bluuebunny/crossref_metadata_2025 huggingface dataset:
Solvation and metal ion effects on structure and reactivity of phosphoryl compounds. 2. β,γ‐Substituted alkylphosphonic esters
Authors: Belciug Marie‐P. , Modro Tom A. , Wessels Philippus L.
Published: January 2005

NMR spectroscopic and conformation analyses ware carried out for five dimethyl β,γ‐disubstituted propylphosphonates, YCH2CHXCH2PO3Me2, in five solvents and in acetone containing sodium and magnesium ions. Conformational preferences observed for the rotation about the C1C2 bond are determined by the attractive interactions between the oxygen containing substituents X (X = OH, OMe) and the phosphoryl group, and are enhanced by the metal ions, presumably via chelation effects. The rotation about the C2C3 bond yields statistical distribution of the rotamers. For Y = benzoyl, there was no evidence for a competition of the carbonyl group for the intramolecular interactions with X or with a metal ion.
Conformational properties of disulphide bridges. 1. CS Rotational potential in ethyl hydrodisulphide
Authors: Görbitz Carl Henrik
Published: January 2005

The potential for CS rotation in ethyl hydrodisulphide, CH3CH2SSH, has been studied with ab initio methods. All stationary point structures were fully optimized at the HF/6–31G* and MP2/6–31G* levels. MP2/6–31G* molecular geometries were used in subsequent single‐point energy calculations with several basis sets and including various amounts of electron correlation up to the MP4SDQ/6–311+G(2d,p) level of theory. Zero‐point vibrational energies and thermal corrections were calculated and used to obtain relative values for ΔH0 and ΔH298. The stability order for the three energy minima is gauche − ⩾ gauche + < trans (positive disulphide chirality, CSSH torsion ≈ 89°). The results have implications for force field calculations on disulphide bridge conformations in peptides and proteins.
Hydrogen bonding. 31. Construction of a scale of solute effective or summation hydrogen‐bond basicity
Authors: Abraham Michael H.
Published: January 2005

The β scale of solute hydrogen‐bond basicity, formulated from 1:1 hydrogen‐bond complexation constants in tetrachloromethane, has been used to set up a scale of effective or summation hydrogen‐bond basicity, appropriate for the situation in which a solute is surrounded by solvent molecules. The method is based on the equation,

where SP is, in this work, a set of solute water–solvent partition coefficients in a given system. The explanatory variables are solute parameters as follows: R2 is an excess molar refraction, π is the solute dipolarity/polarizability, Σα and Σβ2 are the effective solute hydrogen‐bond acidity and basicity and Vx is McGowan's characteristic volume. Various equations are established using β in the equation, and then amended β values are back‐calculated and new Σβ values obtained. It is found that for most solutes, the effective basicity Σβ is invariant over the systems used to within an experimental error of around 0·03 units. About 350 Σβ values obtained from two or more experimental log P values are listed, together with values for homologous series and a number of singly determined values. For some specific solutes, such as sulphoxides, alkylanilines and alkylpyridines, Σβ2 is not constant, and an additional solute basicity denoted as Σβ is needed in order to deal with partitions from water to solvents that are partially miscible with water, such as isobutanol and octanol. Values of Σβ, and where possible Σβ also, are listed for 80 additional solutes.
Low temperature enol–enol association of stable 2,2‐diarylethenols
Authors: Eventova Irina , Nadler Ella B. , Frey Joseph , Rappoport Zvi
Published: January 2005

The low temperature 1H NMR spectra of 2,2‐bis(3,5‐dibromomesityl)ethenol in CS2–CD2Cl2 (3:7) display new signals which indicate the presence of four enol species whose OH is hydrogen bonded. Oligomerization to intermolecularly hydrogen‐bonded enol dimers or tetramers is suggested.
Protonation studies on isomeric tetrafluoro‐2,11‐dithia[3·3]cyclophanes, tetrafluoro[2·2]metaparacyclophane and their corresponding non‐fluorinated analogs. Comparison with fluorinated and non‐fluorinated[2·2]cyclophanes: A stable ion and semi‐empirical (AM1, PM3) investigation
Authors: Laali Kenneth K. , Houser John J. , Filler Robert , Kong Zhengrong
Published: January 2005

Protonation of tetrafluoro‐2,11‐dithia [3·3]paracyclophane and tetrafluoro‐2,11‐dithia [3·3]metaparacyclophane in high‐acidity super acid media, namely FSO3H.SbF5 (1:1) ‘magic acid’–SO2CIF, gave their corresponding acidic disulfonium ions. Additional ring protonation to give a disulfonium–monoarenium trication was not observed. With the non‐fluorinated 2,11‐dithia [3·3] cyclophanes, the disulfonium ions can be ring protonated in equilibrium to give a dynamic disulfonium–monoarenium trication. Tetrafluoro [2·2]‐metaparacyclophane is monoprotonated at the meta ring and gives a complex mixture of conformational isomers. Multinuclear magnetic resonance data on the cyclophane precursors and their derived cations are compared and analysed. The energies, conformations and charge distributions of the isometric fluorinated and non‐fluorinated [2·2] ‐ and dithia [3·3] cyclophanes were calculated by the AM1 and PM3 methods, respectively. In all but one case the cyclophane arenium ions predicted by theory to be energetically most favoured are those observed in solution under stable ion conditions. In agreement with experiment, the instabilities of S,S,C‐cyclophane trications are also theoretically predicted.
Study of the association of 2‐oxoindolines in carbon tetrachloride solutions by IR spectra, dipole moments and average molecular weight measurements
Authors: Koll Aleksander , Rospenk Maria , Stefaniak Lech , Wójcik Jacek
Published: January 2005

Dipole moments, average molecular weights and IR spectra of 2‐oxoidoline (I), 2‐oxo‐3‐methylindoline (II) and 2‐oxo‐3,3‐dimenthylindoline (III) and their N‐methyl‐substituted analogues were studied in CCl4 solutions at various concentrations. It was shown that the molecules with an NB bond associate exclusively to cyclic dimers. The values of the association constants (K) were determined by IR spectrometry. Other methods gave too broad intervals of possible K values. The spectroscopic association constants of compounds I–III are very similar, 380, 350 and 370 dm3 mol−1, respectively. The vs(NH) band shift on association is 275 cm−1.
Cryptate acidity scales. Solvent polarity effect on ion‐pair and free ion acidity of organic compounds
Authors: Antipin Igor S. , Gareyev Roustam F. , Vedernikov Andrey N. , Konovalov Alexander I.
Published: January 2005

[2.1.1] Cryptated lithium salts of a wide range of CH‐acids were studied spectrophotometrically and conductimetrically in solvents of low polarity. Such salts exist in media of different dielectric constants in the form of cryptand‐separated ion pairs which are identical in their spectral and conductance properties with common solvent‐separated ion pairs. No formation of higher ionic clusters was observed for the studied ions at concentrations below 10−3 mol−1 even in the benzene. The relative equilibrium acidities in benzene, N‐methylmorpholine and tetrahydrofuran for a number of CH‐acids are reported. Solvent polarity does not influence the cryptate ion‐pair acidity of the investigated compounds. The relative free ion acidities were calculated for some of the studied acids in different solvents by combination of the ion‐pair acidity values with the conductimetric data on dissociation constants of the cryptate ion pairs. A linear relationship between the free ion acidity and the reciprocal of the dielectric constant was found. An extrapolation method is suggested for the calculation of gas‐phase acidity from the data obtained in solution.
Mechanism of the hydride transfer reaction of leuco crystal violet with cyanomethylene acceptors
Authors: Nishimura Norio , Zaman Khan M. , Yamamoto Shunzo
Published: January 2005

In the hydride transfer of Leuco Crystal Violet to form the Crystal Violet, cation, the role of cyanomethylene acceptors was found to be essentially different from that of p‐benzoquinones, both previously believed to act as π‐acceptors in the same manner.
Competition between decarboxylation and isomerization in the C3H5O energy surface. Justification of the experimental results by molecular orbital calculations on the solvated ions
Authors: Rajadell Fernando , Planelles Josep , Tomás Francisco , Asensio Gregorio , Miranda Miguel A. , Sabater María J.
Published: January 2005

In constrast with recent molecular orbital calculations on the decarboxylation of O‐protonated 2‐oxetanone, this experimental work indicates that no decarboxylation of this cation occurs in sulphuric acid solution up to 150°C, but instead a clean isomerization to protonated acrylic acid takes place. Parallel theoretical work shows that the gas‐phase model is too crude to account successfully for the experimental facts obtained in acidic media. However, the latter are well reproduced when the effect of the solvent is taken into account. The present findings do not necessarily invalidate the reaction mechanism currently accepted to explain the rate enhancement and change of stereochemistry accompanying the decarboxylation of 3,4‐disubstituted 2‐oxetanones under acid catalysis.
Electronic structure of the disilenyl radical anion
Authors: Schoeller Wolfgang W. , Busch Thilo
Published: January 2005

According to quantum chemical calculations, the anion radical of disilene adopts a syn‐ or anti‐pyramidal geometry. The former is not a stable entity on the electronic hypersurface and it decreases further in energy by rotation of the Si–Si bond. Energy optimization of various structural alternatives reveals for the anion radical an Si–Si bond slightly longer than a single bond. The anti geometry prefers a C2h geometry whereas the syn geometry resembles a silylene coupled with a silylene radical anion. The fragmentation of disilene into two silylene units is less exothermic for the neutral species than for the anion (radical), as examined by the ab initio calculations.
Newer aspects of electron transfer in electrophilic Aromatic Nitration. Radical pairs as reactive intermediates by time‐resolved spectroscopy
Authors: Bockman T. Michael , Kochi Jay K.
Published: January 2005

Aromatic nitration is effectively carried out by the deliberate irradiation of the charge‐transfer absorption band (hvCT) of the transient electron donor–acceptor complex [ArH, NO2Y+], where NO2Y+represents common nitrating agents. Time‐resolved spectroscopy demonstrates that the charge‐transfer nitration effected in this way proceeds via an electron‐transfer mechanism, in which the collapse of the reactive triad, [ArH+ ·, NO, Y] is critical to the formation of the Wheland intermediate. Comparative product analyses of toluene and anisole nitrations together with the ‘non‐conventional’ products arising from NO+catalysis,ipsoattack and addition/elimination show that the activation process leading to charge‐transfer (photochemical) nitration is indistinguishable from that leading to electrophilic (thermal) aromatic nitration.
Kinetic studies on the aminolysis of 1‐(trimethylsilyl)ethyl arenesulphonates in acetonitrile and methanol
Authors: Oh Hyuck Keun , Shin Chul Ho , Park Hyoung Yeon , Lee Ikchoon
Published: January 2005

Nucleophilic substitution reactions of 1‐(trimethylsilyl)ethyl arenesulphonates with anilines and benzylamines in acetonitrile and methanol at 65·0°C were studied. The cross‐interaction constants, ρXZ, between substituents in the nucleophile (X) and leaving group (Z) are relatively small (0·10 for XC6H4NH2 in MeCN) but similar to those for other SN2 processes at a secondary carbon atom. This provides further evidence for an approximately constant, loose SN2 transition state at a secondary carbon regardless of the size of the Cα substituent. The transition‐state variations with substituents X and Z are in accord with that expected from the positive ρXZ value observed: a stronger nucleophile and/or nucleofuge leads to an earlier transition state, i.e. a lower degree of bond making and breaking.
Kinetics and mechanism of the imidazole‐catalysed hydrolysis of substituted N‐benzoylimidazoles
Authors: Seoud Omar A. El , Menegheli Paulo , Pires Paulo A. R. , Kiyan Nilo Z.
Published: January 2005

Imidazole (Imz)‐catalysed hydrolysis of benzoate esters proceeds via the intermediate formation of N‐benzoylimidazoles. This paper considers the second step of this reaction, viz., Imz‐catalysed hydrolysis of N‐(4‐X‐benzoyl)imidazoles, X = CH3, H, Cl, CN and NO2, and N‐(disubstituted benzoyl)imidazoles, 2‐chloro‐4‐nitro, 2, 4‐dinitro and 3,5‐dinitro, in water–acetonitrile mixtures (10% or 14%, v/v, in organic solvent). On the basis of catalytic rate constants and the kinetic solvent isotope effect, it is shown that catalysis by Imz is of the general‐base type. Unexpectedly, the hydrolysis of N‐(2,4‐dinitrobenzoyl)imidazole was found to be slower than that of N‐(4‐nitrobenzoyl)imidazole. It is shown that this reactivity order is due to a combination of a steric effect and stabilization of the reactant state due to a donor–acceptor interaction between the Imz moiety and the 2,4‐dinitrophenyl ring.
Micellar catalysis of organic reactions. Part 36. Nucleophilic aromatic substitution reactions in hydroxy functionalized micelles with bulky head groups
Authors: Broxton Trevor. J. , Lucas Mathew
Published: January 2005

The reaction of several nitro activated aromatic halides with hydroxide ions was studied in the presence of hydroxy functionalized micelles containing bulky head groups, e.g. C16H33N+R2CH2CH2OH Br−, where R = Me, Et, Bu. In a biphasic reaction, the aryl halide is first converted into an aryl mincellar ether which subsequently reacts with hydroxide ions to form the phenolic product. Despite the increased nucleophilicity of hydroxide ions as water is squeezed away from the micelle surface by the bulky head groups, no direct reaction of the aromatic substrate with hydroxide ion is detectable. In the second phase of reaction, the breakdown of the aryl micellar ether to form the phenolic product, the order of reactivity in the different micelles is dependent on the steric interactions between substituents ortho to the reaction centre and the head group of the micelle. For compounds having one substituent ortho to the reaction centre, the order of reactivity is Bu > Me > Et, whereas for 2‐chloro‐1,3‐dinitrobenzene, which has two substituents ortho to the reaction centre, the order is Me > Et > Bu.
Bimolecular nucleophilic substitution at a resonance‐stabilized carbenium ion: Elevated value of cross‐interaction constant, imbalanced transition state and the non‐interactive phenomenon
Authors: Lee Ikchoon
Published: January 2005

A characteristic feature in the bimolecular nucleophilic substitution reactions at a resonance‐stabilized carbenium ion has been shown to be an imbalanced transition state resulting in an elevated value of the cross‐interaction constant, which in turn leads to a non‐interactive phenomenon. Examples of the reactions of ring‐substituted phenylethyl and benzhydryl cations are given.
Solvolyses of 4‐methylene‐2ax‐ and ‐2eq‐adamantyl p‐toluenesulphonates: Intermediacy of classical carbocation in the axial and π‐bridged carbocation in the equatorial ptoluenesulphonate solvolyses
Authors: Takeuchi Ken'ichi , Kurihara Yumiko , Okazaki Takao , Kitagawa Toshikazu , Kinoshita Tomomi
Published: January 2005

The rates and products of solvolyses of 4‐methylene‐2ax‐ and ‐2eq‐adamantyl p‐toluenesulphonates (tosylates) (4a‐OTs and 4e‐OTs, respectively) were studied. Compound 4a‐OTs solvolysed more slowly than 2‐adamantyl tosylate (1) in methanol and 2,2,2‐trifluoroethanol (TFE) by factors of 2·3 and 2·5, respectively, at 25°C. However, by taking the inductive decelerating effect of a β‐methylene substituent into account, the rates were revealed to be enhanced by α‐participation by a factor of 50. The products of solvolyses of 4a‐OTs in methanol, 80% acetone and TFE at 100°C were 2ax‐ and 2eq‐alkoxy(or hydroxy)‐4‐methyleneadamantanes (4a‐OR and 4e‐OR, respectively), exo‐4‐alkoxy(or hydroxy)‐5‐methyleneprotoadamantane (exo‐5‐OR) and 5‐[alkoxy(or hydroxy)methyl]‐4‐protoadamantene (6‐OR) with adamantyl to protoadamantyl product ratio of 39:61 (in methanol), 56:44 (in 80% acetone) and 71:29 (in TFE). Despite the nearly symmetric nature of the intermediate cation, the 4a‐OR: 4e‐OR product ratio was essentially constant with 83:17 (in methanol), 85:15 (in 80% acetone) and 82:18 (in TFE). The formation of considerable amounts of 4e‐OR was interpreted as showing the intermediacy of a pair of rapidly equilibrating classical ions. The rates of 4e‐OTs were 2300–4300 times faster than those expected from inductive electron‐withdrawing effect of a β‐methylene substituent. The major product (84·5% in methanolysis and 98·7% in trifluoroethanolysis) was 4e‐OR accompanied by small amounts of 2‐alkoxy‐2,4‐methanoadamantane (9‐OR) and 2‐(alkoxymethyl)‐2,4‐didehydroadamantane (10‐OR), no formation of 4a‐OR having been observed. These results suggested that 4e‐OTs solvolyses via a π‐bridged intermediate cation.
Determination of complex stabilities with nearly insoluble host molecules. Part I. Spectrophotometric detection
Authors: Buschmann Hans‐Jürgen , Cleve Ernst , Denter Ulrike , Schollmeyer Eckhard
Published: January 2005

The determination of stability constants by measuring the increase in solubility of a nearly insoluble host molecule due to complex formation with a soluble guest is discussed. If the host molecule absorbs in the UV–visible region, spectrophotometric measurements are easily performed to obtain the stability constants. The solubilities of the crown ether dibenzo‐18‐crown‐6 and of the cryptand (222BB) in aqueous solutions and the molar absorptivities of these ligands and their complexes with alkali and alkaline earth metal cations were determined. The increase in solubility of these ligands due to complex formation with cations was used to calculate the stabilities of the complexes formed. Even under the assumption that neither the molar absorptivities nor the solubilities are known, the calculated stability constants agree very well with those obtained using the measured values of the molar absorptivities and solubilities. The accuracy of the stability constants even increases with decreasing solubility of the ligands.
SINDO1 study of the photochemical synthesis of azacyclobutadiene from 1,4‐oxazin‐2‐one
Authors: Neumann Frank , Jug Karl
Published: January 2005

The mechanism of the photochemical decomposition of 1,4‐oxazin‐2‐one to azacyclobutadiene and carbon dioxide was investigated by the semi‐empirical MO method SINDO1. The relevant singlet and low‐lying triplet potential energy hypersurfaces were studied and intermediates and transition structures were optimized with limited configuration interaction. The first step transforms 1,4‐oxazin‐2‐one into a bicyclic lactone via internal cyclization. Then a stepwise bond breaking mechanism on the first excited triplet state yields azacyclobutadiene and CO2, whereas a reaction on the first excited singlet state is not common. It is shown that a simultaneous bond‐breaking mechanism on the first excited singlet or on the first excited triplet is possible.
Calorimetric study of the anomeric effect in 2‐carboethoxy‐1,3‐dithianes
Authors: Torres Luis Alfonso , Rojas Aarón , Cuevas Gabriel , Juaristi Eusebio
Published: January 2005

The enthalpic contribution to the anomeric effect in r‐2‐carboethoxy‐trans‐4,trans‐6‐ and r‐2‐carboethoxy‐cis‐4,cis‐6‐dimethyl‐1,3‐dithianes was determined by reaction–solution calorimetry. The enthalpy of solution of both isomers in pure p‐dioxane and the enthalpy of solution and isomerization in the same solvent and in the presence of trifluoroacetic acid were experimentally measured. From these results the corresponding enthalpies of isomerization in solution were calculated, and were found to be ΔHax→eq = −0·16 ± 0·04 kcal mol−1 (−0·67 ± 0·18 kJ mol−1) and ΔSax→eq = −2·68 ± 0·1 cal K−1 mol−1 (−11·2 ± 0·4 J K−1 mol−1). The slightly negative ΔH term nevertheless reflects a substantial anomeric effect owing to the countervailing steric effects in the axial isomer. The significant entropy loss in the equatorial isomer was explained in terms of intramolecular electrostatic effects. The results are in agreement with those obtained from NMR studies of the conformational behaviour of 2‐carboethoxy‐5‐methyl‐5‐aza‐1,3‐dithiacyclohexane.
Reactions of iminodimagnesium reagents with 1,4‐quinones: Their structural factors governing the modes of reactions
Authors: Matsuo Koji , Shiraki Ryuji , Okubo Masao
Published: January 2005

Reactions of benzo‐, naphtho‐ and anthraquinone derivatives (unsubstituted and substituted) with aryliminodimagnesium [ArN(MgBr)2] and aryloxymagnesium of a weak electron‐donating ability were studied. In addition to the reduction products (quinhydrones and hydroquinones), nuclear substitution and condensation
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products were formed. The efficiency of single electron transfer (SET) from ArN(MgBr)2, evaluated by the relative values of the difference between the oxidation and reduction potentials of the reactants (ΔE = Eox – Ered), varies with the electron‐accepting power of quinones. ΔE governs the modes of semiquinone appearance (ESR signals) and the relative amounts of the heat of reactions, reflecting the types and yields of the products. It is concluded that condensation products of both components are produced in the reactions of quinones with the lower SET efficiency by the presence of a fused benzene ring or MeO substituent. The structure–reactivity relationship proposed previously for reactions of various magnesium reagents is extended in the reactions with a variety of quinones even in cases of higher SET efficiency.
Competing solvolytic substitution and elimination reactions via a common irreversibly formed ion–molecule pair intermediate
Authors: Sidhu Harvinder , Thibblin Alf
Published: January 2005

The acid‐catalysed solvolysis of 9‐(2‐phenoxy‐2‐propyl)fluorene in mixtures of water with acetonitrile or methanol at 25°C provides 9‐(2‐hydroxy‐2‐propyl)fluorene, 9‐(2‐propenyl)fluorene, and 9‐(2‐acetamido‐2‐propyl)fluorene or 9‐(2‐methoxy‐2‐propyl)fluorene, respectively. The overall kinetic deuterium isotope effects for the reactions of the hexadeuterated analogue 9‐(1,1,1,3,3,3‐2H6)‐9‐(2‐phenoxy‐2‐propyl)fluorene in 90 vol.% acetonitrile in water were measured as (k + k)/(k + k) = 1·54 ± 0·05, which is composed of the isotope effect k/k = 1·4 ± 0·1 for formation of the substitution products and k/k = 4·0 ± 0·2 for production of 9‐(2‐methoxy‐2‐propyl)fluorene. Similar isotope effects were measured in other solvent mixtures. The results strongly indicate a branched mechanism involving rate‐limiting formation of a common carbocation–molecule pair (with a secondary isotope effect of 1·54), which either undergoes nucleophilic attack by a solvent molecule (with a secondary isotope effect of ca 1), or is dehydronated (isotope effect ca 2·8) by the leaving group or by the solvent. The ion‐molecule pair shows very low selectivities. Thus, in 50 vol.% acetonitrile in water, an acetonitrile molecule is as efficient as a water molecule as a nucleophile towards the ion–molecule pair, kMeCN/kHOH ≥ 1 (ratio of second‐order rate constants). The discrimination between methanol and water is anomalously small, kMeOH/kHOH = 0·7.
Reactions of 1,3‐dihaloadamantanes with diphenylphosphide ions by the SRN1 mechanism. Competition between intermolecular and intramolecular electron transfer reactions
Authors: Lukach Andres E. , Santiago Ana. N. , Rossi Roberto A.
Published: January 2005

The reactivity of 1,3‐dihaloadamantanes with diphenylphosphide ions (Ph2P−) in liquid ammonia was studied. 1,3‐Dichloroadamantane (1a), 1‐bromo‐3‐chloroadamantane (1b) and 1,3‐dibromoadamantane (1c) reacted with Ph2P− ions under photostimulation by the SRN1 mechanism. Irradiation of 1c without Ph2P− ions gave no reaction (<5%). Three products were found: 1,3‐bis(diphenyphosphinyl)adamantane (2), (3‐X‐1‐adamantyl)diphenylphosphine (X = Cl, 3a; X = Br, 3b) and 1‐adamantyldiphenylphosphine (4). Compounds 2 and 4 were formed by intramolecular electron transfer (ET) of the radical anion 3a−· (3b−·); whereas 3a (3b) were formed by intermolecular ET of this radical anion to the substrates. It was observed that the product distribution depends on the substrate and reaction conditions.
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

Substitutent 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

Over 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

Transition 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

Slight 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

Conformational 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

Titration 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

Base‐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

Measured 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

1,3‐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

An 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

Equilibrium 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

Interactions 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

Irradiation 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

Peroxidation 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

The 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

1H 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

A 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

The 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

Im 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

Hänger 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

Solarthermische 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

Fußgängerbrücken 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.