bluuebunny/crossref_metadata_2025 huggingface dataset:
The nitrous acid catalysed nitration of naphthalene. Evidence for a kinetic term that is second order with respect to the aromatic compound
Authors: Leis J. Ramón , Pena M. Elena , Ridd John H.
Published: May 2006

The kinetic equation for the nitrous acid catalysed nitration of naphthalene in aqueous mixtures of sulphuric acid and acetic acid has at least two kinetic terms: one first order with respect to naphthalene and one second order with respect to naphthalene. The orders with respect to nitrous acid and nitric acid vary with the conditions in the way characteristic of the electron transfer mechanism of this reaction. The second-order term with respect to naphthalene is considered to derive from the formation of the dimeric radical cation (ArH)2+•. The acidity dependence of the rate coefficients and the absence of a normal isotope effect in the reaction of naphthalene-d8 are consistent with this interpretation. Keywords: naphthalene, nitration, nitrous acid.
Ligand substitution reactions of pentacyanoferrate(II) complexes with N-heterocyclic cations in aqueous solution
Authors: Macartney Donal Hugh , Warrack Lauren Jean
Published: May 2006

Kinetic and spectroscopic studies have been carried out in aqueous solution on the formation (from Fe(CN)5OH23−) and dissociation of pentacyanoferrate(II) complexes containing 1-(4-pyridyl)pyridinium and the neutral, protonated, and N-methylated forms of 4,4′-bipyridine (BPY), 1,2-bis(4-pyridyl)ethane (BPA), and trans-1,2-bis(4-pyridyl)ethylene (BPE). The pH dependences of the formation kinetics have been analyzed in terms of the specific rate and acid dissociation constants for these ligands. The rate constants (25.0 °C, I = 0.10 M) for the formation of the dinuclear complexes (NC)5FeLFe(CN)56− have been determined for BPY (50 M−1 s−1), BPA (66 M−1 s−1), BPE (95 M−1 s−1), and pyrazine (10 M−1 s−1), and are compared with the respective rate constants for the formation of (NC)5FeLCo(NH3)5. The relationships between the formation rate constants and the size of the ligand, the number of donor sites, and the magnitude and position of charges on the ligand are discussed in terms of an ion-pair dissociative mechanism. Keywords: pentacyanoferrate(II) complexes, N-heterocycles, ligand substitution, kinetics.
On the prediction of the structures of simple and hypervalent hydrides and fluorides containing unpaired electrons
Authors: Baird N. Colin , Kuhn Markus , Lauriston Toyanne M.
Published: May 2006

Abinitio molecular orbital calculations are reported for the optimum structures of many free radicals, including several species in which the central atom is hypervalent, and for some triplet states. Most geometries are close to those predicted by the VSEPR method for molecules with one additional electron, with the unpaired electron occupying a "lone pair" position. Deviations from the ideal geometries of hypervalent radicals can be rationalized in terms of different strengths for repulsions between electrons of different types. Alternatively the geometries can be understood by using the three-centre, four-electron bond theory for the hypervalent species and the σ-conjugation concept for the non-hypervalent molecules. The distortions from ideality of the hypervalent structure occur to allow interaction of the electron pair of the nonbonding MO of the three-center unit to interact with a lobe of the unpaired electron's orbital. Keywords: geometry of free radicals, abinitio MO theory for radicals, VSEPR theory for radicals, sigma conjugation for radicals.
Synthesis, structure, and vibrational spectrum of poly-μ-pyrazine(pyrazine)(trifluoromethanesulfonato-O)copper(I)
Authors: Otieno T. , Rettig S. J. , Thompson R. C. , Trotter J.
Published: May 2006

Orange–brown crystals of composition Cu(pyz)2(CF3SO3) (pyz = pyrazine, 1,4-diazine) were obtained from methanol solutions containing Cu(CF3SO3)2 and pyrazine in approximate 1:2 mole ratio. Single crystal X-ray diffraction and infrared and Raman spectroscopy studies are reported. Crystals of the title compound are triclinic, a = 8.312(2), b = 10.903(3), c = 8.201(2) Å, α = 92.53(2), β = 113.77(2), γ = 91.40(2)°, Z = 2, space group [Formula: see text] The structure was solved by heavy atom methods and was refined by full-matrix least-squares to R = 0.035 and Rw = 0.044 for 2458 reflections with 1 ≥ 3σ(I). The compound contains chains of copper ions linked by bidentate bridging pyrazine ligands. The coordination around each copper is completed by monodentate pyrazine and trifluoromethanesulfonato groups to produce a distorted tetrahedral CuN3O chromophore. Keywords: crystal structure, μ-pyrazine copper(I) triflate polymer, vibrational spectra.
Solid-state motion of OTeF5− compounds: detection by 19F NMR and IR spectroscopy and correlation with the X-ray structure of an orthorhombic crystalline modification of [C14H19N2+][OTeF5−] (C14H19N2+ = protonated 1,8-bis(dimethylamino)naphthalene)
Authors: Kellett Patti J. , Anderson Oren P. , Strauss Steven H. , Abney Kent D.
Published: May 2006

An orthorhombic crystalline modification of [(PS)H+][OTeF5−] was studied by single crystal X-ray diffraction ((PS)H+ = protonated 1,8-bis(dimethylamino)naphthalene): orthorhombic, space group Cmcm, a = 8.607(2), b = 14.048(3), c = 13.365(2) Å, Z = 4, T = −130 °C. The structural parameters for the anion and cation in this modification (Mod-B) are very similar to those for the previously reported triclinic modification of this salt (Mod-A). However, variable temperature IR spectra for Mod-B suggest that the OTeF5− anion is exhibiting a two-site O/Fax interchange above −70 °C. Solid-state 19F NMR spectra suggest that this interchange may be as rapid as 104–105 s−1 at room temperature. Keywords: crystal structure of protonated 1,8-bis(dimethylamino)naphthalene)pentafluorotellurate(VI), polymorphism, solid state 19FNMR.
Aryl-2 et alkyl-2 quinoléines nouvelles isolées d'une Rutacée bolivienne: Galipealongiflora
Authors: Fournet Alain , Vagneur Beatrice , Richomme Pascal , Bruneton Jean
Published: May 2006

Five new 2-aryl and 2-alkyl quinoline alkaloids were obtained from Galipeallongiflora Krause (Rutaceae). These are 2-phenylquinoline 1, 2-(3′,4′-methylenedioxyphenethyl)quinoline 2, 2-(3′,4′-dimethoxyphenethyl)quinoline 3, 2-(3′,4′-methylenedioxystyryl)-4-methoxyquinoline 4, and 2-(1′,2′-pentenyl)-4-methoxyquinoline 6. They are accompanied by three known 2-substituted quinolines, 2-n-amyl-4-methoxyquinoline 5, 2-(3′4′-methylenedioxyphenethyl)-4-methoxyquinoline 7, and 2-phenyl-4-methoxyquinoline 8, and by two known furo[2,3b]quinoline alkaloids: evolitrine and skimmianine. Keywords: Galipealongiflora, Rutaceae, new quinolines.
Identification of isodomoic acid D and two new geometrical isomers of domoic acid in toxic mussels
Authors: Wright Jeffrey L. C. , Falk Michael , McInnes A. Gavin , Walter John A.
Published: May 2006

Isodomoic acids E and F, two new geometrical isomers of the neurotoxin domoic acid, have been found to occur with domoic acid and isodomoic acid D in extracts of toxic mussels. The entire set of geometrical isomers can be prepared in the laboratory by photolysis. Keywords: amnesic shellfish toxin, domoic acid, isodomoic acid, neurotoxin.
Infrared and vibrational circular dichroism intensities of model systems CH3OH, CH3NH2, NH2NH2, NH2OH, and HOOH and the deuterated species, ND2ND2, DOOH, and DOOD: A theoretical study using the vibronic coupling formalism
Authors: Rauk Arvi , Dutler Remo , Yang Danya
Published: May 2006

We have implemented at the abinitio level the vibronic coupling formalism for infrared and vibrational circular dichroism (VCD) intensities of Nafie and Freedman. We report here the results of a series of calculations on model systems. Hydrazine and hydrogen peroxide exist in chiral conformations. Although low barriers to racemization preclude the possibility of optical resolution of the parent compounds, the local gauche configuration at the heteroatom bond is preserved in substituted hydrazines and peroxides and these chiral fragments may contribute significantly to the vibrational chiroptical properties of the molecules in which they appear. The calculated VCD rotational strengths for the chiral gauche forms of both molecules suggest that the torsional mode is very intense in the VCD spectrum. In gauche hydrazine of P chirality, the in- and out-of-phase umbrella motions of the nitrogen atoms, ν6 and ν12, respectively, give rise to an intense CD couplet with negative rotational strength to the long wave length side. In hydrazine, all combinations of NH stretches are relatively weak in the IR but the highest frequency mode of b symmetry, ν8, has moderate positive strength. The in- and out-of-phase OOH bends of HOOH behave as a coupled oscillator system, exhibiting a strong bisignate CD pattern. The signals calculated for the OH stretches are not consistent with a coupled oscillator model. The nearly degenerate stretches result in a single band arising from the more intense negative rotational strength of the antisymmetric mode ν5, in HOOH of P chirality. An alternative model, based on induced charge flow along the chirally disposed skeleton (i.e., Nafie and Freedman's 3-bond electronic current model), explains the calculated sign of the antisymmetric mode, ν5. Significant changes in the VCD patterns due to deuterium substitution in both chiral molecules are discussed. Keywords: vibrational circular dichroism (VCD), abinitio, infrared intensities, molecular orbital, vibronic coupling theory, methanol, methanamine, hydrazine, hydroxylamine, hydrogen peroxide.
Thermodynamics of electrolyte solutions: activity coefficients of NaCl in the NaCl–NiCl2–H2O system at 25, 35, and 45 °C
Authors: Venkateswarlu Ch. , Ananthaswamy J.
Published: May 2006

The activity coefficients of NaCl in the NaCl–NiCl2–H2O system were estimated at 25, 35, and 45 °C and total ionic strengths of 0.5, 1.0, 2.0, and 3.0 m by an EMF method using a Na-ion selective electrode and a silver–silver chloride reference electrode. The Harned coefficients were calculated at all the temperatures studied. At 25 °C the data were analysed using the Pitzer formalism. The osmotic coefficients and the excess free energies of mixing were also calculated at 25 °C. Keywords: activity coefficients, sodium chloride, nickel chloride, Pitzer equations, thermodynamics.
Structural characterization of the capsular polysaccharide and O-chain of the lipopolysaccharide of Actinobacilluspleuropneumoniae serotype 8 (strain 405)
Authors: Altman Eleonora , Brisson Jean-Robert , Perry Malcolm B.
Published: May 2006

The capsular polysaccharide of Actinobacilluspleuropneumoniae serotype 8 (strain 405) was found to be a teichoic acid type polysaccharide composed of 2-acetamido-2-deoxy-D-galactose (two parts), glycerol (one part), and phosphate (one part). From hydrolysis, dephosphorylation, methylation, and 1H and 13C nuclear magnetic resonance studies, the polysaccharide was found to be a high molecular weight linear polymer having the structure:[Formula: see text]The lipopolysaccharide O-antigen of A. pleuropneumoniae serotype 8 was shown to be a high molecular weight unbranched linear polymer of a repeating pentasaccharide unit having the structure identical to that produced by A. pleuropneumoniae serotype 3, as follows:[Formula: see text]Keywords: Actinobacilluspleuropneumoniae polysaccharide, lipopolysaccharide, NMR.
The direct electrochemical synthesis of d10 metal ion derivatives of some anionic bidentate oxygen donors
Authors: Annan Theodore A. , Peppe Clovis , Tuck Dennis G.
Published: May 2006

The electrochemical oxidation of a sacrificial anode (M = Zn, Cd, Hg, In, Sn) in non-aqueous solutions of 3-hydroxyflavone, 1′-hydroxy-2′-acetonaphthone, or 2-ethoxyphenol (= HL) gives ZnL2, CdL2, HgL, InL3, or SnL2 (not all combinations). Adducts with 2,2′-bipyridine (bpy) or N,N,N′,N′-tetramethylethanediamine (tmen) have also been prepared in some cases. Certain zinc and cadmium compounds produce paramagnetic species on exposure to the atmosphere. The structure of Zn(C15H9O3)2tmen has been determined; the crystals are orthorhombic, space group Pbcn, cell dimensions a = 18.817(2) Å, b = 13.886(3) Å, c = 12.567(3) Å, Z = 4, R = 0.0758 for 773 unique observed reflections. The corresponding data for Cd(C12H9O2)2bpy are orthorhombic, space group Pbca, cell dimensions a = 20.338(4) Å, b = 30.473(5) Å, c = 9.316(2) Å, Z = 8, R = 0.0457 for 2623 unique observed reflections. Both molecules have MO4N2 kernels, but each shows significant differences in the two M—O bond lengths to the dioxo ligand, and there are concomitant effects in the C—O and C—C bond distances in the ligands. Keywords: electrochemical synthesis, d10 metal ions, X-ray crystallography, zinc complex, cadmium complex.
Dielectric polarization and molecular interaction. Part III. Styrene, benzene, and ethyl vinyl ether – acrylonitrile – cyclohexane systems
Authors: Saad Gamal R. , Naoum Magdi M. , Rizk Hanna A.
Published: May 2006

The apparent molecular polarization at infinite dilution of acrylonitrile in constant-ratio mixtures of styrene, benzene, and ethyl vinyl ether in cyclohexane leads to the inference of the equilibrium constants of the association process between the acid and the base, and the dipole moments of the complexes formed. The results are mainly consistent with the concept of dipole–dipole interaction. Keywords: dielectric polarization, molecular interaction, styrene, acrylonitrile, cyclohexane.
Antineoplastic agents. 193. Isolation and structure of the cyclic peptide hymenistatin 1
Authors: Pettit George R. , Clewlow Paul J. , Dufresne Claude , Doubek Dennis L. , Cerny Ronald L. , Rützler Klaus
Published: May 2006

The Western Pacific Ocean sponge Hymeniacidon sp. was found to contain a cyclo-octapeptide designated hymenistatin 1 (1) active against the P388 leukemia cell line (ED50 3.5 μg/mL). Structural determination was accomplished utilizing high field NMR (400 MHz) and mass spectral techniques (FAB MS/MS) followed by chiral gas chromatographic analysis to establish the absolute configuration (all S-amino acids). Keywords: cytostatic, Hymeniacidon, hymenistatin, cyclo-octapeptide, P388 leukemia.
Elemental analysis of the marine biological reference material LUTS-1 by instrumental neutron activation, graphite furnace atomic absorption spectroscopy, and anodic stripping voltammetry
Authors: Motkosky Norine , Fernando Angelo Ransirimal , Kratochvil Byron
Published: May 2006

Marine biological reference material LUTS-1, lobster heptopancreas, was analyzed for elemental homogeneity using graphite furnace atomic absorption, anodic stripping voltammetry, and neutron activation analysis. Analysis of samples taken from 12 bottles of LUTS-1 bottled on two different days showed no statistical differences at the 95% confidence level for within-bottle variance for a large number of elements. Differences were observed for between-day variances for aluminum, iron, cobalt, sodium, chlorine, bromine, and iodine, but not at a level sufficient to affect utility as a reference material. Keywords: neutron activation analysis, graphite furnace atomic absorportion spectroscopy, anodic stripping voltammetry, elemental analysis, marine biological reference material.
Medium effect on reactions between anions carrying different charges
Authors: Rodriguez Amalia , Burgos Francisco Sánchez , Burgess John , Carmona Carmen
Published: May 2006

The reactions between hexacyanoferrate(II)–peroxodisulphate, hexacyanoferrate(III)–sulphite, hexacyanoferrate(III)–iodide and peroxodisulphate–iodide have been studied in methanol–water mixtures at different methanol concentrations. The results have been analyzed using different approaches, namely electrostatic theory, the δμθ initial state – transition state analysis, the decomposition of free energy of activation into enthalpic and entropic terms and, finally, correlation between reactivity and the Grunwald–Winstein parameter, Y. The four reactions show similar behaviour and a correlation between reactivity and the charge of the reactants apparently does not exist. The effect of solvent polarity on the encounter formation processes seems to be an important factor in determining reactivity. However, the influence of solvent on electron transfer process within the encounter complexes cannot be discarded. Keywords: Solvent effects, kinetics, methanol, activation parameters, thermodynamic functions of transfer.
The acetolysis of reduced Acaciasaligna (syn. cyanophylla) gum exudate
Authors: Charlson Alexander J. , Stephen Alistair M. , Ravenscroft Neil
Published: May 2006

Carboxyl groups in Acaciasaligna (syn. cyanophylla) gum were reduced by reacting the gum propionate with diborane. Acetolysis of the reduced gum afforded a mixture of oligosaccharide acetates. After de-O-acetylation, 3-O-(α-D-galactopyranosyl)-L-arabinose, 3-O-(β-D-galactopyranosyl)-D-galactose, and 4-O-(α-L-rhamnopyranosyl)-D-glucose were isolated from the mixture. Keywords: acetolysis, Acacia gums.
W-shape and solution non-randomness: hydrocarbons + tetraalkoxysilanes
Authors: Andreoli-Ball Lina , Patterson Donald
Published: May 2006

The excess heat capacity, [Formula: see text], has been measured through the concentration range at 25 °C for cyclohexane + Si(OR)4 where R = methyl (Me), ethyl (Et), propyl (Pr), and butyl (Bu). [Formula: see text] is negative and concave upward for the last three oxysilanes. For R = Me [Formula: see text] has a region of zero curvature, which, on reducing T to 2 °C, becomes concave downward, i.e., its concentration dependence becomes W-shaped. [Formula: see text] has also been obtained at 15 °C for Si(OMe)4 mixed with the following hydrocarbons of increasing molecular size: cyclooctane, cis-decalin, dicyclohexyl, n-hexadecane, 2,2,4,4,6,8,8-heptamethylnonane, 2,6,10,14-tetramethylpentadecane, and 2,6,10,15,19,23-heptamethyltetracosane (at 25 °C). All [Formula: see text] values for these systems present a W-shape, which, with increasing hydrocarbon size becomes increasingly pronounced and moves to lower hydrocarbon concentration. [Formula: see text] behaviour correlates with degrees of non-randomness in the systems as quantified by the concentration–concentration correlation function Scc which is predicted using the Flory–Huggins theory with a group-interaction model and assuming interaction between molecular surfaces. Differences between [Formula: see text] for n-C16 and highly branched C16 isomer, 2,2,4,4,6,8,8-heptamethylnonane, are attributed to liquid state orientational order and higher molecular surface/volume ratio for the normal isomer. Keywords: excess heat capacity, W-shape, non-randomness, hydrocarbons, tetraalkylsiloxanes.
Characterization of asphaltenes from Cold Lake heavy oil: variations in chemical structure and composition with molecular size
Authors: Semple Kathleen M. , Cyr Natsuko , Fedorak Phillip M. , Westlake Donald W. S.
Published: May 2006

Asphaltenes from Cold Lake oil were separated into fractions by size exclusion chromatography to study the molecular size dependence of chemical structural characteristics. Each fraction was analyzed by proton NMR and UV–visible spectroscopy and by elemental analyses for C, H, N, S, Ni, and V. The results indicated that the nature of the average chemical structures in Cold Lake asphaltenes were not uniform throughout the total range of molecular sizes but varied significantly; some changed gradually and some changed quite abruptly. Especially in the very low molecular-weight fraction, the size of aromatic moieties seems to be much smaller, the contents of both aromatic carbon and aromatic hydrogen are higher, the C/H ratio is higher, and the porphyrin and vanadium concentrations are much higher than in heavier fractions. Keywords: Cold Lake asphaltenes, size fractionation, vanadium, proton NMR, porphyrin, chemical structural analysis.
13C CP/MAS NMR spectra of poly-β-D(1 → 4) mannose: mannan
Authors: Marchessault R. H. , Taylor M. G. , Winter W. T.
Published: May 2006

13C solid-state NMR spectra (CP/MAS) were recorded for mannan polymorphs: mannan I ("ivory nut") and mannan II (Codiumfragile). These pure mannans, poly-β-(1 → 4) mannopyranose, show a single resonance for each carbon of the chemical repeat unit. This is the expected result based on the crystalline P212121 and 1222 symmetries. The T1 values were also recorded and the results are compared to those for cellulose obtained from the literature. The much lower mobility of C-6, even in noncrystalline mannan, compared to native cellulose is attributed to the epimeric difference at C-2 between cellulose and mannan allowing intramolecular hydrogen bonding in the former but not the latter. Keywords: mannan, solid state NMR, T1 relaxation, polysaccharide, ivory nut.
Application of FT-Raman spectroscopy to the study of selected organometallic complexes and proteins
Authors: Barnett Steven M. , Dicaire François , Ismail Ashraf A.
Published: May 2006

The study of colored organometallic complexes by dispersive Raman spectroscopy has been limited due to fluorescence or photodecomposition caused by the visible laser used as the excitation source. As a solution to this problem, FT-Raman spectroscopy with a near-infrared laser source has been useful in lowering fluorescence or photolysis in these samples. To investigate the utility of this technique, we have obtained and assigned the FT-Raman spectra of a series of arene chromium tricarbonyl complexes and of cyclopentadienyl manganese tricarbonyl. Some bands previously unobserved by dispersive Raman spectroscopy were seen, including a band assigned to a 13CO satellite in the spectrum of methylbenzoate chromium tricarbonyl. In addition, FT-Raman data for bovine serum albumin (BSA) and Protein-A are presented. Keywords: FT-Raman spectroscopy, metal carbonyl, proteins, organometallics, near infrared.
Xα local spin density calculations of the 1:1 hydrogen-bonded complexes formed by water, ammonia, and hydrogen fluoride
Authors: Vauthier E. C. , Barone V. , Minichino C. , Fliszàr S.
Published: May 2006

The dissociation energies of the 1:1 hydrogen-bonded complexes formed by NH3, H2O, and HF were computed in the LCGTO-Xα local spin density approximation using extended basis sets. Attention was given to the appropriate selection of α. The order of stability of the various complexes reflects well their acid–base properties, in general agreement with experimental data and refined post Hartree–Fock computations. Keywords: hydrogen bonds, Xα method, local spin density method.
Average local ionization energies on the molecular surfaces of aromatic systems as guides to chemical reactivity
Authors: Sjoberg Per , Murray Jane S. , Brinck Tore , Politzer Peter
Published: May 2006

The average ionization energy, [Formula: see text], is introduced and is demonstrated to be useful as a guide to chemical reactivity in aromatic systems. [Formula: see text] is rigorously defined within the framework of self-consistent-field molecular orbital (SCF-MO) theory and can be interpreted as the average energy needed to ionize an electron at any point in the space of a molecule. An abinitio SCF-MO approach has been used to calculate [Formula: see text] at the 6-31G* level, using STO-3G optimized geometries. [Formula: see text] has been computed on molecular surfaces defined by the contour of constant electronic density equal to 0.002 electrons/bohr3, for a series of aromatic systems. This surface [Formula: see text] provides site specific predictions for preferred positions of electrophilic aromatic substitution. Relative reactivity toward electrophiles increases as the magnitudes of the smallest [Formula: see text] values [Formula: see text] for these systems decrease. An excellent relationship, with a correlation coefficient of 0.99, has been found between the Hammett constants and [Formula: see text]; this allowed us to predict the values of these constants for the substituents NHF and NF2, for which they were previously not known. Keywords: average local ionizations energy, chemical reactivity, electrophilic aromatic substitution, molecular surfaces, Hammett constants.
Stabilization of iron-catalysed hydrogen peroxide decomposition by magnesium
Authors: Abbot John , Brown Douglas G.
Published: May 2006

Catalytic decomposition of alkaline hydrogen peroxide by iron can be retarded by introduction of magnesium ions. This effect has been studied to evaluate the possibility of stabilization via formation of an iron–magnesium complex species. Under alkaline conditions, magnesium reacts with initial hydrolysis products of Fe3+ to produce a colourless complex species, in which the metal centres are probably linked through oxy or hydroxy bridges. This species is produced when the Mg:Fe molar ratio exceeds 6:1, and this ratio is also significant when magnesium is introduced during peroxide decomposition experiments. The evidence suggests that complex formation is an important factor in producing stabilization, and cannot be disregarded in favour of an alternative explanation where superoxide radicals combine with Mg2+ to produce magnesium dioxide. Keywords: hydrogen peroxide, kinetics, iron, magnesium, stabilization.
The crystal structure and chemistry of the "red salt" α-trisilver-amidoselenate, Ag3NSeO3
Authors: Kocman Vladimir , Szymański Jan T.
Published: May 2006

The crystal structure of the α-modification of the "red salt" of trisilver amidoselenate, Ag3NSeO3 has been determined from MoKα X-ray diffraction data, and refined to R = 3.19% from 337 unique absorption-corrected observed reflections. The structure is rhombohedral, R3c (No. 161), with (hexagonal) a = 8.462(4), c = 11.372(6) Å. The formula unit has point symmetry 3 (C3), with Se and N on the threefold axis, each approximately tetrahedrally coordinated; Se by three oxygen atoms (at 1.651(8) Å and N (at 1.70(2) Å, and N by Se and three silver atoms (at 2.164(8) Å. The bond of 1.70(2) Å between Se and N is particularly short. The Ag3NSeO3 structural unit bridges to three adjacent units via short Ag … O contacts of 2.23(1) Å to form a three-dimensional network. Keywords: trisilver amidoselenate, selenium–nitrogen bond, X-ray structure.
The chain carriers of intermolecular hydrogen abstraction in the photobromination with N-bromosuccinimide; modifications of the chain carriers
Authors: Zhang Yu-Huang , Dong Ming-Hua , Jiang Xi-Kui , Chow Yuan L.
Published: May 2006

The photoinitiated bromination of 1-chloropentane with N-bromosuccinimide (NBS) in the presence of Br2 is compared with that of NBS and 1,1-dichloroethene to generate the succinimidyl radical (S•) and with that of Br2 + K2CO3 to generate the bromine atom (Br•) as the chain carriers. The relative reactivities of intermolecular H-abstraction (r-values) shown by the NBS + Br2 system decrease to levels lower than those shown by either S• or Br• chain propagations at lower temperatures under appropriate conditions: these observations indicate the presence of a new chain propagating species that is assumed to be the bromine radical complex (BRC) modified from S• or Br• reversibly under the experimental conditions. Supporting evidence for the equilibria of S•, Br•, and BRC in the photodecomposition of NBS + Br2 is discussed. The unusual H-abstraction reactivity of Br• toward the methyl hydrogens of 1 -chloropentane was noted; this serves as independent support for the presence of a new radical propagation species. Keywords: N-bromosuccinimide, succinimidyl radical, photobromination, bromine radical complex.
Kinetics of the detritiation of [10,10-3H] mianserin and related compounds in highly basic media
Authors: Jones John R. , Wickenden David A. , Funke Carel W. , Kaspersen Frans M.
Published: May 2006

A recently developed procedure (Buist etal., J. Chem. Soc. Perkins II, 169 (1990)) has been used to study the kinetics of detritiation of [10,10-3H] mianserin and two other structurally related compounds in highly basic media. Over the H− range 16–20 the stereoselectivity of the exchange process keq/kax for all three compounds is virtually constant (5 ± 1). The apparent increase at still higher basicities is associated with distinct curvature in the log k vs. H− plots, indicating the onset of another mechanism. Partial detritiation studies at higher levels of radioactivity, when combined with 3H nmr spectroscopic analysis, confirms the greater lability of the C-10 (or C-9) equatorial hydrogen. Keywords: detritiation, high basic media, mianserin.
A simple synthetic route to 4-methylcamphor: mechanistic aspects
Authors: Clase J. Andrew , Li Diana L. F. , Lo Linda , Money Thomas
Published: May 2006

A mechanism proposed for the rearrangement of (+)-2-methylenebornane (8) to a mixture of (+)-4-methylisobornyl acetate (5) and its enantiomer (ent-5) is supported by evidence obtained by investigating the rearrangement of deuterated 2-methylenebornane. Keywords: 4-methylcamphor, Wagner–Meerwein rearrangement, 6,2-hydride shift, 9-bromo-4-bromomethylcamphor.
The base-catalysed anomerization of dinitrophenyl glycosides: evidence for a novel reaction mechanism
Authors: Berven Leise A. , Dolphin David , Withers Stephen G.
Published: May 2006

The mechanism of base-catalysed anomerization of per-O-acetylated 2,4-dinitrophenyl-β-D-glucopyranoside in dimethylsulfoxide has been investigated using a variety of techniques. A mechanism involving proton abstraction at C-1 was eliminated by the absence of proton exchange at that center and the measurement of a secondary deuterium kinetic isotope effect for the 1-deuterio substrate. A mechanism involving phenolate departure and recombination is rendered unlikely on the basis of remote substituent effects on the reaction rate and by the absence of any exchange of the phenyl moiety with added phenolate. A mechanism involving nucleophilic aromatic substitution initiated by an attack of the dimethylsulfinyl anion to generate a glucosyl oxyanion intermediate that anomerizes and recombines with the reactive aryl intermediate is consistent with the observations. This mechanism is further supported by the observation of a purple Meisenheimer complex intermediate and by the observed exchange between the substrate containing a labelled sugar moiety and added unlabelled 2,3,4,6-tetra-O-acetyl-β-D-glucopyranose. Keywords: glycoside, anomerization, reaction mechanism.
An asymmetric synthesis of 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN)
Authors: Charlton James L. , Koh Kevin , Plourde Guy L.
Published: May 2006

An asymmetric synthesis of 2-amino-6,7-dimethoxy-1,2,3,4-tetrahydronaphthalene, which can be converted to the title compound by a literature procedure, is described. The synthesis, starting from 2-amino-4,5-dimethoxybenzoic acid and the acrylate of S-methyl lactate, was accomplished in eight steps in 11% overall yield and > 97% optical purity. Keywords: o-quinodimethanes, Diels–Alder, asymmetric, cycloaddition, induction, diastereoselective, ADTN.
The metabolites of Talaromycesflavus: Part 2. Biological activity and biosynthetic studies
Authors: Ayer William A. , Racok Julie S.
Published: May 2006

The soil fungus Talaromycesflavus (Klöcker) Stolk and Samson is an effective biological control for Verticillium wilt of eggplant (Solanummelongena L.), a disease caused by the fungus Verticilliumdahliae Kleb. The water soluble metabolites isolated from the broth when the fungus is grown in liquid still culture, hydroxymethylmaltol (1), 5,6-dihydro-3,5-dihydroxy-6-hydroxymethyl-2H-pyran-2-one (3), and D-glucono-1,4-lactone (5), do not show antifungal activity against V. dahliae; however, hydrogen peroxide displays this inhibition. This peroxide has been detected in the T. flavus broth and it, together with 5, is the product of the glucose oxidase catalyzed oxidation of D-glucose. The results of the search for this antifungal activity are presented. [1-13C] Labelled sodium acetate was utilized as a precursor for studying the biosynthesis of the T. flavus metabolites 4,6-dihydroxy-5-methylphthalide (7), methyl 4-carboxy-5-hydroxyphthalaldehydate (8), and talaroflavone (9). The incorporation of label into these compounds is discussed. Keywords: Talaromycesflavus, fungal metabolites, hydrogen peroxide, biosynthesis, talaroflavone.
NMR relaxation of tin-119 in pentacoordinate and hexacoordinate environments
Authors: Grindley T. Bruce , Curtis Ronald D. , Thangarasa Rasiah , Wasylishen Roderick E.
Published: May 2006

Carbon-13 and 119Sn nuclear magnetic resonance spin-lattice relaxation times at 8.48 T and at 4.70 T have been measured at several temperatures for a number of 2,2-di-n-butyl-1,3,2-dioxastannolane derivatives, most of which were prepared from monosaccharide derived diols. The tin nuclei in these compounds, which are at either pentacoordinate or hexacoordinate sites, have 119Sn T1values that are relatively short, between 13 ms and 300 ms. At 8.48 T, chemical shift anisotropy is the only important mechanism for relaxation of the 119Sn nuclei in these compounds. However, at 4.70 T, the 1H,119Sn dipole–dipole mechanism also contributes slightly. The principal components of the 119Sn chemical shift tensor for the pentacoordinate site in the dimeric structure of methyl 4,6-O-benzylidene-2,3-dibutylstannylene-α-D-glucopyranoside (1) were determined directly from the static 119Sn spectrum of a powdered sample and indirectly from slow spinning CP/MAS spectra using both the Herzfeld–Berger procedure and computer simulation. The tin shift tensor for 1 in the solid state is non-axially symmetric with η = 0.49 ± 0.03. The chemical shift anisotropy determined from the solid state experiments, 748 ± 20 ppm, was in good agreement with the value obtained from the solution T1data, 720 ± 50 ppm. For compounds containing both pentacoordinate and hexacoordinate tin atoms, the 119Sn relaxation data indicate that the tin chemical shift anisotropies for the hexacoordinate sites are approximately 1.6 times those of the pentacoordinate sites. Keywords: 119Sn NMR, NMR relaxation, stannylene acetals, 1,3,2-dioxastannolanes.
Modelling complex solution equilibria. I. Fast, worry-free least-squares refinement of equilibrium constants
Authors: Potvin Pierre G.
Published: May 2006

The methods of Steepest Descent, Gauss–Newton (first-order Taylor series) and Newton–Raphson (second-order Taylor series) least-squares iteration were examined in the context of the refinement of estimated equilibrium constants β in solution. Under certain conditions, all three can produce corrections to the parameters that overshoot the global minimum and diverge therefrom, owing to the shape of the parameter surface. The latter two methods are problematic when the β parameters are overestimated, or when their logarithms are underestimated, whence a useful approximation to the analytical second and higher derivatives was found for any data type. This reduces an exact, infinite-order Taylor series expression of any observable to a simple first-order expression. As illustrated with experimental pH data, faster, more reliable refinement results without overshoot or divergence problems, and without resort to computationally onerous algorithms, such as the Marquardt–Levenberg, Fletcher–Powell, or Hartley–Wentworth methods. Keywords: equilibrium constants, least-squares, Gauss–Newton, complex solution equilibria.
Structural studies of organoboron compounds. XLII. 4,6-Bis(1-cyano-1-methylethyl)-2-mesityl-1,3-dioxa-4,6-diaza-2-boracyclohexane
Authors: Kliegel Wolfgang , Lubkowitz Gottfried , Rettig Steven J. , Trotter James
Published: May 2006

The condensation of N,N′-bis(1-cyano-1-methylethyl)-N,N′-dihydroxymethanediamine and mesitylboronic acid gives 4,6-bis(1-cyano-1-methylethyl)-2-mesityl-1,3-dioxa-4,6-diaza-2-boracyclohexane in good yield. Crystals of the latter compound are orthorhombic, a = 15.825(1), b = 17.958(1), c = 14.014(1) Ǻ, Z = 8, space group Pbca. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.037 and Rw = 0.051 for 2844 reflections with 1 ≥ 3σ(I). The molecule has a six-membered cycloboronate structure featuring the first structurally characterized BONCNO ring. The ring was found to have a C-envelope conformation in the solid state. Bond lengths include: O—B(sp2) = 1.356(2) and 1.370(2), and C(aryl)—B = 1.560(2) Ǻ. Key words: organoboron compound, boron compound, crystal structure.
Tetrahalogenomercury(II) complexes. A study of their electronic spectra, formation constants, ionic strength, and cation effects
Authors: Griffiths Trevor R. , Anderson Richard A.
Published: May 2006

The unambiguous electronic absorption spectra of HgX42−(X = Cl, Br, or I) in methanol have been determined for the first time. The spectra were resolved into their component Gaussian bands, and assigned using a correlation diagram for Td symmetry. The effects of second-order spin–orbit coupling account satisfactorily for the observed band intensities and splittings. The formation constants, K4, as a function of ionic strength were computed over the complete spectral range by multiple linear regression analysis and were, extrapolated to infinite dilution, 0.4 ± 0.1 for Hgl42− and HgBr42−, and 5.7 ± 0.2 for HgCl42−. Significant cation effects were observed upon the spectra of HgX2 with added X−, and may partially be explained in terms of the extended Debye–Hückel equation. Key words: electronic absorption spectra, mercury(II) halide complexes, formation constants, ionic strength, Gaussian band analysis.
Crown ether complexes exhibiting unusual 1:2 macrocycle salt ratios: X-ray crystal structures of cyclohexano-15-crown-5•2LiOPh, cyclohexano-15-crown-5•2NaOPh, and 15-crown-5•2NaOPh
Authors: Watson Kimberly A. , Fortier Suzanne , Murchie Michael P. , Bovenkamp John W.
Published: May 2006

The crystal structures of three crown ether complexes (cyclohexano-15•crown-5•2LiOPh (1): Li2O7C26H36, Mr = 474.45, λ(CuKα)filt. = 1.5418 Å; cyclohexano-15-crown-5•2NaOPh (2): Na2O7C26H36, Mr = 504.54, λ(CuKα)filt. = 1.5418 Å; and 15-crown-5•2NaOPh (3): Na2O7C22H30, Mr = 452.46, λ(CuKα)mono. = 1.5418 Å) have been determined by X-ray diffraction at room temperature (T = 298 K). Complex 1 crystallizes in space group P21/n with a = 15.032(5), b = 13.332(2), c = 13.533(3) Å, β = 106.28(2)°, V = 2603.2 Å3, and Z = 4. Complex 2 crystallizes in space group P21/n with a = 15.94(1), b = 12.194(7), c = 14.068(7) Å, β = 102.84(5)°, V = 2666.3 Å3, and Z = 4. Complex 3 crystallizes in space group P21/c with a = 12.451(3), b = 17.263(3), c = 12.659(6) Å, β = 115.74(3)°, V = 2450.8 Å3, and Z = 4. The three structures were solved by direct methods and refined by full-matrix least-squares calculations to residuals, R, of 0.080, 0.069, and 0.081 for complexes 1, 2, and 3, respectively. In each structure the macrocycle forms a 1:2 host–guest complex with the alkali metal. Each structure exhibits a dimer of the general form (MAC•2MOPh)2 where M = Li+ and Na+ for structures 1 and 2, respectively, and MAC = cyclohexano-15-crown-5, while M = Na+ and MAC = 15-crown-5 for structure 3. In each case the two metals of the monomeric unit have different crystallographic and chemical environments. In structures 1 and 3 one alkali metal is coordinated to all five oxygens of the macrocycle and to one of the phenoxide oxygens, while the other alkali metal is coordinated to only one oxygen of the macrocycle and to three phenoxide oxygens. In structure 2, one alkali metal is coordinated to only four of the five oxygens of the macrocycle and to two of the phenoxide oxygens, while the other alkali metal is coordinated to only one oxygen of the macrocycle and to three phenoxide oxygens (as in 1 and 3). Key words: crown ether, host–guest complexes, X-ray crystallography.
1-n-Propyl- and 1-n-butyl-3-hydroxy-2-methyl-4-pyridinone complexes of group 13 (IIIA) metal ions
Authors: Simpson Linda , Rettig Steven J. , Trotter James , Orvig Chris
Published: May 2006

The aluminum, gallium, and indium tris(ligand) complexes of 3-hydroxy-2-methyl-1-n-propyl-4-pyridinone and 1-n-butyl-3-hydroxy-2-methyl-4-pyridinone have been prepared and characterized. All six compounds were prepared in a one-pot synthesis from maltol, n-propyl-, or n-butylamine and an appropriate metal(III) salt, and were completely characterized (IR, FAB-MS, 1H NMR, 27Al NMR, elemental analysis). Three of the six complexes were studied by single-crystal X-ray diffraction and were found to form trihydrates, unlike their 1-methyl and 1-ethyl analogues, which formed dodecahydrates. The n-butylpyridinone complex Al(C10H14NO2)3•3H2O (Al(nbp)3•3H2O) and n-propylpyridinone complexes Al(C9H12NO2)3•3H2O (Al(npp)3•3H2O), and Ga(C9H12NO2)3•3H2O (Ga(npp)3•3H2O) were essentially isostructural, crystallizing in the space group [Formula: see text] with the following crystal parameters for Al(nbp)3•3H2O (Al(npp)3•3H2O, Ga(npp)3•3H2O): a = 15.885(1) (15.328(1), 15.367(2)) Å, c = 7.280(8) (7.231(2), 7.256(2)) Å, Z = 2. the data were refined by using 1280 (1377, 1802) reflections with I > 3σ(I) to R andRw values of 0.047 (0.057, 0.055) and 0.061 (0.077, 0.081), respectively. They form fac geometries with infinite chains of hydrogen bonds parallel to the c axis similar to those found in the 1-methyl and 1-ethyl complexes. They lack the hexagonal channels of water molecules which characterized the dodecahydrates and are therefore not exoclathrates. Key words: metal complexes, hydroxypyridinone ligands, exoclathrate, hydrogen bonding, crystal structures.
Excess partial molar enthalpies in the water-rich region of the isobutyric acid – water system
Authors: Siu William W. Y. , Wong Terrance Y. H. , Chao Lisa C. F. , Koga Yoshikata
Published: May 2006

The excess partial molar enthalpies of isobutyric acid (IBA), HmE(IBA), and those of H2O, HmE(H2O), were measured in aqueous solutions of IBA. The temperature dependence of HmE(IBA) at the infinite dilution suggested that the structure enhancement of the solvent H2O by IBA is weaker than those by tert-butanol (TBA) or 2-butoxyethanol (BE). The concentration dependence of HmE(IBA), and that of the enthalpic IBA–IBA interaction, N{∂HmE(IBA)/∂nB}, shows that there are two distinct mixing schemes bounded at about xB = 0.03, before reaching the two phase separation. Namely, the IBA–IBA interaction is repulsive below this boundary, while above this boundary it becomes attractive leading eventually to phase separation at a higher concentration. The transition between the two schemes is associated with a peak(negative) anomally in the fourth derivative of the free energy, N2 {∂2HmE(IBA)/∂nB2}.Key words: excess partial molar enthalpies, isobutyric acid – water, transition in mixing scheme.
Stereochemical studies of the photochemical cycloaddition reaction of alkenes with N-benzoylindole and N-carboethoxyindole; evidence for biradical intermediacy
Authors: Hastings David J. , Weedon Alan C.
Published: May 2006

The stereochemistries of the 2 + 2 cycloaddition products obtained from the photochemical addition reaction between N-benzoylindole or N-carboethoxyindole and the alkenes cyclopentene, cyclohexene, cycloheptene, cis- and trans-2-butene, and cis- and trans-4-octene are examined. The structures of the products are shown to be consistent with a photo-cycloaddition mechanism involving the intermediacy of triplet 1,4-biradical species. The quantum yields of adduct formation between N-benzoylindole and both cis- and trans-octene were measured as a function of alkene concentration. The results suggest that cis-octene reacts with the indole derivative's triplet excited state with a rate constant of (1.7 ± 0.3) × 107 M−1 s−1. The results are also consistent with the immediate products of this reaction being 1,4-biradicals, 98% of which revert to the ground state indole derivative and alkene, and only 2% of which proceed to cycloadduct. In marked contrast, the same treatment suggests that trans-octene reacts with the triplet excited state of N-benzoylindole with a rate constant estimated to be in the range of 1 × 106 and 6 × 105 M−1 s−1, and it appears that the 1,4-biradicals formed revert much less efficiently to the starting materials; it is estimated that between 67 and 100% of the 1,4-biradicals proceed to cycloadducts. In the reaction with cis-octene biradical reversion leads to the formation of trans-octene ("Schenk isomerization"); the quantum yield of this process is determined to be 0.074 ± 0.004, which may imply that approximately 75% of the biradicals collapse to cis-alkene and 25% collapse to the trans isomer. Key words: indole, photocycloaddition, 1,4-biradicals.
Triterpenoid glycosides from the Northeastern Pacific marine sponge Xestospongia vanilla
Authors: Morris Sandra A. , Northcote Peter T. , Andersen Raymond J.
Published: May 2006

Isoxestovanin A (6), xestovanin B (7), xestovanin C (8), dehydroxestovanin A (9), epidehydroxestovanin A (10), dehydroxestovanin C (11), and secodehydroxestovanin A (12), seven new triterpenoid glycosides, have been isolated from the Northeastern Pacific sponge Xestospongia vanilla. The structures of the metabolites were determined by a combination of spectroscopic analysis and chemical degradation. The aglycone of 6 has the new isoxestovanane triterpenoid carbon skeleton. Compounds 7, 8, and 11 are the first X. vanilla triterpenoid glycosides known to contain three monosaccharide subunits. Key words: triterpenoid, glycoside, sponge, Xestospongia.
Thermolysis of α-hydroperoxyalkyl diazenes. Spin trapping of radical intermediates and spin trapping kinetics
Authors: Mathew Lukose , Osei-Twum Emmanuel Y. , Warkentin John
Published: May 2006

α-Hydroperoxyalkyl diazenes (Me2C(OOH)N=NR, 1, R = CH2CF3, CH2CH2OMe, CH(Me)2, CMe3, CH2Ph, Ph, CH2CH2OPh, and c-C3H5CD2) decompose in benzene, at 50 °C or less, by a mechanism involving free radical (R•) intermediates. The radicals were trapped with 1-methyl-4-nitroso-3,5-diphenylpyrazole, 2, to afford spin adducts (nitroxyls) that were observed by ESR spectroscopy. When the solvent was ethyl vinyl ether, radicals from 1 (R = CH2CH2OPh) were trapped by the solvent and the adduct radicals so formed were spin trapped by 2. These observations support free radical mechanisms for thermolysis of 1 and for the hydroxyalkylations that occur when 1 are decomposed in solutions containing enol ethers or other unsaturated substrates. The ring-opening of cyclopropylmethyl radicals (cpm) to 3-butenyl radicals was used to estimate the rate constant for radical trapping by 2. For cpm the rate constant is given by log kcpm = (10.7 ± 0.4) − (3.9 ± 0.5)/θ where θ = 2.3 RT kcal mol−1. At 25 °C, the spin trapping rate constant has the value 6.9 × 107 M−1 s−1. Key words: hydroperoxyalkyl diazenes; radicals, spin trapping; spin trapping, rate constant.
Vibrational circular dichroism of (S,S)-[2,3-2H2]oxirane in the gas phase and in solution
Authors: Freedman Teresa B. , Spencer Kevin M. , Ragunathan N. , Nafie Laurence A. , Moore Jeffrey A. , Schwab John M.
Published: May 2006

Vibrational circular dichroism (VCD) spectra of an exceptionally simple chiral three-membered ring molecule, (S,S)-[2,3-2H2]oxirane, have been obtained for samples in the gas phase and in C2Cl4 and CS2 solution. The VCD band contours for the gas phase samples of this molecule with C2 symmetry follow those of the absorption spectra, with the modes of A symmetry species giving rise to bands with two sharp Q-branch maxima on either side of a distinct minimum, and the modes of B symmetry species giving rise to bands with a strong central Q-branch. The signs and relative intensities of the VCD bands can be understood in terms of chiral perturbation by the deuterium substitution of the vibrational modes of the parent oxirane molecule, which mixes modes of A1 and A2 symmetry, and modes of B1 and B2 symmetry, thus introducing angular charge oscillation about the direction of linear charge oscillation. Key words: vibrational circular dichroism, oxirane, chiral perturbation.