed when examined by cyclic voltammetry. Key words: phthalocyanines, cofacial, binuclear, cyclic voltammetry.