bluuebunny/crossref_metadata_2025 huggingface dataset:
Machinability Analysis for 3-Axis Flat End Milling
Authors: Li Ye , Frank Matthew C.
Published: May 2006

This paper presents a method for geometric machinability analysis. The implementation of the strategy determines the machinability of a part being processed using a plurality of 3-axis machining operations about a single axis of rotation for setup orientations. Slice file geometry from a stereolithography model is used to map machinable ranges to each of the line segments comprising the polygonal chains of each slice. The slices are taken orthogonal to the axis of rotation, hence, both two- and three-dimensional (2D and 3D) machinability analysis is calculated for perpendicular and oblique tool orientations, respectively. This machinability approach expands upon earlier work on 2D visibility analysis for the rapid manufacturing and prototyping of components using CNC machining.
Charge localization in polymeric metal-oxide-semiconductor capacitors
Authors: Marinov O. , Deen M. J. , Iniguez B. , Ong B.
Published: May 2006

Charge enhancement of organic materials that was deduced more than two decades ago by means of capacitance-voltage (C-V) measurements meets with difficulties and controversial explanations when using the classical semiconductor theory. Experimentally, it has been found that the charge localization results in nonstationary and frequency-dependent behavior in the C-V measurements. Detailed analyses of the experimental results provide insight on how to interpret the quantities for concentration and relaxation time of charges in organic semiconductors. The experimental results indicate that the charge localization and conduction occur mainly in the bulk of the poly(3,3‴-didodecyl-quaterthiophene) (PQT) semiconducting polymer.
Terahertz quantum cascade lasers: Fabrication, characterization, and doping effect
Authors: Ban D. , Wächter M. , Liu H. C. , Wasilewski Z. R. , Buchanan M. , Aers G. C.
Published: May 2006

The terahertz gap, lying roughly between 300GHz (0.3THz) and 30THz in the electromagnetic spectrum, exists because the frequencies generated by semiconductor devices based on transistors and lasers do not overlap. Generation of coherent terahertz radiation has traditionally involved either extending electronic techniques to higher frequencies or extending photonic sources to longer wavelengths. In both cases, the efficiency drops rapidly as the frequency approaches the terahertz region. We recently fabricated GaAs∕AlGaAs quantum cascade lasers, in which a high-confinement metal-metal waveguide was employed and fabricated using In–Au metallic bonding technique. The devices demonstrated lasing operation at a wavelength of around 104.6μm (or about 2.9THz in frequency). In this article, we first present the fabrication and electrical and optical characterizations of the terahertz quantum cascade lasers. We then characterized a set of terahertz quantum cascade lasers with otherwise identical device parameters but the doping concentration. The δ-doping density for each period was varied from 3.2×1010 to 4.8×1010cm−2. We observed that both the lasing threshold and the free carrier absorption caused the waveguide loss increase monotonically. Interestingly, however, the observed maximum lasing temperature displayed an optimum at a doping concentration of 3.6×1010cm−2.
Compositional effect on the dielectric properties of high-k titanium silicate thin films deposited by means of a cosputtering process
Authors: Brassard D. , Sarkar D. K. , El Khakani M. A. , Ouellet L.
Published: May 2006

We report on the successful growth of high dielectric constant (high-k) titanium silicate TixSi1−xO2 thin films of various compositions (0⩽x⩽1) at room temperature from the cosputtering of SiO2 and TiO2 targets. The developed process is shown to offer the latitude required to achieve not only a precise control of the film composition but an excellent morphology (i.e., dense films with low roughness) as well. The Fourier transform infrared and x-ray photoelectron spectroscopy characterizations have evidenced the presence of Ti–O–Si type of atomic environments, which is the fingerprint of the titanium silicate phase. The titanium silicate films are found to exhibit excellent dielectric properties with very low dielectric losses [tan(δ)<0.02] regardless of their composition. The dielectric constant of the films is found to increase with their TiO2 content from 4 (for pure SiO2 films) to 45 (for TiO2). On the other hand, increasing the TiO2 content of the films is also shown to degrade significantly their leakage current. Nevertheless, titanium silicate films with almost equiatomic composition (x∼0.45) are found to exhibit an excellent trade-off between a high-k value (∼18) and low leakage current (∼5×10−7A∕cm2 at 1MV∕cm). Finally, the compositional dependence of the dielectric properties of the TixSi1−xO2 films is discussed in terms of bonding states and optical band gap.
Probing the composition of Ge dots and Si∕Si1−xGex island superlattices
Authors: Baribeau J.-M. , Wu X. , Lockwood D. J.
Published: May 2006

We use analytical transmission electron microscopy to map the composition of Ge dot and Si∕Si1−xGex island structures grown on (001) Si by molecular beam epitaxy or ultrahigh vacuum chemical vapor deposition. Energy-dispersive x-ray spectroscopy reveals that nominally pure Ge dots grown by molecular beam epitaxy at 650°C exhibit considerable intermixing with the average Ge composition typically increasing from nearly zero at the base to about 50% at the top of the dot. In pyramid shaped dots, the Ge composition increases linearly up to the top of the dot, while for dome dots, a saturation of the incorporation rate is seen beyond a distance of 7nm from the substrate interface. Probing of Si∕Si1−xGex island superlattices also reveals large Si∕Ge intermixing with a Ge accumulation at the crest and Ge depletion at the troughs of the islands. These results are corroborated by x-ray diffraction and Raman scattering measurements.
Computational analysis of solid-vapor equilibria for ZnS and SrS phosphor synthesis conditions
Authors: Wiedemeier Heribert
Published: May 2006

The synthesis and postgrowth treatment of luminescent materials frequently involve solid-vapor interactions. Because the underlying multispecies, multireaction equilibria are rather complex, quantitative correlations between synthesis conditions, vacancy concentrations, and emissive properties for predictive purposes are not readily available. In order to support the development of predictive guidelines, a quantitative, computational analysis of the vapor phase over ZnS and SrS in the absence and presence of nonconstituent species has been performed. The results confirm the complexity of these systems and show quantitatively the effects of impurity gases on the critical metal to nonmetal ratio in the ambient atmosphere. The data also reveal the temperature ranges in which desirable M∕X ratios are available for given experimental conditions. The combined results of such computations are useful as reference data for the optimization and definition of reproducible synthesis conditions, and for the estimation of the type and magnitude of vacancies.
Reactions of Fe3(CO)12 and Fe(CO)5 with functionalized alkynes in CH3OHKOH solution  The crystal structure of Fe2(CO)6(PPh3)[µ-η3-(H2CCCH2)]
Authors: Gervasio Giuliana , Marabello Domenica , Sappa Enrico , Secco Andrea
Published: May 2006

The complex Fe2(CO)7(H2CCCH2) (1) is obtained starting from a variety of functionalized alkynes that can release C3 fragments during the reactions with Fe3(CO)12 or Fe(CO)5 in CH3OHKOH solutions. Owing to the oily nature of 1, it was reacted with triphenylphosphine, and the structure of the resulting title compound (1a) has been determined by X-ray analysis. Complex 1a shows a new coordination mode of a H2C=C=CH2 ligand to a diiron centre and is, to our knowledge, unprecedented. We have also found that 1 (and 1a) are by-products, and not intermediates in the formation of complex 3. A discussion of the possible reaction pathways is given.Key words: iron carbonyls, alkynols, CO and methanol activation, acetyl and acetate complexes, allenyl iron complexes, crystal structure.
Differential reactivity of carbohydrate hydroxyls in glycosylations. III. Structure, stability, and reactivity of 2′-deoxycytidine model compound-BF3 complexes
Authors: Tang Ting-Hua , Whitfield Dennis M. , Douglas Stephen P. , Krepinsky Jiri J. , Csizmadia Imre G.
Published: May 2006

The possibility of complexation between an acidic promoter like BF3 and the alcoholic aglycone has been studied as a key mechanistic intermediate in glycosylation reactions operating via an electron-deficient anomeric carbon. The BF3 complexes at O3′,O4′,O5′, and N3 of the model compound of 3′-,4-N-diacetyl-2′-deoxyribocytosine (1) have been studied. The last regioisomer of the four BF3 complexes was found to have the weakest hydrogen bond and yet was the most stable isomer energetically. Furthermore, the O5′ nucleophilicity of this isomer was the most pronounced, as evaluated by Bader-type charge density topological analysis at the HF/6-31G**//HF/3-21G level of theory. This indicates that such a complex will make glycosylation at the O5′ oxygen possible.
A highly stereoselective preparation of syn-1,2-diols from secondary α-allenic alcohols via iodocyclization. Synthesis of (±)-threo-L-factor and (±)-disparlure
Authors: Friesen Richard W. , Giroux André
Published: May 2006

The iodocyclization of the N-tosyl carbamates 5 of secondary α-allenic alcohols 4 is described. The reaction sequence involves the addition of iodine to the terminal olefin of the allene moiety of 5 resulting in the formation of an isomeric mixture of Z and E diiodides 6 and 7. This initial process is followed, in the same pot, by reaction of the diiodides with silver carbonate. Hydrolysis of the resulting trans imino carbonates 8 provides syn-1,2-diols 10 in a highly diastereoselective (>50:1) and regioselective (>20:1) fashion. Oxazolidinones 9 are minor products that arise from the participation of the carbamate nitrogen atom in the cyclization reaction. The regioselectivity of the cyclization reaction from 6/7 to 8 is highly dependent upon the reaction solvent with the optimum results being obtained in ether/MeCN mixtures (10:1 to 50:1) or CH2Cl2. The synthetic utility of the vinyl iodo diols 10 is demonstrated by the synthesis of (±)-threo-5-hydroxy-4-decanolide (17) ((±)-threo-L-factor) and (±)-cis-7, 8-epoxy-2-methyloctadecane (24) ((±)-disparlure). The key step in each synthesis involves the conversion of the vinyl iodide moiety of 10 into the alkynes 15 or 18 (from 10j and 10k, respectively) via a high-yielding dehydrohalogenation reaction.
On the positional reactivity order in the sulfur trioxide sulfonation of benzene, halogenobenzenes, halogenonaphthalenes, and chloroanthracenes
Authors: Cerfontain Hans , Zou Yousi , Bakker Bert H. , Griendt Freek van de
Published: May 2006

The reaction of sulfur trioxide with benzene, the four halogenobenzenes, and six halogenonaphthalenes in dichloromethane as solvent, and with four chlorinated anthracenes in nitromethane as solvent, has been studied by analysis of the resulting mixture of the sulfo derivatives with 1H NMR. The sulfonation of benzene yields initially benzenesulfonic acid and subsequently the 1,3-disulfonic acid (1,3-S2). The initial sulfonation of the four halogenobenzenes yields ≥ 98% of the 4-S. Their subsequent sulfonation gives the 2,4-S2 and in the case of fluoro- and iodobenzene also the 2,4,6-S3. Monosulfonation of 1-fluoronaphthalene yields only the 4-S, whereas the three other 1-halogenonaphthalenese yield in addition some 5-S. Further sulfonation on any of the four 1-halogenonaphthalene-4-sulfonic acids yields a mixture of the 2,4-S2 and 4,7-S2, and eventually also some 2,4,7-S3, whereas the 1-halogeno-5-sulfonic acids give the corresponding 5,7-S2. Sulfonation of 2-chloro- and 2-bromonaphthalene yields initially 85% 8-S and 15% 4-S, which are subsequently converted into the 6,8-S2 and 4,7-S2, respectively. On reaction with 1.0 mol-equiv. of SO3, 2-chloroanthracene gives the 9-S, and 9-chloroanthracene gives an 18:82 mixture of the 4-S and 10-S. Both 1,5- and 1,8-dichloroanthracene yield initially the 4-S and subsequently the 4,8-S2 and 4,5-S2, respectively.
Peptide models. IX. A complete conformational set of For-Ala-Ala-NH2 from ab inito computations
Authors: Perczel András , McAllister Michael A. , Császár Pál , Csizmadia Imre G.
Published: May 2006

This paper reports the complete set of minimum energy (i.e. stable) conformations of For-Ala-Ala-NH2 (containing three peptide bonds), computed by ab initio geometry optimization. A more confident structure and relative energy comparison of the 49 different relaxed geometries was possible using the HF/3-21G method as compared to empirical (force field) methods. The analysis of the different conformers resulted in a useful structure database, incorporating 30 different β-turns, which will have some relevance to the description of peptide folding. Spectroscopic (e.g. NMR) consequences of the reported ab initio calculations are also discussed, and a 3D-structure assignment method for proteins is provided.
Nuclear magnetic resonance study of complexes formed between conjugated Schiff bases and carboxylic acids
Authors: Le-Thanh H. , Vocelle D.
Published: May 2006

The NMR spectra of the proton located in a N+---H---−O bond formed in the reaction of a conjugated Schiff base (SB) with a carboxylic acid (HA) in a non-polar solvent indicate that this proton is rapidly exchanging with other protons present in the system. For this study, two SB were used: all-trans-retinylidene tert-butylamine (RBA) and trans, tran-2,4-heptadienylidene tert-butylamine (HBA), and five carboxylic acids having pKa's between 0.66 and 4.87.1H NMR spectra were taken at 293 and 233 K in CDCl3 of systems of SB:HA using different ratios of acid to SB. Results indicate that the rapid proton exchanges are temperature and concentration dependent. Finally, the results are related to the natural pigments rhodopsin and bacteriorhodopsin.
Synthèse et études structurales de nouveaux complexes polyhydroxamiques du fer(III) potentiellement utilisables en imagerie par résonance magnétique nucléaire.Deuxième partie: Études des complexes formés in situ à partir d'acides hydroxamiques. Relaxivité du complexe Fe(III) – N,N′-bis(acétylhydroxyamino-3′ propyl)-(amino-4″ butyloxy)-4 pyridine-2,6 dicarboxamide
Authors: Chaubet Frédéric , Van Duong Kiem Nguyen , Courtieu Jacques , Gaudemer Alain , Gref Aurore , Crumbliss Alvin L. , Caudle Matthew T.
Published: May 2006

Numerous analogous dihydroxamates of rhodotorulic acid have been prepared, and their mode of complexation of iron(III) and the stability of these complexes studied as a function of pH, for comparison with ferrioxamine B and the complex of acetohydroxamic acid with iron(III). This study has shown that the stoichiometry of the complexes is 2:3 (at a pH of 7) and 1:1 (at a pH of 2), and that, in spite of a complete complexation, their electrochemical behaviour varies greatly with pH. The very strong complexation constant observed (log β110 = 21.3) at a pH of 1 with the ligand N,N′-bis(acetylhydroxamino-3′ propyl)-pentyloxy-4 pyridine-2,6-dicarboxamide confirms the strong complexation capacity of iron(III) by these complexes. The high relaxivity of the complex of iron(III) with the N,N′-bis(acetylhydroxyamino-3′ -propyl)-(amino-4″ -butyloxy)-4-pyridine-2,6-dicarboxamide suggests that this product could be a useful contrasting agent for nuclear magnetic imaging. [Journal translation]
Nouvelle méthode de synthèse de pyrroles sous micro-ondes
Authors: Oussaid Boualem , Garrigues Bernard , Soufiaoui Mohammed
Published: May 2006

The dehydrogenation of pyrrolidines by manganese dioxide, under microwave irradiation, provides a mild method for the preparation of substituted pyrroles.
Deprotonation energetics of some nucleosides in water from EMF measurements
Authors: Ganguly Sonali , Kundu Kiron K.
Published: May 2006

The deprotonation constants of uridine (K1 and K2), thymidine (K1 and K2), cytidine (K1 and K2), guanosine (K1, K2, and K3), and xanthosine (K1, K2, and K3) have been obtained in water from EMF measurements of Harned-Ehler type cells comprising H2 and Ag-AgI electrodes at different temperatures. The pK values were substituted in the temperature equation: pK = AT−1 + B + CT and A, B, and C were obtained by the method of least squares. Related thermodynamic quantities viz. ΔG0, TΔS0, and ΔH0 were obtained from coefficients A, B, and C of the respective nucleosides. Keywords: deprotonation energetics, uridine, thymidine, cytidine, guanosine, xanthosine.
New extracellular fatty acids in culture filtrates of Sporothrixflocculosa and S. rugulosa
Authors: Choudhury Streeti R. , Traquair James A. , Jarvis William R.
Published: May 2006

Two new and rare unsaturated, extracellular fatty acids were identified in the culture filtrates of the biocontrol fungi, Sporothrixflocculosa and S. rugulosa. 16-Methyl-9E-nonadecenoic acid (1) and (Z,Z)-10,14-eicosadienoic acid (2) were characterized on the basis of infrared (IR) and nuclear magnetic resonance spectra (13C and 1N NMR), and gas chromatographic – mass spectrometric data (GC–MS). The structure of compounds 1 and 2 was confirmed by oxidative degradation to the known standards, dimethyl azelate, dimethyl succinate, dimethyl sebacate, and methyl caproate. Keywords: unsaturated fatty acids, fungi.
Theoretical and experimental barriers to internal rotation in 2,6-difluorobenzaldehyde and 2,4,6-trifluorobenzaldehyde. Relatively low barriers
Authors: Schaefer Ted , Takeuchi Craig S. , Bernard Guy M. , Hruska Frank E.
Published: May 2006

The free energies of activation at 110 K for rotation about the exocyclic C—C bonds in 2,6-difluorobenzaldehyde and 2,4,6-trifluorobenzaldehyde, in dimethyl ether solutions, are 18.8 ± 0.5 and 20.0 ± 0.5 kJ mol−1, respectively, as determined from 19F{1H} dynamic nuclear magnetic resonance measurements. For the parent compound ΔG≠ is 32.2 kJ mol−1 in the same solvent. These free energy barriers, the lowest available for benzaldehyde derivatives, are likely a result of steric and electrostatic repulsions between the C+—O− and C+—F− bonds. Computations of the spectroscopic barrier in the 2,6-difluoro compound at various levels of molecular orbital theory imply that the barrier is predominantly twofold, with a fourfold component of opposite sign, whose magnitude is about 10% of the twofold component. A correlation-gradient computation, MP2/6-31G*, finds a barrier height of 18.6 kJ mol−1 for this compound, lower by 3.0 kJ mol−1 than found with the 6-31G* basis and 2.9 kJ mol−1 with 6-31G**. Similar computations are compared for the parent compound and the 4-fluoro, 2,4,6-trifluoro, and 3,5-difluoro derivatives. Linear relationships exist between the computed spectroscopic barriers (ΔE values at absolute zero for the free molecules) and the free energy barriers for benzaldehyde and the four fluoro derivatives; the theoretical barriers utilize 6-31G** and correlation-gradient MP2/6-31G* procedures. For the 2,6-difluoro derivative, the computed frequencies of the torsional motions about the exocyclic C—C bond yield spectroscopic twofold barriers. These barriers are much lower than the computed energy differences between the planar and perpendicular conformers, perhaps because the negative fourfold components flatten the potential at its minimum. A rough estimate of the relationship between ΔG≠ and ΔE0 for the 2,6-difluorobenzaldehyde suggests that the solvent increases the internal barrier by only about 3 kJ mol−1. By way of contrast, the AM1 barriers, scaled by a factor of 1.9 (as previously recommended) range from 17.3 to 22.6 kJ mol−1, the ΔG≠ values from 18.8(5) to 34.4 kJ mol−1, and the MP2/6-31G* (correlation-gradient) barriers span 18.6 to 36.8 kJ mol−1 for benzaldehyde and the four fluorine derivatives. It seems likely that the internal barrier in benzaldehyde is considerably larger than that modeled on torsional frequencies. Keywords: Free energies of activation, internal rotational barriers in 2,6-difluoro- and 2,4,6-trifluorobenzaldehyde; molecular orbital computations, internal rotational barriers in 2,6-difluoro- and 2,4,6-trifluorobenzaldehyde; correlation gradient computations on internal barriers in benzaldehyde and four of its fluorine derivatives.
Polarographic studies of indium(III) in aqueous medium of sodium azide
Authors: Tokoro Roberto , Bertotti Mauro , Angnes Lúcio
Published: May 2006

In this paper, some aspects of the polarography of indium(III) in azide/hydrazoic acid buffer are presented. The electrode process is mostly governed by diffusion in the complexing medium. Nevertheless, for controlled-potential coulometry (involving a large time window) a catalytic process was observed, ascribed to the reaction of the hydrazoic acid and indium amalgam. The stability of azide complexes of In(III) was studied polarographically, and the formation of four mononuclear species was confirmed by computational analysis of data obtained from polarograms of In(III) in different azide concentrations. The values of the global constants obtained are: β1 = 3.7 × 103 M−1, β2 = 8.6 × 105 M−2, β3 = 5.0 × 107 M−3, β4 = 2.1 × 109 M−4, in a constant ionic strength medium held at 2.0 M with NaClO4. These data are in good agreement with those obtained by potentiometry. The value found for E0′ of the In(III)/In couple was −512.2 mV vs. calomel electrode (3 M NaCl), the mean diffusion coefficient of the complex being (6.1 ± 0.2) × 10−6 cm2 s−1 in perchlorate medium (T = 25 °C). Keywords: polarography, indium(III), azide, stability constants, catalytic wave.
Stereospecific reactions of aryltellurium(IV) trichlorides with 3-cyclohexene-1-methanol and 3-cyclohexene-1,1-dimethanol: the X-ray crystal structure of 2′,4′-dimethoxyphenyl(trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichloride
Authors: Borecka Bozena , Cameron T. Stanley , Malik M. Azad , Smith Barry C.
Published: May 2006

Aryltellurium(IV) trichlorides react with 3-cyclohexene-1-methanol and 3-cyclohexene-1,1-dimethanol to give aryl(trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichlorides and aryl(cis-1-hydroxymethyl-trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichlorides. The structure of 2′,4′-dimethoxyphenyl(trans-6-oxabicyclo[3.2.1]oct-4-yl)tellurium(IV) dichloride has been determined by single crystal X-ray analysis. The compound C15H20Cl2O3Te, Mr = 446.8, crystallizes in the monoclinic space group P21/c with a = 9.626(2) Å, b = 15.768(5) Å, c = 11.176(3) Å, β = 92.08(1)°, V = 1695.2(5) Å3 and D(calc) = 1.751 for Z = 4. Tellurium is bonded axially to two chlorines and equatorially to aryl carbon and carbon from the cyclohexyl portion of oxabicyclo[3.2.1]oct-4-yl in a pseudo-trigonal-bipyramidal arrangement having an equatorial lone pair of electrons. Bond lengths are Te—Cl(1) = 2.492(3) Å, Te—Cl(2) = 2.539(3) Å, Te—C(4) = 2.175(10) Å, and Te—C(9) = 2.090(10) Å. Keywords: organotellurium(IV), 3-cyclohexene-1-methanol, 3-cyclohexene-1, 1-dimethanol.
Spectrophotometric determination of free, ionized, metal ion concentrations in solution by an indicator increment method. Application to the determination of free Mg2+ with calmagite
Authors: Ren Hongji , Kratochvil Byron , He Xiwen
Published: May 2006

A spectrophotometric method, called the Indicator Increment Method, is described in which varying amounts of a complexing indicator are added to a sample and the visible spectra recorded after each addition. Through multiple equilibrium calculations the free metal ion concentration, as well as total metal and total ligand concentrations, can be estimated in systems that contain a single metal ion and one ligand. The technique was tested successfully for the determination of free magnesium in the presence of NTA, phosphate, or oxalate using calmagite as indicator. The method requires the approximate conditional stability constant of the metal–ligand complex to be known beforehand. Keywords: ionized magnesium, indicator increment method, spectrophotometry.
An NMR study of mixed, tartrate-containing TiIV complexes
Authors: Potvin Pierre G. , Fieldhouse Benjamin G.
Published: May 2006

The reactions of amines and amino alcohols with diisopropyl or diethyl R,R- or S,S-tartrate and Ti(OiPr)4 were examined by 1H and 13C NMR to obtain and characterize nonfluxional complexes with the tartrate units in novel binding modes. The mildly acidic 8-hydroxyquinoline and N-phenyl-N-benzoylhydroxylamine selectively formed the products of a double OiPr substitution, Ti2(tartrate)2(ligand)2(OiPr)2, and the products of double tartrate substitution, Ti(ligand)2(OiPr)2, while 2,4-pentanedione formed only the latter Basic amino alkanols formed diastereomerically pure Ti2(tartrate)2(aminoalkoxide)(OiPr)3 species. N,N-Dimethyl-2-aminoethanol (Hdmae) also and uniquely formed monomeric Ti(tartrate)2(Hdmae)2 species that could be described as doubly zwitterionic. Secondary or tertiary amines formed triply C2-symmetric Ti3(tartrate)4(amine)2(OiPr)4 assemblies. Some minor components were believed to be μ-OiPr species. All mixed complexes except Ti(tartrate)2(Hdmae)2 contained chelating and bridging tartrate units, without coordination by ester carbonyls. A nonchelating, nonbridging tartrate unit was also present in the amino alcohol cases. Primary amines, aromatic amines, and hydrazines all failed to provide identifiable complexes. As well, N,N-dibenzylhydroxylamine failed to generate in solution the complex that had previously been characterized by X-ray crystallography. Amidst the rich chemistry of TiIV-tartrate systems, the evident selectivities in product formation were ascribed to macro-ring closures that are specifically directed by the electronic nature of the addend. Transient OiPr-bridged intermediates were also implicated. Keywords: mixed TiIV alkoxides, chiral TiIV alkoxides, enantiospecific complexation.
Peroxydation de l'acide linolénique en présence des fibres d'amiante ou de némalite. Résultats préliminaires avec des cellules épithéliales
Authors: Levasseur-Acker Germaine , Zalma Roger , Copin Edith , Fournier Jeanine , Pezerat Henri , Jankowski Roger
Published: May 2006

Linolenic acid (LH) peroxidation in aqueous medium is studied in the presence of fibers of chrysotile (UICC A and B), crocidolite UICC, and némalite. Preliminary studies on the peroxydation of epithelial cell membranes were also performed. Radicals (L•, LOO•) formed from the peroxidation process are detected by EPR with a spin trapping agent, POBN. The activity of these iron(II)-containing fibers is related to the formation of activated oxygen species, particularly of some highly electrophilic species (•OH or perferryl and ferryl entities). The presence of these species is shown by EPR with a spin trapping agent, DMPO, in the presence or in the absence of certain iron(II) chelators (phosphate ions, ADP, ATP,.AMP, EDTA). The formation and the nature of activated oxygen species depend on the nature both of the inorganic material and of the chelator. It is not possible to directly extrapolate the results obtained with linolenic acid to the peroxidation of epithelial cell membranes. Keywords: asbestos, peroxidation, iron chelators, activated oxygen species, EPR.
Étude par CNDO/2 des mouvements de rotation des unités moléculaires constituant le squelette des polymères du type: PEK, PEEK et PEEKK
Authors: Verot S. , Battesti P.
Published: May 2006

Diphenyl ether (DPE) and diphenyl ketone (DPK) are the molecular constituents of polymers such as PEK, poly(ether–ketone); PEEK, poly(ether–ether–ketone), and PEEKK, poly(ether–ether–ketone–ketone). A conformational analysis of DPE and DPK is examined by means of semiempirical calculations using CNDO/2 (Complete Neglect of Differential Overlap). The modelization is first run disregarding electronic interactions and steric effects, then introducing them in the calculations. We found that the absolute minimum energy conformer has torsional angles (θ1,θ2) = (30°,150°) for the DPE (with and without interactions) and (θ1,θ2) = (90°,90°) for the DPK (without interactions). Considering the polymer chains of PEK, PEEK, and PEEKK, the motion of DPE is expected to be easier than that of DPK in regions of local motions and for equivalent steric effects. Keywords: conformational analysis, diphenyl ether, diphenyl ketone, CNDO/2, rotational barriers.
A density functional study on olefin insertion and hydrogen transfer in the reaction between Cl2Ti+–ethyl and ethylene. Possible implications for the stereochemistry and chain termination in olefin polymerization
Authors: Fan Liangyou , Harrison Daryll , Deng Liqun , Woo Tom K. , Swerhone David , Ziegler Tom
Published: May 2006

Calculations based on density functional theory have been carried out on the reaction between Cl2Ti+–ethyl (1) and ethylene. In this study 1 was taken as a model for the cationic metallocenes of group-4 elements, which have been developed by Kaminsky and Brintzinger as efficient catalysts for the polymerization of olefins. The ground state structure of 1 has a β-agostic conformation in which a single Cβ—H bond is directed towards the metal center. It was assumed that this conformation also serves as a model for the resting state of the growing chain attached to the cationic group-4 metallocenes between insertions. Two paths were considered for the reaction between 1 and ethylene. The first (2) has ethylene approaching the agostic Cβ—H bond, whereas ethylene in the second approach (3) attacks the Ti—Cα link from the side opposite to the Cβ—H bond. The front-side attack (2) results in a transfer of hydrogen from the β-carbon of ethyl to ethylene and represents a chain-terminating step with an activation energy of 5.3 kcal/mol. It was not possible to locate a path leading to olefin insertion from the front-side attack (2). The back-side attack (3) resulted readily in insertion with an activation energy of 3.9 kcal/mol. The study made use of full transition state optimization as well as a tracing of the reaction paths by the intrinsic reaction coordinate (IRC) method of Fukui. Previous investigations have all assumed that olefin insertion takes place via a front-side approach (2) based on the known stereochemistry of α-olefin polymerization. The present study suggests that insertion takes place by a back-side approach (3), and this suggestion is discussed in connection with the known stereochemistry of olefin polymerization. Keywords: Ziegler–Natta, olefin polymerization, density functional.
Routes to dicationic ruthenium(II) nitrile complexes containing chelating diphosphine ligands: X-ray analyses of Ru(dppb)(MeCN)42+ 2PF6−(dppb = Ph2P(CH2)4PPh2) and trans-RuCl2(MeCN)4
Authors: Fogg Deryn E. , Rettig Steven J. , James Brian R.
Published: May 2006

Routes to the Ru(PP)(RCN)42+ 2PF6− complexes (PP = Ph2P(CH2)4PPh2, dppb; R = Me, Ph) from the Ru(II) precursors Ru2Cl4(PP)2, and Ru2Cl4(PP)(PPh3) have been established. The X-ray crystal structure of the dppb-MeCN derivative (3) is described, and the steric effect of the diphosphine ligand is examined by comparison with the crystal structure of trans-RuCl2(MeCN)4 (5), which is also presented. Crystals of 3 are triclinic, a = 13.845(3), b = 14.419(2), c = 12.984(3) Å, α = 108.75(2)°, β = 116.40(1)°, γ = 68.58(1)°, and Z = 2, space group [Formula: see text] and those of 5 are orthorhombic, a = 13.058(2), b = 11.790(2), c = 8.562(1) Å, and Z = 4, space group Pbca. The structures were solved by the Patterson method and refined by full-matrix least-squares procedures to R = 0.039 and 0.024 (Rw = 0.046 and 0.028) for 9973 and 1131 reflections with I ≥ 3σ(I), respectively. Crystallographic and infrared data suggest that the nitrile ligands function solely as σ-donors in these ruthenium(II) complexes. Keywords: ruthenium, nitriles, diphosphine ligands.
Reductions by ferrocytochrome c peroxidase: 5. Kinetics of ferricyanide reduction
Authors: Cheung Eddy , English Ann M.
Published: May 2006

The kinetics of reduction of ferricyanide by yeast ferrocytochrome c peroxidase (CPPII) were investigated as a function of ionic strength in phosphate buffers at pH 7.0 and 25 ± 1 °C. The observed bimolecular rate constant (k12) is 8.4 × 104 M−1s−1in 0.1 M phosphate. The dependence of the reaction rate on ionic strength indicates a change of −9 on the protein at pH 7.0, which is in good agreement with the total charge of −11 estimated for CCPIIfrom its amino acid content. Substituting k12at infinite ionic strength [Formula: see text] into the Marcus cross relation yields an electron self-exchange rate constant [Formula: see text] for the FeIII/FeIIcouple of CCP of 7.2 × 10−5 M−1s−1. This value is over four orders of magnitude higher than that calculated for the FeIV/FeIIIcouple of CCP from literature data for cross-reactions with ferrocyanide at pH 7.0. Possible reasons for the large difference in the two CCP [Formula: see text] values are discussed. Literature data also allowed [Formula: see text] values for various other heme proteins to be determined from their cross-reactions with ferricyanide. The calculated rate constants vary by eight orders of magnitude, and the variation of [Formula: see text] with protein structure suggests that the redox reactivity of ferrous heme proteins towards ferricyanide is dependent on the spin state and coordination of iron, as well as on the accessibility of the heme. Keywords: cytochrome c peroxidase, ferricyanide, Marcus cross relation, electron self-exchange.
The controlled synthesis of triosmium chain clusters
Authors: Lewis Jack , Moss John R.
Published: May 2006

The reaction of H2Os3(CO)12 with ClSnPh3 in the presence of diethylamine gives exclusively the pentametallic chain compound 1,3-ax,ax-Ph3SnOs(CO)4Os(CO)4Os(CO)4SnPh3. A similar reaction with ClAuPPh3 gives 1,3-ax,eq-Ph3PAuOs(CO)4Os(CO)4Os(CO)4AuPPh3. The di-tin compound reacts with HCl to give 1,3-ax,ax-Cl3SnOs(CO)4Os(CO)4Os(CO)4SnCl3. These products are obtained in high yield and the reactions are selective. Keywords: osmium carbonyl chain cluster complex.
Synthesis of 3-substituted tetrahydroisoquinolines by acid-catalyzed cyclization of p-toluenesulfonamides of N-benzyl aminoacetaldehyde derivatives
Authors: Ponzo Viviana L. , Kaufman Teodoro S.
Published: May 2006

The acid-catalyzed cyclization of p-toluenesulfonamides of N-benzyl aminoacetaldehyde, or their acetalic counterparts, usually yields 1,2-dihydroisoquinolines. However, cyclization of intermediates bearing a substituent α to the carbonyl group affords 3-substituted 2-p-toluenesulfonyl tetrahydroisoquinolin-4-ol derivatives, capable of further transformation into the related 1,2,3,4-tetrahydroisoquinolines. Keywords: 3-substituted tetrahydroisoquinolines, MY336-a analog, acid-catalyzed cyclization, tetrahydroisoquinolin-4-ol derivatives.
2-Acylidene-3,5-diaryl-2,3-dihydro-1,3,4-thiadiazoles and related compounds: a question of hypervalent S … O interactions
Authors: Pandya Naresh , Basile Anthony J. , Gupta Ajay K. , Hand Patrick , Maclaurin Cindy L. , Mohammad Taj , Ratemi Elaref S. , Gibson Martin S. , Richardson Mary Frances
Published: May 2006

not available
Crystal and molecular structure of Z- and E-1,2-dichloro-1,2-bis(2-chlorophenyl)ethylene. An X-ray and NMR study
Authors: Antolini Luciano , Folli Ugo , Iarossi Dario , Mucci Adele , Sbardellati Silvia , Taddei Ferdinando
Published: May 2006

The crystal structures of the title compounds were determined by single crystal X-ray diffraction techniques. The molecule of the Z isomer, which crystallizes in the monoclinic space group C2/c with Z = 4 in a cell of dimensions a = 14.891 (2), b = 10.780(2), c = 8.769(1) Å, β = 97.47(2)°, V = 1395.7(7) Å3 has crystallographic twofold symmetry. The E form crystallizes in the orthorhombic space group Pbca with a = 11.730(1), b = 6.932(1), c = 16.841(1) Å, V = 1369.4(2) Å3 and Z = 4. Its molecules have crystallographically dictated [Formula: see text] symmetry. In both isomers the phenyl rings are roughly perpendicular to the average ethylene plane. The atoms characterizing this plane show significant deviations from planarity in the Z isomer. Marked bond-angle distortions at the ethene carbons of both structures are observed. The 1H and 13C NMR spectra of the compounds were measured and, particularly in the case of the 1H chemical shifts, fall into two quite separate spectral regions. At low temperature, two conformational isomers, those with different relative orientation of the C—Cl bonds of the phenyl rings, are observed in the spectrum of each compound. Keywords: chlorostilbenes, overcrowded molecules. X-ray structure, conformations, NMR spectroscopy.
Structure–sweetness relationships for fructose analogs. Part III. 3-Deoxy-D-erythro-hexulose (3-deoxy-D-fructose): composition in solution and evaluation of sweetness
Authors: Szarek Walter A. , Rafka Robert J. , Yang Te-Fang , Martin Olivier R.
Published: May 2006

As part of continuing studies on the structural features responsible for the intense sweetness of D-fructose, 3-deoxy-D-erythro-hexulose (3-deoxy-D-fructose, 1) was prepared, its solution composition was determined, and its taste was evaluated. In aqueous solution, 3-deoxy-D-fructose exists as a complex mixture of five tautomeric forms in which the β-D-pyranose form is preponderant (52.5% at 22 °C) and the α-D-pyranose form is the least abundant (5%). Quite remarkable is the behavior of the open-chain keto form of 1: its content increases from 7.5% at 22 °C, to 36% at 82 °C, and to 47% at 97 °C, making this form the preponderant one at high temperatures. 3-Deoxy-D-fructose was found to be sweet, albeit probably not as sweet as D-fructose. The hydroxyl group at C-3 is thus not an essential function of the glycophore of D-fructose. The significance of this result is discussed in relation to the evidence already available and the divergent theories that have been proposed to explain the origin of the sweet taste of D-fructose. Keywords: 3-deoxy-D-erythro-hexulose (3-deoxy-D-fructose), sweetness, solution composition.
Photophysical studies of hydrophobically functionalized polyionene systems in aqueous solutions
Authors: Uznanski P. , Pecherz J. , Kryszewski M.
Published: May 2006

Complexes of polyionene and anionic dyes and surfactant were prepared by exchange reaction of counterions in the parent polymer. Fluorescence studies were conducted to observe some characteristic behaviour of these complexes in water Polyionenes with aromatic hydrocarbons adopt an open conformation and there is no evidence for interpolymer interactions. On the contrary, polyionenes with long aliphatic counterions easily form microdomains in aqueous solution due to hydrophobic interaction as evidenced by an increase in excimer emission. Microdomains have an interpolymer nature, as confirmed by measurements of energy transfer from naphthalene to pyrene moieties. Polyionenes with SDS counterions interact with external surfactant molecules and form polymer–surfactant aggregates. Electrostatic repulsions between aggregates dominate on hydrophobic interaction between polymer chains and are responsible for conformational changes. Keywords: hydrophobically modified polymers, surfactants, complexation, fluorescence, nonradiative energy transfer
Unusual behavior of the conductivity of LiNO3 in tert-butanol: ion clustering or ion-pair aggregation
Authors: Zhao Yixing , Freeman Gordon R.
Published: May 2006

The electrical conductance of LiNO3 in tert-butanol–water mixed solvents changes gradually from "normal" in pure water to "abnormal" in pure tert-butanol. In water the measured specific conductance increases with increase of temperature, and in tert-butanol the conductance decreases with increase of temperature. In pure tert-butanol, the electrical conductances of NH4ClO4 and LiClO4 increase with the salt concentration and temperature at lower temperatures, but decrease at higher temperatures. The molar conductivity Λ0(10−4 S m2 mol−1) in tert-butanol at 300 K is 5.0 for NH4ClO4 and 4.0 for LiClO4. Both activation energies EΛ0 are 17 kJ mol−1, which gives an unusual correlation between Λ0 and viscosity η(mPa s): [Formula: see text] The values of Λ0 for NH4NO3 and LiNO3 in tert-butanol could not be measured, because ion aggregation is significant even at the lowest concentrations required to obtain conductances sufficiently above that of the solvent. The measured temperature coefficient of LiNO3 conductance in tert-butanol is negative. Ion clustering of nitrate salts is attributed to poor solvation of the planar NO3− ions by the globular tert-butanol molecules. Ion aggregation in tert-butanol increases with increasing T, due to the relatively rapid decrease of the value of εT. Corrections are listed for reaction kinetics parameters for nitrate salts in pure tert-butanol solvent reported in Can. J. Chem. 73, 392 (1995). Keywords: tert-butanol, ion-pair aggregation, lithium nitrate, electrical conductance, solvent effects.
Rhenium and technetium complexes from pentadentate (N3O2) and tetradentate (N2O2) Schiff base ligands
Authors: Luo Hongyan , Liu Shuang , Rettig Steven J. , Orvig Chris
Published: May 2006

The rhenium(V) and technetium(V) complexes: [ReO(apa)], [{ReO(epa)}2O], and [TcOCl(epa)] have been prepared from a potentially pentadentate N3O2 ligand, N,N′-3-azapentane-1,5-diylbis(3-(1-iminoethyl)-6-methyl-2H-pyran-2,4(3H)-dione) (H3apa), or a potentially tetradentate N2O2 ligand, N,N′-ethylene-diylbis(3-(1-iminoethyl)-6-methyl-2H-pyran-2,4(3H)-dione) (H2epa). The N2O2 complexes were synthesized in low yields. There was also evidence indicating that H2ppa, N,N′-propylene-diylbis(3-(1-iminoethyl)-6-methyl-2H-pyran-2,4(3H)-dione), hydrolyzed in the course of coordination, forms a rhenium complex of the bidentate monoprotic (O,O) dehydroacetate (dha−), [Re(dha)Cl2(OPPh3)(PPh3)]•EtOH. The structure of this complex was determined by X-ray crystallography. Crystals of [Re(dha)Cl2(OPPh3)(PPh3)] EtOH (C44H37Cl2O5P2Re•C2H6O) are monoclinic, a = 11.064(2), b = 24.167(2), c = 16.6687(8) Å, β = 98.779(7)°, Z = 4, space group P21/c. The structure was solved by the Patterson method and refined by full-matrix least-squares procedures to R = 0.030 and Rw = 0.025 for 6942 reflections with I ≥ 3σ(F2). Keywords: dehydroacetic acetate based Schiff base ligands, rhenium, technetium, N3O2 Schiff base, N2O2 Schiff base, oxygen transfer.
Irreversible inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A synthase from yeast by F-244 and (RS)-β-butyrolactone
Authors: Bell Karen , Saepudin Endang , Harrison Paul
Published: May 2006

The inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase from yeast was compared for both F-244 (1) and (RS)-β -butyrolactone (5). F-244 exhibits irreversible inhibition with an IC50of 8 nM, similar to that reported for the rat liver enzyme, while the binding constant (1/K1) and inactivation rate constant (kinact) similar to values reported for the human cytoplasmic enzyme. (RS)-β-Butyrolactone (5) also irreversibly inhibits the enzyme, but with much lower efficiency (IC502 mM). The values for K1(9 mM) and kinact(0.0078 s−1) for 5 were determined. The results show that kinactfor 5 and 1 differ by a factor of only 2.5, while K1for 5 is higher by a factor of 1.8 × 105. Hence, the β-lactone ring is shown to be the sole essential structural feature in 1 for irreversible inactivation of HMG-CoA synthase; however, the remaining functionality enhances the binding of 1 to the enzyme relative to 5. Key words: HMG-CoA synthase, F-244, butyrolactone, irreversible, inhibitor.
Influence of vanadium(V) complexes on the catalytic activity of ribonuclease A. The role of vanadate complexes as transition state analogues to reactions at phosphate
Authors: Leon-Lai Chui Har , Gresser Michael J. , Tracey Alan S.
Published: May 2006

The interactions of vanadate and its complexes of uridine, 5,6-dihydrouridine, and methyl β-D-ribofuranoside with bovine pancreatic ribonuclease A (RNase A) (EC 3.1.27.5) were studied by51V NMR spectroscopy and enzyme kinetics. From kinetic studies, it was found that neither inorganic vanadate nor the methyl β-D-ribofuranoside–vanadate complex significantly inhibited the RNase A catalyzed hydrolysis of uridine 2′,3′-cyclic monophosphate. The NMR binding studies were in full agreement with the kinetics studies and showed that neither inorganic vanadate nor the methyl β-D-ribofuranoside–vanadate complex was bound tightly by the enzyme. Approximate binding constants were (5.0 ± 1.0) × 10−7 M and (3.0 ± 0.6) × 10−6 M for the uridine–and 5,6-dihydrouridine–vanadate complexes, respectively. An induced-fit mechanism is suggested, in which the pyrimidine subsite of the active site of RNase A must be fully occupied for the enzyme to be able to tightly bind the transition state or transition state analog. Calculation of the binding energies of vanadate complexes in ribonuclease, phosphoglycerate mutase, and phosphoglucomutase revealed an excess of binding energy over the analogous phosphate derivative of about 25 kJ/mol for all enzymes, even though the binding constants themselves varied by about six orders of magnitude. This energy represents about 40% of that expected to be available for a trigonal-bipyramidal transition state and requires a reassessment of the role of vanadate as a transition state analogue for phosphate transfer. Key words: vanadate, ribonuclease, transition state, binding constants, phosphate analogues, kinetics.
Synthesis and study of Pt(II)–nitrile complexes. Multinuclear NMR spectra and crystal structures of compounds of the types [Pt(R-CN)Cl3]− and cis and trans-Pt(R-CN)2Cl2
Authors: Rochon Fernande D. , Melanson Robert , Thouin Eryk , Bensimon Corinne , Beauchamp André L.
Published: May 2006

Complexes of the type [Pt(R-CN)Cl3]− were synthesized and studied by 1H, 13C, and 195Pt NMR spectroscopies. The 2J(195Pt-13C) coupling constants are about 240 Hz. The signals due to the cyano carbon atoms of the coordinated ligands were observed at higher fields than those of the free ligands. The shielding on the cyano carbon increases as the alkyl chain lengthens and also when the branching increases. The 195Pt signals of all the complexes were observed around −2000 ppm. The crystal structures of (NMe4)[Pt(C3H7-CN)Cl3] (1) and (NMe4)[Pt(p-HO-C6H4-CN)Cl3] (2) were determined. Compound 1 is monoclinic, P21/c, a = 8.384(5), b = 15.336(19), c = 11.759(9) Å, β = 99.52(6)°, Z = 4, R = 0.054, and wR = 0.051. Crystal 2 is tetragonal with a = 16.222(6), c = 12.052(5) Å, Z = 8, R = 0.059, and wR = 0.044. The Pt—CL bond trans to the nitrile ligand is shorter than normal (2.276(3) Å for 1 and 2.264(7) Å for 2) while the two other bonds are normal (2.293(4), 2.287(3) Å for 1 and 2.320(7), 2.275(8) Å for 2). The Pt—N bonds are 1.97(1) Å (1) and 1.92(2) Å (2) and the segments Pt-N≡C-C are linear. Disubstituted compounds were also synthesized and studied by multinuclear NMR. The 195Pt signals of the cis isomers were observed at lower fields than those of the trans isomers (Δ ≈ 65 ppm), while the 13C signals of the cyano carbons of the trans isomers were observed at lower fields than those of the cis compounds (Δ ≈ 0.6 ppm). The cis complexes isomerize to the trans compounds upon heating. The crystal structures of cis-(3) and trans-Pt(C2H5-CN)2Cl2 (4) and also of cis-Pt(p-HO-C6H4-CN)2Cl2 (5) were determined. Crystal 3 is monoclinic, P21/c, a = 7.506(5), b = 9.539(5), c = 14.823(7) Å, β = 92.31(4)°, Z = 4, R = 0.050, and wR = 0.042. The trans isomer 4 is monoclinic with the Pt atom on an inversion centre, P21/c, a = 5.149(4), b = 9.394(8), c = 10.944(10) Å, β = 97.84(7)°, Z = 4, R = 0.017 and wR = 0.020. Finally, compound 5 is triclinic, P-1, a = 7.464(3), b = 10.712(6), c = 12.291(5) Å, α = 75.63(4)°, β = 75.63(4)°, γ = 80.32(4)°, Z = 2, R = 0.045, and wR = 0.056. The Pt—Cl bond distances for the cis isomers are 2.269(5), 2.270(4) Å for 3 and 2.274(2), 2.279(3) Å for 5 while they are 2.289(3) Å for the trans isomer (4). The Pt—N bonds are 1.962(14), 1.988(11) Å (3), 1.972(7), 1.976(7) Å (5) and 1.969(5) Å for 4. Key words: platinum, nitrile, NMR, isomerization, crystal structure.