bluuebunny/crossref_metadata_2025 huggingface dataset:
Reaction of Cp2(CH3)Ta=CH2 with Ru3(CO)12: deoxygenative coupling of carbon monoxide and methylene units to give a heteronuclear cluster-bound 4-cumulene ligand
Authors: Proulx Grant , Hollander Frederick J. , Bergman Robert G.
Published: May 2006

The mechanisms of reactions that deoxygenate carbon monoxide (CO) and convert it into longer-chain hydrocarbons are not well understood. A reaction is reported between an early metal methylidene complex and a late transition metal carbonyl species that results in CO deoxygenation along with coupling of the CO carbon to methylidene groups and other CO carbons. The Schrock tantalum-methylene complex (η5-C5H5)Ta(=CH2)(CH3) reacts with the trinuclear metal carbonyl species Ru3(CO)12 to yield the cluster complex Cp2(CH3)Ta(µ-O)Ru3(C4H4)(CO)9. This material contains a 4-cumulene ligand that bridges the three late-metal centers. Also formed in this reaction is the unstable free tantalum oxo species, (η5-C5H5)Ta(=O)(CH3). A crystal structure of the TaRu3 cluster is reported along with a proposed mechanism for this unusual carbon–carbon bond-forming reaction. Keywords: deoxygenation, carbon monoxide, alkylidene, coupling, cluster.
Tin and lead derivatives of a Schiff base – bis(orthoquinone) ligand
Authors: McGarvey Bruce R. , Ozarowski Andrzej , Tian Zhigang , Tuck Dennis G.
Published: May 2006

The condensation of two molecules of 3,5-di-tert-butylcatechol (dbc) with ammonia under oxidizing conditions is believed to proceed via a keto-imine radical. Further condensation, and subsequent reaction with tin(II) or lead(II) halides, yields complexes of identical composition, M[dbqdi]2, where dbqdi is 3,5-di-tert-butyl-1,2-quinone-1-(2-hydroxy-3,5-di-tert-butylphenyl)imine anion; this name is used without prejudice as to the actual structure of the ligand. The two compounds differ in crystallographic and magnetic properties, and ESR measurements show that the tin compound has a triplet ground state. Both involve six-coordination of the metal in a distorted environment; tin is in the +IV state, but lead is +II, and there are corresponding differences in the form of the ligand. For SnIV(dbqdi)2, the crystal parameters are orthorhombic, space group C2221, a = 19.597(4), b = 24.375(4), c = 23.755(6) Å, Z = 8, and R = 0.073 for 1405 unique reflections. For PbII(dbqdi)2, monoclinic, space group P21/n, a = 14.56(1), b = 25.967(9), c = 15.13(1) Å, β = 99.81(7)°, Z = 4, R = 0.065 for 4911 unique reflections. The different complexation behaviour is shown to be a consequence of the redox properties of the two metallic elements. Keywords: tin, lead, ESR, coordination chemistry, electron transfer.
Excess properties for cyclohexanone + butanols at 298.15 K
Authors: Riggio Roque , Martinez Hector E. , Salas Norma Z. De , Toigo Miriam D. De , Ramos Juan F.
Published: May 2006

Densities, viscosities, and refractive indexes of the binary systems cyclohexanone + n-butanol, + sec-butanol, and + 2-methyl-1-propanol have been measured at 298.15 K and atmospheric pressure, over the whole composition range. The excess values of molar volume, viscosity, Gibbs free energy of activation of viscous flow, and internal pressure were calculated from experimental measurements. Based on the variations of the excess functions with composition, conclusions about the molecular interactions in these kinds of mixtures were obtained. Keywords: excess properties, binary mixtures, butanols, cyclohexanone.
Ionic conductivity in the water-rich region of aqueous 2-butoxyethanol
Authors: Koga Yoshikata , Loo Virginia J. , Puhacz Kataryna T.
Published: May 2006

Ionic conductivities of HCl, KOH, and KCl were measured in aqueous solutions of 2-butoxyethanol (BE) at 25 °C. The quantity, Λj′ = σ/xj, which is almost proportional to the molar conductivity, was extrapolated to the infinite dilution xj → 0. σ is the conductivity and xj is the mole fraction of j(= HCl, KOH, or KCl). The plots of 0Λj′, the value of Λj′ extrapolated to infinite dilution, against xBE showed a change in slope at xBE = 0.0175. The previous work from this laboratory indicated that the mixing scheme changes qualitatively at the same locus, xBE = 0.0175. By mixing scheme we simply mean the way in which BE and H2O molecules mix with each other. Assuming additivity in 0Λj′ in terms of constituent ions, those values for H+OH− were calculated. Plots of [Formula: see text] thus calculated as a function of xBE in the water-rich region, 0 < xBE < 0.0175, suggest that the hydrogen bond probability decreases in the bulk of solution, as xBE increases. Keywords: aqueous 2-butoxyethanol, ionic conductivities, mixing schemes, hydrogen bond network.
A photo-oxidation of croconic acid into oxalic acid
Authors: Fabre Paul-Louis , Castan Paule , Deguenon Diane , Paillous Nicole
Published: May 2006

Croconic acid, H2C5O5, is readily oxidized. This may be attested by decolorization of the solutions and by observation of oxalic acid complexes. The oxidation products are identified by 13C NMR spectroscopy as oxalic and mesoxalic acids, and experimental conditions are specified. The oxidation process requires dioxygen and photons. In parallel, the electrochemical oxidation of croconic acid is studied in aqueous media and in acetonitrile. A potential–pH diagram is drawn. Keywords: croconic acid, oxalic acid, oxidation, electrochemistry, photochemistry.
Poly N-(2-cyanoethyl)pyrrole as a selective film for the thickness-shear-mode acoustic wave sensor
Authors: Deng Zhiping , Stone David C. , Thompson Michael
Published: May 2006

Poly N-(2-cyanoethyl)pyrrole films have been synthesized by electrochemical polymerization and characterized by cyclic voltammetry, scanning electron microscopy, and X-ray photoelectron spectroscopy. Polymeric coatings prepared on the surface of a thickness-shear-mode acoustic wave sensor have been used to examine response selectivity to a number of gas-phase probe molecules. The responses of the poly N-(2-cyanoethyl)pyrrole based sensor are compared with the parent polypyrrole device and rationalized in terms of the molecular interactions between probes and polymer films. The polar cyano functionality enhances interactions with analytes such as acetonitrile. Keywords: gas sensor, thickness-shear-mode acoustic wave sensor, poly N-(2-cyanoethyl)pyrrole film, polypyrrole film, conducting polymer.
The thioformamidyl group as an α-substituent in carbocations
Authors: Bertone M. , Vucković D.L.J. , Cunje A. , Rodriquez C.F. , Lee-Ruff E. , Hopkinson A.C.
Published: May 2006

Abinitio: molecular orbital calculations at MP2(FULL)/6-311G(d,p) or MP2(FULL)/6-31G(d,p) are reported for carbocations RR′CCHO+, RR′CCHS+, RR′CCONH2+, and RR′CCSNH2+where R and R′ are H, CH3, C-C3H5, and C6H5. Primary (R=R′=H), secondary (R=H, R′=alkyl or phenyl), and tertiary (R′=R′=CH3) ions prefer the cyclic oxiranyl or thiiranyl structure 9, with the open structure 8 being a transition structure for ring opening. Tertiary carbocations with R=R′=phenyl or cyclopropyl and the 9-thioformamidyl-9-fluorenyl cation have the open structure. Isodesmic reactions show CONH2 to be weakly stabilizing in the methyl cation, and CSNH2 has a larger stabilizing effect, roughly equivalent to that of a methyl group. An α-thioamide substituent is less stabilizing in the ethyl cation and even less stabilizing in the isopropyl cation. In C6H5CHCSNH2+ the CSNH2 group is slightly destabilizing and, by extrapolation, is more destabilizing in Ar2CCSNH2+. The rearrangement of the α-thioformamidyl-benzyl cation to benzothiophene is calculated to have a low barrier, 15.4 kcal/mol at HF/6-31G(d,p). Keywords: molecular orbitals, destabilized carbocations, rearrangement.
On the relationship between the order parameter and the shape of orientation distributions
Authors: Lafrance Claude-Paul , Nabet Anne , Prud'homme Robert E. , Pézolet Michel
Published: May 2006

The molecular orientation is generally expressed by an "order parameter," [Formula: see text] which depends on both the angular position and the shape of the orientation distribution. This parameter is an average made over all orientations of the structural units studied in a sample and, consequently, a given [Formula: see text] value can correspond to different orientation distributions. In this article, model distributions are used to show the relationship between the shape, width, and angular position of the center of the orientation distribution on the [Formula: see text] coefficient, for the case where the distribution of the molecular chains exhibits cylindrical symmetry with respect to the reference direction. A significant difference is observed between the order parameters calculated for distributions of Gaussian and Lorentzian shapes with similar width at half-height. The variation of the [Formula: see text] coefficient as a function of the width at half-height, W1/2, and of the position of the center of the distribution, θC, is analyzed. Figures showing the range of W1/2–θC coordinates that can correspond to a given [Formula: see text] value are presented. As an example, the influence on the order parameter of the disorder between the different domains of phospholipid samples (mosaic spread) and of the conformational disorder in the acyl chains of these molecules is also studied. This example permits the evaluation of the magnitude of the errors that can be introduced in calculations of the tilt angle of the molecular chains in the case of distributions of finite widths or of bimodal character. Keywords: orientation, orientation function, phospholipid bilayers, conformational disorder, mosaic spread.
Aggregation of squaraine dyes: Structure–property relationships and solvent effects
Authors: McKerrow Andrew J. , Buncel Erwin , Kazmaier Peter M.
Published: May 2006

In characterizing the UV–visible absorption properties of a series of seven anilino class squaraine dyes in dimethyl sulfoxide (DMSO) – water mixtures, spectral features characteristic of aggregation were observed. These included hypsochromic and hypochromic shifts of the absorption maximum, relative to the nonassociated state of these dyes. Previously aggregation of this class of squaraine dyes had only been reported in studies of Langmuir–Blodgett (LB) films prepared with squaraine dyes. In the present study two distinctly different and novel solution dye aggregates, designated as type A and type B, were identified on the basis of their characteristic absorption properties. The type A aggregate was characterized by an absorption maximum that was hypsochromically and hypochromically shifted, in comparison with that of the nonassociated dyes, and had a secondary absorption occurring at approximately the same wavelength at which nonassociated squaraine dyes absorb. These spectral features are believed to be the result of a "face-to-face" alignment of molecules in the aggregate. The type B aggregate featured a broad absorption typically from 525 to 700 nm. Based on similarities between the spectral features of type B aggregates and LB films of surfactant squaraines, the alignment of molecules in the solution aggregate was believed to be such that the electron-deficient squarylium moieties interacted with the electron-rich dialkylamino phenyl moieties. Certain squaraine dyes that were investigated as part of this study were found to form both type A and type B aggregates, depending upon the composition of the DMSO–water medium. Typically, the type A aggregate formed preferentially in media of intermediate DMSO content (50–70% DMSO) and the type B aggregate in more water-rich media (<20% DMSO). In the intervening region it was possible to monitor spectroscopically a dynamic conversion from the type B aggregate to the type A aggregate. An examination of structure–property relationships indicated that dyes with less hydrophobic N-alkyl substituents formed only type A aggregates. Squaraine dyes with more hydrophobic N-alkyl substituents were found to form type B aggregates in water and type A aggregates in some DMSO–water mixtures. A model of squaraine aggregation was proposed in which the type A squaraine aggregate was "thermodynamically" preferred while the type B aggregate was "kinetically" preferred. The stability of the type B aggregate was proposed to be enhanced by increasing hydrophobicity of the N-alkyl substituents and decreased by increasing amounts of DMSO in the solvent system. Keywords: squaraine dyes, aggregation, crystallochromy, solvent effects.
Investigation of mesophase transitions in liquid crystals using inverse gas chromatography
Authors: Price Gareth J. , Shillcock Ian M.
Published: May 2006

Inverse Gas Chromatography, IGC, has been used to study phase transitions in three low molar mass liquid crystals and one side-chain liquid crystalline polymer. This extension of previous work has shown that transitions between all of the various mesophases can be detected and their temperatures measured with an accuracy and precision comparable to that of more conventional techniques. For one of the compounds, 4,4′-(n-hexyloxy)cyanobiphenyl, the kinetics of solidification from the supercooled nematic phase have been measured. The process took place in two stages which could be interpreted in terms of a modified version of the Avrami model of polymer crystallization. An explanation of the results is that the solid support is providing a large number of nucleation sites for the initial part of the solidification, which is completed in a slow, diffusion-limited process. However, further work is required to confirm this mechanism. Keywords: inverse gas chromatography (IGC), liquid crystals, mesophase transitions, activity coefficients, retention volumes.
Synthesis and characterization of a series of poly(α,ω-alkyldiynes) and copoly(α,ω-alkyldiynes)
Authors: Kennedy J.C. , MacCallum J.R. , MacKerron D.H.
Published: May 2006

A series of poly(α,ω-alkyldiynes) and copoly(α,ω-alkyldiynes) were synthesized by catalytic Glaser coupling reactions. Two routes were investigated and developed. These routes were chosen in anticipation that high molar mass, linear and polydisperse, polymers would be achieved. GPC (gel permeation chromatography) curves revealed that this was possible for the majority of polymers synthesized by the first route. However, a wide range of molar masses were obtained by the second route with a significant low molar mass tail present in almost all GPC curves. Very low Mn (number average molar mass) values by GPC gave correspondingly very high Mn values by end-group analysis (EGA). Hence, it was proposed that the low molar mass tail was a consequence of cyclic structures. From these results and observations a mechanism for the polymerization of the α,ω-alkyldiynes was proposed. Keywords: poly(α,ω-alkyldiynes), Glaser coupling, molar mass.
Mechanical behaviour of polymer mixtures in the phase separation region
Authors: Kuleznev V.N. , Kandyrin L.B.
Published: May 2006

The liquid–liquid phase transition triggered by changes in the composition of polymer mixtures in solution or melt is often accompanied by "critical" opalescence, which signals the appearance of a microemulsion in the mixture. The viscosity of the polymer mixture in this region is characterized by a sharp minimum, observed, as a rule, over an extremely narrow range of concentration. Depending on the concentration of the solution, the type of polymer, and the solvent, the viscosity may decrease by a factor of 8–10. On transition from micro- to macro-separation, viscosity rapidly increases back to the original level. Changes in the composition of the mixture can alter the concentration at which phase separation occurs, but the minimum in viscosity invariably corresponds to the moment of phase separation. The critical opalescence region represents the formation of phase particles up to 80–100 nm in size, and this corresponds to the point of viscosity drop. This effect is due to the appearance of thermodynamically stable microemulsions in the polymer mixture, in the region between the binodal and the spinodal in the phase diagram. These emulsions are characterized by lower molecular interaction of incompatible polymers in the highly developed interfacial layer. Extremal changes at the point of phase separation are also observed for other mechanical characteristics of polymer mixtures in solutions or melts, for example, G′ and G″ dynamic moduli or complex viscosity η*. Keywords: polyblends, critical phenomena, viscosity, emulsions, phase separation.
Mechanism of acid-catalyzed photoaddition of methanol to 3-alkyl2-cyclohexenones
Authors: Schuster David I. , Yang Jie-Min , Woning Jan , Rhodes Timothy A. , Jensen Anton W.
Published: May 2006

Contrary to a previous report, it is concluded that formation of methanol adducts to 3-methyl-2-cyclohexenones and of deconjugated enones on irradiation of the enones in acidified solutions proceeds via protonation of the intermediate enone π,π* triplet excited state and not by protonation of a relatively long-lived ground state trans-cyclohexenone. A rate constant for protonation of the triplet state of 3-methyl-2-cyclohexenone by sulfuric acid of 1.7 × 109 M−1 s−1 was determined by laser flash photolysis in ethyl acetate. Based on quantum efficiencies of product formation, a rate constant of ca. 108 M−1 s−1 was estimated for protonation of the enone triplet by acetic acid, which is too small to cause measurable reduction in the triplet state lifetime in the mM concentration range used in the preparative studies. The intermediate carbocation can be trapped by methanol, or revert to starting enone or the exocyclic deconjugated enone by loss of a proton. Since products revert to starting materials in an acid-catalyzed process, there is an acid concentration at which the yields of products are optimal. This concentration is ca. 6 mM for acetic acid, but is only 0.1 mM for p-toluenesulfonic or sulfuric acids. Product formation could be quenched using 1-methylnaphthalene and cyclopentene as triplet quenchers; in the latter case, formation of [2 + 2] photoadducts was observed to compete with formation of methanol adducts. Quenching rate constants were determined by laser flash studies. Keywords: laser flash photolysis, kinetic absorption spectroscopy (KAS), photoacoustic calorimetry (PAC), protonation of triplet states, trans-cyclohexenones.
Energy migration and transfer in styrene-containing polymers with inhibited excimer formation
Authors: Ghiggino Kenneth P. , Smith Trevor A. , Haines David J. , Wilson Gerard J.
Published: May 2006

Steady-state and time-resolved fluorescence spectroscopic techniques have been used to examine energy transfer and the migration of photoexcitation energy in styrene-containing polymer systems in which excimer formation is inhibited. In particular, copolymers of styrene and p-tert-butyl styrene with acenaphthylene (ACE) acceptor chromophores have been studied. Energy transfer efficiencies between styrene moieties and ACE chromophores incorporated as in-chain traps have been determined and are higher than predicted for a single-step Förster dipole–dipole energy transfer process. The results are discussed with reference to the possible mechanisms for excitation energy transport in these polymers. Keywords: polystyrene fluorescence, energy migration, energy transfer, excimer.
Cation binding and conformation of octafunctionalized calix[4]resorcinarenes
Authors: Fransen John R. , Dutton Philip J.
Published: May 2006

The preparations of four calix[4]resorcinarene esters and two calix[4]resorcinarene amides are reported. The logarithms of association constants have been determined by the picrate extraction method for a variety of metal cations, including alkali metal, alkaline earth, lanthanum, and silver cations. Strong binding toward silver ion, and selectivity amongst the alkaline earth cations, was observed for the octa-α-(diethyl acetamide) 3c. Flattened cone conformations have been established for both C-undecylcalix[4]resorcinarene octa-α-(methyl acetate), 3a, and octa-α-(diethyl acetamide), 3c, in chloroform solutions at 200 K. Dynamic nuclear magnetic resonance was used to identify rapid conformational interconversions of both 3a and 3c corresponding to pseudorotation between flattened cone conformations. The silver complex of 3c also exhibits pseudorotation; however, the activation barrier was observed to increase from 42 kJ mol−1 in the free ligand to 49 kJ mol−1 in the complex. The NMR data indicate that the metal cation binding occurs in a symmetrical fashion using two pairs of binding sites that are disposed across the macrocyclic ring from one another. Considering these data it is concluded that four carbonyl functions are involved in the binding of a metal cation in a 1:1 complex with the ligand. Key words: resorcinarene, picrate, association constant, dynamic NMR, pseudorotation.
A kinetic study on singlet molecular oxygen (O2(1Δg)) generation and quenching by dihydroxynaphthalenes
Authors: Luiz Marta , Soltermann Arnaldo T. , Biasutti Alicia , Garcia Norman A.
Published: May 2006

A kinetic study on the aerobic (O2(1Δg)-mediated) photooxidation of a series of dihydroxynaphthalenes was carried out. Members of this family of compounds include important environmental contaminants. The interaction with O2(1Δg), determined by both time-resolved phosphorescence and static methods, yields a quenching rate constant in the range of 105–109 M−1 s−1, depending on the solvent polarity, pH, and substitution pattern of the dihydroxynaphthalene. According to experimental evidence, the mechanism of the interaction seems to be mediated by an encounter complex with a considerable charge transfer component. From a mechanistic point of view this characteristic of DHN indicates a behaviour very similar to that of simple substituted phenols and dihydroxybenzenes. Quantum yields for DHN photooxidation (Φr) indicate that these substrates are excellent candidates for O2(1Δg)-mediated degradation under environment conditions. Furthermore, these compounds proved to be good self-sensitizers upon irradiation at 337 nm. Quantum yields (ΦΔ) for the production of O2(1Δg) were determined to be on the order of 0.25 in benzene. Key words: dihydroxynaphthalenes, photooxidation, quenching, sensitization.not available
In situ preparation of cyclopropanones with the bicyclo[3.1.0]hexan-6-one skeleton (substituted examples of the Loftfield intermediate). Confirmation of a very facile intramolecular alkyl cyclopropanone – dienol rearrangement
Authors: Sorensen T. S. , Sun F.
Published: May 2006

Four substituted bicyclo[3.1.0]hexan-6-ones (cyclopropanones) were prepared in situ, starting from the corresponding 2,6-dibromocyclohexanone and reductively removing the bromine atoms with the organometallic salt PPN+Cr(CO)4NO− The reaction is essentially instantaneous at −78 or−100 °C, and can be conveniently carried out in an NMR tube for easy characterization of the products by low-temperature 1H and 13C NMR spectroscopy. The1,5-di-tert-butyl and 1-tert-butyl analogs were thermally stable to ca. 0 °C, but the 1-tert-butyl-5-methyl and 1-tert-butyl-5-ethyl derivatives were extremely labile, rearranging at ca. −80 °C into a cross-conjugated enol, where the methyl (or ethyl) substituent was converted into an exomethylene group. These enols were also characterized as in situ species using 1H and 13C NMR spectroscopy, and by allowing the enol → α,β-unsaturated ketone rearrangement to take place at about 25 °C. The mechanism of the enol formation was investigated using a 1 -tert-butyl-5-CD3 analog, and the kH/kD ratio for enol formation was determined to be 6 ± 2. From this, the rate-determining step in the enol formation was postulated as a C-H → H-O transfer of a hydrogen atom in a cyclohexyl oxyallyl intermediate. The 1,5-di-tert-butylbicyclohexanone shows dynamic 1H NMR line broadening, the origin of which is also proposed to involve a cyclohexyl oxyallyl intermediate. Key words: cyclopropanone, oxyallyl, bicyclo[3.1.0]hexan-6-one, dienol, sigmatropic rearrangement.
AM1 semiempirical study of benzopyrroles as dienes for Diels-Alder reaction
Authors: Jursic Branko S.
Published: May 2006

A qualitative approach for quick evaluation of the reactivity of dienes such as cyclopentadiene, furan, pyrrole, indole, and isoindole for Diels–Alder reactions was applied. As qualitative methods, the frontier molecular orbital (FMO) energy gap and the Hammond postulate through bond orders and energy of the reaction were evaluated for such dienophiles as ethylene, 1,3-dioxa-4-cyclopentene, and maleic anhydride. It was demonstrated that pyrrole and benzopyrroles are poor dienes in comparison with cyclopentadiene and furan. Because the reaction is controlled by the HOMO of the diene there are special cases when isoindole can react as a diene, while the loss of indole aromaticity in the course of the reaction precludes the cycloaddition. Geometries of transition structures and the accompanying activation energies were calculated for pyrrole, indole, and isoindole as dienes. The stereoselectivity of the reaction is discussed and compared with experimental results. Key words: benzopyrroles as dienes, Diels–Alder cycloaddition, Hammond postulate.
Conformational analysis and 2D NMR assignment strategies for lignin model compounds. The structure of acetoguaiacyl-dehydro-diisoeugenol methyl ether
Authors: Garver Theodore M. , Maa Karen Joy , Marat Kirk
Published: May 2006

The resolved 1H and 13C chemical shifts for the lignin model trimer acetoguaiacyl-dehydro-diisoeugenol methyl ether have been completely assigned using one- and two-dimensional NMR techniques. Identification of spin families and assignments within the spin families were made using routine one-dimensional experiments for chemical shifts and multiplicities, and COSY and heteronuclear correlation experiments to determine connectivities. NOESY, HOESY, and long-range heteronuclear correlation experiments were then used to determine the juxtaposition of the assigned groups and to provide a confirmation of assignments. Steady state NOE difference and time-dependent NOESY and HOESY experiments were then used to provide experimental measures of the trimer conformation. Semiempirical molecular orbital methods were used to calculate minimum energy structures and energy barriers for bond rotations within the molecule. We have found that for the trimer-size molecule the NOE experiments provide a less detailed picture of the molecular conformation than the molecular orbital results. A comparison of NOE and MO results with crystal structures from the literature indicates the extent to which side-chain and methoxyl torsion angles may be estimated using each method. The correlation between NOE and MO predictions for interatomic distances apparently is related to the narrow minima for β-O-4 torsion angles for which the different results confirm each other. Broad minima or low-energy barriers for torsion angle rotations may yield minimized structures by computational methods that are not experimentally justified. Key words: semiempirical molecular orbital calculation, 2D NMR, conformation, lignin model, nuclear Overhauser effect.
Acyl cyanides as carbonyl heterodienophiles: application to the synthesis of naphthols, isoquinolones, and isocoumarins
Authors: Connors Richard , Tran Elisabeth , Durst Tony
Published: May 2006

Irradiation of 2-methylbenzoyl cyanide (3a) in acetonitrile solution results in the formation of its dimer, which upon loss of HCN gives rise to the cycloadduct 7a. The dimerization also proceeds efficiently with derivatives of 3a giving adducts 7b and 7c. When 2-methylaroyl cyanides are photolyzed in the presence of a more reactive acyl cyanide the mixed adducts 8a–e are obtained in excellent yields. The cycloadducts 7a–c and 8a–e react with carbon and nitrogen nucleophiles by a tandem addition–cyclization sequence furnishing substituted naphthols (10a and 10b) and isoquinolones (11a–d), respectively. Isocoumarins 12a and 12b were prepared from the adducts 8a and 8e by treatment with potassium tert-butoxide in THF. Key words: naphthols, isoquinolones, isocoumarins, synthesis of; acyl cyanides; hetero Diels–Alder.
Kraft pulp bleaching using dimethyldioxirane: stability of the oxidants
Authors: Bouchard J. , Maine C. , Berry R.M. , Argyropoulos D.S.
Published: May 2006

Dimethyldioxirane (DMD) is a cyclic peroxide made by oxidizing acetone with peroxymonosulfate (PMS) in water buffered at pH 7.5 using sodium bicarbonate. It has been shown that DMD generation can be achieved in situ within a pulp suspension allowing very selective TCF bleaching of kraft pulp. This process involves simultaneous generation of DMD, reaction of PMS and DMD with residual lignin, and spontaneous decomposition of both oxidants. The first part of this work is a kinetics study of the decomposition of PMS and DMD as a function of pH under conditions similar to those for in situ bleaching. The effect of chelation as well as the effect of transition metal ions on decomposition rate was also investigated. DMD is very sensitive to pH and its half-life is very short under alkaline conditions. The presence of any transition metal ion that can be involved in a one-electron transfer is detrimental to DMD stability. However, fast reaction of DMD with pulp almost counteracts the effect of metal ions. Key words: bleaching, peroxymonosulphate, dimethyldioxirane, metal ions, kinetics, decomposition, kraft pulp.
The use of bisphthalonitriles in the synthesis of side-strapped 1,11,15,25-tetrasubstituted phthalocyanines
Authors: Leznoff Clifford C. , Drew David M.
Published: May 2006

Nucleophilic aromatic substitution reactions of 3-nitrophthalonitrile yield 3-hydroxyphthalonitrile and 3-neopentoxyphthalonitrile, the latter of which condensed to 1,8,15,22-tetraneopentoxyphthalocyanine as a mixture of isomers. Bisphthalonitriles such as 1,3-bis(2′,3′-dicyanophenoxy)-2,2-dipentylpropane, 1,3-bis(2′,3′-dicyanophenoxy)-2,2-diethylpropane, 1,3-bis(2′,3′-dicyanophenoxy)-2,2-dioctylpropane, and 1,3-bis(2′,3′-dicyanophenoxy)-2-methyl-2-trityloxymethylpropane all gave bis-crown-like 1,11,15,25-tetrasubstituted phthalocyanines as pure compounds when treated with lithium octoxide in 1-octanol at 196 °C. A host of nine other bisphthalonitriles including 1,5-bis(2′,3′-dicyanophenoxy)-3-oxapentane, 1,1-bis(2′,3′-dicyanophenoxymethyl)cyclohexane, 1,2-bis(2′,3′-dicyanophenoxymethyl)benzene, and 2,5-bis(2′,3′-dicyanophenoxymethyl)furan did not dimerize to mononuclear phthalocynaines. The "gem dimethyl" effect was suggested as a reason for the successful macrocyclizations. Key words: nucleophilic aromatic substitution, phthalonitriles, bisphthalonitriles, 1,11,15,25-tetrasubstituted phthalocyanines.
Cd(II) complexes with phthalic acids: solution study and crystal structure of cadmium(II) phthalate hydrate
Authors: Vaz José Luis Lucas , Duc Gérard , Petit-Ramel Michelle , Faure René , Vittori Olivier
Published: May 2006

The acid–base properties of phthalic acid (H2L) have been determined by simultaneous spectroscopic, potentiometric, and polarographic measurements. Stability constants of the complexes CdHL+, CdL, CdL22−, CdL34−, CdH−2L24− and their distribution versus pH were determined at 25 °C and ionic strength in 0.1 M NaNO3. The advantages of each method are pointed out in such a way as to describe more precisely the interactions between cadmium and phthalic acid over a wide range of ligand and metal concentrations. The crystal structure of the [CdL] complex has been established by single-crystal X-ray diffraction. Cd(C8H4O4)•H2O is orthorhombic, space group Pbca, a = 23.463(5), b = 9.293(2), c = 7.730(1) Å, Z = 8, and ρ = 2.323 g cm−3. The structure was refined to a final R value of 0.054 for 3231 unique reflections with I > 3σ(I). The crystal is a three-dimensional polymeric network in which the Cd atom is seven-coordinated. The first ligand coordinates in a bidentate carboxylate (O,O′) mode (Cd—O = 2.372(5) and 2.445(5) Å); a second ligand is part of a seven-membered ring formed with its two carboxylate groups (Cd—O = 2.317(5) and 2.341(5) Å); two other phthalic ligands make one coordination bond (Cd—O = 2.255(5) and 2.449(5) Å) while the seventh bond is a water molecule (Cd—O = 2.333(6) Å). Key words: crystal structure, cadmium complexes, phthalic acid complexes.
Formal transfers of hydride from carbon–hydrogen bonds. Attempted generation of H2 by intramolecular protonolyses of the activated carbon–hydrogen bonds of dihydrobenzimidazoles
Authors: Brunet Philippe , Wuest James D.
Published: May 2006

Protonolyses of carbon–hydrogen bonds can occur under suitable conditions to produce carbocations and H2. In an effort to accelerate these fundamental reactions, we have attempted to make them intramolecular by devising compounds in which carbon–hydrogen bonds designed to be particularly good formal donors of hydride are held in close proximity to acidic sites. Dihydrobenzimidazoles 4 and 11 are compounds of this type, since the carbon–hydrogen bonds at C2 are activated as formal donors of hydride by adjacent lone pairs in a dihydroaromatic ring, and acidic anilinium and carboxylic acid groups are held nearby. Unfortunately, this proximity does not lead to the formation of H2 by protonolysis; instead, other reactions intervene when compounds 4 and 11 are subjected to pyrolysis. Key words: intramolecular protonolysis of carbon–hydrogen bonds, formal donors of hydride, generation of H2, dihydrobenzimidazoles.
Ground- and excited-state properties of some naphthoflavyliums
Authors: Elhabiri Mourad , Figueiredo Paulo , George Florian , Fougerousse André , Brouillard Raymond , Merlin Jean-Claude , Cornard Jean-Paul
Published: May 2006

A series of five, structurally related, substituted 2-phenyl-benzopyrylium (flavylium) salts were synthesized and characterized by NMR, absorption, and fluorescence techniques. Their hydration and deprotonation constants were obtained through thermodynamic and relaxation kinetic methods. Metallic complexation with the two compounds possessing a catechol group and its effect on the fluorescence intensity was also studied. The ground state properties of the five pigments are correlated with the theoretical data collected through AM1 molecular orbital calculations. Key words: naphthoflavylium synthesis, hydration and deprotonation constants, fluorescence, metallic complexation, molecular orbital calculations.
Detection of Newcastle disease virus using an evanescent wave immuno-based biosensor
Authors: Lee William E. , Thompson H. Gail
Published: May 2006

A fibre-optic-based evanescent wave biosensor has been used for the detection of virus. Polyclonal antibody directed against the model analyte, Newcastle disease virus, was covalently immobilized on aminosilane-coated quartz fibres, which served as the capture and concentration element of the sensor system. Radiolabelling studies demonstrated a high degree of antibody attachment to the quartz solid support. Maximal coverage could be attained with antibody solution as low as 10 μg/mL and carbonyldiimidazole as the cross-linking agent. Fluorescein-labelled anti-NDV served as the detector antibody in a sandwich format. Assay times were approximately 15 min with a limit of detection of about 2 ng of purified virus in 0.2 mL sample volume. The biosensor detection system possesses several desirable characteristics such as a limited number of mechanical components, a multiple use active surface, and low baseline variation, which indicate that it has the potential to serve as an on-line continuous monitoring device of an automated or semi-automated detection system. Key words: fibre optic, biosensor, fluorescence, evanescence, immunoassay, antibody, Newcastle disease virus.
Excess partial molar enthalpies of alkane-mono-ols in aqueous solutions
Authors: Tanaka Steven Hiroshi , Yoshihara Hikari Infinity , Ho Alice Wen-Chi , Lau Frankie W. , Westh Peter , Koga Yoshikata
Published: May 2006

Excess partial molar enthalpies, HAE, of methanol, ethanol, and 1- and 2-propanols in aqueous solutions were measured directly, accurately, and in small increments in mole fraction at 25 °C. From these data, the solute–solute enthalpic interactions, HAAE≡N(∂HAE/∂nA), were evaluated for each alcohol. These data indicate that three distinctively different mixing schemes, I, II, and III exist, as was the case for aqueous 2-butoxyethanol previously studied in our laboratory. The transition from mixing scheme I to II appears to take place gradually within a small composition range. As the hydrophobic moiety becomes smaller from 2-butoxyethanol to methanol, the locus of the transition moves to a higher value in mole fraction of the alcohol. At the same time, the range of transition becomes wider and the solute–solute enthalpic interaction weaker. Key words: excess partial molar enthalpies in aqueous solutions, methanol, ethanol, 1-propanol, 2-propanol, enthalpic interaction, transition of mixing scheme.
Rhenium complexes ofP,P,P′,P′-tetrakis(o-hydroxyphenyl)diphosphinoethane
Authors: Luo Hongyan , Orvig Chris
Published: May 2006

Preparation of the hydrochloride salt of a new potentially hexadentate ligand precursor P,P,P′,P′-tetrakis(o-hydroxyphenyl)diphosphinoethane dihydrochloride (abbreviated H4P2O4•2HCl) is described. From H4P2O4•2HCl, neutral [Re2O2Cl2(PPh3)2(μ-P2O4)] and dianionic [Re2O2Br4(μ-P2O4)]2−dinuclear rhenium(V) complexes were synthesized. The complexes have been characterized by elemental analysis, infrared spectroscopy, mass spectrometry, and1H/31P{1H} NMR spectra.31P{1H} NMR revealed that only one isomer, presumably the anti, was present for [Re2O2Cl2(PPh3)2(μ-P2O4)], and that two isomers, both anti and syn isomers, were observed for [Re2O2Br4(μ-P2O4)]2−. The coligands (PPh3for the former, Br−for the latter) of both complexes underwent ligand exchange with pyridine. Key words: rhenium, P,P,P′,P′-tetrakis(o-hydroxyphenyl)diphosphinoethane, dimer, isomers.
Analysis of magnesium–carbon bonding in magnesium anthracene systems
Authors: Stegmann Ralf , Frenking Gernot
Published: May 2006

Ab initio calculations at the MP2/3-21G(*) level of theory have been carried out for the magnesium–anthracene complexes 9,10-magnesiumanthracene•3H2O (1) and the 9-methyl (2), dimethyl (3), and 9,10-bis(methylsilyl) (4) substituted derivatives. The theoretically predicted geometries of the anthracene ligands are also reported. The calculated geometries of 1–4 are in very good agreement with experimental values for the corresponding THF complexes. The Mg—C9,10 bonds of the bridged structures are rather long and the anthracene ligands are folded by ~40° along the C9–C10 line in the complexes. Analysis of the electronic structure shows clearly that the Mg—C9,10 bonds should be considered as purely ionic. This is revealed by topological analysis of the electron density distribution and its associated Laplacian. The electron density at the Mg—C9,10 bond critical points ρ(rb) is very low and the Laplacian [Formula: see text] and the energy density Hb have positive values. The ionic nature of the Mg—C9,10 bond is also indicated by the natural bond order (NBO) analysis, which gives a Lewis structure with two lone pairs at C9 and C10 but no Mg—C9,10 bonds. The NBO method gives a charge donation from Mg to the anthracene ligand of nearly two. The theoretically predicted NMR chemical shifts using the GIAO method give 13C resonances for the complex 1 and for anthracene and anthracene dianion that are in good agreement with experimental values. Key words: magnesium–anthracene complexes, ab initio calculations, analysis of magnesium–carbon bonding.
Topological analysis of the electron localization function applied to delocalized bonds
Authors: Savin A. , Silvi B. , Colonna F.
Published: May 2006

What is a local viewpoint of delocalized bonds? We try to provide an answer to this paradoxical question by investigating representative conjugated organic molecules (namely, allyl cation, trans-butadiene, and benzene) together with reference nonconjugated systems (ethylene and propene) by means of topological analysis of the electron localization function ELF. The valence attractors of the ELF gradient field are classified according to their synaptic order (i.e., connections with core attractors). The basin populations [Formula: see text] (i.e., the integrated density over the attractor basins) and their standard deviation, σ, have been calculated and are discussed. The basin populations and their relative fluctuations, defined as [Formula: see text] are sensitive criteria of delocalization. In the case of well-localized C—C or C=C bonds, λ ~0.4, whereas for delocalized bonds λ increases to about 0.5. Another criterion of delocalization is provided by the basin hierarchy, which is defined from the reduction of the localization domains. For most systems, delocalization occurs not only for neighboring carbon-carbon disynaptic attractor basins, but also for nearest neighbor disynaptic protonated attractor basins. Key words: electron localization function, topological analysis, delocalization, population analysis.
A theoretical study of some hydrogen-bonded complexes using the theory of atoms in molecules
Authors: Tang Ting-Hua , Cui Yun-Ping
Published: May 2006

π-Type hydrogen-bonded complexes consisting of hydrogen halide HX (X = Cl, F) and the carbon–carbon triple or double bond of vinyl acetylene (1-buten-3-yne, HC≡C-CH=CH2) have been studied. The vinyl acetylene molecule contains two possible π-bonding sites (C≡C and C=C). It offers three possible structures of [Formula: see text] that comprise two T-type bonds to C≡C (endo and exo approaches) and one T-type bond to C=C (perpendicular approach). The optimized geometries and the hydrogen-bond stabilization energies, based on MP2(FULL)/6-311 ++ G(d,p)//6-31G(d,p) calculations, indicate that the π-type hydrogen bond to a C≡C triple bond leads to a more stable complex than for an analogous bond to C=C. The calculated global minima for the complexes with HF and HCl correspond to the H—X moiety lying along a bisector of the C≡C triple bond in the endo approach, predictions that are in good agreement with the reported FTMS results. The topological properties of the electron density distributions of these two systems have been analyzed in terms of the theory of atoms in molecules. The nature of π-type hydrogen bonds has also been discussed using the Laplacian of the electron density, [Formula: see text] The complexes [Formula: see text] and [Formula: see text] as well as the hydrogen-bonded complex consisting of 2-butyne (CH3-C≡C-CH3) and HCl were also studied for comparison. Key words: ab initio calculation, hydrogen bonding, topological analysis of electron density, vinyl acetylene, 2-butyne.
Topological analysis of the experimental electron density
Authors: Tsirelson Vladimir G.
Published: May 2006

Methods of topological analysis of the experimental electron density reconstructed from X-ray diffraction data are described. Their advantages and drawbacks are discussed and the results for organic and inorganic crystalline solids are presented. Key words: topological analysis, experimental electron density.
Molecules in high-intensity laser fields
Authors: Nguyen-Dang T.-T. , Châteauneuf F. , Manoli S.
Published: May 2006

The separability of a dressed molecule, a composite molecule + quantized radiation field system, at high field intensities is examined. Various forms of the Hamiltonian describing the dressed molecule are reviewed and are used to assess the zeroth-order separability of the dressed system. A new high-order adiabatic separation between the strongly coupled quantized field and molecular subsystems is derived. Qualitative manifestations of laser-induced molecular structures are discussed within this high-order adiabatic representation. Key words: dynamics, dressed molecule, intense laser field, adiabatic separation, laser-induced molecular structure.
On the performance of molecular model core potential orbitals in spin-orbit and electron correlation studies
Authors: Krause Dietmar , Klobukowski Mariusz
Published: May 2006

The role of improved parametrization and accurate basis sets in model core potentials was studied in calculations of the spin-orbit coupling constants (in PH, AsH, and SbH) and of the electron correlation effects (in P2, As2, and Sb2). An effective method of identifying and removing the intruder quasi-core orbitals from the virtual orbital space was proposed in connection with the post-Hartree–Fock calculations. The results demonstrated that (a) the flexible valence basis sets allow evaluation of the spin-orbit effects without resorting to any scaling techniques and (b) the intruder quasi-core orbitals, even if left imbedded in the virtual space, have negligible effect on the values of the electron correlation energy. Key words: effective core potentials, basis sets, spin-orbit effects, electron correlation.
Carbon-13 chemical shifts of alkene carbons in 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles and related benzothiazoles and -selenazoles, and their relationship to other push-pull alkenes
Authors: Mueller Jennifer L. , Gibson Martin S. , Hartman J. Stephen
Published: May 2006

Carbon-13 chemical shifts of alkene carbons are observed in the ranges 78–109 ppm (Cα) and 154–164 ppm (Cβ) for a series of 11 2-acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles, 3 2-acylidene-3-alkyl-2,3-dihydrobenzothiazoles, and 2 2-acylidene-3-alkyl-2,3-dihydrobenzoselenazoles of known geometry, indicating appreciable charge polarization in these compounds as in other push–pull olefins. Substitution that promotes more extensive charge delocalization results in the Cα signal shifting to the higher-frequency end of the chemical shift range. The observed shifts are compared with those calculated according to the Pretsch scheme. Key words: carbon-13 NMR, chemical shifts, push–pull olefins, 1,3,4-thiadiazoles, benzothiazoles, benzoselenazoles.
New investigation of the reaction of 2-amino-2-oxazolines with isocyanates and isothiocyanates. Evidence of a transposition during the second step
Authors: Bosc Jean-Jacques , Jarry Christian , Ouhabi Jamal , Laguerre Michel , Carpy Alain
Published: May 2006

The reaction of some 2-amino-2-oxazolines with isocyanates and arylisothiocyanates yields 3-monosubstituted or 2,3-disubstituted 2-iminooxazolidines depending on the experimental conditions and on the nature of the electrophile reactants. The structures of a mono- and a disubstituted derivative were established by X-ray analysis. The chemical behaviour of the compounds was investigated by molecular and quantum mechanics calculations. An intramolecular transposition during the second step of the reaction was found. Key words: 2-amino-2-oxazolines, iso(thio)cyanates, transposition. X-ray structure, MOPAC calculations.