bluuebunny/crossref_metadata_2025 huggingface dataset:
Capacité calorifique de polyélectrolytes en solution
Authors: Daoust Hubert , Thanh Hoa Le , Ferland Pierre , St-Cyr Daniel
Published: May 2006

Heat capacities of polyelectrolytes in aqueous solutions have been measured at 25 °C using a Picker-type dynamic micro-calorimeter. The precision of the apparatus is such that it is possible to calculate the apparent molar heat capacity [Formula: see text] of the solute, even at high dilution. The polymers that were studied include polymethacrylic acid (PMAH) and polyacrylic acid (PAH), as well as its sodium salt and the sodium salt of the polystyrènesulfonate. For the salts, the results show that the value of [Formula: see text] decreases rapidly with dilution to low concentrations; this result is in perfect concordance with a relation derived from the theory of Lifson and Katchalsky. A study of the variation of [Formula: see text] at a constant concentration, with the extent of the neutralization of the acids PMAH and PAH has also been conducted. As has been shown previously, the chain of the PMAH undergoes a conformational transformation when the level of neutralization gets close to 25%. This transformation implies an increase of the value of [Formula: see text] [Journal translation]
Quantum mechanical calculation of thermodynamic functions of solvation of ammonium ions in water
Authors: Contreras Renato , Klopman Gilles
Published: May 2006

The application of an extended version of the generalized Born formula including dielectric saturation effects, implemented within the SCF-CNDO/2 approximation, provides a complete set of data concerning the thermodynamics of solvation of some ammonium ions in water. The calculated free energies of solvation are in good agreement with experimental data. An estimation of the entropy and enthalpy of solvation is also given and satisfactory qualitative trends are obtained.
Potentiel d'erreur et seconde quantification: application à la corrélation
Authors: Laforgue Alexandre
Published: May 2006

We reexamine an error potential method. The so-called "perturbative error correction" is analysed on the pattern of the correlation problem. In the whole C.I. space the idea of a perturbation correction of each w.f., cannot lead to a linear operator. But a restriction of this process to the uncorrelated q-configurations only permits proposing a q-operator which is of linear form in the whole C.I. space. Summing over q we obtain a Cler operator adapted to an r-repetitive method which performs the whole C.I. without diagonalization. On the other hand Cler is determined by a rapidly convergent, recurrent process. Its r-times product has as a limit a correlation operator adapted to all the electronic configurations. Now, the second quantization formalism expands under a polynomial form both the cler and the correlation operator. Both these operators can be expanded in clusters. Then the excitation matrix appears to be a sum of an infinite set of r-matrices which can be determined by an ordinary perturbation.
Hydrogen bridges and electron transfer in biomolecules. Study of a possible mechanism on a model charge–recombination system
Authors: Re Giuseppe Del , Peluso Andrea , Minichino Camilla
Published: May 2006

It is suggested that internal electron transfer in biomolecules such as multiheme proteins can take place along H-bridge/peptide-link chains by a proton-relay process followed by radicalic return to the initial condition. The plausibility of such a process is assessed by (a) reference to published suggestions and evidence; (b) estimates of vibrational characteristics and of the free energies of the various states of a model system consisting of three H-bridges and one peptide group. It appears that the proposed mechanism is highly efficient and very fast, at least if the H-bonds formed by a peptide group evolve in a concerted manner.
Nucleoside analogs involving modifications in the carbohydrate ring: nuclear magnetic resonance spectroscopic studies
Authors: Szarek Walter A. , Pinto B. Mario , Iwakawa Masaharu
Published: May 2006

The concomitant use of 1H nmr and 13C nmr spectroscopy as a probe of structure, stereochemistry, and conformation of several nucleoside analogs derived from 1-oxa-4-thiacyclohexane is described. The 1H nmr spectroscopic properties of an acyclic nucleoside analog derived from uridine are also described.
The structure and resonance Raman spectra – structure correlations for N-benzoyl-DL-alanine ethyl dithioester
Authors: Angus R. H. , Carey P. R. , Lee H. , Storer A. C. , Varughese K. I.
Published: May 2006

An X-ray crystallographic analysis of N-benzoyl-DL-alanine ethyl dithioester demonstrates that it takes up a B-type conformation comparable to that found for N-acylglycine dithioesters in the crystalline state. B-conformations are characterized by small N—C—C—S(thiol) torsional angles resulting from an [Formula: see text](thiol) interaction. In the present case the N—C—C—S torsional angle is −23° and the [Formula: see text] distance 2.830 Å. A Raman spectroscopic study of a single crystal and a resonance Raman (RR) spectroscopic study of powdered N-benzoyl-DL-alanine ethyl dithioester establish the RR signature for the compound in its B-form. The RR spectra of the dithioester in CCl4 and H2O/CH3CN, aided by 13C=S substitution, provide information on the RR signatures of B, A, and C5 type conformers in solution. The A and C5 signatures for the N-acylalanine dithioester resemble closely those for an N-acylglycine dithioester but the B signature is perturbed markedly compared to that of a glycine analog.
Étude cinétique de l'ouverture thermique de la liaison C—C d'aziridines et d'époxydes dipôles-1,3 potentiels: I. Méthode d'étude expérimentale
Authors: Derdour Aïcha , Texier Fernand
Published: May 2006

The thermolysis of the 2-cyanoaziridines (1), 2-alkoxycarbonylaziridines (2), 2-arylaziridines (3), and 2,2-dicyano-3-aryloxiranes (4) leads to a rupture of the carbon –carbon bond yielding an azomethine ylide and the ylide of a carbonyl. The reaction of these ylides of azomethine with methyl acetylene dicarboxylate (MADC) leads to the formation of a 3-pyroline, which is transformed, according to the substituants, to a 2-pyrroline or to pyrrole. The addition of the ylides of carbonyl leads to the formation of dihydrofurans. Through the kinetic treatment of the addition of these heterocyclic compounds (1 to 4) to MADC, it is possible to determine the rate constants for the opening of the C—C bond (k1). In the case of the aziridines 1, the rates have been determined by ir while hplc has been used in the other cases. Relative to the heterocyclic compounds, the order of the experimental rate constants (kex) is always equal to one. In the cases of theN-cyclohexyl-2-cyano-3-alkylaziridines and of the N-cyclohexyl-2-carbomethoxy-3-phenylaziridine, kex varies with the concentration of MADC and this implies that the rate constants for the cycloaddition of the ylide of azomethine and its reclosing to give aziridine are similar. In the other cases, kex is independent of the concentration of MADC and this implies that the heterocyclic compounds are slowly transformed into 1,3-dipoles, followed by a rapid cycloaddition, [Formula: see text]. [Journal translation]
Interaction entre le groupe carbonyle et les hétéroatomes des acétates d'éthyle α-substitués
Authors: Guihéneuf Georges , Bénard Christian , Lachkar Abdeljalil , Luçon Maryvonne
Published: May 2006

Infrared spectra of ethyl α-aminoacetates show three bands in the carbonyl region. Solvent effects show that this is not due to Fermi resonance. These spectra can be interpreted using a model that considers the interactions between the nitrogen lone pair and the carbonyl group. The existence of an equilibrium between the conformers caused by the different orientations of the nitrogen lone pair with respect to the carbonyl group can be forecast. This model enables the interpretation of the carbonyl absorptions of the α-substituted (methoxy and ethoxy) ethyl acetates.
ipso Nitration. XXVI. Nitration of 1,2-dimethyl-4-nitrobenzene. Formation and reactions of adducts
Authors: Fischer Alfred , Henderson George N. , Iyer Lokanathan M.
Published: May 2006

Nitration of 1,2-dimethyl-4-nitrobenzene in a mixture of acetic anhydride and trifluoroacetic anhydride gives the diastereoisomers of 4,5-dimethyl-2,4-dinitrocyclohexa-2,5-dienyl acetate (50%), in addition to the 1,2-dimethyldinitrobenzenes. In moderate and more strongly acid conditions the adduct gives 4-nitro-, 3,4-dinitro-, and 3,5-dinitro-o-xylene. In neutral and weakly acid solutions 4-nitro- and 3,5-dinitro-o-xylene are formed by a radical and a sigmatropic pathway. The adduct reacts facilely with nucleophiles by allylic substitution to give a 5,6-dimethyl-6-nitrocyclohexa-2,4-dienyl derivative which, in many instances, undergoes a second allylic substitution to a new 4,5-dimethyl-2,4-dinitrocyclohexa-2,5-dienyl derivative. Depending on the nature of the introduced substituent, these 2,4- and 2,5-dienyl products may eliminate nitrous acid, under the reaction conditions, to give the corresponding aromatic compound. The dienyl acetates undergo acid-catalysed transesterification to the corresponding dienols.
Synthesis and protonation studies of a meso-unsubstituted surfactant porphyrin
Authors: Ringuet Michel , Gagnon Jacques
Published: May 2006

A meso-unsubstituted surfactant porphyrin has been synthesized. Spectroscopic studies show that intermolecular protonation occurs between the side-chain carboxyl groups and the inner nitrogen atoms of the porphyrin, leading to the formation of a porphyrin dication. In 5 × 10−5 M chloroform solution, 27% of the porphyrin molecules are in the dicationic form and they are self-associated with some remaining molecules in the free base form.
Open-chain nitrogen compounds. Part VIII. 1-(2′-Acetylphenyl)-3-alkyltriazenes with reactive substituents in the alkyl group: synthons for five- and six-membered nitrogen heterocycles
Authors: Vaughan Keith , LaFrance Ronald J. , Tang York , Hooper Donald L.
Published: May 2006

The synthesis of a series of 1-(2′-acetylphenyl)-3-alkyltriazenes, with electron-withdrawing α-substituents in the alkyl moiety, is described. The structure of the products from the reaction of 2-acetylbenzenediazonium ion (1) with methylamine has been reinvestigated using high field (360 MHz) nmr spectroscopy. The 2-acetylphenyltriazenes undergo cyclization to five-or six-membered heterocycles, depending on the substituent present in the alkyl group. Attack by N-3 of the triazene on the ortho-acetyl group leads to a 4-methylenetriazine, which can undergo oxidation to a benzotriazinone or acid-catalysed rearrangement to a 4-alkylaminocinnoline. The reaction of 1 with alkylamines also affords red products, which have been identified as 4-arylazomethylenetriazines and fully characterized by 1H and 13C nmr spectroscopy. The formation of these azo compounds is consistent with the enamine character of the 4-methylenetriazines. The introduction of a CO2Et group at the α-position of the alkyl moiety provides a competing cyclization pathway to give a five-membered heterocycle, a 5-hydroxytriazole. In this case the cyclization pathway to the six-membered heterocycle is favoured, whereas the introduction of a CN group can lead to exclusive triazole formation. The reaction of the nitrile (2c) with alumina in chloroform leads to a new heterocycle, a quinazolinotriazole; in DMSO solution, 2c prefers the alternative pathway and gives only the 3-cyanomethyl-4-methylenetriazine. Introduction of a benzoyl group in the alkyl moiety gives an unstable triazene; it reacts with alumina in chloroform to give a triazole, which in turn rearranges to a novel diazoalkane.
Proton magnetic resonance spectrum of cellulose triacetate. Temperature effects, the rotational conformation of the 6-acetoxymethyl group, and computer modelling of methyl 4,6-di-O-acetyl-β-glucopyranoside
Authors: Rao Vanga S. , Sauriol Françoise , Perlin Arthur S. , Viet M. T. Phan
Published: May 2006

Pronounced narrowing of the resonance signals in the 400-MHz 1H nmr spectrum of cellulose triacetate in CDCl3 between 25 and 50 °C, as well as shielding and deshielding changes in 1H chemical shifts, suggest that thermal disruption of intermolecular aggregates is accompanied by a conformational modification. Analysis of the spin–spin coupling patterns clearly evident at 50 °C indicates that although there is a reversal in the chemical shifts of H-6R and H-6S relative to those for acetylated D-glucopyranose derivatives, the rotational conformations of the exocyclic 6-acetoxymethyl groups of the polymer and the model compounds all favor RHS and RCS rotameric forms. Supporting evidence for this conclusion is obtained from the 2-dimensional 1H spectrum of a trisaccharide, O-β-D-glucopyranosyl-(1 → 3)-O-β-D-glucopyranosyl-(1→ 4)-O-β-D-glucopyranose undecaacetate. Computer-generated models of methyl 4,6-di-O-acetyl-β-glucopyranoside are examined in relation to the stereochemistry of 6-acetoxymethyl groups.
Synthesis and characterization of Pt(II) complexes containing the bidentate pyrazolylgallate ligand, [Me2Gapz2]−
Authors: Nussbaum Sophia , Storr Alan
Published: May 2006

The complexes [Me2Mpz2]Pt(Me)L (where L = CO, Ph3P, or PhC≡CPh for M = Ga; L = PhC≡CPh for M = B) have been synthesized by the direct reaction of the ligands Na[Me2Mpz2] with Pt(Me)Cl(COD) followed by the addition of ligand L. The complexes have been shown to be fluxional in solution by variable temperature 1H nmr methods.
Use of peak height for quantification in solvent extraction/flow injection analysis
Authors: Sweileh Jamal A. , Cantwell Frederick F.
Published: May 2006

The following equation is derived which expresses peak height (P.H.) in terms of sample concentration injected (CSAMP), detector sensitivity (SF), dilution due to band dispersion (Rv), fraction extracted [Formula: see text], flow rates of carrier (FCarrier), organic phase (Fo), and compensating solvent (Fc), and flow rate of organic extract through the membrane phase separator (FM):[Formula: see text]Each term in the equation is evaluated experimentally to validate the equation.
Metal ion biomolecule interactions. PART VII. Synthesis, spectroscopic properties and crystal structure investigations of some bis(theophyllinato)tetraaquocadmium(II) compounds
Authors: Buncel Erwin , Kumar Rajesh , Norris Albert R. , Beauchamp André L.
Published: May 2006

The synthesis, 1H and 13C nmr and infrared spectra of the complex bis(theophyllinato)tetraaquocadmium(II), [CdC14H22N4O5], and its N,N-dimethylformamide solvates are reported. The crystals containing two molecules of N,N-dimethylformamide per complex belong to the monoclinic space group P21/c with a = 10.891(3), b = 13.696(7), c = 10.110(2) Å, β = 109.03(4)°, ρc = 1.595 g cm−3, and Z = 2. Least-squares refinement of the structure has led to a final value of the conventional R factor of 0.044 for 261 variables, using 1299 nonzero MoKα reflections. The complex consists of monomeric [CdTh2(H2O)4] units. The cadmium lies on an inversion center. The geometry of the atoms bonded to cadmium is roughly octahedral with the two theophyllinate ions trans to each other and coordinated through N(7). The Cd—N(7), Cd—O(13), and Cd—O(14) distances of 2.300(5) Å, 2.346(5) Å, and 2.336(5) Å, respectively, are normal. The hydrogen bonding in the crystals is complex, with all the ligand donor and acceptor atoms participating. N,N-Dimethylformamide is not coordinated to cadmium, but participates in the hydrogen bonding system. The nmr spectra are consistent with theophyllinate ion being similarly bonded to cadmium in dimethyl sulfoxide solution.
The indirect determination of trace reactive silicon in waters by neutron activation analysis
Authors: Oltmann Phillip , Ryan Douglas E.
Published: May 2006

Reactive silicon in waters was determined indirectly by counting 101Tc after neutron activation of molybdosilicic acid (MSA) extracted with methyl isobutyl ketone (MIBK). The detection limit (defined as twice the standard deviation for the blank) was 0.2 ng mL−1. The relative standard deviation was 3% at the 10 ng mL−1 level and 30% at the 1 ng mL−1 level. Phosphate, arsenate, and ferric ions did not interfere by themselves. Interference from the reduction of molybdophosphoric acid by tannic acid was eliminated through the use of dichromate. The salt effect normally encountered in the analysis of sea water was examined with particular reference to silicate content of reagent salts used in preparing synthetic sea water.
1H nuclear magnetic resonance spectral parameters of toluene. Implications for conformational analysis and isotope shifts
Authors: Schaefer Ted , Sebastian Rudy , Penner Glenn H.
Published: May 2006

A precise analysis of the 1H nmr spectrum of toluene as a dilute solution in carbon disulfide yields a revised set of spectral parameters. The chemical shift of the para proton lies 12.6 ppb to low frequency of that of the ortho protons at 300 K. The ring proton chemical shifts are discussed and compared with 1H and 3H shifts observed in carbon tetrachloride. The long-range couplings between methyl and ring protons can be said to be quantitatively understood in terms of σ and σ–π electron transmitted mechanisms. The changes observed in these three couplings in phenylacetaldehyde can be quantitatively reproduced in terms of these mechanisms and also illustrate how these changes are direct measures of the conformational preferences in this molecule.
Correlations between carbon-13 nuclear magnetic resonance and X-ray crystallography for pyrroles containing electron-withdrawing groups
Authors: Paine III J. B. , Dolphin D. , Trotter J. , Greenhough T. J.
Published: May 2006

The 13C nmr chemical shifts of benzyl 4-ethyl-3,5-dimethylpyrrole-2-carboxylate (1), methyl E-3-(3-ethyl-4,5-dimethylpyrrol-2-yl)-2-cyanopropenoate (2), and methyl E-3-(5-(benzyloxy)carbonyl-3-ethyl-4-methylpyrrol-2-yl)-2-cyanopropenoate (3) have been compared to their X-ray crystallographic structures. The 13C nmr chemical shifts were determined by direct comparison of a series of closely related homologs.Crystal data for 1, 2, and 3 are as follows: 1, monoclinic, P21/c, a = 14.934(2), b = 6.674(2), c = 15.269(2) Å, β = 101.96(1)°, Z = 4; 2, monoclinic, P21/n, a = 7.3030(3), b = 13.478(1), c = 12.985(1) Å, β = 97.48(1)°, Z = 4; 3, monoclinic, P21/c, a = 11.157(2), b = 13.109(2), c = 14.068(1) Å, β = 115.47(1)°, Z = 4. The structures were determined with diffractometer data by direct methods, and refined by full-matrix least-squares techniques to R = 0.052, 0.040, 0.038 for 862, 2032, 1483 reflexions, respectively. The molecules are approximately planar, except for deviations of the phenyl rings and of the terminal carbon atoms of the C3-ethyl groups from the molecular planes. The bond lengths in the pyrrole rings differ from those in pyrrole itself, as a result of the presence of the strongly electron-withdrawing sustituents; exocyclic bond lengths also exhibit differences from normal values.The competitive effects of the various electron withdrawing groups have been correlated to change in bond lengths, 13C chemical shifts, and the chemical reactions of the pyrrolic nucleus.
Monolayers of mono- and dioxyethylene dodecyl ethers at air–water and alkane–water interfaces
Authors: Aveyard Robert , Carr Neil , Slezok Helen
Published: May 2006

Surface pressure–area isotherms have been obtained for mono- and dioxyethylene dodecyl ethers both spread at the air–water surface and adsorbed at the heptane–water interface and results are compared with those for straight chain alkanols. At the heptane–water interface it appears that the ethyleneoxy groups are disposed parallel to the interface in fairly dilute films. The dual hydrophilic/hydrophobic nature of isolated CH2CH2O groups is demonstrated by showing that they give a negative contribution to the standard free energy of adsorption from both water and heptane to the heptane–water interface.
Structural determination of the group K capsular polysaccharide of Neisseriameningitidis: a 2D-NMR analysis
Authors: Michon Francis , Brisson Jean Robert , Roy René , Jennings Harold J. , Ashton Fraser E.
Published: May 2006

The capsular polysaccharide antigen of Neisseriameningitidis group K was isolated by Cetavlon precipitation and purified by ion-exchange chromatography. The structure of the K polysaccharide was determined to a large extent by comprehensive proton and carbon-13 nuclear magnetic resonance (nmr) studies. In these studies one-dimensional and two-dimensional experiments were carried out directly on the K polysaccharide. The K polysaccharide is composed of the following repeating unit: -4)β-D-ManpNAcA(1→3) [4-OAc]β-D-ManpNAcA(1→. Except for the one-bond couplings between their anomeric carbons and protons [Formula: see text], all the nmr spectroscopic evidence was consistent with both 2-acetamido-2-deoxy-D-mannopyranosyluronic acid residues adopting the 4C1 (D) conformation and having the β-D-configuration. This ambiguity in [Formula: see text] is probably due to through-space electronic effects generated by the presence of contiguous carboxylated sugar residues in the K polysaccharide. The O-acetyl substituents of the K polysaccharide are essential for its antigenicity to group K polysaccharide-specific antibodies.
Heterogeneous reaction of nitric acid with methyl sulfide
Authors: Singleton Donald L. , Irwin Robert S. , McKenney Donald J. , Paraskevopoulos George
Published: May 2006

The reaction of gaseous mixtures of CH3SCH3 and HNO3, studied by infrared spectroscopy and gas chromatography, was found to occur heterogeneously. The products detected were NO2 and CH3S(O)CH3, and approximately one NO2 was formed for each CH3SCH3 consumed. The initial rate of formation of NO2 was first order in CH3SCH3 and zero order in HNO3. The mechanism is discussed, and an upper limit is determined for the rate constant of the homogeneous reaction.
Radiolabelling with organomercury compounds. Part 1. The synthesis and structure of 6-halocholest-5-en-3β-ols
Authors: Flanagan Richard J. , Charleson F. Peter , Synnes Eyvind I. , Wiebe Leonard I. , Theriault Yvon X. , Nakashima Thomas T.
Published: May 2006

The synthetic route to 6-chloromercuricholest-5-en-3β-ol has been studied and the yield improved from 2% to 20%. The reaction of 6-chloromercuricholest-5-en-β-ol with fluorine, chlorine, bromine, and iodine has been examined. 6-Bromocholest-5-en-3β-ol and 6-chlorocholest-5-en-3β-ol have been directly prepared from 6-iodocholest-5-en-3β-ol using cuprous halides. A new intermediate, 6-tosyloxycholest-5-en-3β-ol, has also been synthesized by the action of silver tosylate on 6-iodocholest-5-en-3β-ol. The structures of the compounds were proven by a combination of high-resolution 1Hmr and 13Cmr. A new crystal modification of 6-chloromercuricholest-5-en-3β-ol has been found.
Influence of surface micellar head group neutralized in hexadecyltrimethylammonium bromide (CTAB) and hydroxide (CTAOH) on the dehydrochlorination of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (DDD)
Authors: Otero C. , Rodenas E.
Published: May 2006

The basic dehydrohalogenation reactions of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (DDD) have been studied in cationic micelles of hexadecyltrimethylammonium bromide (CTAB) and hexadecyltrimethylammonium hydroxide (CTAOH). Different theoretical approaches are discussed, considering the fraction of micellar head group neutralized, β, as a constant, or supposing different kinds of variation in its value. The following two empirical expressions for β have been found: β = 0.8 + 0.5 ([NaOH] + [KBr]) for DDT, and β = 0.8 + 1 ([NaOH] + [KBr]) for DDD, and this is the best way to explain the experimental results.
Backbone-modified oligopeptidic bioregulators. The synthesis and configuration of thioamide, amidoxime, cyanoamidine, and amidrazone analogs of the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (f-Met-Leu-Phe-OR)
Authors: Sauvé Gilles , Rao Vanga S. , Lajoie Gilles , Belleau Bernard
Published: May 2006

Reaction conditions for the synthesis of thioamide, amidoxime, and N-substituted amidine analogs of the peptide bond are described. Several new amidine analogs of the chemotactic peptide f-Met-Leu-Phe-OR were synthesized using the thioamides as precursors. The assignment of the E/Z configuration was accomplished by nuclear magnetic resonance. The biological activity of these analogs is briefly described.
Chemistry of 4-trimethylsilyl-3-dialkylaminocrotonate esters and the cycloaromatization reactions with enamines
Authors: Kang G. J. , Chan T. H.
Published: May 2006

Methyl 4-trimethylsilyl-3-dialkylaminocrotonate is synthesized by the silylation of methyl 3-dialkylaminocrotonate. It reacts with carbonyl electrophiles at its γ-position. The unusual regiochemistry of this reaction is studied and rationalized. It reacts with enamines derived from acyclic ketones to give aromatic compounds in a 3C + 3C combination and with enamines derived from cycloketones of 5- to 8-membered rings to give aromatic compounds in a 4C + 2C combination. A mechanism for this cycloaromatization reaction is proposed.
Equilibrium and spectroscopic studies of the ternary system: L-histidine, copper(II), and the native sequence peptide representing the copper(II)-transport site of human serum albumin
Authors: Tabata Masaaki , Sarkar Bibudhendra
Published: May 2006

Equilibrium and spectroscopic studies of Cu(II)-transfer of native sequence tripeptide, L-aspartyl-L-alanyl-L-histidine-N-methyl amide (AAHNMA), representing the Cu(II)-transport site of human serum albumin (HSA), and L-histidine (L-His) are reported. The equilibria in the ternary system, M–A–B (M = Cu(II), A = anionic form of AAHNMA, and B = anionic form of L-His) have been investigated by analytical potentiometry in I = 0.2 [(Na+,H+) (Cl−,OH−)] at 25 °C. The ternary system shows the presence of five mixed ligand complexes: MH2AB, MHAB, MAB, MH−1AB, and MH−2AB. The species distribution and their stability constants were evaluated by the mathematical analysis of the potentiometric data. The species were further confirmed by their individual spectra computed from the absorption measurements. At physiological pH, the equilibrium studies reveal the presence of 13% of MH−1AB (λmax = 530 nm.ε = 90 M−1 cm−1) and 3% MAB (λmax = 595 nm, ε = 97 M−1 cm−1). The combined results of equilibrium and spectroscopic studies indicate the mixed ligand complex CuH−1AB formed by deprotonation of peptide nitrogen as an important intermediate in the Cu(II)-transfer reaction. The stability constant of CuH−1AB is compared to those of other tripeptides which were designed to mimic the specific Cu(II)-transport site of human albumin.
Isomérisation des radicaux insaturés. III. Radicaux α,α,β-,α,β,γ- et α,α,γ-triméthallyles
Authors: Deslauriers Hélène , Collin Guy J.
Published: May 2006

α,α,β-, α,β,γ-, and α,α,γ-trimethallyl radicals have been generated in the 147.0–nm gas phase photolysis of 2,3,3-trimethyl-1-butene, 3,4-dimethyl-2-pentene, and 2,4-dimethyl-2-pentene, respectively. Under these conditions, the majority of allyl radicals have an internal energy sufficient for further decomposition: they give rise to the formation of various 1,3-dienes and small amounts of either 1,2- or 2,3-dienes. An internal sigmatropic 1,4-hydrogen atom transfer process is part of the proposed mechanism to explain such products. Moreover, the fragmentation of the trimethyl substituted allyl radicals involves the split of one β(C—C) bond, then one β(C—H), and, to a lesser extent, one central C—CH3 bond.
Studies related to the synthesis of dimethyl tetracyclo[5.2.1.02,6.03,8]decane-7,8-dicarboxylate
Authors: Camps Pelayo , Aliaga José , Figueredo Marta , Ortuno Rosa María , Gómez Antonio Gil de , Santos María Teresa , Castané Joan , Feliz Miguel
Published: May 2006

A synthesis of dimethyl tetracyclo[5.2.1.02,6.03,8]decane-7,8-dicarboxylate, 3, whose key step implies the formation of the C1—C2 bond by regioselective intramolecular C—H insertion of a carbene generated from dimethyl (1R,2S,6R,7S)-10-oxotricyclo[5.2.1.02,6]decane-2,6-dicarboxylate, 1 via its tosylhydrazone, is described. Attempts to synthesize diester 3 or compounds containing its carbon skeleton, by forming the same C—C bond, starting from dimethyl (1R,2S,6R,7S)-4-oxotricyclo[5.2.1.02,6]decane-2,6-dicarboxylate, 2, or the corresponding anhydride, 13, are also included.
Phase diagram measurements and thermodynamic analysis of the PbCl2–NaCl, PbCl2–KCl, and PbCl2–KCl–NaCl systems
Authors: Gabriel Armand , Pelton Arthur D.
Published: May 2006

The phase diagrams of the PbCl2–NaCl and PbCl2–KCl systems have been measured by the cooling curve technique. In addition, a number of measurements at compositions in the ternary system PbCl2–KCl–NaCl have been made. The binary data have been analysed along with other available thermodynamic data for the binary systems in order to obtain mathematical expressions for the binary thermodynamic properties. Various estimation procedures were then used to calculate the Gibbs energy of the ternary liquid from the binary expressions, and the ternary phase diagram calculated therefrom was compared to the experimental ternary points. In the case of the Conformal Ionic Solution equation, agreement was within 3 °C at all measured points. In the case of two "geometric" estimation techniques, the Kohler and Toop equations, agreement was within 10 °C but could be brought to within 3 °C by the addition of only one small adjustable ternary correction term. The technique of coupled thermodynamic/phase diagram analysis permits a large reduction in the amount of experimental work necessary to measure the ternary diagram. In addition, the ternary thermodynamic expressions developed can be used to calculate all thermodynamic properties of the ternary liquid.
The crystal structure of a chiral m-cyclophane
Authors: Chan T. H. , Kang G. J. , Belanger-Gariepy F. , Brisse F. , Steliou K.
Published: May 2006

The crystal structure of a chiral m-cyclophane, the α-isomer of (S)-α-methoxy-α-(trifluoromethyl)phenylacetate ester of 13-(methoxycarbonyl)-14-hydroxy-17-methyl-[11]-m-cyclophane (C30H37O5F3, FW = 534.62) has been solved by direct methods and refined by a least-squares procedure to a final R = 0.048 for 2756 independent reflections. The crystals belong to the orthorhombic system, a = 10.8077(13), b = 13.676(3), c = 19.060(4) Å, V = 2817.2 Å3, Z = 4, and the space group is P212121. The absolute configuration of the chiral plane, using the S-configuration of the α-methoxy-α-(trifluoromethyl)phenylacetate ester as reference, is found to be S, in agreement with the X-ray result. The 14-membered ring has the slightly distorted rectangular conformation that has been described by Dale as the rectangular diamond-lattice conformation [3434]. The distortions, also observed in similar 14-membered rings, have been successfully approximated by molecular mechanics calculations.
Preparation, infrared spectra, and crystal structure of two modifications of the (thymidinato-N3)methylmercury(II) complex
Authors: Guay France , Beauchamp André L.
Published: May 2006

Reaction of CH3HgOH with thymidine (HT) yielded the neutral CH3HgT complex crystallizing as a hydrated or an anhydrous material, depending on preparation conditions. Both forms were examined by X-ray diffraction. The anhydrous variety is monoclinic, space group P21, a = 4.798(6), b = 14.270(8), c = 10.390(4) Å, β = 102.74(9)°, and Z = 2 molecules per cell. The structure was refined on 1552 nonzero MoKα reflections to a conventional R factor of 0.034. The hydrated form belongs to the orthorhombic space group P212121, a = 10.484(3), b = 14.633(3), c = 18.538(5), Z = 8. The structure was refined on 1816 nonzero MoKα reflections to R = 0.036. In both forms, the CH3Hg+ ion is linearly bonded to the deprotonated N(3) site of thymidine. The water molecules and hydroxyl groups in the ribose unit participate in a hydrogen bonding network, in which the carbonyl groups are involved as acceptors. The infrared spectra of the two forms differ significantly only by the absorptions due to the water molecules. By comparing with the spectrum of thymidine, diagnostic regions for complexation with deprotonated thymidine have been proposed
The crystal field strength of the nitro ligand and the chemistry of the hexanitrocobaltate(III) anion
Authors: Buist Richard J. , Au-Yeung Steve C. F. , Eaton Donald R.
Published: May 2006

The chemical and spectroscopic properties of the hexanitrocobaltate(III) anion are not in accord with the classification of the N-bonded nitrite ion as a strong field ligand. The nitro groups are rapidly displaced by other ligands, including water, and in dilute aqueous solution spontaneous reduction to Co(II) occurs. Comparison of solid state and solution vibrational and 59Co nmr spectra demonstrates that the principal species in solution is the same as in the solid. All ligands are N-bonded. However, within 2 or 3 min of dissolution new species appear. An electron transfer mechanism for ligand exchange is suggested. It is shown that the band at 480 nm arises from the first d–d transition of the pentanitroaquacobaltate(III) ion and not from a hexanitrocobaltate(III) transition. The composition of the aged solutions has been studied by 59Co, 14N, and 17O nmr. At least 10 different species are apparent in the 59Co spectra. They have been assigned to mixed nitro/nitrito/aqua ions. Electron transfer can also lead to the formation of Co2+ and NO3−, ions, both ions being detected by 14N and 17O nmr spectroscopy. The Co complexed nitro ligand has been detected for the first time in the 14N nmr spectrum. Analysis of 59Co chemical shifts shows that the crystal field strength of the nitro ligand falls steadily with the number of nitro groups in the molecule. The cis groups are four times more effective than the trans groups in causing this change. The cyano ligand shows the opposite behaviour — the crystal field strength increases with substitution and trans groups have a larger effect than cis groups. The reactions of the hexanitrocobaltate(III) ion with ethylenediamine and with cyanide ions have been studied by 59Co nmr. Mixed nitro complexes are formed with ethylenediamine but mixed nitrito complexes predominate with cyanide.
Study of the main extracted species of cobalt(II) ion with dodecylthioglycolic acid as extractor
Authors: Beneitez P. , Ortiz S. J. , Ortega J.
Published: May 2006

The distribution of cobalt(II) between water and an organic extractant (dodecylthioglycolic acid) dissolved in kerosene has been studied using a tracer of 60Co. The aqueous media wee always constituted by buffer solutions (acetic acid – sodium acetate) and the ionic strength was adjusted with sodium nitrate to 1 mol dm−3. The main species of cobalt(II) originating in the organic phase has been isolated and characterized on the basis of elemental analysis, thermal gravimetric analysis, and ir spectra.
Reactivity of electron-rich binuclear hydrides. Synthesis, structure, and reactions of [{(i-Pr)2PCH2CH2P(i-Pr)2}Rh]2(μ-H)(μ-OC6H5)
Authors: Fryzuk Michael D. , Jang May-Ling , Jones Terry , Einstein Frederick W. B.
Published: May 2006

The reaction of phenol with the binuclear rhodium hydride [{(i-Pr)2PCH2CH2P(i-Pr)2}Rh]2(μ-H)2 generates [{(i-Pr)2PCH2CH2P(i-Pr)2}Rh]2(μ-H)(μ-OC6H5), 2, which was characterized by both solution spectroscopic techniques and a single crystal X-ray diffraction study. The structure was solved using Patterson and Fourier methods and refined to R = 0.019 (Rw = 0.027) for 6062 reflections; crystals of the compound are triclinic, space group [Formula: see text], with a = 9.018(2), b = 12.988(2), c = 17.558(3) Å, α = 99.42(1), β = 92.62(2), γ = 91.85(2)°, and Z = 2. Further reaction with excess phenol leads to the mononuclear complex (η5-C6H5O)Rh{(i-Pr)2PCH2CH2P(i-Pr)2}•2C6H5OH, which contains two phenol units hydrogen bonded to the oxygen of the η5-cyclohexy]dienylone ligand. The presence of the bridging phenoxide ligand in 2 results in reduced reactivity as compared to the starting binuclear hydride dimer.
The chemistry of peptides related to metabolites of Trichoderma Spp. 3. An unusual dehydration reaction of 2-methylalanine peptides after electron impact ionization
Authors: Jamieson W. D. , Taylor A.
Published: May 2006

Peptides of 2-methylalanine, where this residue is not the C-terminus, give molecular ions of low abundance on electron impact ionization, but abundant ions are formed by loss of the elements of water from the molecular ion. The dehydration reaction does not occur in permethylated derivatives, nor under the milder ionization conditions in the chemical ionization source. The reaction does not appear to be time or temperature dependent, but depends on the steric hindrance of the NH group of the 2-methylalanine residue. The reaction occurs at sites where 2-methylalanine is not the C-terminal amino acid. In the cases of two pentapeptides where the dehydration reaction was of high probability, the correct sequence was found by semi-quantitative measurements of reaction rates in the first field-free region of a double focussing mass spectrometer.
Adsorption isotherms for coadsorption studies from solution
Authors: Nikitas Panaghiotis , Pappa-Louisi Adriani
Published: May 2006

The mixed adsorption of k adsorbates from solution on energetically homogeneous surfaces is studied by means of lattice statistical thermodynamics. The adsorption layer is considered to behave as a non-electrolyte bulk solution composed from k adsorbates and solvent. On the basis of this assumption the adsorption isotherms and their state equations are determined by means of the lattice theory of strictly regular solutions and Flory–Huggins statistics. The thermodynamic restrictions imposed when these models have to be applied to real systems are also determined and discussed.
The structure of [Ir2(CO)2(μ-H)(μ-CO)(Ph2PCH2PPh2)2][BF4] and comparisons with its rhodium analogue and other related doubly bridged A-frames
Authors: Sutherland Bruce R. , Cowie Martin
Published: May 2006

The structure of [Ir2(CO)2(μ-H)(μ-CO)(DPM)2][BF4] (DPM = Ph2PCH2PPh2) has been determined crystallographically. It crystallizes in the monoclinic space group P21/n (a = 13.7740(6) Å, b = 15.277(2) Å, c = 23.581(2) Å, β = 97.015(5)°, V = 4924.9 Å3, Z = 4) and on the basis of 6628 unique observations and variation of 272 parameters, the structure converged to R = 0.031 and Rw = 0.043. This metal–metal bonded complex is similar to its rhodium analogue, having the two essentially identical Ir centres bridged by the carbonyl, hydride, and two diphosphine ligands. The major difference between this compound and the rhodium species (in the solid state) relates to the orientation of the phenyl groups of the diphosphine ligands; in the iridium complex four of these groups block the sites adjacent to the bridging hydride ligand, whereas these sites are relatively unobstructed in the rhodium analogue. This difference may result in different coordination sites for substrate molecules and therefore in different chemistry.
Mechanism of formation of serine β-lactones by Mitsunobu cyclization: synthesis and use of L-serine stereospecifically labelled with deuterium at C-3
Authors: Ramer Shawn E. , Moore Richard N. , Vederas John C.
Published: May 2006

The ring closure of N-benzyloxycarbonyl-L-serine (1) under Mitsunobu conditions (Ph3P, dimethyl azodicarboxylate, −78 °C) to give the corresponding β-lactone (2) is shown by deuterium and oxygen-18 labelling studies to proceed by hydroxy group activation, in contrast to analogous cyclizations of more hindered β-hydroxy acids, which usually occur by carboxy group activation. Samples of 1 stereospecifically labelled with deuterium at C-3 were prepared by hydrogenation of (Z)-2-acetamido-3-methoxyacrylic acid (9) with deuterium, followed by selective Acylase I deacetylation of the 2S isomer, removal of the protecting groups, and N-acylation of the resulting L-serine with benzyl chloroformate. Mitsunobu cyclizations of this 3R deuterated N-acyl serine, of the [hydroxy-18O] analog 1g, and of the [carboxy-18O] derivative 1f show that lactonization occurs with inversion of configuration at C-3, loss of the hydroxy oxygen, and retention of the carboxy oxygens. Similar labelling experiments demonstrate that aqueous sodium hydroxide opens the β-lactone ring by exclusive attack at the carbonyl to regenerate 1, whereas acidic hydrolysis proceeds primarily by attack of water at the C-3 methylene group of 2. This information allows interconversion of L-serines that are stereospecifically labelled at C-3 with hydrogen isotopes and affords access to other labelled (β-substituted alanines.
Synthesis and conformational analysis of 2-arylseleno-1,3-dithianes. Crystal and molecular structure of 2-(4-methoxyphenylseleno)- and 2-(4-trifluoromethylphenylseleno)-1,3-dithiane
Authors: Pinto B. Mario , Sandoval-Ramirez Jesus , Sharma R. Dev , Willis Anthony C. , Einstein Frederick W. B.
Published: May 2006

The synthesis of 2-(4-methoxyphenylseleno)-1,3-dithiane 3 and 2-(4-trifluoromethylphenylseleno)-1,3-dithiane 5 from 2-chloro-1,3-dithiane 1 and the corresponding sodium arylselenolates is described. Nuclear magnetic resonance spectroscopic investigation of the products indicates that the compounds exist predominantly in a conformation in which the arylseleno moiety adopts an axial orientation. X-ray crystallographic investigation indicates that the 1,3-dithiane ring exists in the chair conformation with the arylseleno moiety in the axial orientation. Compound 3 is orthorhombic, space group P212121 with a = 5.449(2) Å, b = 9.217(2) Å, c = 24.860(3) Å, V = 1248.5 Å3, Z = 4. The structure was refined to R = 0.038 for 689 reflections with I > 2.3σ(I). Compound 5 is monoclinic, space group C2/c, with a = 28.628(7) Å, b = 5.246(2) Å, c = 21.342(5) Å, β = 121.12(1)°, V = 2743.8 Å3, Z = 8. Its structure refined to R = 0.064 for 966 reflections with I > 2.3σ(I).
The crystal structure of an enol silyl ether
Authors: Brisse F. , Thoraval D. , Chan T. H.
Published: May 2006

The crystal structure of the enol silyl ether compound, C28H33NO3Si, 1, has been established by direct methods and refined to a final R value of 0.053 for 1825 observed reflections. The crystals belong to the monoclinic system, a = 8.683(5), b = 12.689(5), c = 24.585(11) Å, β = 105.33(4)°, and the space group is P21/c. The C(sp2)—C(sp2) bond distance is normal but the Si(1)—O(1)—C(1) angle with a value of 132.5(4)° is larger than usual. This is attributed to an interaction between Si(1) and O(2) of the N-acetate group, d(Si … O) = 3.320(4) Å. There is a distortion of the tetrahedral coordination around the silicon atom since the bond angles involving Si vary from 102.8(4) to 112.6(5)°. All three phenyl rings and the N-acetate group are planar. The two phenyls at C(3) are inclined by [Formula: see text] with respect to the ethylenic group constituted of the two C(sp2) atoms and their four substituents. The molecules are held in the crystal by van der Waals forces only.
Electron donor–acceptor complexes between naphthylamines and methyl viologen in aqueous sodium dodecyl sulphate solution
Authors: Bertolotti S. G. , Cosa J. J. , Gsponer H. E. , Hamity M. , Previtali C. M.
Published: May 2006

The electron donor–acceptor (EDA) interaction between methyl viologen (MV2+) and 1-naphthylamine (1NA), 2-naphthylamine (2NA), and N,N-dimethyl-1-naphthylamine (DMA) was studied in water and in aqueous sodium dodecyl sulphate (SDS). The experimental values of the association constants in water were 8.9, 9.8, and 2.8 M−1 for 1NA, 2NA, and DMA, respectively. In the presence of SDS the observed values were very much higher and strongly dependent upon the detergent concentration. The enhancement in the interaction is due to an increase in the local concentration of the partners in the micellar pseudophase. MV2+ itself was shown to strongly interact with the micelles. A new absorption is present at the red tail of the spectrum of MV2+ in the presence of SDS micelles. An association constant of 1700 M−1 was obtained from this absorption.
Preparation and X-ray crystal structures of the arsenic pentafluoride adducts of benzo-2,1,3-thiadiazole and benzo-1,2,3-thiadiazole
Authors: Apblett Allen , Chivers Tristram , Richardson John F.
Published: May 2006

The reaction of arsenic pentafluoride with benzo-2,1,3-thiadiazole, 1, or benzo-1,2,3-thiadiazole, 2, in liquid SO2 gave 1:1 adducts that were characterized spectroscopically (infrared and 13C nmr) and by X-ray crystallography. Crystal data: for 1•AsF5, monoclinic, space group P21/n, a = 6.932(1), b = 9.113(1), c = 15.136(2) Å, β = 98.035(7)°, V = 946.8(2) Å3, Z = 4; for 2•AsF5, monoclinic, space group P21/a, a = 7.573(2), b = 13.101(2), c = 9.514(3) Å, β = 90.95(2)°, V = 943.9(4) Å3, Z = 4. The coordination of AsF5 to one of the nitrogen atoms in 1 introduces asymmetry in the heterocyclic ring, with the longer bond lengths being associated with the coordinated nitrogen atom, d(S—N) = 1.633(5) and 1.577(6) Å, d(C—N) = 1.364(7) and 1.339(8) Å. The quinonoid character of the benzene ring is still apparent in the adduct. In 2•AsF5, the AsF5 molecule is coordinated to the nitrogen atom that is bonded to carbon.
Synthesis of (±)-albicanyl acetate and (±)-isodrimenin by the electrophilic cyclization of olefinic allylsilanes
Authors: Armstrong Rosemary J. , Harris Francis L. , Weiler Larry
Published: May 2006

The allylsilane 3 was synthesized and cyclized with Lewis acids or mercuric trifluoroacetate to give 5 or 6, which were converted into albicanyl acetate (7) and isodrimenin (12).[Formula: see text]
Steric activation in prototropic reactions of pyrazine derivatives. II. The Brønsted relation
Authors: Nagarajan K. , Lee T. W. S. , Perkins R. R. , Stewart Ross
Published: May 2006

The rate of deprotonation of the 2-methyl group in 1,2,3-trimethylpyrazinium ion by carboxylate, aniline, and pyridine bases has been measured in D2O. Carboxylate ions containing bulky groups near the reaction centre, e.g. ortho benzoates, react faster than predicted by the Brønsted equation that correlates the reactions of unhindered carboxylates. Anilines and pyridines, on the other hand, show conventional steric effects. A tentative explanation for the activation engendered by groups adjacent to the carboxylate centre is based on the known effect that high concentrations of organic electrolytes have on the strengths of carboxylic acids but not of amines. Since ortho carboxylate ions have their relative basicities increased by an alkyl-rich environment, it is argued that the reactive methyl groups of the substrate might provide such an interaction in the transition state.
Activation volumes for ester hydrolysis via elimination–addition
Authors: Isaacs Neil S. , Najem Tariq S.
Published: May 2006

Esters that have an acidic proton α to the carboxyl group (or in a vinylogous situation) and a good leaving group may undergo hydrolysis by elimination to an intermediate ketene, which rapidly hydrates. Examples are found in p-hydroxybenzoate, malonate, and acetoacetate esters of nitrophenols. The evidence for this mechanism (E1cb type) includes the independence of rate with pH in the region of dissociation of the acidic proton and, in particular, positive volumes of activation that contrast sharply with negative values typical of the more usual BAc2 mechanism of hydrolysis.
The hydrolysis of (π-C6H6)Cr(π-C6F5CO2C2H5): an unexpected decarboxylation
Authors: McGlinchey Michael J. , Nguyen Hao
Published: May 2006

The attempted basic hydrolysis of the ester sandwich compound (C6H6)Cr(C6F5CO2Et) did not yield the expected carboxylic acid but instead produced (C6H6)Cr(C6F5H) in good yield together with traces of (C6H6)Cr(C6HF4OMe). Attempts to trap a benzyne intermediate were unsuccessful and the mechanism of decarboxylation is discussed in terms of internal chelation at the chromium centre.