bluuebunny/crossref_metadata_2025_split huggingface dataset:
Kinetic ESR study on the reaction of vitamin E radical with vitamin C and its lipophilic derivatives in cetyltrimethylammonium bromide micelles
Authors: Liu Zhong‐Li , Han Zheng‐Xu , Yu Kai‐Chao , Zhang Ya‐Li , Liu You‐Cheng 
Published: January 2005

AbstractThe reaction between α‐tocopheroxyl radical (VE˙) and ascorbic acid (VC) and its lipophilic derivatives ascorbyl‐6‐caprylate (VC‐8), 6‐laurate (VC‐12) and 6‐palmitate (VC‐16) was studied by stopped‐flow ESR spectroscopy in cetyltrimethylammonium bromide (CTAB) micelles. The second‐order rate constants for the reaction were found to be 9 · 0, 3 · 0, 0 · 7 and 0 · 03 × 105 1 mol−1 s−1 for VC, VC‐8, VC‐12 and VC‐16, respectively, indicating a remarkable influence of the aliphatic side‐chain on the reactivity. The lifetime of the reaction intermediate, ascorbate radical anion, was greatly enhanced by the lipophilic side‐chain, being 0 · 4, 5 and 110 s for VC‐8−., VC‐12−. and VC‐16−. respectively. Kinetic analysis shows that the inter‐ and intramicellar diffusion may be the rate‐limiting steps for the reaction carried out in micelles.
Comparative study of aromaticity in five‐membered rings containing S, SO and SO2 groups
Authors: Rozas Isabel 
Published: January 2005

AbstractOwing to the importance of the concept of aromaticity, different indices have been developed to try to quantify this property. The possible π delocalization through an XSOnX group (X = C, N; n = 0, 1 or 2) could explain heteroaromaticity in rings containing the moiety. For that reason, the aromaticity of five‐membered sulphurcontaining rings with different oxidation numbers (S, SO and SO2) and including no (thiophene), one (isothiazole) or two (1,2,5‐thiadiazole) adjacent atoms was investigated. Ab initio calculations were carried out to determine geometrical parameters (planarity of the ring, bond length and bond order), electronic structure (charge distribution and dipolar moment) and the participation of d‐orbitals of sulphur. According to these calculations, only compounds with S(II) can be considered to be aromatic, whereas compounds bearing S(IV) or S(VI) are better described as ylides.
Chemiexcitation of an anthraquinone derivative in the thermal cycloreversion of a dewar‐anthraquinone derivative
Authors: Miki Sadao , Kagawa Hiroyuki , Yoshida Zen‐Ichi 
Published: January 2005

AbstractDewar‐anthraquinone (1a) and 1,2,3‐tri‐tert‐butyl‐5,8‐Dewar‐anthraquinone (1b) were synthesized. Thermodynamic parameters for their cycloreversion to the corresponding anthraquinones indicate that thermal population of the excited states of the products via non‐adiabatic valence isomerization is energetically feasible. On thermolysis of 1b, the formation of a detectable amount of 1,2,3‐tri‐tert‐butyl‐1,4‐Dewar‐anthraquinone (3) was observed. The excited singlet state of 1,2,3‐tri‐tert‐butylanthraquinone (2b) produced via the non‐adiabatic process is responsible for the formation of 3.
Cyclopropylmethoxycarbene: A kinetic limit on the 1,2‐carbon migration
Authors: Moss Robert A. , Jang Eun G. , Fan Hong , Włostowski Marek , Krogh‐Jespersen Karsten 
Published: January 2005

AbstractCyclopropylmethoxycarbene undergoes ambiphilic‐nucleophilic intermolecular reaction with alkenes and methanol, but its intramolecular chemistry (1,2‐carbon migration) is suppressed (k < 3 × 103 s−1) by the α‐methoxy substituent.
Hammett reaction constants for irreversible electroreduction of iodobenzenes in non‐aqueous solvents
Authors: Jaworski Jan S. , Kacperczyk Anna , Kalinowski Marek K. 
Published: January 2005

AbstractPolarographic half‐wave potentials for the electrochemical reduction of a series of para‐substituted iodobenzenes in acetonitrile, acetone, benzonitrile, dimethylformamide, dimethyl sulphoxide and methanol obey Hammett equations and the reaction constant found, ρ = 0·36 ± 0·02, is independent of the solvent used. The results obtained are discussed on the basis of a mechanism of the electrode process.
Laser flash photolysis study of 10, 10‐dimethyl‐9‐anthrylidene, a carbene with nearly degenerate singlet and triplet states
Authors: Admasu Atnaf S. , Platz Matthew S. 
Published: January 2005

Abstract10, 10‐Dimethyl‐9‐anthrylidene (DMA) was studied by laser flash photolysis (LFP) techniques. LFP of the requisite diazo precursor in pentane or carbon tetrachioride produced transient spectra attributed to the hydrocarbon and monochlorinated diarylmethyl radicals, respectively. In the presence of pyridine and oxygen, ylides were formed upon LFP of 10, 10‐dimethyl‐9‐diazoanthrone. However, ylides are not formed in acetonifrile or acetone. The carbene itself was not detected by transient absorption spectroscopy. The absolute rate constants for reaction of DMA with pyridine, methanol and triethylsilane are 1·96 × 109, 1·8 × 1010, and 1·0 × 108 I mol−1 s−1, respectively. It is concluded that the singlet and triplet states of DMA are essentially degenerate.
Solution and solid‐state aggregation properties of 4‐mercaptopyridine‐4‐thiopyridone
Authors: Etter Margaret C. , MacDonald John C. , Wanke Randall A. 
Published: January 2005

AbstractThe hydrogen‐bond pattern of 4‐mercaptopyridine‐4‐thiopyridone in solution and in the solid state consists of extended chains of 4‐thiopyridone associated through NH…S hydrogen bonds. The solution structure in chloroform was studied by UV–visible spectrometry and vapor pressure osmometry. A tautomerization constant of 1 · 8, favoring 4‐thiopyridone, and a self‐association constant of 2600 ml−1 were determined. Over 50% of the sample is aggregated near saturation with significant amounts (>15%) of the sample present as oligomers containing four or more 4‐thiopyridone monomer units. In its crystal structure, 4‐mercaptopyridine‐4‐thiopyridone is composed of infinite chains of 4‐thiopyridone associated by NH…S hydrogen bonds between glide related molecules, with N…S = 3 · 219(3) Å, H…S = 2 · 42(4) Å, N—H…S angle = 175(4)° and H…S = C angle = 96(1)°. Relationships between solution aggregation, crystal nucleation, and crystal propagation are discussed. Crystal data are as follows: space group = P21/c, a = 7 · 183(5) Å, b = 6 · 132(5) Å, c = 11 · 618(7) Å, β = 90 · 51(5)°, Z = 4, Dc = 1 · 44 g cm−3, 1623 reflections, R = 0 · 050, Rw = 0 · 072.
Heats of formation of organic molecules by ab initio calculations. Aldehydes and ketones
Authors: Schmitz Lawrence R. , Motoc I. , Bender Charles , Labanowski Jan K. , Allinger Norman L. 
Published: January 2005

AbstractA bond and group equivalent scheme that allows the calculation of heats of formation for aldehydes and ketones from ab initio 6–31G* energies has been developed. For a group of 24 aldehydes and ketones, the RMS error for the calculated heat of formation was 0 · 46 kcal mol−1. Heats of formation have been predicted for an additional seven compounds for which the experimental values are believed to be either in error or unknown. There are some problems with the norbornanones.
Experimental and theoretical study of the influence of the solvent on asymmetric diels—alder reactions
Authors: Cativiela C. , Garcia J. I. , Mayoral J. A. , Royo A. J. , Salvatella L. , Assfeld X. , Ruiz‐Lopez M. F. 
Published: January 2005

AbstractRate, endo/exo and diastereofacial selectivities of the Diels–Alder reaction between cyclopentadiene and (—)‐menthyl acrylate were measured in a series of organic solvents and organic–aqueous mixtures. Regression analyses show that a model with the α empirical solvent parameter accounts for most of the changes in rates and diastereofacial selectivities, whereas in the case of the endo/exo selectivity, inclusion of the π* and δ parameters is needed. Theoretical calculations carried out on the model reaction between methyl acrylate and cyclopentadiene using dielectric continuum and supermolecule models do not clearly show the effect of the different solvation mechanisms on rate. However, they agree with the above‐mentioned influence of α and π* on the endo/exo and diastereofacial selectivities.
Theoretical studies on nitrile‐forming elimination reactions
Authors: Lee Ikchoon , Park Hyoung Yeon , Lee Bon‐Su , Kong Byung Hoo , Lee Byung Choon 
Published: January 2005

AbstractThe base‐promoted nitrile‐forming elimination reactions of YCH=CHCβH=NOCH=CHZ (Y=OCH3, H or Cl and Z=H or NO2) were studied by the AM1 MO theoretical method with Cl− as a base. The reaction is found to proceed by an E1cB‐like E2 mechanism in which Cβ—H bond cleavage is more advanced than N—O bond breaking. The syn‐elimination has a more E1cB‐like transition state (TS) than the anti elimination, which is attributed to the structurally favourable nN—ó * (Cβ—H) charge‐transfer interaction. An electron‐withdrawing Y substituent lowers the activation barrier by stabilizing negative charge developed on Cβ in the TS. An electron‐withdrawing substituent in the leaving group (Z = NO2) tends to enhance the anti relative to the syn elimination process by depressing the δ*(N—O) level, which in turn makes the nć—δ*(N—O) interaction more effective. The YCH=CH— and —CβH=N fragments are perpendicular in the TS, which is stabilized by delocalization of negative charge developed on the Cβ atom.
Reduction of 1‐benzyl‐3‐cyanoquinolinium ion by phosphonate: Solvent effects in propan‐2‐ol–water mixed solvents
Authors: Lee In‐Sook Han , Kim Sun Hee , Kreevoy Maurice M. , Lee Chang Kiu , Lee Chang Hee 
Published: January 2005

AbstractRate constants (k2) were determined for the reduction of 1‐benzyl‐3‐cyanoquinolinium ion by phosphonate dianion in mixed solvents consisting of propan‐2‐ol and water. The reduction product was mostly 1‐benzyl‐3‐cyano‐1,4‐dihydroquinoline with a trace of the 1,2‐isomer. The solvent properties were varied by increasing the fraction of water in the mixed solvent, which increases the polarity of the solvent. Increasing the proportion of alcohol makes the solvent a better electron pair donor. The higher fraction of propan‐2‐ol in the mixed solvent gives rise to a substantial increase in k2. A quantitative comparison of the solvent effect on this reaction with the solvent effects on related reactions suggests a solvent Brønsted α value of 0·45. This suggests that the PO3− unit of the transition state interacts only weakly with the solvent although the final oxidized product is phosphate.
Alkylation of pyridinium‐N‐phenoxide betaine dyes with iodomethane: Substituent, solvent and temperature dependence
Authors: Linert Wolfgang , Strauss Bernhard , Herlinger Erwin , Reichardt Christian 
Published: January 2005

AbstractThe rate constants of the pseudo‐first‐order methylation reaction of seven substituted pyridinium‐N‐phenoxide betaine dyes by iodomethane, were determined by UV–visible spectrophotometry in nine different solvents. The influence of substituents on the position of the long‐wavelength, intramolecular charge‐transfer absorption band of the unsubstituted betaine dye can be described by a modified Hammett equation. The solvent‐dependent absorption constant ρA correlates satisfactorily with the acceptor properties of the nine solvents used. The substituent‐dependent alkylation rate constants (In k) correlate linearly with the Hammett substituent constants, whereas the solventdependence of in k follows a more complicated pattern. Temperature‐dependent measurements of the rate of alkylation exhibit an isoenthalpic behaviour for all the betaines in all the solvents investigated, i.e. the Arrhenius activation energy is almost constant for all differently substituted betaines in a given solvent. Temperature‐dependent measurements of the alkylation rate of the unsubstituted betaine in different solvents led to an isosolvent relationship, i.e. the corresponding Arrhenius plots show a common point of intersection. According to the theory of isokinetic relationships, this isosolvent behaviour is interpreted in terms of a resonant energy exchange between the reacting species and the surrounding heat‐bath system, which in this particular case seems to be the betaine molecule itself.
Direct measurement of the energy barrier for 1,2‐chlorine atom migration in α‐methyl‐α‐chlorobenzyl(chloro)crbene
Authors: Liu Michael T. H. , Chateauneuf John E. 
Published: January 2005

AbstractThe first direct determination of a 1,2‐chlorine atom shift in a chlorobenzylcarbene was achieved by nanosecond laser flash photolysis. Arrhenius activation parameters of Eact = 3.39 ± 0.14 kcal mol−1 and log [A(s−1)] = 10.98 ± 0.14 were obtained for 1,2‐chlorine migration in α‐methyl‐α‐chlorobenzyl(ch1oro)carbene. The lifetime of this carbene is considerably longer than previously estimated and the measured Eact, is in excellent agreement with that determined by product analysis.
Concerning the development of scales of solvent ionizing power based on solvolyses of benzylic substrates
Authors: Kevill Dennis N. , D'Souza Malcolm J. 
Published: January 2005

AbstractBased on analysis of recently reported measurements, it is suggested that, provided a sufficiently large set of specific rate measurements in a wide variety of solvent types is available, the negligible to moderate improvements in the correlations of other types of benzylic chlorides, which are gained by use of a benzylic chloride (in place of 1‐adamantyl chloride) as the standard substrate, do not justify the development of new specialized scales of solvent ionizing power. A similar conclusion is reached concerning solvent ionizing power scales developed for use with benzylic p‐nitrobenzoates. However, such scales could be helpful when specific rates of solvolysis in only a limited variety of solvents are available, e.g. when a change in mechanism severely limits the range of operation of one of the pathways. Contrary to a previous claim, variations in ion‐pair return could be the cause of deviations from Grunwald‐Winstein plots for solvolyses of benzhydryl chloride in fluorinated alcohol solvents; such effects could be coupled, at least in part, with variations in aromatic ring solvation.
Quantitative aspects of chirality. II. Analysis of dissymmetry function behaviour with different changes in the structure of the model systems
Authors: Kuz'min V. E. , Stel'makh I. B. , Yudanova I. V. , Pozigun D. V. , Bekker M. B. 
Published: January 2005

AbstractThe efficiency of the developed method of dissymmetry functions was studied using various model systems. The alteration of the dissymmetry function was analysed with systematic variations of the bond lengths, valence angles and masses of atoms in model tetrahedra. The behaviour of the dissymmetry function was studied for conformationally labile systems and chiral polyhedra. In general, it was found that in all cases the alteration of the dissymmetry function is in agreement with the speculative representations of the changes in degree of chirality.
Quantitative aspects of chirality. III. Description of the influence of the structure of chiral compounds on their twisting power in the nematic mesophase by means of the dissymmetry function
Authors: Kutulya L. A. , Kuz'min V. E. , Stel'makh I. B. , Handrimailova T. V. , Shtifanyuk P. P. 
Published: January 2005

AbstractFor a number of chiral α,β‐unsaturated carbonyl compounds, cyclohexanone and cyclohexenone derivatives, existing essentially in the form of single molecular conformation, the correlation dependences were established between the values of the calculated dissymmetry functions (DF) with respect to atomic masses and atom refractions, and the induction effectiveness of helical ordering in the nematic mesophase (twisting power). Such correlative dependencies, as the examples show, allow one to predict safely enough the twisting power of new substances with the ‘fixed’ molecular conformation. Types of dissymmetry functions and kinds of correlative dependences, the most acceptable for these reasons, were revealed. Dissymmetry functions formed on the basis of atom refractions (polarizabilities) describe the influence of the molecular structures of chiral compounds on the twisting power with higher accuracy than those that characterize the dissymmetry of the atomic masses distribution, according to the important role of the dispersion forces in the formation of helical ordering in liquid crystalline systems. The components characterizing the distribution dissymmetry of atomic refractions and masses with respect to the long and two shorter axes as the peculiar characteristics of biaxiality of chiral molecules describe the influence of the molecular structure on the twisting power with the same accuracy as the general DF do. Thus influence of chiral dopant molecules on the interactions between molecules in the uniaxial mesophases is negligible within the limits of the DF method. Based on the calculations for model systems it is established that the presence of the non‐linear cinnamoyl fragment has a decisive influence on the molecular dissymmetry of the chiral α,β‐unsaturated ketones. The alkyl groups influence the molecular dissymmetry only slightly, but it is important that their presence ensures the stabilization of the chiral conformer with the defined helicity of the cinnamoyl fragment.
Effect of hexadecyltrimethylammonium bromide micelles on the rate of oximolysis of esters
Authors: Zanette Dino , Chaimovich Hernan 
Published: January 2005

AbstractTwo amphiphilic oximes, 10‐phenyl‐10‐hydroxyiminodecanoic acid (oxime II) and 4‐(9‐carboxynonanyl)‐1‐(9‐carboxy‐1‐hydroxyimino nonanyl)benzene (oxime III) were synthesized. The pKa values of oximes II and III and acetophenoxime (oxime I) and the rates of oximolysis of p‐nitrophenyl acetate (NPA) and p‐nitrophenyl octanoate (NPO) were determined in the presence and absence of micellar hexadecyltrimethylammonium bromide (CTAB). The rates of oximolysis increased by up to 3 × 104‐fold in the presence of CTAB. Quantitative analysis of micellar effects, using an ion‐exchange pseudo‐phase model, allowed the determination of the second‐order rate constants for the reactions of oximes I‐III with NPA and NPO in the micellar pseudo‐phase. The calculated rate constants in the micellar pseudo‐phase were lower than those in water, demonstrating that the rate enhancements were due to substrate concentration in the micelles. Comparison of the rate constants in micelles and water suggests that the sites of reaction of oximes I‐III with NPO and NPA are similar to those in bulk aqueous solution. Micellar incorporation of the hydrophobic oximes II and III does not lead to a major change in the nucleophilicity of the oximate anion.
Oxidation of indole by N‐sodio‐N‐chlorobenzenesulphonamide (chloramine‐B) in alkaline medium catalysed by os(viii): A kinetic and mechanistic study
Authors: Venkatesha B. M. , Ananda S. , Mahadevappa D. S. 
Published: January 2005

AbstractThe kinetics of oxidation of indole (In) and 5‐substituted indoles (OCH3, Br and Cl) by chloramine‐B (CAB) were studied in alkaline medium with Os(VIII) as catalyst at 303 K. At low [In]0 the rate law rate = k [CAB] [Os(VIII)] [In]0 is obeyed, which changes to rate = k [CAB] [Os(VIII)] [OH−]−1 at higher substrate concentrations. Variation of ionic strength has no effect on the rate and the dielectric effect is negative. The reaction was studied at different temperatures and activation parameters were evaluated. Hammett correlation of substituent effects indicated a linear free energy relationship with ρ+ = − 1.0, showing the formation of an electrondeficient transition state. From enthalpy–entropy relationships and Exner correlations, the isokinetic temperature β was found to be 330 K, indicating enthalpy as a controlling factor. The mechanism assumes the formation of a complex between oxidant and Os(VIII) at high and low [In]0. Proton inventory studies in H2O‐D2O mixtures showed the involvement of a single exchangeable proton of OH− ion in the transition state.
Molecular mechanics of bridged ferrocene derivatives: Conformational energy surfaces of [3]‐, [4]‐ and [45] ferrocenophanes
Authors: Rudziński Jerzy M. , Ōsawa Eiji 
Published: January 2005

AbstractA molecular mechanical model is presented which allows computational interpretation of stereodynamics in ferrocenophanes by using a simple form of bending potential for angles involving the central iron atom and extended to carbon atoms of different cyclopentadienyl rings. Potential energy surfaces of [3] ‐, [4] ‐ and [45] ferrocenophanes were studied in detail. For [3] ferrocenophane, the calculated energy barrier of the bridge reversal process agrees well with the experimental value. The previous interpretation of a rigid bridge in [4] ferrocenophane is questioned on the basis of the calculated low barriers. The predominance of experimentally indistinguishable enantiomeric pairs may be responsible for the misinterpretation. [45] Ferrocenophane is estimated to interconvert into D5‐symmetric global energy minima over barriers of 13–15 kcal mol−1 through one‐by‐one flipping of five tetramethylene bridges.
Kinetics of deprotonation of triphenyl(2‐substituted‐9‐fluorenyl)phosphonium ions in 50% dimethyl sulfoxide–50% water. Intrinsic rate constants, imbalances and evidence for changes in transition state structure
Authors: Bernasconi Claude F. , Fairchild Douglas E. 
Published: January 2005

AbstractRate constants (kB1 and kBH−1) for the reversible deprotonation of triphenyl(2‐Z‐9‐fluorenyl)phosphonium ions (Z = H, Br, NO2) by piperidine, morpholine, n‐butylamine, 2‐methoxyethylamine, glycine ethyl ester, cyanomethylamine, OH− and water were determined in 50% Me2SO–50% (v/v) water at 20°C. Brønsted αCH (variation of carbon acid) and β values (variation of amine), and intrinsic rate constants [log k0 = log kB1/q when pKBHa ‐ pKCHa + log(p/q) = 0] were obtained. αCH decreases with increasing basicity of the amine whereas βB decreases with increasing acidity of the carbon acid. These trends, which imply changes in the transition‐state structure with reactivity, can be described by the interaction coefficient pxy = ∂βB/∂pKCHa = ∂αCH/ ‐ ∂pKBHa = 0.03 (primary amines) and 0.01 (piperidine/morpholine). αCH is smaller than βB, indicating an imbalance due to a lag in the delocalization of the negative charge into the fluorenyl moiety at the transition state. The influence of the Ph3P+ group on the intrinsic rate constant is analyzed in terms of possible contributions by inductive/field (I), resonance (R), polarizability (P) and steric (S) effects. Using 9‐carbomethoxyfluorene as a reference, it is shown that the stronger electron‐withdrawing I effect of the Ph3P+ group relative to the COOMe group enhances log k0 substantially; the fact that the R effect of Ph3P+ is weaker than that of COOMe also contributes to an increase in k0, and so does the P effect of the phosphorus. All these increases are virtually completely offset by the rate‐retarding S effect of the bulky Ph3P+ group. A similar analysis for the Me2S+ derivative studied by Murray and Jencks [J. Am. Chem. Soc. 112, 1880 (1990)] leads to similar conclusions except that the still smaller R effect is probably one of the main reasons why k0 for the Me2S+ derivative is more than ten times higher than for the Ph3P+ derivative; another potential reason is a difference in the steric effect.