bluuebunny/crossref_metadata_2025_split huggingface dataset:
The meta versus para substituent effect in the gas phase: Separation of inductive and resonance components
Authors: Decouzon Michèle , Exner Otto , Gal Jean‐Francois , Maria Pierre‐Charles 
Published: January 2005

AbstractSubstitutent effects of acceptor groups were measured for the gas‐phase basicities of some substituted benzonitriles, methyl benzoates and acetophenones, and for gas‐phase acidities of some substituted benzoic acids. Substitutents considered were NO2, CN, SO2CH3, SO2F, CF3, CH2Cl, COCH3 and COOCH3, always in the meta and para positions. By combination with the literature data, a general conclusion is drawn that the substituent effects are proportional in the two positions and almost equal (the para:meta ratio is 1·06). No sign of any resonance effect was detected within the accuracy of the approach. The substituent effects in solution were recalculated statistically and the result was similar with a higher ratio, 1·09–1·19. It follows that the conjugation of acceptor groups, particularly of NO2 as a typical example, has been overestimated by current theories when compared with the much stronger conjugation of typical donors. Moreover, the inductive effect is propagated more effectively from the para than from the meta position. Many correlations in the literature, neglecting this situation, have been in fact controlled by the more complex, and more striking, behaviour of donors, which has obscured the simpler and more monotonous effect of acceptors, usually less represented in the sample.
An unusual parabolic dependence of rate on acidity in the fischer indole reaction
Authors: Hughes David L. 
Published: January 2005

AbstractOver the H0 acidity range from + 2 to −8, four different mechanistic variations occur in the Fischer indole reaction, leading to an unusual parabolic dependence of the rate constants on the acidity of the medium. In strongly acidic media kinetic isotope effects indicate that the rate‐determining step is deprotonation to form the ene–hydrazine tautomer, whereas in weakly acidic media the tautomerization is so rapid that [3,3]‐rearrangement becomes rate limiting. At an intermediate acidity, the deprotonation is rate determining, with the rate being strongly dependent on the concentration of the anionic counter ion.
AB initio study of Diels–Alder reactions of 1,3,4‐oxadiazole with ethylene, acrylonitrile, maleonitrile, fumaronitrile and 1,1‐dicyanoethylene. Inverse order and ratio of endo/exo reactivity
Authors: Jursic Branko S. , Zdravkovski Zoran 
Published: January 2005

AbstractTransition structures for the Diels–Alder reactions of 1,3,4‐oxadiazole with ethylene, acrylonitrile, maleonitrile, fumaronitrile, and 1,1‐dicyanoethylene were located with ab initio molecular orbital calculations with the RHF/3–21G theoretical model. The activation energies were calculated by single‐point calculations with the RHF/6–31G* and MP2/6–31G* theory levels on the RHF/3–21G geometries. Geometries of two transition structures and the corresponding reactants were generated at the MP2/6–31G* and the energies evaluated at the MP3/6–31G* level. The values obtained are comparable to those generated by MP2/6–31G*//3–21G. The asynchronicity of the transition structures follow the same trend as the corresponding all‐carbon Diels–Alder reactions with buta‐1,3‐diene and cyclopentadiene, but the activation energies are almost in opposite correlation. This behavior is explained by the oxadiazole nitrogen lone pair repulsion with the π‐orbitals of the nitrile groups.
Solvation effects on the relative basicity of propylamines
Authors: Headley Alland D. , Starnes Stephen D. , Cheung Eric T. , Malone Patricia L. 
Published: January 2005

AbstractSlight structural moiecular variations are known to affect different properties of compounds. In solution, different solute‐solvent interactions are known also to alter the properties of numerous compounds. Quantitative structure‐activity relationships (QSAR) are used regularly to analyze and predict the variations of different properties of compounds that are caused by structural variations and significant solute‐solvent interactions. The relative basicities of n‐propylamine, dipro;ylamine and tripropylamine were determined in nine different solvents from potentiometric titrations. QSAR that were developed from these experimental basicity values were used to evaluate the type and significance of the solute‐solvent interactions. The important interactions that influence basicity variations for the propylamines studied are dipolarity‐polarizability interaction between the solute and the solvent and hydrogen bonds from the propylammonium ions to basic solvents. The role of hydrogen bonds from the propylamines to acidic solvents is minor.
The solvent effect on the conformational equilibria of 4‐substituted cyclohexenes
Authors: Došen‐Mićović Ljiljana , Šolaja Bogdan 
Published: January 2005

AbstractConformational preferences have been measured for 4‐X‐cyclohexenes (X = Br, Cl, CHO, COOH) in a series of solvents with increasing polarity. The conformational equilibrium exhibits a very weak solvent dependence. ΔG varies at most by 0·25 kcal mol−1 (1 kcal = 4·184 kJ) shifting from a non‐polar mixture (CS2–CDCI3) to highly polar (CD3)2SO. The reaction field theory was used to calculate the variation of ΔG with solvent polarity. The calculated solvent shifts δΔG are small and in agreement with the values determined experimentally.
Enthalpies of micelle formation by hexadecyltrimethylammonium bromide in aqueous solution containing pentanol
Authors: Bach Jason , Blandamer Michael J. , Burgess John , Cullis Paul. M. , Tran Phuong , Soldi L. Giorgio , Rao K. Chowdoji , Subha M. C. S. , Kacperska Anna 
Published: January 2005

AbstractTitration calorimetric data show a dramatic change from endo‐ to exothermic deaggregation when pentanol–hexadecyltrimethylammonium bromide (CTAB) mixed solutions are injected into an aqueous solution containing pentanol. The results are interpreted in terms of a change in the structures of the aggregates in solution from simple CTAB micelles to mixed amphiphilic microheterogeneities when pentanol is added.
1,3‐Hydron transfer in some 5‐ and 7‐substituted 1‐methylindenes. Enantioselectivities and enantiomer‐dependent kinetic isotope effects
Authors: Aune Marie , Bergson Göran , Matsson Olle 
Published: January 2005

AbstractBase‐catalysed substrate‐enantioselective 1,3‐hydron transfer (kinetic resolution) was studied in the indene system. A series of 1‐methylindenes substituted in the aromatic ring [5‐methoxy‐ (2), 5‐fluoro‐ (3), 5‐nitro‐ (4) and 7‐nitro (5)] and 1‐methylindene (1) were employed as substrates. The rate constants, the enantioselectivities and the kinetic isotope effects (KIEs) for the enantioselective reactions [(kH/kD)+ and (kH/kD)−] were determined at 20°C using (+)‐(8R, 9S)‐dihydroquinidine as chiral catalyst in the solvent o‐dichlorobenzene. The rate constants vary according to the elecronic effects of the substituents. The primary deuterium KIE, ranging from 4·73 [for (+)‐(S)‐2] to 11·3 [for (−)‐(R)‐5], is correlated with the rate constants as expected on the basis of the Melander–Westheimer postulate. The introduction of a substituent in the aromatic ring decreases the enantioselectivity. All compounds except 5 show the same sense of the enantioselectivity [k+/k− > 1; all substrates used are (+)‐(S)/(−)‐(R)]. The enantiomer dependence of the KIE is most pronounced for 1 [(kH/kD)+ = 5·71 and (kH/kD)− = 6·46] and vanishes for the most acidic substrates (4 and 5).
Prediction of hydrogen bond basicity in nitriles from dipole moments, mesomeric effects and electrostatic potentials
Authors: Herail Martine , Berthelot Michel , Proutiere Alain 
Published: January 2005

AbstractMeasured molecular dipole moments and theoretical percentages of mesomeric zwitterionic forms were used to calculate the attractive electrostatic potentials at short, V(s), medium, V(m), and long, V(I), distances. Values of an attraction power function Φ (H) = A + BV (s) V(m) V(I) was then deduced for 18 nitriles and cyanamide vinylogues or iminologues along the C≡N direction. The satisfactory agreement observed between Φ (H) and the hydrogen bond basicity scale pKHB indicates that the method can be a useful tool for the prediction of hydrogen bonding, needing only simple calculations and allowing easy interpretation.
Intermolecular interactions that determine the regioselectivity in 1,3‐dipolar cycloadditions of N‐methyl‐1,3‐oxazolium‐5‐olates with N‐(phenylmethylene)benzenesulphonamide
Authors: Bonati L. , Ferraccioli R. , Moro G. 
Published: January 2005

Abstract1,3‐Dipolar cycloadditions of N‐(phenylmethylene)benzenesulphonamide with mesionic N‐methyl‐1,3‐oxazolium‐5‐olates give 2,5‐disubstituted imidazole derivatives with high regioselectivity. The intermolecular interactions underlying this regioselectivity were investigated. The conformational and electronic properties of the reagents were characterized separately. The approach in the early stages of the reactions was then modelled by considering the steric and electrostatic molecular interactions. The interaction energies related to different reaction paths were calculated by perturbation molecular orbital (PMO) treatment and compared.
Strain and structural effects on rates of formation and stability of tertiary carbenium ions in the light of molecular mechanics calculations
Authors: Müller Paul , Mareda Jiri , Milin Didier 
Published: January 2005

AbstractAn empirical MM2 force‐field was developed for the calculation of steric or strain energies of carbenium ions, and applied to the rationalization of the rates of solvolysis of bridgehead derivatives. The latter constitute a homogeneous series of model compounds for solvolysis, spanning a rate range of ca 20 log units. Their rate constants correlate with the calculated steric energy differences between bridgehead bromides and the corresponding carbenium ions. The rate constants of tertiary derivatives of general structure may similarly be rationalized in terms of strain changes, although the correlation exhibits more scatter than that for the bridgehead derivatives alone. In the bridgehead series, the relative free energies of activation for solvolysis correlate with the heterolytic bond dissociation energies D° (R+ – Br−) in the gas phase. However, this correlation breaks down when simple mono‐ and acyclic substrates are included. This is attributed in part to the proximity of the leaving group in the transition state of solvolysis, which stabilizes the developing positive charge at the cationic centre in comparison with the charge of the free ion. The significance of the force‐field calculations with respect to the structure of bridgehead carbenium ions was tested by comparison of structural data obtained from ab initio calculations. The structures of cations suffering strong distortions owing to C–C hyperconjugation are poorly reproduced by the molecular mechanics calculations, the parameters of which are based on solvolytic reactivity and not on carbenium ions.
Acidities and homolytic bond dissociation enthalpies of 4‐substituted‐2,6‐di‐tert‐butylphenols
Authors: Bordwell F. G. , Zhang Xian‐Man 
Published: January 2005

AbstractEquilibrium acidities and estimates of homolytic bond dissociation enthalpies (BDEs) in DMSO of the O–H bondsfor α‐tocopherol, 2,6‐di‐tert‐butylphenol, six 4‐substituted‐2,6‐di‐tert‐butylphenols, and ten related phenols are reported. The presence of a 2,6‐di‐tert‐butyl group in a phenol increases its acidity and makes substituent effects on the acidity caused by a para electron‐withdrawing group larger. The BDEs of the O–H bonds in 2,4,6‐tri‐tert‐butylphenol, 4‐methoxy‐2,6‐di‐tert‐butylphenol, 4‐methyl‐2,6‐di‐tert‐butylphenol and α‐tocopherol, estimated by combining their pKHA values with the oxidation potentials of the conjugate anions, Eox(A−), according to the equation BDE = 1·37pKHA + 23·1Eox(A−) + C have been found to agree to within 2 kcal mol−1 with literature values (1 kcal = 4·184 kJ). Introduction of 2,6‐di‐tert‐butyl groups into phenol and six 4‐substituted phenols weakens the O–H bonds by amounts ranging from 3·6 to 10·3 kcal mol−1. These effects are attributed to increases in ground‐state energies which introduce strains that are relieved when homolytic cleavage of the O–H bond forms an oxygen‐centered radical where the odd electron can be delocalized into the benzene ring.
Metal ion complexation by the phosphoryl and the carbonyl groups probed by 17O NMR spectroscopy
Authors: Belciug Marie‐P. , Modro Agnes M. , Modro Tomasz A. , Wessels Philippus L. 
Published: January 2005

AbstractInteractions between compounds containing phosphoryl, carbonyl, or hydroxyl groups (or combinations of these groups) with metal ions (Na+, Mg2+) were probed in CD3CN solutions by 17O NMR spectroscopy. For all phosphoryl and carbonyl substrates a strong high—field shift of the PO and CO signals was observed on the addition of a salt; no shift was observed, however, for the OH group signal, indicating that no chelation takes place in the hydroxyphosphoryl (or hydroxycarbonyl) derivatives. For 2‐hydroxy‐3‐benzoylpropylphosphonic diester the 17O NMR studies, together with conformational analysis based on the 1H spectroscopy, demonstrated the chelation of a metal ion by the PO and CO functions (fermation of an eight–membered cyclic structure) with the OH group involved in the intramolecular hydrogen bonding, but not in the complexation to the metal ion.
Photohydration reaction of 1‐(p‐nitrophenyl)‐5,5‐dimethyl‐1,3‐hexadiyne
Authors: Baek Eun Kyung , Shim Sang Chul 
Published: January 2005

AbstractIrradiation of 1‐(p‐nitrophenyl)‐5,5‐dimethyl‐1,3‐hexadiyne (NDHD) in aqueous sulfuric acid yields conjugated allenyl ketones (1 and 2) and β‐dicarbonyl compounds (3 and 4). The products 1 and 2 are primary photoproducts while 3 and 4 are secondary products formed by thermal hydration of the primary photoproducts showing the maximum quantum yield at H0= −1·0. A mechanism involving a synchronous addition of hydronium ion (H3O+) to a triplet state of NDHD is proposed.
Antioxidant synergism and mutual protection of α‐tocopherol and β‐carotene in the inhibition of radical‐initiated peroxidation of linoleic acid in solution
Authors: Li Zhao‐Long , Wu Long‐Min , Ma Lan‐Ping , Liu You‐Cheng , Liu Zhong‐Li 
Published: January 2005

AbstractPeroxidation of linoleic acid was initiated by azobis(isobutyronitrile) in tert‐butyl alcohol and inhibited by α‐tocopherol, β‐carotene and retinal, either alone or in combination. Significant antioxidant synergism and a novel mutual protection of α‐tocopherol and β‐carotene were found and the possible involvement of retinal in the process is discussed.
Steric substituent constant Ωs based on molecular mechanics calculation. II. Directionally weighted steric constants and their application to the analysis of the steric course of reactions
Authors: Akai Ikuo , Kuroda Shin‐Ichi , Sakakibara Kazuhisa , Hirota Minoru 
Published: January 2005

AbstractThe directionally weighted steric substituent constant Ωjk was introduced to evaluate the anisotropic steric hindrance effect to the reaction centre. The correlation of the rates with a series of directionally weighted Ωjk is useful for characterizing the feature of steric effect. The difference between the hydrolyses of carboxylic esters and carboxamides was shown by this method.
Influence of intramolecular hydrogen bonding on the conformational equilibrium of cis‐3‐N,N‐dimethylaminocyclohexanol compared with trans‐3‐N,N‐dimethylaminocyclohexanol and cis‐ and trans‐3‐N,N‐dimethylamino‐1‐methoxycyclohexane
Authors: de Oliveira P. R. , Ribeiro D. S. , Rittner R. 
Published: January 2005

Abstract1H NMR data show that concentration increase shifts the conformational equilibrium of cis‐3‐N,N‐dimethylaminocyclohexanol (1) (cis‐3‐DACH) from the 1aa conformer, stabilized by an intramolecular hydrogen bond (IAHB), to the 1ee conformer [43% (0.01 M) to 70% (0.40 m), in CCl4], which can form intermolecular hydrogen bonds (IEHB). The percentage of 1ee conformer also increases with the solvent basicity from 36% in C6D12 to 89% in DMSO. The conformational equilibrium of the trans isomer (trans‐3‐DACH) is also dependent on concentration, since 1ae increases from 77% (0.05 M) to 84% (0.40 M) in CCl4 but not with the solvent polarity. The occurrence of an IAHB in cis‐3‐DACH was confirmed by the study of a model compound, cis‐3‐N,N‐dimethylamino‐1‐methoxycyclohexane (2) (cis‐3‐DAMCH), lacking an OH group and presenting a single conformer 2ee (∼95%). The corresponding trans isomer (trans‐3‐DAMCH) behaves similarly to trans‐3‐DACH, since the 2ae conformer occurs as ∼83%, in the studied solvents. The PCMODEL program gave very good coupling constant values for the qualitative analysis of energy changes in the study of concentration and solvent effects, since the energy values obtained for cis and trans isomers of 3‐DACH were in good agreement with the theoretically calculated [B3LYP/6–311+g(d,p) level] values. The averaged calculated energy of the IAHB, for conformer 1aa of cis‐3‐DACH, with optimization of the reference structure and at several levels of theory, is 5.74 kcal mol−1 (1 kcal = 4.184 kJ). Copyright © 2005 John Wiley & Sons, Ltd.
Atom transfer radical polymerization of 6‐O‐methacryloyl‐1,2;3,4‐di‐O‐isopropylidene‐D‐galactopyranose in solution
Authors: Meng Jian‐Qiang , Du Fu‐Sheng , Liu Ya‐Shu , Li Zi‐Chen 
Published: January 2005

AbstractA detailed exploration of the atom transfer radical polymerization (ATRP) of a sugar‐carrying monomer, 6‐O‐methacryloyl‐1,2;3,4‐di‐O‐isopropylidene‐D‐galactopyranose (MAIPGal) was performed. The factors pertinent to ATRP, such as initiators, ligands, catalysts, and temperature were optimized to obtain good control over the polymerization. The kinetics were examined in detail when the polymerization was initiated by methyl 2‐bromoisopropionate (2‐MBP), ethyl 2‐bromoisobutyrate (2‐EBiB), or a macroinitiator, [α‐(2‐bromoisobutyrylate)‐ω‐methyl PEO] (PEO–Br), with bipyridine (bipy) as the ligand at 60 °C or by 2‐EiBB with N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) as the ligand at room temperature (23 °C). The effects of the catalysts (CuBr and CuCl) were also investigated. We demonstrate that the successful ATRP of MAIPGal can be achieved for 2‐EBiB/CuBr/bipy and 2‐MBP/CuCl/bipy at 60 °C and for 2‐EBiB/CuBr/PMDETA at room temperature. The initiation by 2‐EBiB at room temperature with PMDETA as the ligand should be the most optimum operation for its moderate condition and suppression of many side reactions. Chain extension of P(MAIPGal) prepared by ATRP with methyl methacrylate (MMA) as the second monomer was carried out and a diblock copolymer, P(MAIPGal)‐b‐PMMA, was obtained. Functional polymers, poly(D‐galactose 6‐methacrylate) (PGMA), PEO‐b‐PGMA, and PGMA‐b‐PMMA were obtained after removal of the protecting groups. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 752–762, 2005
Luminescent polymer containing Eu chelates with different neutral ligands
Authors: Liang Hao , Chen Biao , Guo Fuquan , Guan Junbo , Zhang Qijin , Li Zhenzhong 
Published: January 2005

AbstractThe luminescence properties of Eu chelates with different neutral ligands in poly(methyl methacrylate) (PMMA) have been investigated and the influence of the neutral ligand was analyzed. According to the fluorescence emission spectra, the Judd–Ofelt parameters Ω2 and Ω4 of Eu chelates containing PMMA have been calculated and the radiative properties are also presented. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Große Spannweiten
Authors: Plieninger Sven 
Published: January 2005

AbstractIm nachfolgenden Artikel soll über eine Reihe von Bauwerken berichtet werden, die aufgrund ihrer Nutzungsanforderungen große Spannweiten aufweisen.Immer sind große Flächen stützenfrei zu überspannen und damit die Aufgabenstellung ähnlich, und dennoch entsteht eine große Bandbreite an Tragwerkslösungen, die dieses Themengebiet für uns Ingenieure so spannend macht.Bei den vorgestellten Bauten handelt es sich um Projekte, die in den vergangenen Jahren für Messenutzung (Bild 1) sowie den Bau und Betrieb von Flugzeugen entstanden sind oder sich derzeit in der Planung und Umsetzung durch unser Büro befinden.
Beitrag zur Optimierung von Hängeranschlüssen stählerner Stabbogenbrücken
Authors: Teich Stephan , Graße Wolfgang 
Published: January 2005

AbstractHänger von Stabbogenbrücken sind häufig veränderlichen Beanspruchungen infolge Verkehrslasten ausgesetzt. In Abhängigkeit von Spannungsspielzahl und Spannungsniveau kann deshalb die Gefahr des Ermüdungsbruchs bestehen.Als maßgebend sind in dieser Hinsicht die Anschlüsse der Hängerstangen an Bogen bzw. Versteifungsträger anzusehen, da sich hier durch Geometrieänderung und durch die Kerbwirkung der Anschlußschweißnähte ein ungleichförmiger Spannungszustand einstellt. Die maximale Spannungsspitze im Kerbgrund wird durch die Tiefe und Schärfe der Kerben bestimmt. Um eine ausreichende Ermüdungssicherheit zu gewährleisten, ist es notwendig, diese rißerzeugenden Spannungsspitzen zu minimieren und möglichst einen homogenen Spannungsverlauf über das gesamte Bauteil zu erzeugen. Mit Hilfe einer parametergestützten Gestaltoptimierung und anschließender Topologieanpassung soll die Gefahr von Ermüdungsbrüchen vermindert werden. Für die unter diesen Gesichtspunkten entworfene Anschlußkonstruktion wird die Kerbwirkung, ausgedrückt durch die Kerbwirkungszahl Kf, mittels FE‐Analyse ermittelt. Abschließend wird eine analytische Formel für Kf entwickelt, welche die Anwendung des Kerbspannungskonzeptes für Hängeranschlüsse ermöglicht.
20 Jahre Entwicklung von solarthermischen Kraftwerken – Warum benötigen wir dringend die Nutzung solarer Energiequellen?
Authors: Schiel Wolfgang , Bergermann Rudolf 
Published: January 2005

AbstractSolarthermische Kraftwerke müssen schon bald einen wesentlichen Beitrag zum drastisch steigenden Energiebedarf leisten. Sie können dies auch, denn die Entwicklung der verschiedenen Technologien ist sehr weit fortgeschritten. Der letzte aufwendige Schritt der großtechnischen Erprobung steht aber noch aus.Der Beitrag beschreibt die Kraftwerkskonzepte, an deren Entwicklung Schlaich Bergermann und Partner wesentlichen Anteil hat: das Aufwindkraftwerk, das Dish‐Stirling System zur dezentralen Stromerzeugung, die europäische Version der Parabolrinnen‐Kraftwerke sowie das Turm‐Solarkraftwerk, für das neue Heliostaten entwickelt wurden.
Weit und krumm – Zum Entwurf gekrümmter, seilgestützter Fußgängerbrücken
Authors: Keil Andreas 
Published: January 2005

AbstractFußgängerbrücken sind besondere Strukturen. An sie werden weniger restriktive Anforderungen als an die großen Brücken für Straße und Eisenbahn gestellt.Dies ermöglicht gekrümmte Grundrißverläufe, die sich aus der geometrischen Forderung nach einer harmonischeren Anbindung an bestehende Wegenetze und nach flüssigeren Verkehrsführungen ergeben. Insbesondere bei größeren, für Seiltragwerke prädestinierten Spannweiten ist es möglich, diese Krümmung für effiziente Tragstrukturen zu nutzen, ohne auf die Leichtigkeit und Transparenz von Seilbrücken verzichten zu müssen. Interessante und spannende Tragwerke sind möglich, und mit der dritten Dimension erhalten diese Brücken eine eigene, räumliche Dynamik.