bluuebunny/crossref_metadata_2025_split huggingface dataset:
Conformational properties of disulphide bridges. 1. CS Rotational potential in ethyl hydrodisulphide
Authors: Görbitz Carl Henrik 
Published: January 2005

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractProtonation 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

AbstractDipole 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

Abstract[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

AbstractIn 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

AbstractIn 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

AbstractAccording 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

AbstractAromatic 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

AbstractNucleophilic 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

AbstractImidazole (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

AbstractThe 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractReactions 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

AbstractThe 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

AbstractThe 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.