bluuebunny/crossref_metadata_2025 huggingface dataset:
Charge and energy redistribution in sulfonamides undergoing conformational changes. Hybridization as a controlling influence over conformer stability
Authors: Breneman Curt M. , Weber Lawrence W.
Published: May 2006

The redistribution of charge and electronic kinetic energy was studied during rotation about the S—N bonds of sulfonamide and fluorosulfonamide. The rotational potentials and electronic topological features of both compounds were evaluated at the HF/6-3 1G* level of theory and their electron densities partitioned into atomic contributions using FASTINT, an updated version of the PROAIM program. The results indicate that the stability of each rotamer is strongly dependent upon the hybridization of the sulfonamide nitrogen. The hybridization of the nitrogen was determined by examination of the positions and magnitudes of the electrostatic and Laplacian minima in the nonbonded region of the sulfonamide nitrogen atom. Independent assessments of hybridization were made using nitrogen pyramidalization altitudes. The rotational barriers in these compounds were found to arise mainly from energetic penalties resulting from adding electrons to already electron-rich sulfonyl oxygens while removing electron density from other more electronegative atoms. The fluorine-substituted analogue provided an example in which the sulfur and oxygen atoms were much less electron rich, causing an enhancement of the nitrogen rehybridization effects. The extent of covalent bonding between pertinent pairs of atoms in sulfonamide and fluorosulfonamide was assessed throughout the rotational pathway using the BONDER program. In contrast with much existing dogma, all of these findings were consistent with the same general model of charge and energy flow that has been shown to determine the internal rotational barriers in amides. Key words: sulfonamide, electron density analysis, rotational barrier, hybridization, atoms-in-molecules calculations.
Preparation, spectroscopic properties, and ion chemistry of 3-diazo-1,1,1-trifluoro-2-propanone
Authors: Strazzolini Paolo , Giumanini Angelo G. , Gambi Alberto , Verardo Giancarlo , Cerioni Giovanni
Published: May 2006

The title compound (1) is prepared in satisfactory yield by reacting CH2N2 with trifluoroacetic anhydride in Et2O; the d-analog was obtained by exchange with D2O. 1H, 13C, 15N, 17O, and 19F NMR spectra of 1 were studied, as well as its IR spectrum. A single isomer is present corresponding to the more stable Z configuration. The structural assignment was made on the basis of quantum mechanical calculations, which revealed that the Z form is some 13.4 kJ mol−1 more stable than the E form and the activation energy for the E → Z transition is 64.2 kJ mol−1. Mass spectra under different experimental conditions were recorded and breakdown pathways of the parent ion of 1 charted. Key words: 3-diazo-1,1,1-trifluoro-2-propanone; 3-d-3-diazo-1,1,1-trifluoro-2-propanone; structure; 1H, 13C, 15N, 17O, and 19F NMR; EI-MS.
Determination of the visible spectra of electrode reaction products in strongly absorbing media by diffusion-controlled chronoabsorptometry
Authors: Jones David H. , Hinman A. Scott
Published: May 2006

Chronoabsorptometry under diffusion-controlled conditions has been applied to determination of the difference in molar absorptivity between electrode reaction products and reactants. The technique allows complete determination of the ultraviolet–visible spectra of reaction products that are stable on the time scale of a few hundred milliseconds. The technique was implemented with a reflectance cell that employs quartz light pipes to minimize optical absorbance of the bulk solution without obstructing the current path between counter and working electrodes. The spectrum of one-electron oxidized chloro-(5,10,15,20-tetraphenylporphinato)Fe(III) is determined with the new technique and compared with that obtained by conventional thin-layer spectroelectrochemistry. Key words: spectroelectrochemistry, chronoabsorptometry, thin-layer spectroelectrochemistry.
Electroréduction de dérivés dinitrés aromatiques. V. influences du matériau d'électrode et de systèmes rédox sur les réductions du 4,4′-dinitrodibenzyle et de l'acide 4,4′-dinitrostilbène-2,2′-disulfonique
Authors: Martre Anne-Marie , Mousset Guy , Cosoveanu Veronica , Danciu Virginia
Published: May 2006

This work concerns the electrochemical and chemical reductions of 4,4′-dinitrodibenzyl and 4,4′-dinitrostilbene 2,2′-disulfonic acid under various experimental conditions. On the one hand, the electrochemical reduction is realized on Sn, monel, and Hg electrodes with or without the presence of a redox couple, and on the other hand, the chemical reduction is performed by mean of an electrochemically generated reducing agent (Sn0). According to the type of redox couple used (Ti3+/Ti4+ or sn0/sn2+), important changes in the nature and the ratio of reaction products are observed. The best conditions for obtaining di- or monoamines are defined, they correspond to 90% yield for the diamines, 70% and 50%, respectively, for 4-amino-4′-nitrodibenzyl and for 4-amino-4′-nitrostilbene-2,2′-disulfonic acid. The electrochemical behavior of the dinitro derivatives and of TiOSO4, TiCl4, and SnCl2, used as precursors of the redox couples is studied under conditions of macroscale electrolyses (5 N H2SO4–EtOH 50/50, 2.5 N H2SO4, and 10% HCl). Key words: 4,4′-dinitrodibenzyl, 4,4′-dinitrostilbene 2,2′-disulfonic acid, electrochemistry, reduction, Ti3+/Ti4+ and Sn0/Sn2+ redox systems.
Synthesis of chemically and functionally diverse scaffolds from pentaerythritol
Authors: Hanessian Stephen , Prabhanjan Hubli , Qiu Dongxu , Nambiar Sudhir
Published: May 2006

Pentaerythritol (2,2-bis-hydroxymethyl-propane-1,3-diol) was converted into a series of mono-, di-, and trisubstituted derivatives, comprising allyl ethers and amino-alkyl ethers, by systematic chemical manipulation of the hydroxy groups. The remaining hydroxymethyl group in the case of the trisubstituted analog was functionalized with ether groups bearing terminal ω-carboxyl or ω-alkene groups. These derivatives are versatile templates and scaffolds for single, double, or triple substitution with appropriate ligands forming amides and esters, and allowing the attachment of the ω-alkene or ω-carboxyl group to solid support for combinatorial chemistry. Key words: molecular diversity, scaffolds, templates and tris(2-aminoethyl)-pentaerythritol.
The chemistry of thujone. XIX. Acid-promoted ring cleavage of thujone-derived cyclopropylcarbinols
Authors: Kutney James P. , Chen Yong-Huang , Rettig Steven J.
Published: May 2006

Thujone-derived cyclopropylcarbinols can cleave via three distinct pathways: the exo cleavage, the endo cleavage, and the interesting cyclopropylcarbinyl rearrangement. Factors determining which pathway is adopted under specific conditions are discussed. A short sequence leading to synthesis of the sesquiterpene (+)-β-cyperone is described. Key words: thujone, cyclopropylcarbinols, cyclopropylcarbinyl rearrangement, β-cyperone synthesis.
Stereoselectivity of directed epoxidations of 22-hydroxy-Δ23-sterol side chains
Authors: Back Thomas G. , Baron Denise L.
Published: May 2006

A series of 22-hydroxy-Δ23-sterols comprising (3β,5α,6β,22S)-6-methoxy-3,5-cyclo-25,26,27-trinorcholest-23-en-22-ol (6), (3β,5α,6β,22S,23E)-6-methoxy-3,5-cyclo-26,27-dinorcholest-23-en-22-ol (7), 3β,5α,6β,22R)-6-methoxy-23-methyl-3,5-cyclo-25,26,27-trinorcholest-23-en-22-ol(10),3β,5α,6β,22S,23Z)-6-methoxy-3,5-cyclo-26,27-dinorcholest-23-en-22-ol (11), and 3β,5α,6β,22R)-6-methoxy-3,5-cyclo-26,27-dinorergost-23-en-22-ol (12) were subjected to epoxidation with m-chloroperbenzoic acid and tert-butyl hydroperoxide in the presence of either vanadyl acetoacetonate or molybdenum hexacarbonyl, and the threo:erythro ratios of the products were determined. The results are of relevance for the synthesis of sterols with oxygenated side chains, such as brassinolide (1). The oxidations of 6 and 7 were erythro selective with all three oxidants, especially with the vanadium-catalyzed system. Peracid oxidation of the 22-tert-butyldimethylsilyl ether (8) and 22-pivaloate (9) of alcohol 7 showed similar erythro selectivity to that of the parent compound 7. Allylic alcohol 10 gave exclusively the erythro epoxide with all three oxidants, while 11 and 12 were threo-selective under all three conditions. Molecular modeling indicated that erythro selectivity in the vanadium-catalyzed epoxidation of 10 was consistent with a destabilizing interaction (A(1,2) strain) between the gem-methyl and C(21) methyl groups in the conformation required for formation of the threo-isomer. The threo-selective peracid oxidations of 11 and 12 were attributed to A(1,3) strain between the cis-methyl groups and C(20) in the conformation required for formation of the erythro-epoxide. The differences in the calculated energies of conformations leading to the threo- and erythro epoxide diastereomers of substrates containing no gem or cis substituents proved too small to permit reliable prediction of diastereoselectivity. Key words: 22-hydroxy-Δ23-sterols, brassinosteroids, allylic alcohols, epoxidation, diastereoselectivity
5-Perfluoroalkyldipyrromethanes and porphyrins derived therefrom
Authors: Wijesekera Tilak P.
Published: May 2006

Acid-catalyzed condensation of pyrrole and perfluoroalkyl aldehydes produced, in a single step, α,β-unsubstituted 5-perfluoroalkyldipyrromethanes, which were isolated and characterized for the first time. These dipyrromethanes were used as the key intermediates in the synthesis of trans-perfluroalkylporphyrins as well as meso-tetrakis(perfluoroalkyl)porphyrins. The two-step procedure described here provides access to a wide variety of porphyrins with trans perfluoroalkyl groups. Key words: 5-perfluoroalkyldipyrromethanes, perfluoroalkylporphyrins, electron-deficient porphyrins.
Folding strain stereocontrol in cyclohexane ring formation by means of an intramolecular ester enolate alkylation reaction
Authors: Tokoroyama Takashi , Kusaka Hisashi
Published: May 2006

Diastereoselectivity in the cyclization of ethyl 7-bromo-2-methylheptanoates with an additional substituent at various positions in the chain, by LDA treatment, was investigated in connection with the concept of folding strain stereocontrol. Cyclization of 3-, 4-, and 6-methyl-substituted substrates revealed high selectivity, which demonstrates the prevalence of folding strain stereocontrol and the usefulness of this approach for stereoselective ring construction. In particular, reactions of the latter two substrates resulted in the stereodivergent preparation of diastereomeric 1,3-dimethylcyclohexanecarboxylates. In the case of the 5-mefhyl-substituted substrate, the selectivity of ring closure was only moderate.1H and13C NMR spectroscopic data were useful for determining the conformation of 1 -methylcyclohexanecarboxylate derivatives. The origin of the diastereoselectivity was examined through the qualitative comparison of the strain in the diastereomeric folding in the transition state. Various factors that might affect stereoselectivity were examined in the cyclization of 5-substituted substrates to better understand this concept. As predicted, the selectivity increased as the substituent became bulkier: Ph < Me ≈ Et < i-Pr < t-Bu. The effects of other factors — solvent, base counter cation, and leaving group — on selectivity agree with results predicted from the reactivity–selectivity relationship. Key words: folding strain stereocontrol; diastereoselectivity in ring-closure reaction; remote asymmetric induction; ethyl 2-methylcyclohexanecarboxylate derivatives,1H and13C NMR; stereoselective synthesis of substituted cyclohexane derivatives.
Annulated derivatives of 2,2′-biimidazole, 2-(2′-imidazolyl)benzimidazole, and 2,2′-bibenzimidazole
Authors: Ames James R. , Houghtaling Melissa A. , Terrian Deborah L. , Mitchell Timothy P.
Published: May 2006

Preparation of bisannulated salts of 2,2′-biimidazole, 2-(2′-imidazolyl)benzimidazole, 2,2′-bibenzimidazole, and annulated salts of 1,1′-dimethyl-2,2′-biimidazole, 1,1′-dimethyl-2-(2′-imidazolyl)benzimidazole, and 1,1′-dimethyl-2,2′-bibenzimidazole is accomplished by direct alkylation of the parent azaheterocycle with excess 1,n-dihaloalkane. Discussions of the product conformations use electronic absorption spectra and NMR. The redox potentials of these salts are measured in DMF and CH3CN, and become increasingly more negative and less reversible as the systems become less planar. Keywords: electrochemistry, bridged azabiaryl salts.
The complexation of glucose and other monosaccharides with cyclodextrins
Authors: Hacket Frank , Coteron Jose-Miguel , Schneider Hans-Jörg , Kazachenko Vladimir P.
Published: May 2006

Equilibrium constants of complexes between β-cyclodextrin and glucose, galactose, or mannose have been determined by fluorometric competition titrations in water to be as low as K = 1 – 0.5 M−1, in sharp contrast to recently published values for glucose around K = 400 M–1. The pentoses ribose, xylose, and arabinose show association constants between 6.3 and 1.5 M−1, in agreement with published values. Preliminary computer-aided molecular modelling studies suggest that the small values observed for the hexoses are not due to steric hindrance of an intracavity inclusion but to a smaller number of intermolecular hydrogen bonds and to some strain as a result of cyclodextrin deformation. Calorimetric measurements of the glucose–β-cyclodextrin complex show, again in contradiction with the literature, an endothermic reaction, or a free enthalpy value close to zero. Key words: cyclodextrin complexes, sugar complexation, glucose complexation, calorimetry, computer-aided molecular modelling.
Metal complexes of propane-1,2,3-triamine. Potentiometric and calorimetric investigations
Authors: Cini Renzo , Sabatini Antonio , Vacca Alberto , Zanobini Fabrizio
Published: May 2006

The complexes formed by the ions Co2+, Ni2+, Cu2+, and Zn2+ with the ligand propane-1,2,3-triamine (L1) in 0.15 mol dm−3 aqueous NaCl at 25 °C have been studied by potentiometry and calorimetry (ligand to metal molar ratio, 1–3; pH range from 3.0 to 6.5–11.0). Stability constants have been obtained from potentiometric data by means of the least-squares program HYPERQUAD for the following species: [M(L1)]2+ and [MH(L1)]3+ for all four metal cations; [M(L1)2]2+ and [MH(L1)2]3+ for Ni2+, Cu2+, and Zn2+; [MH2(L1)2]4+ for Ni2+ and Cu2+; and [M(OH)(L1)]+ for Cu2+ only. Stabilities increase in the order Zn < Co < Ni < Cu. The complexes of L1 have approximately the same stability as the corresponding ones formed by the related triamines 2-methyl (2-methylamine) propane-1,3-diamine (L3) and 2-ethyl (2-methylamine)propane-1,3-diamine (L4). The stability constants have been used to obtain, from the calorimetric titration experiments, the formation enthalpies of all the metal complexes except those for cobalt(II) as well as the corresponding entropy changes. The thermodynamic quantities are compared with the corresponding values previously found for L3, in order to derive information about the coordination ability of the ligand. The analysis suggests that the unprotonated ligand forms a fac-tridentate complex in most cases, but for some species, such as [Cu(L1)2]2+, [CuH(L1)2]3+, and [Zn(L1)]2+, the thermodynamic quantities indicate that at least one of the nitrogen atoms of the triamine is either loosely bonded or not coordinated to the metal ion. A gas-phase molecular mechanics analysis shows that the strain energies of the fac-[Co(L1)2]2+ and -[Ni(L1)2]2+ symmetrical (s) and unsymmetrical (u) isomers are very similar. If the same solvation energy is assumed for the two isomers, each isomer will be present in roughly equal amounts in solution. Keywords: propane-1,2,3-triamine, potentiometry, calorimetry, complexation, cobalt, nickel, copper, zinc.
1H NMR study of paramagnetic rhenium(III) monomers with nitrogen heterocycles. Crystal structures of ReCl3L2(PPh3) complexes (L = pyridine, 3-picoline, and 1-methylimidazole)
Authors: Pearson Céline , Beauchamp André L.
Published: May 2006

Rhenium(III) monomers of the type ReCl3Ln(PPh3)3−n, where n = 1,2, and 3, were prepared for simple nitrogen heterocyclic ligands L and characterized mainly by 1H NMR and IR spectroscopies. Most of the 1H NMR signals of these paramagnetic compounds were assigned by using methylated pyridines. A mer-cis octahedral geometry slightly distorted by triphenylphosphine was determined by X-ray diffraction for three ReCl3L2(PPh3) complexes: L = pyridine, triclinic, [Formula: see text], a = 11.379, b = 13.532, c = 18.160 Ǻ, α = 80.16°, β = 89.57, γ = 86.27°, Z = 4, R = 0.076; L = 3-picoline, triclinic, [Formula: see text], a = 10.557, b = 12.580, c = 13.153 Ǻ, α = 66.80°, β = 67.46°, γ = 75.89°, Z = 2, R = 0.021; L = 1-methylimidazole, monoclinic, P21/c, α = 15.437, b = 9.436, c = 18.607 Ǻ, β = 92.23°, Z = 4, R = 0.046. The same structure was deduced from 1H NMR data for these compounds in solution, whereas the other types of complexes were found to exist as the mer,trans-ReCl3L(PPh3)2 and mer-ReCl3L3 isomers. In contrast with these complexes, which contain the mer-ReCl3 core, the starting material ReCl3(benzil)(PPh3) is found to be the fac isomer in solution. The typical low-field shifted pattern of 1H NMR signals for PPh3 is not greatly perturbed by changes in the number or arrangement of the remaining ligands in the coordination sphere, but the signals for protons on conjugated systems directly bonded to the metal atoms are much more affected by the unpaired electrons. Keywords: paramagnetic NMR, crystal structures, rhenium(III) complexes, imidazole, pyridine, triphenylphoshine.
Keto–enol tautomerism, NMR spectra, and H–D exchange of 4-hydroxycoumarins
Authors: Traven Valery F. , Negrebetsky Vadim V. , Vorobjeva Larisa I. , Carberry Edward Andrew
Published: May 2006

4-Hydroxycoumarin 1, 4,5-dihydroxycoumarin 2, and 4,7-dihydroxycoumarin 3 undergo H–D exchange at the C(3) atom of the lactone ring. Although only the 4-hydroxy-2-chromenone tautomeric forms are seen in the 1H and 13C NMR spectra of compounds 1–3, the equilibrium between the 4-hydroxy-2-chromenone and 2,4-chromandione forms is suggested to be the key step in the H–D exchange reaction. 4,5-Dihydroxycoumarin shows the highest rate of the reaction, since H-bonding between 5-hydroxyl and 4-keto functional groups can provide relative stability to the 5-hydroxy-2,4-chromandione tautomeric form, a probable intermediate of the exchange. NMR spectra and tautomeric transformations of 3-(4-methoxyphenylazo)-4-hydroxycoumarin 4 and 3-acetyl-4-hydroxycoumarin 5 are also discussed. The stabilities of different tautomeric forms of compounds 1–5 have been evaluated by MNDO calculations. Keywords: 4-hydroxycoumarin derivatives, keto-enol tautomerism, H–D-exchange.
Étude expérimentale et modélisation des réactions d'oxydation du n-pentane et du cyclopentane
Authors: Simon V. , Simon Y. , Scacchi G. , Baronnet F.
Published: May 2006

The reduction of automotive emissions and the reformulation of gasoline composition require a better understanding of the combustion mechanism, in particular aiming at the determination of the influential reactions which are responsible for the onset of knock in spark ignition engines. We have studied the oxidation mechanism of two hydrocarbons of different research octane number: n-pentane (62) and cyclopentane (100). This work follows a previous study on the oxidation of n-heptane and isooctane. The experimental study was performed at 873 K in a jet-stirred flow reaction vessel. The reaction mechanisms of n-pentane and cyclopentane were analysed on a purely kinetic basis, derived from our experimental results, and were finally modelled by using a programme of simulation of reaction mechanisms. The different behaviour of these two hydrocarbons with respect to autoignition phenomena is mainly dependent on the relative concentration of resonance-stabilized radicals in the reaction medium. Keywords: oxidation reaction, n-pentane, cyclopentane, modelling, autoignition.
Synthesis of regiospecifically substituted 2-hydroxybenzocyclobutenones
Authors: Lear Yvonne , Durst Tony
Published: May 2006

Regiospecifically substituted 2-hydroxybenzocyclobutenones were synthesized from the corresponding TBS protected 2-bromo-mandelate esters via halogen–metal exchange, cyclization, and subsequent deprotection. Keywords: 2-hydroxybenzocyclobutenones, halogen–metal exchange, benzocyclobutenediones, 2-bromomandelate esters.
Electrochemical reduction of nickel(II) dithiocarboxylates at the mercury electrode
Authors: Safavi A. , Fotouhi L.
Published: May 2006

The reduction mechanisms of a series of nickel(II) dithiocarboxylate complexes have been investigated in dimethyl sulphoxide at the mercury electrode. Various electrochemical techniques, including polarography, cyclic voltammetry, chronoamperometry, and controlled potential coulometry, were employed. The reduction of the complexes of the acid derivatives of 2-aminocyclopentene-1-dithiocarboxylate (ACD) proceeds initially by an ECE mechanism (electron transfer – chemical reaction – electron transfer) followed by a one-electron irreversible process. The nature of the complete electrode reaction suggests a metal-centered reduction. The nickel complexes of the ester derivatives of ACD underwent a one-electron reduction that was subject to a follow-up catalytic reaction (EC′ mechanism) and the original complex is regenerated through this regeneration reaction. Keywords: reduction, nickel(II) dithiocarboxylate, mercury electrode.
Novel boron chelate complexes from the reaction of salicylaldehydes, tertiary amines, and diphenylborinic or phenylboronic acid. Crystal and molecular structures of two new types of chelated organoborate salts
Authors: Kliegel Wolfgang , Metge Jörg , Rettig Steven J. , Trotter James
Published: May 2006

The one-pot reaction of equimolar amounts of salicylaldehyde, diphenylborinic acid anhydride, and a tertiary amine in the presence of an alkanol (R′OH) led to the addition of R′OH to the aldehyde group and the formation of an O,O-acetal moiety within the chelate anion with the charge balanced by an ammonium cation arising from the tertiary amine. Exchanging the diphenylborinic acid in the three-component reaction system for phenylboronic acid did not give the analogous adduct or chelate but one additional mole equivalent of phenylboronic acid (anhydride) was incorporated, leading to a polycyclic anion containing pyroboronate and acetal boronate functions with an associated ammonium cation. Crystals of 1-methylpiperidinium 4-ethoxy-2,2-diphenyl-1,3-dioxa-2-borata-1,2,3,4-tetrahydronaphthalenate, 6d, are monoclinic, a = 9.522(1), b = 17.703(1), c = 14.974(1) Å, β = 98.275(10)°, Z = 4, space group P21/n, and those of 1-methylpiperidinium 6,10-epoxy-4-methoxy-6,8-diphenyl-5,7,9-trioxa-8-bora-6-borata-5,6,7,8,9,10-hexahydrobenzocyclooctenate, 10e, are monoclinic, a = 7.968(1), b = 19.3707(4), c = 15.9400(5) Å, β = 100.186(5)°, Z = 4, space group P21/n, The structures were solved by direct methods and refined by full-matrix least-squares procedures to R = 0.050 and 0.033 (Rw = 0.040 and 0.032) for 2815 and 3529 reflections with I ≥ 3σ(F2), respectively. Compounds 6d and 10e are both representative of new classes of organoboron chelates. Keywords: salicylaldehyde semiacetal diphenylboron chelate, salicylaldehyde bisboronate, organoboron compound, crystal structure.
Reactions of alkoxyaryloxycarbenes with tethered triple bonds: a new synthesis of substituted benzofurans
Authors: Kassam Karim , Venneri Paul C. , Warkentin John
Published: May 2006

Cyclic 3-alkoxy-3-aryloxyvinylcarbenes can be generated by a highly exo-selective intramolecular cyclization of a dioxycarbene onto a tethered triple bond. Like their dialkoxy counterparts, the 3-alkoxy-3-aryloxyvinylcarbene intermediates are capable of interesting reactions. In contrast to 3,3-dialkoxyvinylcarbenes, which undergo a formal [3 + 2] cycloaddition with highly electron-deficient olefins such as benzylidenemalononitrile, 3-alkoxy-3-aryloxyvinylcarbenes undergo a highly diastereoselective [1 + 2] cycloaddition with benzylidenemalononitrile to give cyclopropylketene acetals. At high temperatures, those cyclopropylketene acetals undergo a clean vinylcyclopropane rearrangement in which the stereochemical integrity of the cyclopropane is retained. Keywords: carbene, alkoxyaryloxy, [1 + 2] cycloaddition, ketene acetal, vinylcyclopropane rearrangement, oxadiazoline.
On the synthesis and the mechanism of formation of halogenated enol lactones
Authors: Kayser Margaret M. , Zhu Jun , Hooper Donald L.
Published: May 2006

The synthesis of halo enol lactones from cyclic anhydrides via lactonization of the corresponding keto phosphoranes provides a direct route to these interesting compounds, which possess important biological properties and are useful intermediates in organic synthesis. In this paper we outline the syntheses of several halo enol lactones and discuss mechanistic consequences of these reactions on the understanding of Wittig reactions with cyclic anhydrides. Keywords: halolactonization, cyclic anhydrides, halo enol lactones, Wittig reaction mechanism.
Synthesis of hyperbranched glycodendrimers incorporating α-thiosialosides based on a gallic acid core
Authors: Meunier Serge J. , Wu Quigquan , Wang Sho-Nong , Roy René
Published: May 2006

Hyperbranched glycodendrimers containing sialic acid residues were synthesized in order to further understand the multivalency effect and its role in carbohydrate–protein interactions. Gallic acid 7 as trivalent core and oligoethylene glycol derivatives as hydrophilic spacers were used to scaffold the dendritic backbones. α-Thiosialoside 16 was conjugated onto N-chloroacetylated dendritic precursors 13,14, and 26 by nucleophilic substitution to afford trivalent 17,18, and nonavalent 27 sialodendrimers. Complete sugar deprotection furnished water-soluble α-thiosialodendrimers 21,22, and 29, which were used in protein-binding studies. Turbidimetric analysis confirmed the strong potential of sialodendrimers 29 having nine readily accessible sialic acid residues to bind, cross-link, and precipitate two different lectins. Preliminary results indicated that nonavalent α-sialodendrimer 29 had a greater affinity towards dimeric wheat germ agglutinin (WGA) and the lectin from the slug Limaxflavus (LFA) than the corresponding trivalent glycodendrimers 21 and 22. Keywords: carbohydrate, dendrimer, sialic acid, gallic acid, lectin, wheat germ agglutinin, Limaxflavus.
The chemical history of a current
Authors: Laidler Keith J.
Published: May 2006

The electric current was born two centuries ago, and its precise nature is still being elucidated. Various lines of investigation have been pursued. The discovery of electrolysis in 1800 was followed in 1834 by Faraday's laws, and led to the realization that there is a fundamental particle of electricity. In the 1840s, Joule's studies on batteries and electric motors led him to the formulation of the principle of conservation of energy. Clerk Maxwell's theory of electromagnetic radiation owed much to measurements of the speed of an electric current in a good conductor. The existence of ions in solution was not accepted until late in the century, and has led to a much deeper understanding in the whole field of electrochemistry. The electrode processes that occur in a galvanic cell and in electrolysis usually control the rate with which the current flows, and the mechanisms of such processes have presented a great challenge to research workers. Brian Conway has made important contributions to the understanding of electrode processes, and to related electrochemical problems, over a period of more than 40 years. Keywords: electric current, electrochemistry, electrode processes.
The role of crystal imperfections in electrochemical phase formation and growth
Authors: Staikov G. , Lorenz W.J.
Published: May 2006

The role of different crystal imperfections and surface inhomogeneities in the processes of electrochemical phase formation and crystal growth is discussed on the basis of experimental results obtained by electrodeposition of metals on native and foreign single crystal substrates in selected model systems. The major role of screw dislocations in the electrochemical growth of real crystals is demonstrated in the case of silver electrodeposition on silver single crystal faces prepared by the so-called capillary technique. Experimental results show that the electrochemical spiral growth of silver crystal faces with low dislocation density can be used for a preparation of surfaces with a defined and uniform density of monatomic steps. Combined electrochemical and in situ scanning probe microscopy (SPM) studies of underpotential deposition (UPD) of metals on foreign single crystal substrates show that surface inhomogeneities induce a stepwise formation of different low-dimensional (0D, 1D, and 2D) metal phases. The thermodynamic, kinetic, and structural aspects of the stepwise formation of Me phases of different dimensionality in the UPD range are discussed on an atomic level using results of electrochemical and in situ SPM measurements in various systems. Keywords: electrocrystallization, screw dislocations, spiral growth, surface inhomogeneities, underpotential deposition, low-dimensional phases, scanning tunneling microscopy.
A practical approach to modeling the electrical double layer in the presence of specific adsorption of ions
Authors: Pettinger Bruno , Doblhofer Karl
Published: May 2006

Model calculations are presented that yield in a straightforward manner the quantitative dependence of the specific adsorption of ions at electrode surfaces on the applied electrode potential (electrode charge). Furthermore, the double-layer capacitance and the potential at the outer Helmholtz plane (ø2) are obtained. The derivation is based on Devanathan's three-capacitor model for the interfacial electric-potential distribution. A convenient correction function for the ø1 potential accounting for the discreteness-of-charge effect is derived, largely on the basis of recent work by Conway et al. The results are shown to be in very good agreement with published work by Lawrence and Parsons on the double layer between Br− electrolyte and the mercury electrode. Keywords: electrochemistry, specific adsorption, electric double layer, discreteness-of-charge effect.
Electrocatalytic hydrogenation of phenol on various electrode materials
Authors: Martel Anna , Mahdavi Behzad , Lessard Jean , Ménard Hugues , Brossard Louis
Published: May 2006

The electrocatalytic hydrogenation (ECH) of phenol was investigated at room temperature under galvanostatic control in aqueous sulfuric acid solutions on platinized platinum (Pt/Pt) electrodes and on composite Rh/Ni and Ru/Ni electrodes consisting of rhodium or ruthenium chemically deposited on nickel particles dispersed in a lanthanum polyphosphate matrix. The order of electrocatalytic activity at a current density (based on the geometric area) of 1 mA/cm2 was found to be Ru/Ni > Pt/Pt > Rh/Ni. The efficiency decreased with increased current density. For the Ru/Ni electrodes, the efficiency increased with the percentage of Ru (2.3 to 5%). On Ru 5%/Ni electrodes and at a current density of 5 mA/cm2, cyclohexanol was obtained with a high selectivity of 91 % and a current efficiency of 23% after 94% conversion. In neutral aqueous boric acid containing sodium chloride as supporting electrolyte, the Rh/Ni and Ru/Ni electrodes showed very low activity. The influence of the supporting electrolyte, periodic current control, and temperature was studied in the neutral medium with composite Raney nickel (RaNi) electrodes consisting of Raney nickel particles dispersed in a nickel matrix. At 60 °C and a current density of 10 mA/cm2, the selectivity of cyclohexanol formation was 100% with a 11% current efficiency after 92% conversion with sodium chloride as supporting electrolyte. Keywords: electrocatalytic hydrogenation, phenol, ruthenium on nickel cathodes, rhodium on nickel cathodes, Raney nickel cathodes.
Electrokinetic study of the adsorption of ethyl violet and crystal violet by montmorillonite clay particles
Authors: Schramm Laurier L. , Yariv Shmuel , Ghosh Dipak K. , Hepler Loren G.
Published: May 2006

Electrophoretic mobilities of mixtures of a montmorillonite clay (in different exchangeable metal cation forms) and increasing amounts of two cationic dyes (ethyl violet and crystal violet) were measured. Electrophoretic mobilities were found to vary between −60 × 10−5 and +40 × 10−5 cm2 s−1 V−1. For both the dyes, the degree of saturation at which the isoelectric point (IEP) occurs, decreases with increasing valency of the metal cations. An effort was made to connect the IEP, maximum flocculation, and dye adsorption parameters. An important adsorption parameter is the transition saturation (TS), the saturation beyond which adsorption by organophilic attractions occurs in addition to adsorption by electrical and (or) π-interactions. It was found that maximum flocculation occurs before the IEP for all the exchangeable cations tested, but the IEP is reached at similar saturations to the TS for most of the cations. These results have been interpreted in terms of different types of adsorption phenomena and particle associations. Keywords: clay, adsorption, dye, ion exchange, electrokinetic charge.
Excess molar volumes and viscosities of binary mixtures of propylene carbonate with (C5–C8) n-alkanols at 298.15 K
Authors: El-Banna Mohamed M.
Published: May 2006

Excess molar volumes (VE) and changes in viscosity (Δη) have been determined for binary liquid mixtures of propylene carbonate (PC) with (C5–C8) n-alkanols at 298.15 K and atmospheric pressure, over the whole composition range. The positive VE corresponding to negative Δη, for all the mixtures studied, suggests the presence of specific interactions, namely, involving the intermolecular hydrogen bonds. The results are correlated by means of a Redlich–Kister type equation. The fitting of viscosities to the parameter (N12) of Nissan–Grunberg or to the (H12) parameter of Hind et al. provides models for the dynamic viscosities; the two-parameter (v12 and v12) McAllister equation and the three parameters (a, b, and c) Heric–Brewer provide models for the kinematic viscosities, allowing us to gain greater knowledge of the interactions between unlike molecules. However, this analysis suggests that the McAllister model fits the data better than the others. Keywords: excess volumes, propylene carbonate, n-alkanols, liquid mixtures, hydrogen bonding.
NEURAL SYNCHRONIZATION VIA POTASSIUM SIGNALING
Authors: POSTNOV DMITRY E. , RYAZANOVA LUDMILA S. , MOSEKILDE ERIK , SOSNOVTSEVA OLGA V.
Published: May 2006

Using a relatively simple model we examine how variations of the extracellular potassium concentration can give rise to synchronization of two nearby pacemaker cells. With the volume of the extracellular space and the rate of potassium diffusion as control parameters, the dual nature of this resource-mediated coupling is found to be responsible for the coexistence of competing patterns of in- and anti-phase synchronization between identical cells. Cell heterogeneity produces significant modifications of the dynamical regimes in the control parameter plane. By comparison with conventional gap junctional coupling, potassium signaling gives rise to considerable changes of the cellular response to external stimuli.
NOTES ON FIVE-DIMENSIONAL KERR BLACK HOLES
Authors: SAKAGUCHI MAKOTO , YASUI YUKINORI
Published: May 2006

The geometry of five-dimensional Kerr black holes is discussed based on geodesics and Weyl curvatures. Kerr-Star space, Star-Kerr space and Kruskal space are naturally introduced by using special null geodesics. We show that the geodesics of AdS Kerr black hole are integrable, which generalizes the result of Frolov and Stojkovic. We also show that five-dimensional AdS Kerr black holes are isospectrum deformations of Ricci-flat Kerr black holes in the sense that the eigenvalues of the Weyl curvature are preserved.
A NOVEL FRAMEWORK FOR SIGNAL REPRESENTATION AND SOURCE SEPARATION: APPLICATIONS TO FILTERING AND SEGMENTATION OF BIOSIGNALS
Authors: CLIFFORD GARI D.
Published: May 2006

A general technique for representing quasi-periodic oscillations, typical of biomedical signals, is described. Using energy thresholding and Gaussian kernels, in conjunction with a nonlinear gradient descent optimization, it is shown that significant noise reduction, compression and turning point location is possible. As such, the signal representation model can be considered a form of correlated source separation. Applications to filtering, modelling and robust ECG QT-analysis are described.
Queens on a chessboard
Authors: Ginat David
Published: May 2006

Can you place N queens on an NxN chessboard such that none of them will threaten the others? It is easy find such a placement for an 8x8 board. It is also simple to develop a recursive, backtracking program to produce such a placement for any N (N>3). However, the recursive execution may be very costly. Can you offer an efficient, systematic way for generating such a placement for any N?
String enhancements to ANSI Standard Fortran (part 1)
Authors: Lambird Robert J.
Published: May 2006

A specification concerning string enhancements to ANSI Standard Fortran is presented for purposes of discussion and evaluation. American National Standards Institute Fortran Specification X3.9--1966 is used as the base line language. Linear graphs are used to formally define and otherwise demonstrate the proposed language extensions. Illustrative, short examples of the proposed string manipulation capabilities are presented. Reference is made to other extended Fortran languages, e.g., Fortran V.
A limited-character APL symbolism
Authors: McMurchie Tom
Published: May 2006

The desire to process APL inputs from limited-graphic peripherals has forced many people to consider the problem of representing the APL character set by some combination of a smaller set of characters. One of these attempts, by Dr. Charmonman, was presented in the July 10, 1970 edition of the APL Quote-Quad. This representation, though certainly a start, has a number of serious flaws which are described below. In an attempt to design a more acceptable notation, the following rules were proposed as a guideline.
On the structure of dictionaries for compilers
Authors: Busam Vincent A.
Published: May 2006

The format of a dictionary (symbol table) adequate for a PL/I compiler is presented. The dictionary contains two types of entries: name and attribute. Methods of storing name entries are discussed. Emphasis is placed on the contents and structure of the attribute entries necessary to handle structures in a block environment. Extensions of the method that allow one-pass compilation are discussed. The relevance of the dictionary structure to other higher-level language compilers is explained.
Efficient data accessing in the programming language Bliss
Authors: Wile David S. , Geschke C. M.
Published: May 2006

The specification of data structure in higher-level languages is isolated from the related specifications of data allocation and data type. Structure specification is claimed to be the definition of the accessing (addressing) function for items having the structure. Conventional techniques for data structure isolation in higher-level languages are examined and are found to suffer from a lack of clarity and efficiency.The means by which data structure accessors may be defined in Bliss, the specification of their association with named, allocated storage, and their automatic invocation by reference to the named storage only, are discussed. An example is presented which illustrates their efficient implementation and their utility for separating the activities of data structure programming and algorithmic programming.
Cylinders
Authors: Weston Paul E. , Taylor Stephen M.
Published: May 2006

A form of list structure is described which permits an efficient representation of relational data structures. The general notions of the PLEX and of ring structures, because of their proven value, have been used as a basis; but by systematically treating the array structure of the PLEX as an implicit form of linkage which is complementary to the explicit links carried by pointers, a new form of linked data structure emerges, which is called CYLINDER. While CYLINDERS are built up from two or more simple rings of pointers, they characteristically exhibit a multiplicity of closed search paths, which are usable in the construction of data representations. Examples of CYLINDER applications are discussed.
Indeterminate (0 ÷ 0) check in APL
Authors: Pettus C.
Published: May 2006

One unique feature
1.2
of the APL system is the definition
(0 ÷ 0) = 1
. Although there is obvious merit in this decision, in real analysis
(0 ÷ 0)
is indeterminate. If one wishes to be alerted to the fact that a denominator is zero and does not want the result 1 if the numerator is also zero, one can replace the expression [EQUATION] with [EQUATION] Then if
X
and
Y
are both zero, a Domain Error will be reported instead of yielding.
Algorithms
Authors: Fletcher Robert P.
Published: May 2006

This function integrates the function F over the N-dimensional cube with sides (-1,1) using the method of Sag and Szekeres (1). M represents the number of intervals each side is divided into for the integration. The function operates on M points at a time. Thus F must have an argument which is an NxM matrix, the columns of which represent points in N-space, and return a result which is an M-vector. A function returning the hyperbolic tangent is required: the one given will not give DOMAIN ERROR for the '*' primitive.
Hybrid compilation
Authors: Loka Raghavendra Rao
Published: May 2006

Compilers normally use the same technique to translate all routine level units regardless of the complexity, or the cost/benefit ratio of the effort. This would lead to unnecessary increase in compile time if there are too many simple routines. This note describes a hybrid scheme used in a hardware design compiler to overcome this problem. The compile times have been reduced by 10% across all the inputs-in the domain of hardware design compilation with this technique. The note also describes what the original goal was, and the compromises made to deal with reality.
A High-Performance Team
Authors: Beevers Philip
Published: May 2006

You work in the product development group of a software company, where the product is often compared with the competition on performance grounds. Performance is an important part of your business; but so is adding new functionality, fixing bugs, and working on new projects. So how do you lead your team to develop high-performance software, as well as doing everything else? And how do you keep that performance high throughout cycles of maintenance and enhancement?
People are more adaptable than HCI would have us believe
Authors: Weiss Aaron
Published: May 2006

There is no shortage of opinions on what makes a good user interface, whether the subject is Microsoft Windows, Macintosh, or Web sites. In fact, it is some people's profession to theorize on the placement of menu bars, the correct size for icons, the proper length of a line of text, what shape the mouse should take, and which hand you should use it with. These people are known as UI (User Interface) experts and their field is known as HCI, or Human-Computer Interaction. They know what you like.
Seven reasons to shave your head and three reasons not to
Authors: Patterson David A.
Published: May 2006

Many of these monthly missives have addressed the different ways ACM is working to change the image of computing professionals. This month, in the spirit of April Fool's Day, we root out the ACM President's effort to change that image one person at a time---starting at the top.
Calendar
Authors: O'Hanlon Charlene
Published: May 2006

O’Reilly Emerging Technology
But, Having Said That, ...
Authors: Kelly-Bootle Stan
Published: May 2006

A persistent rule of thumb in the programming trade is the 80/20 rule: "80 percent of the useful work is performed by 20 percent of the code." As with gas mileage, your performance statistics may vary, and given the mensurational vagaries of body parts such as thumbs, you may prefer a 90/10 partition of labor. With some of the bloated code-generating meta-frameworks floating around, cynics have suggested a 99/1 rule - if you can locate that frantic 1 percent. Whatever the ratio, the concept has proved useful in performance tuning.
Improving the energy behavior of block buffering using compiler optimizations
Authors: Kandemir M. , Ramanujam J. , Sezer U.
Published: May 2006

On-chip caches consume a significant fraction of the energy in current microprocessors. As a result, architectural/circuit-level techniques such as block buffering and sub-banking have been proposed and shown to be very effective in reducing the energy consumption of on-chip caches. While there has been some work on evaluating the energy and performance impact of different block buffering schemes, we are not aware of software solutions to take advantage of on-chip cache block buffers.This article presents a compiler-based approach that modifies code and variable layout to take better advantage of block buffering. The proposed technique is aimed at a class of embedded codes that make heavy use of scalar variables. Unlike previous work that uses only storage pattern optimization or only access pattern optimization, we propose an integrated approach that uses both code restructuring (which affects the access sequence) and storage pattern optimization (which determines the storage layout of variables). We use a graph-based formulation of the problem and present a solution for determining suitable variable placements and accompanying access pattern transformations. The proposed technique has been implemented using an experimental compiler and evaluated using a set of complete programs. The experimental results demonstrate that our approach leads to significant energy savings. Based on these results, we conclude that compiler support is complementary to architecture and circuit-based techniques to extract the best energy behavior from a cache subsystem that employs block buffering.