bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis and thermal study of tris(ammonium) hexafluoro metallates(III) of some rare earths
Authors: Patwe S. J. , Wani B. N. , Rao U. R. K. , Venkateswarlu K. S.
Published: May 2006

Fluorination of oxides of rare earths using ammonium hydrogen fluoride at room temperature gave new compounds [NH4]3MF6•nH2O, where M = Y, La, Nd, and Pr. In the case of cerium, [NH4]4CeF8 is formed. The course of thermal decomposition of all these compounds is studied by thermogravimetry/differential thermal analysis (TG/DTA) and X-ray powder diffraction (XRD). Keywords: hexafluoro metallates, rare earths, ammonium hydrogen fluoride, thermal decomposition, X-ray diffraction.
Synthesis and X-ray structure determination of the Ir–Ag–Ir donor–acceptor metal–metal bonded complex [{(η5-C5Me5)(CO)2Ir}2Ag][BF4]
Authors: Einstein Frederick W. B. , Jones Richard H. , Zhang Xiaoheng , Sutton Derek
Published: May 2006

The title complex (2) has been synthesized from the reaction of (η5-C5Me5)Ir(CO)2 (1) with AgBF4. Complex 2 crystallizes in the monoclinic space group P21/c with a = 12.355(2) Å, b = 13.713(2) Å, c = 16.672(3) Å, β = 97.70(1)°, and Z = 4. The structure was solved by using 3131 observed reflections (I ≥ 2.5σ(I)) in the range 0° < 2θ < 50° to final discrepancy indices of RF = 0.032 and RwF = 0.038. The cation may be described as consisting of two molecules of 1 bound to Ag+ by donor–acceptor Ir → Ag bonds. The Ir—Ag—Ir skeleton is nearly linear (angle 173.67(6)°). The Ag—Ir bond lengths are equal (2.659(1) Å), and the two molecules of 1 are disposed mutually trans, to give the cation approximate (noncrystallographic) C2h symmetry. Keywords: complex, iridium, silver, metal-metal bond, X-ray, crystal, structure, carbonyl, donor-acceptor, pentamethylcyclopentadienyl.
Metal pyrazolate polymers. Part 1. Synthesis, structure, and magnetic properties of the [Cu(pz)2]x polymer
Authors: Ehlert M. K. , Rettig S. J. , Storr A. , Thompson R. C. , Trotter J.
Published: May 2006

A novel and reliable method for the synthesis of polybis(μ-pyrazolato-N,N′)copper(II), [Cu(pz)2]x(where pz = N2C3H3), is presented. Single crystals of the polymer suitable for X-ray study have been grown under carefully controlled conditions. Crystals of polybis(μ-pyrazolato-N,N′)copper(II) are orthorhombic, a = 7.917(1), b = 11.491(2), c = 7.778(1) Å, Z = 4, space group Ibam. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.029 and Rw = 0.038 for 507 reflections with I ≥ 3σ(I). The crystal structure consists of infinite double-bridged polymeric chains. The Cu atom has a D2 distorted tetrahedral coordination geometry with Cu—N = 1.957(2) Å and N—Cu—N = 94.3(1)–139.5(1)°. Magnetic susceptibility studies (4.2 to 299 K) on this material reveal very strong antiferromagnetic exchange between copper(II) centres in the extended linear chains. The magnetic data have been successfully analyzed according to the isotropic Heisenberg model for linear chains with −J = 78 cm−1 and g = 2.12. Keywords: crystal structure, copper(II) pyrazolate chain polymer, antiferromagnetic exchange.
Sclerodin and related compounds from a plant disease causing fungus. Scleroderris yellow
Authors: Ayer William A. , Kamada Masayuki , Ma Yu-Ting
Published: May 2006

The metabolites of several strains of a fungus that gives rise to the disease of conifers known as Sirococcus shoot blight have been studied. The metabolites are similar to those isolated from Gremmeniellaabietina, the causative agent of Scleroderris canker of pine. Sclerodin (1), scleroderolide (2), and Scleroderris blue (3) have been isolated, along with the known lactone 4, trypethelone (6), and a new purple compound named scleroquinone (12). Another new compound, related to Scleroderris blue (3) and Scleroderris green (7); which we have named Scleroderris yellow, is shown to possess structure 8. The sclerodin (1) isolated in these studies is not optically pure, in some cases being nearly racemic. It is noted that the optical purity may be a function of the culture medium employed. Scleroderolide (2) strongly inhibits the germination of lettuce seeds. In addition to the metabolites noted, squalene, methyl oleate, stearic acid, ergosta-4,6,8(14),22-tetraene-3-one, and ergosterol endoperoxide were also isolated from the cultures. Keywords: Sirococcus, phenalenones, Scleroderris yellow, scleroquinone, sclerodin.
Photosensitized (electron transfer) carbon–carbon bond cleavage of radical cations: the 2-phenylethyl ether and acetal systems
Authors: Arnold Donald R. , Lamont Laurie J.
Published: May 2006

The scope of the photosensitized (electron transfer) carbon–carbon bond cleavage involving radical cations has been defined for 2-phenylethyl ethers and acetals. The thresholds for reactivity of the monophenylethyl and gem-diphenylethyl derivatives are compared. While the radical cation of methyl 2,2-diphenylethyl ether (7) cleaves to give ultimately diphenylmethane (2) and dimethoxymethane (8), the radical cation of methyl 2-phenylethyl ether (9) was stable under these conditions. In contrast to the lack of reactivity of the radical cation of 9, the radical cations of methyl 2-phenyl-2-propyl ether (11), methyl 2-phenylcyclopentyl ether (13), and 2-phenylmethyl-1,3-dioxolane (16) cleave. Cleavage in the monophenylethyl series is limited to formation of a carbocation at least as stable as the secondary α-oxyalkyl or di-α-oxyalkyl. The basis for predicting this type of reactivity of radical cations is defined. The rate of carbon–carbon bond cleavage is increased by increasing the oxidation potential of the molecule, by decreasing the carbon–carbon bond strength, and (or) by decreasing the oxidation potential of that fragment that will become the carbocation. The results obtained from the reactions of 2-diphenylmethyl-1,3-dioxolane (14) and 2-phenylmethyl-1,3-dioxolane (16) cast doubt on the published oxidation potential for the 1,3-dioxolan-2-yl radical. Keywords: photochemistry, radical cation, electron transfer, bond cleavage, radical.
Synthesis and monolayer spreading behaviour of surface-active compounds containing electron- and hole-transporting groups. 1. Fluorenylidene derivatives
Authors: Taylor J. W. , Sloan C. P. , Holden D. A. , Kovacs G. J. , Loutfy R. O.
Published: May 2006

Surface-active derivatives containing fluorenylidene groups were synthesized, with the goal of creating thin-film devices incorporating monolayers of electron-transporting materials. The new compounds were studied in monolayers at the air–water interface, and were further characterized by cyclic voltammetry. The spreading behaviour of long-chain esters of a fluorenylidene malononitrile carboxylic acid was relatively insensitive to the cross-sectional area of the hydrocarbon chain, but was more strongly affected by changes in the polarity of the hydrophilic groups. Conditions for Langmuir–Blodgett deposition of multilayers of two of the new compounds on titanium-coated polyester film were also explored. Keywords: monolayers, Langmuir–Blodgett films, fluorene, surfactants.
Alkali metal salt solvation study in 1,1,3,3-tetramethylurea using nuclear magnetic resonance spectroscopy
Authors: Ciolino Laura Ann , Barker Barbara Jane
Published: May 2006

The solution behavior of a series of alkali metal tetraphenylborates, perchlorates, and iodides in 1,1,3,3-tetramethylurea (TMU) has been studied by proton nuclear magnetic resonance (NMR) spectroscopy. TMU has a simple proton NMR spectrum consisting of one singlet at 2.75 ppm. Alkali metal tetraphenylborates and perchlorates were observed to be dissociated in TMU. Sodium iodide, however, was observed to be associated in TMU. Present results are compared with those from other NMR investigations in TMU and related nonaqueous solvents. Keywords: alkali metal salt solvation, tetramethylurea, nuclear magnetic resonance, solute–solvent interactions.
Biosynthesis of mollisin by Mollisiacaesia
Authors: McInnes A. Gavin , Walter John A. , Wright Jeffrey L. C. , Vining Leo C. , Ranade Neelima , Bentley Ronald , McGovern Edward P.
Published: May 2006

The 2H nuclear magnetic resonance spectrum of mollisin from cultures of the fungus Mollisiacaesia supplemented with [2-2H3]acetate was consistent with biosynthesis of the metabolite via a single-chain polyketide. The presumption that chain scission accompanies dichlorination of a reactive methylene group in such a polyketide was supported by the detection of chloroperoxidase activity with monochlorodimedone as a substrate, and failure to detect chlorinating activity when the substrate was acetoacetyl coenzyme A. However, a decisive choice between single-chain and two-chain polyketide intermediates cannot be made with information provided by the techniques used so far. Keywords: biosynthesis, mollisin, deuterium labelling and NMR; deuterium labelling and NMR, mollisin biosynthesis; labeling of mollisin by deuterium, NMR analysis; mollisin biosynthesis, deuterium, labeling and NMR; NMR deuterium, mollisin biosynthesis.
Topological investigations of the volumetric and enthalpic effects in some binary mixtures of non-electrolytes
Authors: Singh Prem P. , Bhatia Meenu
Published: May 2006

Volumetric and enthalpic effects have been measured at 308.15 K for pyridine (A) + aniline, o-toluidine, formamide, or N,N-dimethylformamide (B) mixtures and the same have been rationalized by graph–theoretical arguments to suggest that while aniline in the pure state exists as an equilibrium mixture of monomers and dimers, o-toluidine exists mainly as dimers. This analysis has further yielded information about the state of association in formamide and N,N-dimethylformamide that is consistent with the existing views on their nature of association. This analysis has further yielded information about the molecular entities in these mixtures that has been supported by their infrared spectral studies. The energetics of the various interactions that result in the formation of these A + B mixtures have also been investigated and the results have been rationalized by graph–theoretical arguments. Keywords: topological, association, solvation, energetics, volumetric, enthalpic effects.
A study on excess functions and thermodynamic parameters of some bromoform- containing binary mixtures
Authors: Joshi Shrikant S. , Aminabhavi Tejraj M. , Shukla Shyam S.
Published: May 2006

Densities and viscosities have been determined for six binary mixtures of bromoform, aniline, n-hexane, benzonitrile, butyronitrile, and benzylnitrile at different mole fractions and temperatures. The excess volumes VE, apparent excess values of viscosity δη, excess Gibbs energy of activation of flow ΔG*E, and partial molar volumes Vi have been calculated from the experimental data. Furthermore, attempts were made to calculate theoretically the excess isobaric thermal expansion coefficient δα, of the mixtures using the refractive index mixture rules. These results are in good agreement with the experimental data obtained from densities. Various excess functions and thermodynamic parameters have been used in the discussion of results to understand the nature and type of interactions in binary mixtures. Keywords: excess volume, refractive index mixture rules, interactions, bromoform, density, viscosity.
Alkylammonium lead halides. Part 2. CH3NH3PbX3 (X = Cl, Br, I) perovskites: cuboctahedral halide cages with isotropic cation reorientation
Authors: Knop Osvald , Wasylishen Roderick E. , White Mary Anne , Cameron T. Stanley , Oort Michiel J. M. Van
Published: May 2006

Methylammonium lead(II) halides, CH3NH3PbX3 (X = Cl, Br, I), have been investigated by single-crystal X-ray diffraction, 2H and 14N nmr, adiabatic calorimetry, and other methods. The chloride (CL) has transitions at 171.5 and 177.4 K, the bromide (BR) at 148.4, 154.2, and 235.1 K, and the iodide (IO) at 162.7 and 326.6 K. The respective entropies of transition (J K−1 mol−1) are 11.0 and 5.1 for CL; 8.7, 3.4, and 5.3 for BR; and 16.1 and 1.9 for IO. The highest-temperature phase, phase I, of each halide is of the cubic (Pm3m) perovskite type. The cation in CL(I) and BR(I) could not be localized in the electron density maps; the thermal motion of the halogen atom is highly anisotropic. The ln T1(2H) vs. T−1 plots (N-deuterated samples as well as CD3NH3PbCl3) show significant departures from linearity: the temperature variation of T1(2H) in BR(II) and IO(II) can be represented by functions of the type ln T1(H) = k0 − k2T−2, which give adequate analytical representations of T1(2H) and T1(14N) in phase I as well. On cooling, BR(II) and IO(II) exhibit small quadrupole splittings QS(2H), which can be represented to a high degree of correlation by QS(2H) = k(Ttr − T)n, i.e. they appear to exhibit critical behaviour with respect to T. The 14N nmr results indicate that the C—N bond in phase I reorientates in an isotropic potential at a rate approaching that of the freely rotating methylammonium ion. Below phase I this motion takes place in an increasingly anisotropic potential in BR(II) and IO(II) and is essentially arrested in CL(II), BR(III), and IO(III). The temperature dependence of the activation energy Ea for the cation reorientation and other aspects of the non-Arrhenius behaviour are discussed, and the CH3NH3PbX3 perovskites are compared with the corresponding (CH3NH3)2TeX6 halides, utilizing preliminary 2H nmr results on (CH3ND3)2TeBr6. The electrical conductivity, between 0 and 95 °C, of CH3NH3PbI3 increases with temperature and exhibits no discontinuity at Ttr = 326.6 K; the activation energy for the conduction process is estimated as ~0.4 eV. Keywords: crystal structure, MeNH3PbCl3 and MeNH3PbBr3; heat capacity, MeNH3PbX3 (X = Cl, Br, I); methylammonium lead halides; solid-state NMR, 2H and 14N; spin-lattice relaxation, non-Arrhenius behaviour.
Solubility of nonpolar gases in 2,6-dimethylcyclohexanone
Authors: Gallardo Maria Asuncion , Lopez Maria Del Carmen , Urieta Jose Santiago , Losa Celso Gutierrez
Published: May 2006

Solubility measurements of He, Ne, Ar, Kr, Xe, H2, D2, N2, CH4, C2H4, C2H6, CF4, SF6, and CO2 in 2,6-dimethylcyclohexanone at temperatures 273.15 to 303.15 K and at a gas partial pressure of 101.33 kPa are reported. Standard changes in Gibbs energy, enthalpy, and entropy for the dissolution process at 298.15 K are also presented. Results for both solubility and thermodynamic functions are compared with those for cyclohexanone and 2-methylcyclohexanone. The scaled particle theory is used to obtain the effective Lennard–Jones (6,12) pair potential parameters for 2,6-dimethylcyclohexanone and, from these, the values it predicts for the solubility of the studied gases in the solvent are obtained. Keywords: gas solubility, Henry coefficient, 2,6-dimethylcyclohexanone, thermodynamic functions of solution, non-polar gases.
2,2′-Bipyridyl adducts of tungsten oxide fluorides: preparation and characterization of WOF4•bipy and WO2F2•bipy; crystal structure of WO2F2•bipy
Authors: Arnaudet Lucile , Bougon Roland , Ban Buu , Charpin Pierrette , Isabey Jacques , Lance Monique , Nierlich Martine , Vigner Julien
Published: May 2006

WOF4•bipy was prepared from the reaction of WOF4 with 2,2′-bipyridyl (bipy) in the melt or in solution in CH2Cl2 or CH3CN.The adduct, which is not moisture-sensitive at room temperature, was characterized by elemental analysis, X-ray powder data, and vibrational spectroscopy. WOF4•bipy is almost insoluble in the usual organic solvents, and the species which were observed in CH2Cl2 or CH3CN by 1H NMR and Raman spectroscopy resulted from the dismutation of WOF4•bipy into WO2F2•bipy and WF6•bipy.This transformation in solution was revealed by the slow growing of crystals identified as WO2F2•bipy and by the 1H NMR spectrum which displayed lines assigned to the independently synthesized and characterized species WF6•bipy. Unlike the WOF4 adduct, the tungstyl adduct is moisture-sensitive, its crystal structure showed that the tungsten atom is surrounded by a very distorted octahedron of ligands with each nitrogen of the bipy ligand trans to one oxygen atom and the two fluorine atoms trans to each other. The space group, unit cell parameters, and R factor for C10H8F2N2O2W are as follows: monoclinic, P21/n (No. 14) (alternate setting of the standard space group P21/c), a = 8.637(1) Å, b = 13.823(2) Å, c = 9.526(2) Å, β = 109.98(2)°, V = 1068.8(6) Å3, Z = 4, and R = 0.044 for 806 (I > 3σ(I)) of 1471 total reflections and 94 parameters. Keywords: 2,2′-bipyridyl adducts, tungsten oxide tetrafluoride, tungsten dioxide difluoride.
Selective reduction of less reactive carbonyl groups in the presence of diborane and sodium bisulfite on silica gel
Authors: Chihara Teiji , Wakabayasi Tamie , Taya Kazuo , Ogawa Haruo
Published: May 2006

The less reactive carbonyl group of a mixture of reducible groups of carbonyl compounds was preferentially reduced with diborane on silica gel by first forming an adduct of the more reactive carbonyl group with sodium bisulfite. For example, 4-acetylbenzaldehyde could be converted to 4-(1-hydroxylethyl)benzaldehyde in 93% selectivity; in a mixture of 4-phenyl-acetophenone and 4-phenylbenzaldehyde, biphenylethanol was preferentially formed in 95% yield with 16% yield of 4-phenylbenzyl alcohol. Silica gel and sodium bisulfite were essential for this selective reaction. Aliphatic and aromatic aldehydes and unhindered cyclohexanones could be selectively protected by this method. Keywords: sodium bisulfite, silica gel, carbonyl compound, selective reduction, diborane.
Studies on complexes of VO(II), Mn(II), Cu(II), Ni(II), Co(II), Zn(II), Fe(III), and UO2(II) with hydrazone β-diketone ligands
Authors: Abdel-Moez Mohamed S. , Stefan Shaker L. , El-Behairy Mostafa M. , Mashely Mohamed M. , El-Shetary Basheir A.
Published: May 2006

The stepwise stability constants of VO(II), Mn(II), Cu(II), Ni (II), Co(II), Zn(II), Fe(III), and UO2(II) metal ions with 2-(2-acetylphenylhydrazone)-5,5-dimethyl-1,3-cyclohexane dione (2-APHDm), 3-(2-acetylphenylhydrazone)pentane-2,4-dione (2-APHA), 2-(2-acetylphenylhydrazone)-1-phenyl-1,3-butane dione (2-APHB), 2-(2-acetylphenylhydrazone)-1,3-diphenyl-1,3-propane dione (2-APHDB), and 3-(2-acetylphenylhydrazone)-1,1,1-trifluoropentane-2,4-dione (2-APHTA) were determined at 30 °C by pH titrations in 75% (v/v) dioxane–water solvent. In addition, metal complexes of three related β-diketones, 2-APHA, 2-APHB, and 2-APHDB have been synthesized and characterized by elemental analysis, uv–visible, ir spectra, and conductance in DMF and magnetic susceptibility measurements. The structure of the complexes have been proposed. Keywords: transition metal complexes of hydrazone-β-diketone ligands.
Synthesis of glycan fragments of glycoproteins using peracetylated N-allyloxycarbonyl-β-D-glucosamine and 1,6-anhydro-β-D-mannopyranose derivatives
Authors: Lafont Dominique , Boullanger Paul , Banoub Joseph , Descotes Gerard
Published: May 2006

The disaccharides β-D-GlcNAOC-(1 → 2)-D-Man, β-D-GlcNAOC-(1→ 4)-D-Man, the trisaccharide β-D-GlcNAOC-(1 → 2)-[β-D-GlcNAOC-(1→ 4)]-D-Man, and the tetrasaccharide β-D-GlcNAOC-(1 → 2)-[β-D-GlcNAOC-(1→ 3)]-[β-D-GlcNAOC-(1 → 4)]-D-Man have been synthesized in their peracetylated form, using the Lewis acid catalyzed condensations of 1,3,4,6-tetra-O-acetyl-2-N-allyloxycarbonylamino-2-deoxy-β-D-glucopyranose 1 with properly substituted 1,6-anhydro-β-D-mannopyranose derivatives. The anhydro glycosylation products obtained were then easily transformed into the 4C1 peracetylated derivatives by acetolysis. The N-allyloxycarbonyl groups could be converted into N-acetyl groups in the presence of Pd(0) complexes, followed by reacetylation of the free amino function in high yields. Keywords: N-allyloxycarbonyl, glycosylation, glucosamine, glycan fragments.
On the possibilities of π-ylides
Authors: Langler Richard Francis , Precedo Laura
Published: May 2006

A systematic survey of substituted annulenes, using zeroth order Hückel theory, has uncovered a subclass of such structures which are predicted to have very substantial charge separation. An examination of selected structures by MNDO semi-empirical molecular orbital computations provided support for the Hückel descriptions. Such compounds may be of immediate interest to those pursuing synthetic or mechanistic problems which involve cycloaddition reactions. Keywords: π-ylides.
New total synthesis of phenanthrene alkaloids
Authors: Estévez Juan C. , Villaverde M. Carmen , Estévez Ramon J. , Seuas Julio A. , Castedo Luis
Published: May 2006

We describe a new synthesis of phenanthrene alkaloids based on the photocyclization of stilbene compounds prepared by a simple, efficient new method. It allows the preparation of phenanthrene compounds with a primary amine group in their side chain. Keywords: alkaloid, phenanthrene, stilbene, photocyclization.
Structure of the L1 and L6 core oligosaccharide epitopes of Neisseriameningitidis
Authors: Di Fabio Jose L. , Michon Francis , Brisson Jean-Robert , Jennings Harold J.
Published: May 2006

The core oligosaccharides obtained from the lipopolysaccharides of Neisseriameningitidis serotypes 1 and 6 were isolated by mild acid hydrolysis, and their structures elucidated by combined chemical, biochemical, and physical techniques. Use was made of 500 MHz 1H nuclear magnetic resonance measurements in both one-dimensional and two-dimensional modes as well as of nuclear Overhauser enhancement experiments. To assist in the structural assignments, the purified oligosaccharides were also degraded by chemical and enzymatic procedures to smaller fragments. The L1 oligosaccharide is a biantennary partially O-acetylated heptasaccharide in which the larger antenna terminates in a α-D-Galp1 → 4β-D-Galp1 → 4β-D-Glcp unit, whereas the smallest disaccharide antenna carries phosphorylethanolamine groups at O-3 of its heptosyl moiety. The L6 oligosaccharide is also a biantennary partially O-acetylated heptasaccharide but, unlike the L1 core, is terminated by a β-D-GlcpNAcl → 3β-D-Galp1 → 4β-D-Glcp unit in its larger antenna. The heptosyl moiety in the small disaccharide antenna bears a phosphorylethanolamine group at O-7, unlike O-3 for the L1 determinant. Keywords: lipopolysaccharides, Neisseriameningitidis, oligosaccharide epitopes, structure, NMR spectroscopy.
A novel method of inversion for a class of matrices: application to calculation of two-component plasma viscosities in a magnetic field
Authors: Eu Byung Chan
Published: May 2006

To calculate transport coefficients for a plasma in a magnetic field in a linear approximation it is necessary to invert matrices of fairly high dimensions. In this paper we present a novel method of inverting matrices involved in such problems. The method requires a solution of an inhomogeneous matrix integral equation in the Neumann series and a resummation of the series obtained. It involves inversions of lower order matrices, which can be achieved easily. We then apply the method to calculate viscosities of a two-component plasma subject to a homogeneous magnetic field. The structure of the viscosity matrix (tensor) calculated thereby is shown to be the same as that of a neutral system with an axial symmetry; namely, it can be decomposed into a scalar, a 2 × 2 matrix, and a 3 × 3 matrix. The method developed here can be applied to calculate other transport coefficients of the two-component plasma. Keywords: mathematical method, viscosity, plasmas, transport coefficients, kinetic theory.
The crystal structure of 1,2-dibenzoyl-1-tert-butylhydrazine, a nonsteroidal ecdysone agonist, and its effects on spruce budworm (Choristoneurafumiferana)
Authors: Chan T. H. , Ali A. , Britten J. F. , Thomas A. W. , Strunz G. M. , Salonius A.
Published: May 2006

The crystal structure of 1,2-dibenzoyl-1-tert-butylhydrazine (1), a nonsteroidal ecdysone agonist, has been established by direct methods and refined to a final R value of 0.038 for 2502 observed reflections. The crystals belong to the monoclinic system, a = 10.5114(7), b = 31.557(3), c = 11.1578(8) Å, β = 113.210(6)°, and the space group is P21/n. The N—N bond in 1 was found to adopt a gauche conformation with a dihedral angle of −71.8° or −58.1°. The two amide functions adopt different planar conformations. The compound is found to have a potent effect on the development of spruce budworm (Choristoneurafumiferana) larvae, causing early apolysis and subsequent mortality at 0.2–10 ppm concentration in artificial diet. Keywords: ecdysone, agonist, spruce budworm, 1,2-dibenzoyl-1-tert-butylhydrazine.
Structural requirements for molecular recognition by the Neurokinin receptors
Authors: Lemay François , Hassani Chakib Ameziane , Michel André G.
Published: May 2006

Populations of lower energy conformers have been generated by conformational energy calculations, using empirical energy functions, for the C-terminal pentapeptide of Substance P (CH3-CO-Phe-Phe-Gly-Leu-Met-NH2), Neurokinins A and B (CH3-CO-Phe-Val-Gly-Leu-Met-NH2), and Eledoisin (CH3-CO-Phe-Ile-Gly-Leu-Met-NH2). The C-terminal sequences are greatly conserved in the Neurokinin and Tachychinin families of peptides and are mainly responsible for the molecular recognition. These sequences differ in the nature of the second residue. This substitution interferes with the recognition processes and modulates the specificities. A procedure based on a random generation of conformers, followed by energy minimizations, has been applied to produce the populations of conformers for the three pentapeptides. The conformational properties of the lower energy conformers have been analysed and compared with respect to the substitution of the second residue. Earlier nuclear magnetic resonance data as well as structure–activity relationship results have been interpreted in light of the proposed stable conformers. It is shown that the selectivity towards the NK3 receptor requires a structure with a restrained flexibility introduced at the second N-terminal residue. The selectivity towards the NK1 receptor is shown to be related to the presence of a constrained structure at the Gly residue. These structures suggest further chemical modifications be introduced in order to produce more selective and stable analogues. Keywords: peptide conformation, empirical energy, Neurokinin, molecular recognition.
Conformational analysis and complete assignment of the proton and carbon NMR spectra of ouabain and ouabagenin
Authors: McIntyre Deane D. , Germann Markus W. , Vogel Hans J.
Published: May 2006

The 1H and 13C NMR spectra of the cardenolide ouabain and its aglycon ouabagenin have been completely assigned by two-dimensional NMR techniques, including phase-sensitive COSY and carbon–proton correlation (HETCOR, HMQC, and COLOC) spectra. The major conformer of these two compounds in solution is all-chair as determined from proton–proton coupling constants and is similar to that in the crystal lattice as previously determined by X-ray diffraction. The conformations of the A and D rings of ouabain in water are somewhat different than in DMSO/CDCl3 (2:1). At lower temperatures (−20 °C) signals from two conformers in slow exchange were readily observed in the 13C spectra, with an approximate ratio of 1:7. Molecular mechanics and dynamics calculations indicate that the conformational process responsible for this involves a chair/twist-boat interconversion of the A ring, with the all-chair conformer highly preferred. Keywords: ouabain, conformational analysis, 2-D NMR, molecular mechanics, molecular dynamics.
Self-association of caffeine in aqueous solution: an FT-IR study
Authors: Falk Michael , Gil Manuel , Iza Nerea
Published: May 2006

Spectra of caffeine in D2O solution (0.003 to 0.129 M) were studied by Fourier-transform infrared spectroscopy. Spectral changes in the carbonyl stretching region were interpreted in terms of the self-association equilibria resulting from the stacking of caffeine molecules. Absence of isosbestic points indicated the occurrence of more than two spectroscopically distinct caffeine species. A simple monomer–dimer–polymer model was employed, in which the successive equilibria are governed by two association constants, Kd for the formation of dimers and Kp for the formation of trimers and larger polymers. The value Kd = 158.1 L mol−1 was taken from a recent ultraviolet study by Iza etal. and the value Kp = 27 L mol−1 was derived from the analysis of infrared band profiles sharpened by Fourier self-deconvolution. From these values of Kd and Kp, the weight fractions of monomers, dimers, and polymers were calculated for every caffeine concentration and their individual molar absorptivity spectra were derived. In agreement with the earlier work of Maevsky and Sukhorukov, it was found that stacking association causes shifts of both C=O stretching fundamentals to higher wavenumbers. The monomer-dimer shifts are smaller than the dimer-polymer shifts. Analogous shifts were observed for the stacking self-association of 1,3-dimethyluracil in D2O. The shifts to higher wavenumbers are probably caused by the diminished hydrogen bonding of water to C=O groups of the stacked bases. Keywords: caffeine, self association, infrared spectroscopy, base stacking.
α-(Dialkylamino)-α-pyrrolylacetonitriles. A potpourri of useful synthetic transformations
Authors: Bray Brian L. , Muchowski Joseph M.
Published: May 2006

α-(Dialkylamino)-α-(pyrrol-2-yl)acetonitriles (3) are versatile intermediates that are readily converted into regiochemically pure 2-bromo-6-dialkylamino-1-azafulvenes and 5-bromo-2-acylpyrroles (acyl = CHO, COR). In addition, 3 (R1 = H), 4 (R1 = H), and the β-substituted compound 9 are transformed easily and under mild conditions, via the corresponding α-methylthio compounds (e.g., 7a), into the methyl pyrrole carboxylates 8a, 8b, and 10, respectively. Keywords: α-(dialkylamino)-α-(pyrrol-2-yl)acetonitriles, 5-bromo-2-acylpyrroles, methyl pyrrole carboxylates.
Synthesis and X-ray crystal structure of the 3,5-dimethylpyrazolato copper(I) trimer, [Cu(pz″)]3
Authors: Ehlert Martin K. , Rettig Steven J. , Storr Alan , Thompson Robert C. , Trotter James
Published: May 2006

A moderately high-yield, high temperature synthesis of the title compound is reported. Crystals of tris[(3,5-dimethylpyrazolato-N1,N2)copper(I)] are triclinic, a = 12.360(4), b = 14.637(4), c = 12.358(4) Å, α = 105.29(2), β = 119.83(2), γ = 93.99(3)°, Z = 4 (trimers, two per asymmetric unit), space group [Formula: see text]. The structure was solved by heavy atom methods and was refined by full-matrix least-squares procedures to R = 0.034 and Rw = 0.038 for 3877 reflections with I ≥ 3σ(I). The structure consists of trimeric molecules with an unusually short average Cu—N distance of 1.852 Å. The central Cu3N6 nine-membered rings are planar to within 0.07 Å. The trimer units are linked in pairs about centres of symmetry by weak [Formula: see text] interactions averaging 2.946 Å. Keywords: pyrazolato copper (I) timer, copper (I) pyrazolate.
A characterization and catalytic study of Y zeolite supported Ni/Cu bimetallics
Authors: Coughlan Brendan , Keane Mark A.
Published: May 2006

A wide range of CuNiNaY andCuNiKY zeolites were prepared by ion exchange. The location of both Ni2+ and Cu2+ cations within the zeolite framework was monitored after various stages of thermal treatment and the reduction process of the transition metal ions in a flowing hydrogen atmosphere was investigated and correlated with reduction time and temperature. The metallic phase generated on reduction was characterized by X-ray diffraction line broadening. The catalytic activity of the reduced zeolites in the hydrogenation of benzene is discussed in the light of these physical characterizations.Keywords: zeolites, supported Ni/Cu bimetals, cation location, crystallite size, catalysis.
Electronic effects of heterocyclic substituents. Spectroscopical and theoretical (AM1) study in a series of heterocyclic carboxaldehydes
Authors: Cativiela C. , Garcia J. I.
Published: May 2006

The electronic effects of a series of 18 heterocyclic carboxaldehydes (furans, thiophenes, pyrroles, and pyridines) have been studied by means of the correlation existing between 13C chemical shifts of the carbonylic carbon and calculated total and π charges (AM1). The implications of this theoretical model to explain polar and resonance contributions to the total electronic effect are discussed. Keywords: heterocyclic substituents, electronic effects, 13C NMR, DSP models.
Excited state dipole moments and polarizabilities of some aromatic alkenes and alkynes
Authors: Sinha Hemant K. , Thomson Paul C. P. , Yates Keith
Published: May 2006

Electric field induced changes in the optical absorption spectra (electrochromism) have been used to obtain the excited state dipole moments and polarizabilities of styrene, 2-vinylnaphthalene, 2-naphthylacetylene, and 9-vinylanthracene. Excited state dipole moments of the order of 4–6 debye have been obtained for all these molecules except for 9-vinylanthracene, for which the excited state dipole moment is zero within experimental error. These results support earlier proposals of the involvement of charge transfer excited states in the fast and efficient acid-catalysed photohydration reactions of this type of substrate. 9-Vinylanthracene, on the other hand, under similar conditions reacts differently to give a dimer as the major photoproduct, and its reaction is not subject to acid catalysis. These results shed light on the importance of charge transfer excited states on the efficiency of photohydration reactions. Keywords: electrochromism, excited state, dipole moments, polarizabilities.
The mechanism of the photochemical cycloaddition reaction of N-benzoylindole with cyclopentene
Authors: Disanayaka Bimsara W. , Weedon Alan C.
Published: May 2006

The mechanism of the photochemical cycloaddition reaction between N-benzoylindole, 1, and cyclopentene to give cyclobutane adducts 2 and 3 has been examined. The triplet excited state lifetime and quantum yield of intersystem crossing were determined for 1 as (2.8 ± 0.3) × 10−8 s and 0.39 ± 0.01, respectively, using the triplet counting procedure. In addition, the dependence of the quantum yield of cycloadduct formation upon the concentration of cyclopentene and upon the concentration of excited state quenchers has been determined. The results are used to propose a mechanistic model in which the triplet excited state of 1 reacts with cyclopentene to give a triplet 1,4-biradical intermediate. Following spin inversion the biradical intermediate reverts to the ground state starting materials or proceeds to the products 2 and 3; this partitioning, along with the quantum yield of intersystem crossing, gives rise to a limiting quantum yield of cycloaddition at infinite alkene concentration of 0.061. It is calculated that 84% of the biradical intermediates revert to the starting materials and 16% proceed to cycloadducts. The quantum yield data are also used to calculate two independent values of the rate constant for reaction of the triplet excited 1 with alkene; the values are (1.8 ± 0.1) × 107M−1 s−1 and (4.0 ± 0.8) × 106 M−1 s−1'. Some evidence for self quenching of the triplet excited state of 1 by ground state 1 was also observed. The quantum yield of intersystem crossing and the triplet excited state lifetime of 1 were found to vary with the solvent used; this is discussed in terms of the possible existence of a charge transfer triplet excited state. Keywords: indole, photocycloaddition, mechanism.
Formation and reactivity of 1-pyrrolyl-2,2,2-trifluoroethyl cations
Authors: Allen Annette D. , Kwong-Chip Jean-Marc , Lin Wing Cheung , Nguyen Paul , Tidwell Thomas T.
Published: May 2006

1-(1-Methyl-2-pyrrolyl)-2,2,2-trifluoroethyl p-nitrobenzoate (3) reacts by carbocation formation with an m value for the dependence of rate on the solvent polarity parameter YOTs of 0.56, and a rate 41 times slower than (1-methyl-2-pyrrolyl)methyl p-nitrobenzoate. The products from 3 include significant amounts of material derived from solvent attack at the 5-position of the pyrrole ring. The results indicate a high degree of charge delocalization by the strongly donating pyrrolyl group, resulting in a low k(H)/k(CF3) rate ratio and nucleophilic attack on the ring. 1,1, l-Trifluoro-2-(1-methyl-2-pyrrolyl)propan-2-ol (10) reacted with CF3CO2H via a tertiary carbocation, which led to dimeric products resulting from electrophilic substitution on the pyrrole ring. Keywords: pyrrole, trifluoromethyl substituents, carbocations.
Spectrophotometric determination of peroxydisulphate with o-dianisidine by flow injection
Authors: Shiundu Paul M. , Wade Adrian P. , Jonnalagadda S. B.
Published: May 2006

Precise and accurate knowledge of peroxydisulfate ion concentration is critical in industrial polymer production, where it is extensively used as an activator. Its ability to oxidize in either acidic, neutral, or alkali media also makes it widely applicable in many other areas of chemistry.In this paper we present an optimized spectrophotometric flow injection method for determination of peroxydisulfate. The analyte oxidizes o-dianisidine to form a stable product which has a convenient absorbance maximum at 450 nm. This provides a simple and sensitive alternative to present methods which are more costly, tedious, or complex. Keywords: spectrophotometric method, peroxydisulphate.
The solution-phase photochemistry of 2-trifluoromethylnorbornene
Authors: Nguyen N. , Harris B. E. , Clark K. B. , Leigh W. J.
Published: May 2006

The photochemistry of 2-trifluoromethylnorbornene in pentane solution has been investigated. Direct photolysis with 193 nm light yields 1-trifluoromethyl-2-norcarene in 90% yield, due to formal [1,3]-sigmatropic rearrangement, in addition to three other minor products. Chlorobenzene-sensitized photolysis affords photoreduction products, principally exo- and endo-2-trifluoromethylnorbornane and decane isomers, in addition to several products of higher molecular weight. On the basis of comparisons of the photochemistry of this compound to that previously reported for norbornene and 2-cyanonorbornene, as well as spectroscopic evidence, it is suggested that the lowest excited singlet state (the photoreactive state in solution) in 2-trifluoromethylnorbornene is the π, π* state. The results indicate that trifluoromethyl substitution has the effect of significantly raising the energy of the π,R(3s) Rydberg state in simple alkenes, without altering the high energy (ca. 6.5 eV) or the localized character of the alkene π, π* state. Furthermore, they lend additional support to the view that the carbene-derived products typically formed upon photolysis of alkenes in solution are derived from π,R(3s) excitation. Keywords: Rydberg, photolysis, far-uv, alkene, solution phase, singlet, triplet.
Radical clocks as probes of 1,4-biradical intermediates in the photochemical cycloaddition reactions of 2-cyclopentenone with alkenes
Authors: Rudolph Andreas , Weedon Alan C.
Published: May 2006

not available
On the structure and reactivity of a stabilized phosphorane: methyl (triphenylphosphoranylidene) acetate
Authors: Kayser Margaret H. , Hooper Donald L.
Published: May 2006

In the absence of a source of acidic protons, methyl (triphenylphosphoranylidene)acetate 1 exists as a mixture of the cis and trans isomers even at room temperature. Acidic protons catalyze the interconversion of the two forms and facilitate the reaction of the ylide with active carbonyl compounds such as benzaldehyde or phthalic anhydride. Keywords: stabilized phosphoylide, structure, reactivity.
Reactions of organic anions. 174. Vicarious Nucleophilic Substitution of hydrogen (VNS) in nitrobenzofuroxans and the Boulton–Katritzky rearrangement of the VNS products
Authors: Ostrowski Stanisław , Wojciechowski Krzysztof
Published: May 2006

The Vicarious Nucleophilic Substitution of hydrogen in 4-nitrobenzofuroxan derivatives by carbanions of chloromethyl phenyl sulfone, chloromethyl p-tolyl sulfone, chloromethyl tert-butyl sulfone, and N,N-dimethyl chloromethane sulfonamide proceeds at positions 5 and 7. In some cases the Boulton–Katritzky rearrangement of the products obtained, leading to more stable 7-substituted isomers, was observed. Keywords: carbanions, nitrobenzofuroxans, sulfones, Vicarious Nucleophilic Substitution, Boulton–Katritzky rearrangement.
Kinetics, isotope effects of the reaction of 1-(4-nitrophenyl)-1-nitroalkanes with DBU in tetrahydrofuran and chlorobenzene solvents
Authors: Galezowski Wlodzimierz , Jarczewski Arnold
Published: May 2006

The kinetics of the reaction of[Formula: see text](R = Me, Et, i-Pr; NPNE, NPNP, MNPNP respectively; L is H or D) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) base in tetrahydrofuran (THF) and chlorobenzene (CB) solvents are reported. The products of these proton transfer reactions are ion pairs absorbing at λmax = 460–480 nm. The equilibrium constants in THF were [Formula: see text]and in CB [Formula: see text]for NPNE, NPNP, MNPNP respectively. The thermodynamic parameters of the reactions are also quoted. The substrate reacts with DBU in both THF and CB solvents in a normal second-order proton transfer reaction. In the case of deuteron transfer, isotopic D/H exchange is much faster than internal return. The reactions show low values of enthalpy of activation ΔH* = 14.3, 18.1, 24.2 and 13.0, 15.1, 18.6 kJmol−1 for NPNE, NPNP, and MNPNP in THF and CB respectively, and large negative entropies of activation −ΔS* = 141, 139, 146; 140, 146, 160 J mol−1 deg−1 for the same sequence of substrates and solvents. The kinetic isotope effects are large, (kH/kD)20°c = 12.2, 13.0, 10.1; 12.9, 12.0, 10.2 for the above sequence of substrates and solvents, and show no difference with changes in either steric hindrance of the C-acids or polarity of the solvents. Keywords: proton transfer, kinetic isotope effect.
Charge transfer emissive singlet excited states and photoinduced electron transfer properties in the diarylideneacetone compounds (RCHCH)2CO; R = phenyl, 1- and 2-naphthyl, 3-(N-ethylcarbazoyl), and 4-(C5H5)Fe(C5H4C6H4CHCH(CO)CHCHC6H5)
Authors: Harvey Pierre D. , Gan Liangbing , Aubry Christiane
Published: May 2006

Four diarylideneacetone compounds ((RCHCH)2CO, where the aryl groups are phenyl (dba), 1-naphthyl (1-dNapha), 2-naphthyl (2-dNapha), and 3-(N-ethylcarbazoyl) (dNECa)), and 4-(C5H5)Fe(C5H4C6H4CHCH(CO)CHCH(C6H5) (dba-Fc) have been prepared and characterized. The compounds are found to be fluorescent and photochemically and reversibly electrochemically active. The lowest-energy absorption bands for the diarylideneacetones are assigned to a charge transfer (CT) electronic transition, except for dba-Fc, in which a ferrocenyl ligand field transition assignment is made. The 77 K CT absorption and fluorescence bands are vibrationally structured (vibrational spacings = 1260–1360 cm−1). While the fluorescence decay at 293 K is monoexponential, the excited state fluorescence lifetimes (τF) for the 77 K samples exhibit double exponential decays, the short component being 0.38–0.64 ns and the long one 3.5–10.9 ns. The photophysical results are interpreted in terms of excited state deactivation processes dominated by radiationless pathways that are associated with the presence of fluorescent species with different geometries. Only the dNECa compound is found to be fluorescent in solution at 298 K [Formula: see text]. Cyclic voltammetry and coulometry measurements suggest that a reversible one-electron reduction process and an irreversible higher potential one-electron reduction process take place in the −1 to −2 V vs. SSCE range. In addition, dba-Fc also exhibits an electrochemically reversible one-electron oxidation wave at 0.52 V vs. SSCE centered at the ferrocenyl group. These results together with the spectroscopic electronic data have permitted evaluation of the reduction potentials of the lowest singlet (CT excited states (E1−/*);they range from 1.4 to 2.2 V vs. SSCE, with dba being the strongest photooxidizing agent and dNECa the weakest. Photoinduced intermolecular electron transfer reactions have been investigated by steady state fluorescence techniques and picosecond flash photolysis spectroscopy for dNECa and dba, respectively. The bimolecular deactivation rate constants, kq, for the reductive photoinduced electron transfer reactions of dNECa with diphenylamine (DPA) (kq = (2.65 ± 0.25) × 107 M−1 s−1) and N, N, N′, N′-tetramethylphenylenediamine (TMPM) (kq = (1.38 ± 0.03) × 108 M−1 s−1) have been obtained in THF solutions at 293 K. No fluorescence quenching is observed when oxidative and energy transfer quenchers are used with dNECa. For the non-emissive dba compound at room temperature, picosecond flash photolysis experiments show that quenching of the broad dba transient band (~500 nm) does indeed occur between 5 and 10 ns. Keywords: dibenzylideneacetone, charge transfer, photoelectron transfer.
2R,4S-2-(2′-Methyl-3′-hydroxy-5′-hydroxymethylenepyridine-C4′)-5,5-dimethylthiazolidine-4-carboxylic acid, the product of the reaction of D-penicillamine and vitamin B6
Authors: Faggiani R. , Howard-Lock H. E. , Lock C. J. L. , Orgias R.
Published: May 2006

We have studied the reaction of both D- and L-penicillamine with pyridoxal hydrochloride and examined the products by single crystal X-ray diffraction. The structure of the title compound, A, formed by the reaction of D-penicillamine and pyridoxal was determined. Crystals are orthorhombic, P212121, with cell dimensions a = 9.507(5), b = 19.185(5), c = 7.766(2) Å and Z = 4. The structure was solved by standard methods and refined to R = 0.088, Rw = 0.079 for 2433 independent reflections. A exists as a zwitterion, and bond lengths and angles are normal. In the solid state, A and the corresponding product obtained from L-penicillamine, B, have identical geometrical structure but are of opposite chirality; that is, D-penicillamine produces the 2R,4S diastereomer and L-penicillamine produces the 2S,4R diastereomer (with no S,S and R,R components). In solution, however, NMR spectra show the presence of both pairs of diastereomers (2R,4S and 2S,4S; 2S,4R and 2R,4R). In neutral or alkaline solution there appears to be a rapid epimerization at the thiazolidine carbon atom attached to the pyridoxal moiety. Features of the mass, 1H NMR, vibrational, and electronic spectra are also discussed. Key words: D-penicillamine, vitamin B6, pyridoxal hydrochloride, therapeutic uses, bischemical reactions, structures.
Polycarboxylate crown ethers from meso-tartaric acid
Authors: Fronczek Frank R. , Gandour Richard D. , Fyles Thomas M. , Hocking Philippa J. , McDermid Susan J. , Wotton P. Daniel
Published: May 2006

The synthesis of crown ethers derived from meso-tartaric acid was investigated. The sodium salt of the bis(dimethylamide) of meso-tartaric acid reacted with diethylene glycol ditosylate to give a mixture of 18-crown-6 tetraamide and 27-crown-9 hexaamide crown ethers. The 2R,3S,11S,12R 18-crown-6 isomer crystallized in triclinic space group [Formula: see text] (a = 7.557(2), b = 8.866(2), c = 10.4133(13) Å, α = 94.13(2), β = 95.86(2), γ = 99.26(2)°, R = 0.040 for 2090 observed of 3129 unique reflections). The structures of the remaining products were then assigned from the NMR spectra. The solution conformations of the amide crown ethers were examined by NMR, and provide a rationale for the product distribution obtained. One of the 18-crown-6 isomers and a mixture of the two 27-crown-9 isomers were hydrolyzed to the respective crown ether carboxylic acids, and the stability constants for complexation of cations were determined by potentiometric titration. The meso tetra- and hexacarboxylates are remarkably nonselective and inefficient cation complexing agents, compared to related crown ethers from R,R-(+)-tartaric acid, due to the unfavorable conformational control exerted by the tartaro units. Key words: crown ether synthesis, complexation, crown ether conformation, meso-tartaric acid, crystal structure.
The structure and partial synthesis of imbricatine, a benzyltetrahydroisoquinoline alkaloid from the starfish Dermasterias imbricata
Authors: Burgoyne David L. , Miao Shichang , Pathirana Charles , Andersen Raymond J. , Ayer William A. , Singer Peter P. , Kokke William C. M. C. , Ross Donald M.
Published: May 2006

The structure of imbricatine (1), a cytotoxic metabolite of the starfish Dermasterias imbricata, has been determined by spectroscopic analysis and chemical degradation. Synthesis of model compounds 7 and 18 provided evidence for the absolute configuration and constitution of the benzyltetrahydroisoquinoline substructure of imbricatine. Key words: imbricatine, asteroid, alkaloid.