bluuebunny/crossref_metadata_2025 huggingface dataset:
Model studies of competing hydrolysis and epimerization of some tetrapeptides of interest in amino acid racemization studies in geochronology
Authors: Moir Michael E. , Crawford Robert J.
Published: May 2006

The processes of epimerization of individual peptide units in proteins and the concurrent cleavage of peptide bonds are modelled by heating some tetrapeptide and tetrapeptide derivatives to 148.5 °C in pH 6.8 phosphate buffer. An excess of D-proline was observed during the heating of L-prolyl-L-leucylglycylglycine. The D/L ratio of proline attains a maximum value of 2.1 after 90 minutes. The excess D-proline is attributed to the formation of a 2.3:1 mixture of diketopiperazines cyco-(D-prolyl-L-leucyl) and cyclo-(L-prolyl-L-leucyl). These two species account for most of the leucine and proline in the final mixture after the tetrapeptide is no longer detectable. Only small amounts of prolylleucine can be detected after 50 hours. It is suggested that the above tetrapeptide undergoes internal aminolysis. Leucine in the tetrapeptide glycyl-L-leucylglycylglycine racemizes three times faster than in L-prolyl-L-leucylglycylglycine. This demonstrates that an amino acid residue in a peptide chain does have an effect upon the rate of epimerization of a neighbouring peptide residue. A discussion of the geochemical implications of the results is included.
Ionic reorientations and phase transition in solid IF6AsF6
Authors: Albert Shmuel , Ripmeester John A.
Published: May 2006

19F static and rotating frame spin–lattice relaxation times (T1 and T1ρ) as well as broadline NMR studies have been carried out on solid IF6AsF6 between 213 and 393 K. From the T1– and T1ρ–temperature behaviour, we inferred a phase transition at 350(± 2) K and a supercooled high temperature phase. In both phases the second moments are consistent with a model where both IF6+ and AsF6− reorient about random axes. The T1 and T1ρ data in the low temperature phase can be interpreted in terms of a single frequency factor τ0 = 3.15(± 0.2) × 10−16 s and activation energy Ea = 10.4(± 0.4) kcal/mol for both cation and anion reorientation. At the temperature of the phase transition the ionic-reorientational frequencies become faster by ~ 2 orders of magnitude. In the high temperature phase T1 and T1ρ are essentially independent of temperature which may be associated with a spin–rotation relaxation mechanism.
Protium–deuterium exchange of substituted pyridines in neutral D2O at elevated temperatures
Authors: Werstiuk Nick Henry , Ju Chen
Published: May 2006

H–D exchange of a series of pyridines 1–15, 10H-pyridino[3,2-b][1,4]benzothiazine (16), phenothiazine (17), and aniline has been carried out in neutral D2O at elevated temperatures. The substrates undergo selective exchange and are generally isolated in good yield. Cyanopyridines 4,5, and 6 hydrolyze readily in D2O at elevated temperatures, and the resultant carboxylic acids decarboxylate. This reaction provides a convenient one-step route to selectively labelled pyridines. Possible mechanisms of exchange are presented. Keywords: protium–deuterium exchange, substituted pyridines, temperature effect
Complexes of 18-crown-6 with oxonium ions derived from transition metal chlorides and hydrochloric acid: 18-crown-6.H3O+. FeCl4−, 18-crown-6.H3O+.InCl4−, (18-crown-6.H3O+)2.Pd2Cl62−
Authors: Chênevert Robert , Chamberland Daniel , Simard Michel , Brisse François
Published: May 2006

The macrocyclic polyether 18-crown-6 forms complexes with hydrates of strong acids derived from transition metal chlorides and hydrochloric acid. The following charged-component complexes have been isolated and characterized: 18-crown-6.H3O+.FeCl4−, 18-crown-6.H3O+.InCl4−, and (18-crown-6.H3O+)2.Pd2Cl62−. X-ray diffraction study of the palladium complex is reported. The oxonium ion is located at the center of the macrocyclic ether cavity and the symmetry of the ring is close to D3d. Keywords: 18-crown-6 complexes, oxonium ions, X-ray diffraction, crystal structure
Effect of temperature on cyclic voltammograms during the electrodeposition of lead
Authors: Girgis Magdy , Ghali Edward
Published: May 2006

The electrochemical behavior of lead deposition process in a temperature range between 25 and 75 °C was studied in a medium of lead chloride and ammonium acetate by linear and cyclic sweep perturbation. The study was undertaken on four substrates: Pb, Cu, Ag, and C. An increase in temperature resulted in accelerating the deposition process and promoted dendrite formation. Moreover, different kinetic parameters such as adsorption isotherms, Tafel slopes, and diffusion coefficient were affected as a function of temperature elevation. Surface analytical study performed by Auger electron spectroscopy revealed an increase in peak intensities of Cl and Pb as the temperature increased. Keywords: electrodeposition, cyclic voltammetry, potentiostatic transients, underpotential deposition, temperature effect.
Viscosities of mixtures of chloroform + triethylamine: analysis in terms of three components (A, B, and AB)
Authors: Schutte Richard P. , Liu Terry C. , Hepler Loren H.
Published: May 2006

Viscosities and densities of highly non-ideal mixtures of chloroform (A) + triethylamine (B) have been measured at 25 °C. It has been shown that the Grunberg equation (one adjustable parameter) can account for these viscosities over the whole range of compositions. It has also been shown that a new combination of the ideal associated solution model, based on consideration of an equilibrium of the type A + B = AB, with an ideal viscosity equation (also one adjustable parameter) accounts more accurately for all of the viscosities. This latter treatment also has the advantage of offering a clear pictorial model of the mixtures. Keywords: viscosity of solution, complex formation in solution, chloroform + triethylamine, triethylamine + chloroform.
Vibrational spectral studies of solutions at elevated temperatures and pressures. 11. A Raman spectral study of aqueous iron(III) chloride solutions between 25 and 300 °C
Authors: Murata Katsuo , Irish Donald E. , Toogood Gerald E.
Published: May 2006

The Raman spectra of acidified aqueous iron(III) chloride solutions have been measured between 25 and 300 °C. When Fe3+ concentrations are in the range 0.75 to 1.0 mol kg−1 and Cl−/Fe3+ ratios, R, in the range 3 to 9.5, the dominant species at 25 °C is trans-[Fe(H2O)4Cl2]+; at 300 °C the sole iron-containing species is tetrahedral [Formula: see text]. Conversion of [Fe(H2O)4Cl2]+ into [Formula: see text] appears not to involve intermediate iron species. In the presence of excess chloride the reaction [Formula: see text] is presumed to occur; ΔH for this reaction has been estimated as +65 ± 8 kJ mol−1. In addition to increasing temperature, factors which favour [Formula: see text] over other iron species include increasing acidity, increasing R, and decreasing dielectric constant. Keywords: high temperature aqueous solutions, iron(III) chloride, Raman spectroscopy.
Basicity of substituted 2-phenyl-1,1,3,3- tetramethylguanidines and other bases in acetonitrile solvent
Authors: Leffek Kenneth T. , Pruszynski Przemyslaw , Thanapaalasingham Kohila
Published: May 2006

A series of 30 phenyl substituted derivatives of 2-phenyl-1,1,3,3-tetramethylguanidine was synthesized and the pKa value of each was measured in acetonitrile using a conventional, general purpose glass electrode. The slope of the Hammett relationship, for the para and meta substituted compounds, ρ = 2.18 ± 0.16, is significantly lower than ρ = 2.85 ± 0.06 found for anilines in water, even though the phenyl ring is attached directly to the basic centre in both cases. The influence of electronic and steric effects on the basicity of the compounds is discussed. The solvent effect on the basicity is also discussed, by comparison of the pKa values in acetonitrile and water. The mean value of [Formula: see text] is slightly higher than the value previously observed for ammonia and aliphatic amines, [Formula: see text]. Additionally, the pKa values were measured for seven other cyclic bases, including 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN). Keywords: 2-phenyl-1,1,3,3-tetramethylguanidines, pKa measurements.
Nitrogenous metabolites from the deep water sponge Hexadella sp.
Authors: Morris Sandra A. , Andersen Raymond J.
Published: May 2006

The secondary metabolites of the sponge Hexadella sp., collected by SCUBA (−40 m) and manned submersible (−100 to −200 m) in Jervis Inlet, British Columbia, have been examined. The submersible-collected material contained only the previously reported compound topsentin B2 (1). Sponge material collected in shallower water via SCUBA contained hexadellins A (2) and B (3), two new dibromotyrosine derived metabolites. The structures of 2 and 3 were determined by spectroscopic analysis of the diacetyl derivatives 6 and 7. Keywords: sponge, metabolite, Hexadella, nitrogenous.
Hydrogen isotope exchange between D2 and H2O catalyzed by platinum plate
Authors: Miyamoto Shin-Ichi , Sakka Tetsuo , Iwasaki Matae
Published: May 2006

The reaction rate of hydrogen isotope exchange between D2 and H2O catalyzed by platinum plate is studied. The exchange reaction is described with the kinetic model which is the modification of that for the exchange reaction catalyzed by alumina-supported platinum catalyst. For the comparison of experimental results with this model relative amount of the number of sites for hydrogen adsorption was estimated from the initial rate of hydrogen isotope exchange between H2 and D2 on the same surface. The results show that the kinetic model is applicable for the plate catalyst if the number of the sites for hydrogen absorption, which is very sensitive to the surface state of the catalyst, was estimated not from the macroscopic surface area but from our scheme. Keywords: hydrogen isotope exchange reaction, platinum plate as catalyst.
Cryptowolinol et cryptowolidine, nouveaux alcaloïdes de type dibenzopyrrocoline
Authors: Lebœuf Michel , Cavé André , Ranaivo Anissa , Moskowitz Henri
Published: May 2006

Four quaternary dibenzopyrrocoline alkaloids were isolated from the Neocaledonian Lauraceae Cryptocaryaphyllostemon and C. oubatchensis. Two of them were identified as cryptowoline and cryptaustoline, the only natural representatives of this structural type known up to now. The other two are new products and have been named cryptowolinol and cryptowolidine; their structures were elucidated, and synthesis of cryptowolidine was perfomed. An original feature of cryptowolinol is the unusual presence of an alcoholic hydroxyl on the benzylic carbon; its putative biogenetic origin is discussed. Keywords: Cryptocarya, Lauraceae, dibenzopyrrocoline alkaloids.
Compressibilités de l'eau et du benzène dans tout le domaine de miscibilité du système eau + 2-propanol + benzène
Authors: Hétu Daniel , Lara Jaime , Desnoyers Jacques E.
Published: May 2006

Sound velocities have been measured over the whole miscibility domain of the water + 2-propanol + benzene system. The apparent molar isentropic compressibilities of benzene and water were calculated from these data. The apparent molar compressibilities of benzene and water present large changes in the water-rich and benzene-rich regions, respectively, whereas the changes are smooth in the intermediate region. These results suggest that microtransitions similar to the formation of micelles and inverse micelles occur at the two ends of the phase diagram. The regularity of the intermediate region is consistent with a bicontinuous model of microemulsions. For the water-rich region, the compressibilities of transfer of benzene at infinite dilution, from water to water + 2-propanol mixtures, were simulated with the model of Roux and co-workers. This simulation tends to confirm that the alcohol and the benzene form mixed aggregates. Keywords: compressibility, ternary system, microemulsion, water, benzene, 2-propanol.
Substituent effects at silicon in cations SiX+, HSiX+•, and H2SiX+, and in radicals H2SiX+•
Authors: Hopkinson A. C. , Lien M. H.
Published: May 2006

Abinitio molecular orbital calculations at the 6-31G* level have been used to optimise structures for ions SiX+, HSiX+•, and H2SiX+, and for neutrals HSiX (singlets), H2SiX•, and H3SiX, where X is H, CH3, NH2, OH, F, CN, and NC. Single point calculations at the MP4(SDTQ)/6-31G* level were used to calculate substituent stabilisation energies.The amino group is the strongest π-donor and also is the most stabilising group in the cations, the silylenes, and the silyl radicals. Stabilisation is greatest in ions SiX+. Ions HSiX+• and H2SiX+ are stabilised by similar but smaller amounts, although CN and NC are destabilising in these ions. Substituent stabilisation energies in radicals H2SiX• are almost zero. There is a linear relationship between the stabilisation energies of ions H2SiX+ and the ionisation potentials of radicals H2SiX•, but a similar plot correlating stabilisation energies for ions HSiX+۟• with the ionisation potential of HSiX (singlet) shows considerable scatter. Keywords: silications, silyl radicals, stabilisation energies.
Dioxygen reduction at a graphite electrode modified by mononuclear tetraneopentoxyphthalocyaninatocobalt(II) and related polynuclear species
Authors: Janda Pavel , Kobayashi Nagao , Auburn Pamela R. , Lam Herman , Leznoff Clifford C. , Lever A. B. P.
Published: May 2006

Potentiodynamic data were obtained under argon for the mononuclear species tetraneopentoxyphthalocyaninatocobalt(II) species adsorbed on glassy carbon (GC), ordinary pyrolytic graphite (OPG), and highly oriented pyrolytic graphite (HOPG). Comparative data show that the most convenient material to use is HOPG whereon a limiting monolayer is apparently achieved. Data were also presented for the mononuclear and polynuclear analogues on HOPG under both argon and dioxygen. Comparison is made of the potential data obtained in water and in organic solvent. A pH dependence study shows that the dioxygen reduction potential tracks the Co(II)TNPc/Co(I)TNPc redox couple with a slope of approximately −65 mV/pH for the mononuclear species, in the acid regime, but substantially less than −65 mV/pH for the polynuclear species. There is no pH dependence in the alkaline regime. Possible mechanisms are discussed. Keywords: phthalocyanine, electrochemistry, dioxygen reduction, pH dependence, graphite electrode, cobalt phthalocyanine.
Versatility of squaric acid as ligand. Crystal, molecular structures and magnetic studies of four poly- or dimetallic Cu(II) complexes
Authors: Bernardinelli Gerald , Deguenon Diane , Soules Regis , Castan Paule
Published: May 2006

The synthesis and magnetic data of four copper(II) complexes obtained from squaric acid and bipyridyl or imidazole as ligand are described. Cu(C4O4)(imidazole)2•2H2O (compound I) is made of chains of squarato-1,3-bridged Cu(II) ions. The compound crystallizes in the monoclinic system, space group P21/n, and lattice parameters a = 7.455(2) Å, b = 8.196(2) Å, c = 11.235(2) Å and β = 100.55(2)°. RF = 0.067 for 1076 observed reflections. [Cu2(bipyridyl)2(C4O4)(H2O)4](C4O4H)2-4H2O (compound II) is a C4O4H− salt of a squarato-1,3 bridged centrosymmetric Cu(II) dimeric cation. The compound crystallizes in the triclinic system, space group P-1and lattice parameters a = 9.021(2) Å, b = 9.017(2) Å, c = 12.581(4) Å, α = 98.58(2)°,β = 106.81(2)°, and γ = 106.03(2)°. RF = 0.038 for 2376 reflections. Each copper atom is pentacoordinated in a distorted square-based pyramidal and is linked by two nitrogen atoms of the bipyridyl, one oxygen atom of the squarato bridge, and one oxygen atom of a water molecule in the plane and one water molecule out of plane. [Cu2(bipyridyl)2(C4O4) (H2O)2Br2] (compound III) is a squarato-1,3 bridged copper(II) dimer. The compound crystallizes in the monoclinic system, space group P21/n, and lattice parameters a = 10.480(4) Å, b = 8.503(1) Å, c = 14.590(2) Å, and β = 99.75(2)°. RF = 0.048 for 1423 reflections. Each copper atom is pentacoordinated in a distorted square-based pyramidal and is linked by two nitrogen atoms of the bipyridyl, one oxygen atom of the squarato bridge and one oxygen atom of a water molecule in the plane, and one bromine atom out of the plane.In complexes I, II, and III, the magnetic susceptibility data could be best described by neglecting the exchange coupling. The magnetic data of [Cu(bipyridyl)(C4O4)•-4H2O]n (IV) are best explained in terms of an alternating Heisenberg ferrornagnet. The epr spectra ratify results obtained from magnetic and crystallographic measurements. Keywords: squaric acid, 3,4-dihydroxy-3-cyclobutene-1,2-dion, copper(II) dimers, copper(II) chains, magnetic properties.
Nitrosyl complexes of rhenium. 3. The synthesis and substitution reactions of [ReH(CH3OH)(NO)(PPh3)3]ClO4 derived from the reaction of ReH2(NO)(PPh3)3 with HClO4. Members of the series ReHX(NO)(PPh3)3
Authors: Chen Julie Y. , Grundy Kevin R. , Robertson Katherine N.
Published: May 2006

ReH2(NO)(PPh3)3 reacts with alcoholic HClO4 to yield the solvated cation, [ReH(ROH)(NO)(PPh3)3]ClO4 (R = CH3, C2H5) which can be isolated as an explosive solid (R = CH3 only). In the solid state, the perchlorate counter ion is hydrogen bound to the ligated alcohol, as observed for [ReH(CH3OH)(CO)(NO)(PPh3)2]ClO4. This latter complex also results from the reaction of [ReH(CH3OH)(NO)(PPh3)3]ClO4 with CO under ambient conditions. On reaction with p-tolylisocyanide (RNC), however, substitution is also accompanied by dihydrogen elimination to give [Re(OR)(NO)(CNR)2(PPh3)2]ClO4, irrespective of the mole ratio of the reactants. In contrast, [ReH(CH3OH)(NO)(PPh3)3]ClO4 reacts with coordinating anions with loss of methanol to give air-sensitive ReHX(NO)(PPh3)3 (X = OCH3, F, Cl, Br, I, N3, NCO, SCN), of which only those with X = Cl, Br, N3 were characterized in solution. The compounds ReHX(NO)(PPh3)3 are similar to their osmium analogues in having a labile third phosphine. Thus, reaction with neutral ligands such as CO or RNC leads to the series of neutral rhenium hydrido nitrosyls ReHX(L)(NO)(PPh3)2. Prior to this work, the only known examples from this series were those with L = CO, X = H, F, OCH3. Interestingly, the isomer of ReHF(CO)(NO)(PPh3)2 isolated in this work differs from that previously isolated (from the reaction of [ReF(CO)(NO)(PPh3)3]+ with [Formula: see text]) in having F trans to NO. All structural assignments have been made on the basis of elemental analyses, infrared spectroscopy, 1H and 31P nmr spectroscopy and deuteration studies, where appropriate. Keywords: hydrido, nytrosyl, rhenium, complexes.
Synthesis, solid state 31P CP-MAS NMR infrared and Raman studies of mercury(II) complexes of 1,3,5-triaza-7-phosphaadamantane (PTA)
Authors: Alyea Elmer C. , Fisher Keith J. , Johnson Steve
Published: May 2006

Insoluble mercury(II) complexes of a water-soluble phosphine, 1,3,5-triaza-7-phosphaadamantane (PTA), have been studied by solid state 31P CP-MAS NMR, infrared and Raman spectroscopies. Complexes of the type (PTA)HgX2 show larger coordination chemical shifts but similar J(31P—199Hg) coupling constants to those previously observed for similar trialkylphosphine complexes. These complexes all appear to be dimeric or polymeric. The nitrate complex of the form (PTA)2Hg(NO3)2 has also been isolated and complexes of the type (PTA)4Hg2+ are shown to exist. Keywords: mercury(II) phosphine complexes, solid state 31P NMR studies.
On the partitioning of the viscosity B coefficient and the correlation of ionic B coefficients with partial molar volumes of ions for aqueous and non-aqueous solutions
Authors: Bhattacharyya Moni Mohan
Published: May 2006

A new method of partitioning the viscosity B coefficients has been proposed on the basis of the assumption that ionic B values should be a monotonie function of crystal radii cubed, independent of the sign of the charge, for aqueous and various non-aqueous systems. Ionic B values thus obtained in different solvents by a single method of division provide a basis for uniform comparison among themselves as well as with the existing values. The usual assumption of equal contributions of K+ and Cl− ions has been discarded. The internal consistency of the derived set of Bi values is demonstrated by correlation with the ionic radii, hydrated ionic radii, partial molal volumes, temperature coefficient of equivalent conductance, and partial molal ionic entropy. The salient feature of the set of Bi values is noted by its linear relationship not only with the partial molal volumes but the same is also observed with the ionic radii of ions of different valence and charge types. Eyring's equation for viscosity using Frank–Wen's model of ion–solvent interaction has been utilised for the calculation of void viscosity and electrostriction viscosity. The latter is compared with the corresponding component of partial molal volumes using the Drude–Nernst equation based on the continuum model for solvent. Keywords: ion–solvent model, ionic B values, B coefficients, division of B coefficient, void viscosity, micro viscosity.
Reactions of polar dienes with o-quinones
Authors: Paquet Jacques , Brassard Paul
Published: May 2006

The behaviour of various types of polar dienes towards halogenated ortho quinones has been investigated in a number of representative cases. As compared to the commonly used para analogues, o-quinones provide a wide range of products that indicate a keener response to the nature, number, and position of substituents on both reactants. 3-Halogenated-o-naphthoquinones 1 and 2 react smoothly with a representative vinologous ketene acetal 3, vinylketene acetals 4 and 5, and a monooxygenated diene 6 to provide variously substituted phenanthrenequinones 7–11. Only monooxygenated dienes on the other hand add to o-benzoquinones 14–16 and give convenient syntheses of the corresponding o-naphthoquinones 18–20. Keywords: cycloaddition, o-naphthoquinones, phenanthrenequinones, regiospecificity.
Novel regio- and stereoselective modifications of heparin in alkaline solution. Nuclear magnetic resonance spectroscopic evidence
Authors: Jaseja Mahesh , Rej Rabindra N. , Sauriol François , Perlin Arthur S.
Published: May 2006

Nuclear magnetic resonance spectroscopic evidence is presented in characterizing three new structurally modified forms of heparin. One of these, polymer M-I, represents a conversion of about two-thirds of the α-L-iduronic acid 2-sulfate residues (1) into residues of a 2,3-anhydro derivative (3), through the action of sodium hydroxide. The formation of 3 is attributed to a base-catalysed displacement of the sulfate group of 1 by an intramolecular attack of O-3 on C-2. In more concentrated sodium hydroxide solution, heparin is transformed almost quantitatively into polymer M-II, which differs from it in having residues of (non-sulfated) α-L-iduronic acid (4) in place of 1. It is likely that 3 is an intermediate, and that a selective nucleophilic attack of hydroxide ion at C-2 accounts for the ido configuration in 4. The third modification, giving polymer M-III, is induced when a neutral or weakly alkaline solution of M-I is heated at 70 °C or above, which promotes a different stereochemistry in the hydrolysis of the 2,3-oxirane ring of 3. Hence, in contrast to residues of 4 in M-II, most of the uronic acid residues of M-III appear to have the alternate, α-L-galacto, configuration. As shown by a comparison of beef lung and hog mucosal heparin, the rate at which M-I is converted into M-III is facilitated by the higher level of structural heterogeneity in the mucosal heparin. Whereas the formation of M-I, -II, and -III is accompanied by only moderate depolymerization, these novel polymers retain little of the anti-coagulant and anti-XA activities of the unmodified heparin. Keywords: NMR spectroscopy, heparin, desulfation, anhydroaldoside, base-catalysed displacement.
Isolation and structure of the unusual Indian Ocean Cephalodiscusgilchristi components, cephalostatins 5 and 6
Authors: Pettit George R. , Kamano Yoshiaki , Dufresne Claude , Inoue Masuo , Christie Nigel , Schmidt Jean M. , Doubek Dennis L.
Published: May 2006

The Indian Ocean (South African) marine worm Cephalodiscusgilchristi (Hemichordata phylum) has been found to contain a series of unusual disteroidal alkaloids, designated the cephalostatins, that possess exceptionally potent lymphocytic leukemia (murine P388) cell line inhibitory activity (to ED50 10−9 μg/mL). Two of the less prominent members with P388 ED50 10−3–10−2 μg/mL, cephalostatins 5 (2) and 6 (3), were isolated and assigned structures based on rigorous interpretation of two-dimensional 400-MHz 1H and 13C nuclear magnetic resonance. Keywords: cephalostatins 5 and 6, disteroidal alkaloids, lymphocytic leukemia, cytostatic, Cephalodiscusgilchristi.
Alkaloids of Lycopodiumselago. On the identity of selagine with huperzine A and the structure of a related alkaloid
Authors: Ayer William A. , Browne Lois M. , Orszanska Helena , Valenta Zdenek , Liu Jia-Sen
Published: May 2006

Selagine, an alkaloid of Lycopodiumselago, is shown to be identical with huperzine A (2). A minor alkaloid, C15H18N2O2, isolated from L. selago is found to be 6β-hydroxyhuperzine A (4). Keywords: huperzine A, selagine, Lycopodium alkaloids, Alzheimer's disease, acetylcholine esterase inhibitor.
Rate constants for abstraction of hydrogen from benzene, toluene, and cyclopentane by methyl and ethyl radicals over the temperature range 650–770 K
Authors: Zhang H.-X. , Ahonkhai S. I. , Back M. H.
Published: May 2006

Rate constants for the abstraction of hydrogen from benzene, toluene, and cyclopentane by methyl and ethyl radicals have been measured relative to the corresponding abstraction reaction from ethylene. The method is based on the effect on the rates of formation of methane and ethane of the addition of small quantities of the reactants to the thermal chain reactions of ethylene in the temperature range 650–770 K. Taking the following values of the rate constants for the reference reactions (R = 8.314 J mol−1 K−1):[Formula: see text]the following rate constants were measured:[Formula: see text]The values of the activation energies are discussed in relation to the dissociation energies of the C—H bonds in the reactants. Keywords: kinetics, abstraction, methyl, ethyl, benzene, toluene.
Electron affinities of aza-substituted polycyclic aromatic hydrocarbons
Authors: Dillow Glen W. , Kebarle P.
Published: May 2006

Electron affinities for aza-substituted polycyclic aromatics were determined from measurements of electron transfer equilibria in the dilute gas phase with a pulsed electron high pressure mass spectrometer (PHPMS). These are (in kcal/mol): quinazoline (12.7), quinoxaline (15.8), cinnoline (16.0), acridine (20.3), benzo[c]cinnoline (20.6), pyrido[2,3-b]pyrazine (22.5), phenazine (29.5). Solvation energies of the corresponding radical anions in acetonitrile and dimethylformamide are derived from the gas phase data and literature on electron reduction potentials in solution. An observed linear relationship between the electron affinities and the reduction potentials allows estimates of electron affinities to be made for 12 aza compounds whose EA's are too low to be measured with the present method. Keywords: aza-substituted aromatic hydrocarbons, electron affinities, electron transfer, radical anions, reduction potentials, solvation energies of radical anions, stabilities of radical anions.
127I Mössbauer studies of oxygen bonded iodine complexes. 2. Aryliodine(III)carboxylates; the crystal and molecular structure of bis(trifluoroacetato)pentafluorophenyliodine(III), (C6F5)I(O2CCF3)2
Authors: Bardan Madhumita , Birchall Thomas , Frampton Christopher S. , Kapoor Pratibha
Published: May 2006

The 127I Mössbauer spectra of a series of mono- and diaryliodine(III) carboxylates have been recorded at 4.2 K. The observed parameters are shown to be strongly dependent on the primary bonding arrangement about iodine only and the secondary bonding interactions have little or no effect. The crystal and molecular structure of bis(trifluoroacetato)pentafluorophenyliodine(III), (C6F5)I(O2CCF3)2, has been determined. Crystals are triclinic, of space group [Formula: see text], witsh a = 5.732(2) Å, b = 12.127(4) Å, c = 29.658(9) Å, α = 116.53(2)°, β = 98.02(2)°, γ = 117.09(2)°, V = 1482.0(8) Å3, Dc = 2.33 g cm−3 for Z = 4, and R1 = 0.0420 (R2 = 0.0431) for 2722 reflections with I > 3σ(I). The two independent molecules in the asymmetric unit have essentially the same primary geometry about the central iodine, a distorted T-shaped AX3E2 arrangement. Keywords: 127I Mössbauer, aryliodine(III) carboxylates, crystal structure, (C6F5)I(O2CCF3)2.
Tetramethylammonium bifluoride, crystal structure and vibrational spectra
Authors: Wilson William W. , Christe Karl O. , Feng Jin-an , Bau Robert
Published: May 2006

Single crystals of [N(CH3)4]HF2 were obtained as a by-product during the recrystallization of [N(CH3)4]ClF4 from CH3CN solution. X-ray diffraction data show that [N(CH3)4]HF2 crystallizes in the orthorhombic space group Pmn21 with Z = 2 and unit cell dimensions a = 6.611(5), b = 8.753(5), and c = 5.386(4) Å. The structure was solved by direct methods and refined by least squares to a final R = 0.055 by using 205 independent reflections. The HF2− anions are symmetric, exhibit an unusually short [Formula: see text] distance of 2.213(4) Å, and vibrational frequencies close to those of the free HF2− anion. Keywords: tetramethylammonium bifluoride, crystal structure, Raman spectrum.
Trigonal prismatic versus octahedral coordination geometry of transition metal dithiolates: Comparison between saturated and unsaturated early transition metal tris(1,2-dithiolate) complexes
Authors: Martin Joel L. , Takats Josef
Published: May 2006

Reaction of a stoichiometric amount of H2edt/NaHedt (edt = ethane-1,2-dithiolate) with the appropriate metal amide, followed by cation exchange, gave the complexes [NEt4]2[M(edt)3] (M = Ti, 1; Hf, 2) and [AsPh4] [Nb(edt)3]•CH3CN, 3. The structure of 3 has been determined crystallographically (space group; cell dimensions; Z; unique data (I ≥ 3σ (I); R/Rw): [Formula: see text]; a = 9.095(1), b = 14.609(3), c = 15.639(3) Å and α = 60.09(2), β = 80.07(3), γ = 89.89(2)°; z = 2; 1488; 4.3/4.5. The coordination geometry of 3 is severely distorted from the trigonal prismatic (TP) structure found in the analogous Nb(1,2-S2C6H4)3−, which contains an unsaturated 1,2-dithiolate ligand system. The average twist angle between triangular faces is 33.3°. However, on the basis of VT 1H NMR spectroscopy it is argued that the TP form is at most 9–10 kcal mol−1 less stable than the observed geometry. A comparison between the structures of tris-chelate complexes of the early transition metals containing saturated and unsaturated 1,2-dithiolates and related ligands is made, and the factors thought to be responsible for TP vs. octahedral geometry are reviewed and their relative importance assessed. Keywords: structure, trigonal prismatic, 1,2-dithiolate, early transition metals.
Bromation régiosélective en série aromatique. I: Monobromation en position para de phénols et d'aminés aromatiques par le tribromure de tétrabutylammonium
Authors: Berthelot Jacques , Guette Catherine , Desbène Paul-Louis , Basselier Jean-Jacques , Chaquin Patrick , Masure Daniel
Published: May 2006

The reaction of tetrabutylammonium tribromide (TBABr3) with phenols and aromatic amines in aprotic and non-basic solvents at 20 °C gives exclusively the corresponding para-brominated compounds in high yields. A mechanism involving electrophilic substitution by the tribromide anion Br3− itself is suggested to account for the results, especially the regioselective para bromination. Keywords: bromination, tetrabutylammonium tribromide, phenols, aromatic amines.
Forbeside E: a novel sulfated sterol glycoside from Asteriasforbesi
Authors: Findlay John A. , He Zheng-Quan , Jaseja Mahesh
Published: May 2006

The structure of forbeside E, a minor constituent of the saponin fraction of the starfish Asteriasforbesi, has been identified as the disodium salt of (20R)-5α-pregn-9(11)ene-3β-sulfo-oxy-6α-[O-β-D-4-sulfo-oxyquinovopyranosyl]-20-ol, principally by 2D-COSY, DQ COSY, HCORR, and DEPT experiments. Sodium thornasterol-3-O-sulfate has been isolated as a natural product from the same organism. Keywords: sulfated sterol glycoside, Asteriasforbesi metabolites, sodium thomasterol-3-O-sulfate, asterogenol rearrangement.
Pyrazagaboles, pyrazolyl-bridged heterocycles containing boron and gallium
Authors: Rettig Steven J. , Sandercock Mark , Storr Alan , Trotter James
Published: May 2006

The synthesis and structural characterization of representative and novel pyrazagaboles, boron- and gallium-containing compounds with a central six-membered heterocyclic B—(N—N)2—Ga ring feature, are detailed. Crystals of dimethylgallium-bis(μ-pyrazolyl)dimethylboron are orthorhombic, a = 7.6135(5), b = 16.2917(5), c = 22.331(1) Å, Z = 8, space group Pbca, and those of hydroxy(methyl)galliumbis(μ-pyrazolyl)dimethylboron are orthorhombic, a = 7.6197(3), b = 16.2184(7), c = 22.190(1) Å, Z = 8, space group Pbca. Both structures were solved by conventional heavy atom methods and were refined by full-matrix least-squares procedures to R = 0.054 and 0.040 for 1243 and 1272 reflections with I ≥ 1.5 and 3σ(I), respectively. The crystal structures are isomorphous and both feature six-membered B—(N—N)2—Ga rings having highly asymmetric boat conformations, unlike those observed for the majority of the homobinuclear analogs previously reported. The mean Ga—N and B—N distances in the two compounds are 1.977 and 1.61 Å. Keywords: pyrazagaboles, pyrazolyl-bridged heterocycles, crystal structures, synthesis of pyrazagaboles.
Iodimetric determinations in organic solvents: Determination of manganese oxidation states in methylene chloride solutions
Authors: Perez-Benito Joaquin F. , Brillas Enrique , Arias Conchita
Published: May 2006

An iodimetric technique has been developed to quantitatively analyze the oxidants present in organic solutions. Addition of excess tetrabutylammonium iodide and excess glacial acetic acid results in the formation of tetrabutylammonium triiodide, whose concentration can be measured spectrophotometrically at 295 or 365 nm. Application of this technique to the determination of the final oxidation state of manganese in permanganate reactions in methylene chloride solutions is described. The equilibrium constant for formation of tetrabutylammonium triiodide in methylene chloride has been measured: K = (3.48 ± 0.12) × 104 at 25.0 °C. Keywords: iodimetric determinations, manganese oxidation states, methylene chloride, quaternary ammonium permanganate, quaternary ammonium triiodide.
Acid–base behaviour of the ferricyanide ion in perchloric acid media. Spectrophotometric and kinetic study
Authors: Domingo Pedro L. , García Begoña , Leal José M.
Published: May 2006

A wide study was carried out of the acid–base behaviour of ferricyanide ions in aqueous perchloric acid media in the range 0.1–11.5 M. This study shows that protonation and decomposition of the ferricyanide ions occur simultaneously. The three successive protonation equilibria were found in the acidity ranges 0.1–4.0, 3.5–8.5, and 7.0–11.5 M HClO4, respectively. A kinetic study was also made of the decomposition reaction at 60 °C. The kinetic data are explained by considering a reaction mechanism involving homolytic and heterolytic dissociation steps. The homolytic and heterolytic rate constants corresponding to each of the four protonated species were determined, along with the acid–base protonation constants, pK1 = −6.25 ± 0.10; pK2 = −3.23 ± 0.03; and pK3 = −0.60 ± 0.02. Keywords: acid–base behaviour, protonated species, decomposition, homolytic dissociation, heterolytic dissociation.
Single-ion Setschenow coefficients for several hydrophobic non-electrolytes in aqueous electrolyte solutions
Authors: Perez-Tejeda P. , Maestre A. , Delgado-Cobos P. , Burgess J.
Published: May 2006

Setschenow coefficients have been derived from solubility measurements on cyclohexane, benzene, naphthalene, 1-naphthol, 1,5-dihydroxynaphthalene, and anthracene in alkali halide, tetraalkylammonium bromide, and tetraphenylarsonium chloride aqueous solutions at 298.2 K. Single ion Setschenow coefficients have thence been obtained by an assumption involving extrapolation of the tetraalkylammonium bromide results to zero cation volume. Setschenow coefficients for the tetraalkylammonium cations correlate well with a hydrophobicity parameter based on their transfer chemical potentials from water into 1,2-dichloroethane. Keywords: solubilities, aqueous salt solutions, Setschenow coefficients.
Structural studies of digitoxin and related cardenolides by two-dimensional NMR
Authors: Drakenberg Torbjörn , Brodelius Peter , McIntyre Deane D. , Vogel Hans J
Published: May 2006

The 1H and 13C NMR spectra of the cardenolides digitoxigenin, digoxigenin, digitoxin, and mono- and bis-digitoxigenin digitoxosides have been completely assigned by two-dimensional NMR spectroscopy. The techniques used include phase-sensitive COSY, multiple relay COSY, and carbon–proton correlation (HETCOR and HMQC) spectra. Various aspects of the solution conformation of the steroid moiety of digitoxin and digoxigenin could be determined from coupling constants and NOE difference experiments and they are indicative of an all-chair conformation. The carbohydrate rings in digitoxin and the mono- and bis-digitoxigenin digitoxosides are also in the chair conformation. Keywords: cardenolides, digitoxigenin, digitoxin, 2-dimensional NMR, conformational analysis.
The effects of metal salts on the photosensitized (electron transfer) carbon–carbon bond cleavage of the 2,2-diphenylethyl ether system
Authors: Lamont Laurie J. , Arnold Donald R.
Published: May 2006

Irradiation of an acetonitrile solution of para-dicyanobenzene (1), 3,3-diphenyltetrahydrofuran (2), and magnesium perchlorate leads to the formation of 4,4-diphenyl-1,3-dioxane (7). Similarly, irradiation of an acetonitrile solution of 1, methyl 2,2-diphenylcyclopentyl ether (5), and magnesium perchlorate leads to the formation of 6,6-diphenyl-2-methoxytetrahydropyran (8). The mechanism proposed for these reactions involves formation of the radical cations [Formula: see text] and [Formula: see text] with the first excited singlet state of 1 acting as the photosensitizer (electron transfer). The radical cations [Formula: see text] and [Formula: see text] then cleave to give 1,5 radical cations. Reaction of the cationic site with water, followed by futher oxidation of the radical site by the perchlorate anion, gives the diphenylalkyl carbocation that can cyclize via a six-membered intermediate to the observed products 7 and 8. While the addition of other perchlorate salts (lithium and tetra-n-butylammonium) also lead to the formation of 7 and 8, lithium trifluoroacetate is ineffective. The proposal that water is involved as the nucleophile is supported by incorporation of 17O in the products 7 and 8 when 17O-enriched water was added to the reaction mixture. Keywords: photosensitization, electron transfer, radical cations, bond cleavage, 2,2-diphenylethyl ether.
g-Values for γ-irradiated water as a function of temperature
Authors: Elliot A. John , Chenier Monique P. , Ouellette Denis C.
Published: May 2006

The g-values of primary species formed in the γ-radiolysis of water have been estimated for the temperature range 25–300 °C. The g-values for the hydrated electron, the hydroxyl radical, and molecular hydrogen all increase approximately 50% over this temperature range. The g-values for the minor products, hydrogen peroxide and atomic hydrogen, probably also increase with temperature; however, there is evidence from some of the experimental data which suggests that these g-values may slightly decrease with increasing temperature. Keywords: radiolysis, g-values, temperature dependence.
Dihydride formation versus H2-elimination in the protonation of the heterobimetallic FePt complex (CO)3Fe(μ-H)(μ-PCy2)Pt(PEt3)2
Authors: Jenkins Hilary A. , Loeb Stephen J. , Dick David G. , Stephan Douglas W.
Published: May 2006

The reaction of Li[Fe(CO)4(PCy2)] with trans-PtCl(H)(PEt3)2 results in the formation of the hydride complex (CO)3Fe((μ-H)((μ-PCy2)Pt(PEt3)2, 1. This heterobimetallic, phosphido-bridged complex reacts with one equivalent of HBF4•Et2O to give the complex [(CO)3Fe(μ-H)2((μ-PCy2)Pt(PEt3)2][BF4], 2, which contains two bridging hydride ligands. This species is isolated and fully characterized by 31P{1H} and 1H NMR and infrared spectroscopy. In contrast, 1 reacts with one equivalent of HCl•DMA (DMA = dimethylacetamide) to give the complex (CO)3ClFe(μ-PCy2)Pt(PEt3)2, 3. This species is the result of oxidative addition of HCl with subsequent reductive elimination of H2(g). This complex is fully characterized by 31P{1H} and 1H NMR, infrared spectroscopy and an X-ray crystal structure determination. 3 crystallizes in the space group [Formula: see text] with a = 10.037(4) Å, b = 10.644(3) Å, c = 17.137(9) Å, α = 102.80(3)°, β = 76.74(3)°, γ = 103.99(3)°, V = 1702(1) Å3, and Z = 2. The structure was refined to R = 2.54% and Rw = 2.73% for 4056 reflections with Fo2 > 3σ(Fo2). Keywords: heterobimetallic, hydride, phosphide, protonation.
A chemiluminescence study of the kinetics of reactions of Cr(III) with carboxylate ligands: oxalate, salicylate, and humic acid
Authors: Huizenga Douglas L. , Patterson Howard H.
Published: May 2006

The reaction of Cr(III) with chelate-forming carboxylate ligands can be monitored at the low ligand concentrations typical of natural waters by chemiluminescence analysis of unreacted Cr(III). This technique is utilized here for the measurement of Cr(III) reaction kinetics with oxalate, salicylate, and humic acid over a wider ligand concentration range than previously reported. Variation of the pH and ligand concentrations allowd the determination of rates of reaction by three pathways with different pH dependencies. The magnitude of rate constants of these pathways for Cr(III) with inorganic (Cl−, SCN−) and carboxylate ligands gives insight into the reaction pathways for the environmentally important ligand humic acid. Keywords: Cr(III), oxalate, salicylate, humic acid.
Biomimetic ion transport: on the mechanism of ion transport by an artificial ion channel mimic
Authors: Fyles Thomas M. , James Tony D. , Kaye Katharine C.
Published: May 2006

The influx of cations into vesicles mediated by a synthetic transporter is coupled to proton efflux and may be quantified by a pH-stat technique. The dependence of the transport upon cation type and concentration, upon transporter concentration, and upon temperature has been examined. The synthetic transporter is closely similar to the natural channel forming compound gramicidin, and significantly different from the carrier valinomycin, with respect to the variables examined. Keywords: ion transport, vesicle membrane, channel, gramicidin, transport mechanism.
The solution conformation of the luteinizing hormone-releasing hormone (LHRH) anti-sense and reversed anti-sense peptides
Authors: Otter Albin , Scott Paul G. , Stewart John M. , Kotovych George
Published: May 2006

The LHRH anti-sense peptide (Ser1-Arg2-Ala3-Gln4-Ser5-Ile6-Gly7-Pro8-Val9-Leu10) and the inversed sequence, the LHRH inversed anti-sense peptide, have been studied by 1H and 13C NMR in one and two dimensions. The spectroscopic evidence, obtained in CD3OH/H2O (60/40) solution at pH 3.6, indicates that the first four amino acids of the anti-sense peptide form one complete 310-helical turn followed by an extended conformation for the rest of the peptide. On the time-averaged NMR scale the molecules in the 310-helical conformation are in fast exchange with the molecules that are extended throughout (major conformer). The reversed anti-sense peptide exhibits less discernible features. It appears likely that a small percentage of the molecules form a β-turn in the C-terminal four residues. Another β-turn might be located in the Val2-Ile5 fragment. Both turns occur in very minor concentrations only, and therefore it is impossible to define what kind of β-turns are present. Keywords: LHRH anti-sense, inversed anti-sense, conformation, NMR.
Minor saponins from the starfish Asteriasforbesi
Authors: Findlay John A. , He Zheng-Quan , Blackwell Barbara
Published: May 2006

The structures of three new saponins from the starfish Asteriasforbesi have been elucidated by spectroscopic means. Forbeside F is the hydrate of the monosodium salt of 6α-O-{β-D-fucopyranosyl(1 → 4)-O-[β-D-quinovopyranosyl(1 → 2)]-O-β-D-quinovopyranosyl(1 → 3)-O-6-deoxy-β-D-xylo-hexos-4-ulopyranosyl}-20S-hydroxy-3β-sulfo-oxy-5α-cholest-9(11)-en-23-one. Forbeside G is the hydrate of the monosodium salt of 6α-O-{[β-D-quinovopyranosyl(1 → 2)]-O-β-D-quinovopyranosyl(1 → 3)-O-6-deoxy-β-D-xylo-hexos-4-ulopyranosyl}-20S-hydroxy-3β-sulfo-oxy-5α-cholest-9(11)-en-23-one. Forbeside H is the monosodium salt of 6α-O-{[β-D-quinovopyranosyl(1 → 2)]-β-D-xylopyranosyl(1 → 3)-B-D-quinovopyranosyl}-20S-hydroxy-3β-sulfo-oxy-5α-cholest-9(11)-en-23-one. Keywords: Asteriasforbesi; forbesides F, G, and H; starfish saponins; sterol glycosides.
Tracer diffusion in aqueous sucrose and urea solutions
Authors: Easteal Allan J.
Published: May 2006

Tracer diffusion coefficients for water (as HTO), methanol (14CH3OH), and acetonitrile (CT3CN) in aqueous sucrose (10–25% w/w) and urea (0.25–4 mol L−1) solutions at 298 K have been determined using the diaphragm cell technique. For sucrose solutions tracer diffusion coefficients vary with the 2/3 power of the relative viscosity and the viscosity dependence is the same for the above tracers as for sucrose and oxygen. In urea solutions tracer diffusion coefficients appear to have a solute-specific viscosity dependence, but the solute specificity is largely removed when the effects of proton exchange and urea dimerisation are considered. Keywords: diffusion, water, methanol, acetonitrile, sucrose, urea.
Proton transfer reactions between 4-nitrophenylphenylcyanomethanes and cyclic nitrogen bases in acetonitrile solvent
Authors: Dworniczak Miroslaw , Leffek Kenneth T.
Published: May 2006

3,3,6,9,9-Pentamethyl-2,10-diazabicyclo[4.4.0]dec-1-ene has been synthesized and its proton transfer reactions, together with those of the bases 2,2,6,6-tetramethylpiperidine and 1,2,2,6,6-pentamethylpiperidine, have been studied with the carbon acids 4-nitrophenylphenylcyanomethane and 2-methyl-4-nitrophenylphenylcyanomethane. Equilibrium constants, rate constants, primary deuterium isotope effects, and activation parameters are reported for the reactions in acetonitrile solvent. Values of the pKa in acetonitrile have been measured for the conjugate acid of each base and the Brønsted β value of 0.56 in acetonitrile has been determined for the proton transfer from 4-nitrophenylphenylcyanomethane. Keywords: proton transfer reactions, kinetic isotope effects, cyclic nitrogen bases.