bluuebunny/crossref_metadata_2025 huggingface dataset:
Solvatochromism of dyes. Part I. Solvatochromism of merocyanines. Derivatives of the 7H‐indolo [1,2‐a] quinolinium system. A new model of solvatochromism
Authors: Soroka Jacek A. , Soroka Krystyna B.
Published: January 2005

A new approximate model of the solvatochromism of merocyanines is presented. In solution merocyanines exist in the electronic ground state as a mixture of three different structures. Since only one longest wavelength band is observed in the UV—visible absorption spectra of such merocyanines, their lowest excited states are roughly assumed to be represented by a single structure. The molar fraction of each of the three structures of merocyanines in the ground state is a function of the solvent polarity. This (VBHB) model has been verified using merocyanines prepared from 5‐(hydroxyaryl)‐7,7‐dimethyl‐7H‐indolo [1,2‐a] quinolinium perchlorates. The analysis of the experimental data suggests that the three ground‐state structures of merocyanines are the vinylogous amide (V) and the betaine as the free (B) or hydrogen‐bonded (HB) form.
An ene reaction with a polar transition state as ascertained from solvent effects
Authors: Laszlo Pierre , Teston‐Henry Michelle
Published: January 2005

A kinetic study is reported for the ene reaction between methyl acrylate as the enophile and β‐pinene, with aluminium chloride as catalyst, in a series of solvents varying in polarity. These experiments point to a transition state having pronounced zwitterionic character, in line with earlier suggestions in the literature. An unexpected and significant change of the rate constant with the initial concentrations is also found.
Micellar rate effects on reactions of hydroxide ion with phosphinate and thiophosphinate esters
Authors: Blaskó Andrei , Bunton Clifford A. , Hong Young S. , Mhala Marutirao M. , Moffatt John R. , Wright Sallyanne
Published: January 2005

Cationic micelles of cetyltrimethylammonium chloride, bromide and mesylate (CTACl, CTABr, CTAOMs) speed reactions of OH− with phosphinate and thiophosphinate esters: Ph2PO.OPh (1a), Ph2PO.OC6H4NO2−p (1b), Ph(i‐Pr)PO.OC6H4NO2−p (1c), Ph2PO.SEt (2a), Ph2PO.SPh (2b), (EtO)2PO.SPh (2c) and (EtO)2PS.OC6H4NO2−p (Parathion, 3). First‐order rate constants go through maxima with increasing [surfactant]. The rate—surfactant profiles are fitted quantitatively in terms of a kinetic model that treats the distribution of OH− between aqueous and micellar pseudo‐phases in terms of coulombic and non‐coulombic ion‐micelle interactions. Second‐order rate constants at the micellar surface are lower than in water by factors that range from 0.035 for 1a to 0.7 for 1c. The thiophenyl derivative (2b) is more reactive than the corresponding phenoxy derivative (1a) and it is more reactive than the corresponding thioethyl derivative (2a). Parathion (3) is the least reactive substrate in both water and micelles.
Molecular Mechanics (MM3) calculations on sulfides
Authors: Allinger Norman L. , Quinn Maritza , Rahman Mita , Chen Kuohsiang
Published: January 2005

The structures of eleven sulfide compounds including methanethiol, ethanethiol, dimethyl sulfide, ethyl methyl sulfide, di‐tert‐butyl sulfide, thiacyclopentane, thiacyclobutane, 5‐thiabicyclo [2.1.1] hexane and 7‐thiabicyclo [2.2.1] heptane have been calculated to agree with experimental data, along with the vibrational spectra of the first three of these. The heats of formation of 24 sulfides (including mercaptans) have also been calculated to agree with experimental data. In general, the force field for sulfides seems to be similar in accuracy with that for hydrocarbons.
Molecular mechanics (MM3) calculations on disulfides
Authors: Chen Kuohsiang , Allinger Norman L.
Published: January 2005

An MM3 parameter set was developed for disulfides so as to permit the force field representation of these molecules with respect to geometries, including conformations, vibrational spectra and heats of formation.
Ortho, effect of one methyl group: Conformation of methyl 5‐nitro‐2‐methylbenzoate in crystal and in solution
Authors: Tinant Bernand , Declercq Jean‐Paul , VšEtečKa Václav , Exner Otto
Published: January 2005

The x‐ray structure of methyl 5‐nitro‐2‐methylbenzoate was determined [M,r = 195.17, monoclinic, P,21/c, a, = 7.265(1), b, = 7.749(1), c, = 16.526(3) Å, α; = 91.50(2)., V, = 930.0(3) Å3, Z, = 4, D,x = 1.39 g cm−3, Cu Kα, Λ = 1.5418 Å, μ = 9.56 cm−1, F,(000) = 408, T, = 291 K, R, = 0.081 for 1295 observed reflections]. In contrast to the planar molecule of the corresponding acid, studied previously, the carboxylate group in the ester molecule is twisted through an angle 25° from the ring plane. This torsion angle was estimated to be 65° in benzene solution; the estimation was based on the dipole moment and that of methyl 2‐methylbenzoate. For the corresponding ethyl esters a value of 79° was found in a similar way. Although these results are expressed in terms of one non‐planar conformation, the alternative explanation of an equilibrium of two planar forms is not excluded in solution. The only safe conclusion is that the energy differences are small and the conformation is sensitive both to the conditions and to small changes in structure; a large part of the observed effects of an ortho,‐methyl group are due to reasons other than to the hindered resonance.
Relative homolytic strengths of NH bonds in cyclic and acyclic diacylhydrazides, imides and hydrazoic acid
Authors: Bausch M. J. , David B. , Prasad V. , Wang L.‐H. , Vaughn A.
Published: January 2005

With the aid of a thermochemical cycle consisting of acidity and new redox data in dimethyl sulfoxide (DMSO) solution, homolytic NH bond dissociation energies (ΔBDE values) relative to acetamide (1) (where the NH BDE for 1 is defined as 0 kcal mol−1) were determined for diacetamide (0), biuret (+ 1), 3,3‐dimethylglutarimide (−3), diacetylhydrazine (−16), 4,4‐dimethylpyrazolidine‐3,5‐dione (− 25), 4‐dimethylurazole (−29), hydrazoic acid (−15), succinimide (−15) and 1,2‐dimethylurazole (−13) (all values in kcal mol−1). These ΔBDE data provide (a) additional evidence for the minimal NH bond weakening effects of adjacent carbonyl groups, (b) evidence for relatively large NH bond weakening effects of adjacent —NHC(O)R moieties in both cyclic and acyclic hydrazides and (c) evidence suggesting that urazolyl radicals are more stable than pyrazolidinedionyl radicals, relative to their hydrogenated precursors. Inserting the appropriate acidity and redox data for hydrazoic acid into a thermochemical cycle that includes a constant that permits comparison of DMSO solution BDEs with gas‐phase BDEs yields estimates of 93 kcal mol−1 for the homolytic strengths of the NH bonds present in succinimide and HN3. The DMSO NH BDE determined in this way for H—N3 is in remarkable agreement with a determination of its gas‐phase value, whereas the DMSO NH BDE for succinimide places it intermediate between three published estimates of its gas‐phase value.
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

The 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

Owing 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

Dewar‐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

Cyclopropylmethoxycarbene 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

Polarographic 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

10, 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

The 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

A 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

Rate, 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

The 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

Rate 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

The 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

The 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

Based 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

The 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

For 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

Two 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

The 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

A 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

Rate 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.
Alkali metal ion‐assisted cleavage of crown ether anisoles by toluenethiolate anion
Authors: Cacciapaglia Roberta , Mandolini Luigi , Romolo Francesco S.
Published: January 2005

The effect of alkali metal ions (Li+, Na+, K+, Cs+) on the reactivity of toluenethiolate anion in the demethylation of 2‐methoxy‐1,3‐xylyl‐18‐crown‐5, 2‐methoxy‐1,3‐xylyl‐24‐crown‐7 and the model compounds anisole and 2,6‐dimethylanisole was investigated in dimethylformamide (+3.3 M water) at 60°C. It was found that the metal ion effects are markedly influenced by the presence of the polyether chain in the reaction system. Whereas reactions of the model compounds are slightly inhibited by the presence of alkali metal ions, the latter strongly enhance rate of demethylation of the crown ether derivatives, the observed catalytic factors ranging over nearly three orders of magnitude. These remarkable rate‐enhancing effects are ascribed to a strong transition‐state stabilization by metal ions through cooperation of electrostatic binding with the negative charge developing on the oxygen atom of the methoxy group undergoing nucleophilic attack and coordinative interaction with the polyether chain.
Direct nucleophilic aromatic substitution reactions in the syntheses of anthraquinone derivatives: Chemistry and binding of podands, crown ethers, and a cryptand
Authors: Kim Hyunsook , Schall Otto F. , Fang Junping , Trafton John E. , Lu Tianbao , Atwood Jerry L. , Gokel George W.
Published: January 2005

The direct nucleophilic aromatic substitution reactions of anthraquinones have permitted the syntheses of more than 30 novel podands, crown ethers and lariat ethers. Anthraquinones having (ethyleneoxy)n sidearms were obtained by direct displacement of chloride by the anion of CH3(OCH2CH2)nOH. The ethyleneoxy‐substituted anthraquinones could, in turn, undergo direct replacement by nucleophiles that failed to displace chloride. This approach has been used for the preparation of two‐armed podand derivatives and several novel crown derivatives of anthraquinone. Binding comparisons are presented for several of these new anthraquinones. Direct substitution did not prove successful in the preparation of anthraquinone‐ [2 · 2] ‐cryptand which was obtained by alkylation. The crystal structure of the latter reveals an orientation of ring and anthraquinone appropriate for cation binding, a fact confirmed by cation binding constant measurements.
Supramolecular control of molecular electronic behaviour of TCNQ−. salts
Authors: Grossel Martin C. , Weston Simon C.
Published: January 2005

Crown ether complexation of 1:1 alkali metal TCNQ salts, MTCNQ (M=K, Rb, Tl) leads to structures containing isolated TCNQ dimers. These materials provide valuable models for investigating the spectroscopic and electronic behaviour of TCNQ−. dimer components which are key structural fragments of, for example, organic metals and semiconductors.
Kinetics and mechanism of the dissociation of the Cu2+ and Ni2+ complexes with monooxo‐ and dioxomacrocyclic tetraamines
Authors: Siegfried Liselotte C. , Kaden Thomas A.
Published: January 2005

The kinetics of the acid‐induced dissociation of the Ni2+ and Cu2+ complexes of a monooxo and two dioxo tetraaza macrocycles were studied by stopped‐flow spectrophotometry at single wavelengths and using a photodiode array, For the monooxo derivative the dissociation rate follows the law vd = k1Ccom [H+]/(K2 + [H+]), whereas for the dioxo derivatives the law is vd = k2Ccom [H+]2/(K1 + K2 [H+] + [H+]2). The dissociation kinetics are interpreted by a mechanism in which protonated intermediates are formed in a rapid pre‐equibrium, the rate‐determining step being the dissociation of this species. Comparison of the spectral results from multi‐wavelength stopped‐flow measurements with those acquired in the equilibium study using spectrophotometric titrations allows the nature and structure of these intermediates to be discussed. The order of the dissociation rates in strongly acidic solution, where a plateau in the pH dependence is reached, was determined and it demonstrates the importance of the ring size on one hand and of the number of amide groups on the other.
Heats of formation of enols. The double‐bond stabilizing effect of the hydroxyl group
Authors: Keeffe J. R. , Kresge A. J.
Published: January 2005

Aqueous solution free energies of ketonization have been combined with enthalpies of reduction of the keto forms and other thermochemical data to yield estimates of the heats of formation of the corresponding enols. A double‐bond stabilization term, DOH = 5.4 ± 0.5 kcal mol−1 (1 kcal = 4.184 kJ), is obtained for the OH group. This quantity is close to Hine's double‐bond stabilization parameter for the methoxyl group (4.9 ± 0.2 kcal mol−1). Comparison with available experimental gas‐phase enthalpies of formation suggest that, relative to the keto form, enols are more stable in the gas phase than in water. The differences are unlikely to be due entirely to entropic effects. A wholly enthalpic, gas‐phase double‐bond stabilization term, DOH = 8.1 ± 0.6 kcal mol−1, may be calculated from the available experimental data. This result indicates that the hydroxyl group more effectively stabilizes the CC bond in the gas phase than in aqueous solution. An important limitation is that the results reported bere apply only to simple enols, that is, monofunctional enols not stabilized by extra resonance or hydrogen‐bonding effects.
Partial rate factors in photochemistry: The meta photocycloaddition of methylbenzenes to cyclopentene
Authors: de vaal P. , Lodder G. , Cornelisse J.
Published: January 2005

The partial rate factors of the four modes of meta photocycloaddition of methyl‐substituted benzenes to cyclopentene have been determined by measuring quantum yields of formation of the most important meta photocycloadducts of benzene, toluene and the three xylenes. It is demonstrated that the results can be applied to predict quantum yields of any meta photocycloadduct of methylbenzenes to cyclopentene. The predicted yields agree very well with those determined experimentally, and it is shown that the effects of two or more methyl groups are fully additive. The method is promising for the investigation of mechanisms of photochemical reactions and can be used to predict regioselectivities.
Ionization kinetics of rhodamine b leucocyanide in the presence of poly(vinylbenzyltrimethylammonium chloride)
Authors: Zimerman O. E. , Cosa J. J. , Gsponer H. E.
Published: January 2005

The participation of the carboxylate group in the ionization of Rhodamine B leucocyanide was studied in an aqueous solution of poly(vinylbenzyltrimethylammonium chloride) (PVBTA) or cetyltrimethylammonium bromide (CTAB) at constant temperature and pH 8. The experimental rate constant (kexp) decreases with increasing PVBTA concentration. The results are interpreted in terms of the binding of the ionized form of Rhodamine B leucocyanide (RBCN−) to a single class of completely independent binding sites on the PVBTA. From this model, a value of 1.09 × 106 l mol−1 was obtained for the binding constant. The effect of CTAB is different from that obtained with PVBTA. After a region in which there are large changes in kexp with the CTAB concentration, this constant reaches a zone where its value is independent of the surfactant concentration. The decrease in kexp in the first zone was explained in terms of an ionic association between RBCN− and the cationic heads. In the second zone, it was assumed that a micellar effect operates on the ionization kinetics. In a similar form, the esterification of the carboxylate group leads to a slower ionization rate.
Degradation of atrazine by hydrolysis and by hydroxyl radicals
Authors: Chan Gabriel Y. S. , Hudson Michael J. , Isaacs Neil S.
Published: January 2005

Rates of degradation of the triazine herbicide atrazine are reported, together with an analysis of the sequence of stages involved, using hydroxyl radicals generated by hydrogen peroxide in the presence of UV light and by the pure photochemical reaction. These conditions bring about the displacement of chlorine by hydroxyl, dealkylation of the alkylamino groups and finally deamination of the triazine ring. The significance of these results to water purification is discussed.
Structure of, and internal rotation in, hexakis [4‐(2‐phenylpropan‐2‐yl) phenoxy] benzene, and structural comparison with its precursor phenol 4‐(2‐phenylpropan‐2‐yl)Phenol
Authors: MaCnicol D. D. , Mallinson P. R. , Vallance I.
Published: January 2005

The title compounds (1 and 3, respectively) were studied by X‐ray crystal structure analysis. Both are trigonal, space group R3, with lattice parameters a = 27.718(6), c = 8.161(2) Å, Z = 3 and a = 31.267(7), c = 6.560(2), Z = 18, respectively. The dodecamethyl compound 1 is isomorphous with its parent hexakis(4‐benzylphenoxy)benzene (2), a known host. However, ‘collapse’ along the c‐axial direction has occurred such that the appreciable clathrate cavity of 2 has been replaced by only a very small residual void for 1. Phenol 3 does not pack in discrete [OH]6 hydrogenbonded hexamers; instead, molecules are assembled in infinite chains, linked by OH…︁O hydrogen bonds of length 2.735(3) Å, which are propagated along threefold screw axes. A variable‐temperature CPMAS NMR study of sidechain aryl group rotation in 1 gives a value ΔG≠ of 14.0 ± 0.7 kcal mol−1 (at 42 °C) for the (crystallographically unique) para ‐disubstituted ring, a similar 14 kcal mol−1 free energy barrier being found hindering rotation of this ring, and of the outer phenyl ring of 1, at 9°C.
Stereoelectronic effects at carboxyl oxygen
Authors: Tadayoni B. Mitra , Rebek Julius
Published: January 2005

The recent attempts to evaluate the stereoelectronic effects at carboxyl oxygen using Kemp's triacid derivatives are reviewed. Contexts include metal‐ion chelation, acidity measurements, intramolecular general base catalysis and nucleophilic substitution. These give a range of 0–8 kcal/mol for the effects, and leave the issue open for more incisive experiments.
Biphasic kinetics and flip‐flop behaviour of vesicles of fluorocarbon amphiphiles with a 1,3‐disubstituted glycerol structure
Authors: Liang Kangning , Hui Yongzheng
Published: January 2005

Fluorocarbon amphiphiles with a 1,3‐disubstituted thioglycerol structure form stable unilamellar vesicles. The fluorocarbon bilayer of an amphiphile which has an ammonium salt as head group is an ‘insulator’ for the permeation of hydroxide anions. In the presence or absence of a fluorocarbon nucleophile, hydrolysis of a probe amphiphile carrying a cleavable p‐nitrophenyl ester group in the vesicle under alkaline conditions shows a pattern of biphasic kinetics, in which the fast and slow reaction can be attributed to the hydrolysis of probe molecules at outer and inner surfaces of vesicles, respectively. The fact that the slow rate constant always remains constant at 0.25 min−1 at 25°C, independent of the pH of the system and of concentration of the nucleophile, indicates that the slow process is an outward flip‐flop process of probe molecules within the fluorocarbon domain.
Nucleophilic substitution reactions of methallyl arenesulphonates with anilines and N, N‐dimethylanilines
Authors: Oh Hyuck Keun , Shin Chul Ho , Lee Ikchoon
Published: January 2005

Kinetic studies of the reactions of methallyl arenesulphonates (II) with anilines and N,N‐dimethylanilines in acetonitrile at 45.0°C are reported. The sign and magnitude of the cross‐interaction constants pxz (and βxz) between substituents in the nucleophile (X) and leaving group (Z) suggest that the transition state (TS) is slightly tighter than that for the corresponding reactions of allyl arenesulphonates (I). This is also supported by the observation that the magnitudes of ρx and ρz for II are uniformly greater than those for the reactions of I. These results are in line with the simple MO theory that the 2‐position of the allyl system is inactive electronically. The steric effect of the 2‐methyl group in II causes a rate retardation and a shift of the TS toward a later position along the reaction coordinate with a slight increase in the overall tightness of the TS structure. The large |ρxz| value obtained eliminates the possibility of an SN2′ mechanism.
Gas‐phase ambident reactivity of cyclic enolate anions
Authors: Freriks Ivo L. , de Koning Leo J. , Nibbering Nico M. M.
Published: January 2005

The gas‐phase reactions between cyclic enolate anions and unsaturated perfluorocarbon compounds were studied by Fourier transform ion cyclotron resonance. A correlation is observed between the experimental electron detachment threshold energies, the reaction selectivity and the ring size of the cyclic enolate anions. The results suggest that frontier orbital interactions play an important role in the course of the reactions of cyclic enolate anions, as has been shown previously for the reactions of acyclic enolate anions. In addition, the reaction selectivity of cyclic enolate anions may also be influenced by the charge distribution and by transannular interactions. In comparison with the acyclic enolate anions, the HOMO energy of the small cyclic enolate anions (C4–C5) has decreased and the carbanion character has increased, both of which favour reaction via carbon. The medium ring‐sized cycloketone enolate anions behave very similarly to their acyclic analogues, whereas for the larger ring‐sized cycloketone enolate anions (C8–C12) the reaction via the carbon nucleophilic centre gains importance as a result of the stabilization of the HOMO due to transannular interactions.
Z–E Isomerism of 5‐cyclohexylmethylenehydantoins
Authors: Tan Sau‐Fun , Au‐Yong Po‐Lung , Lee Lay‐Ting
Published: January 2005

5‐Cyclohexylmethylenehydantoin and its 1‐ and 3‐methyl derivatives were studied as the aliphatic analogues of the 5‐benzylidenehydantoins to examine the effects of replacing a benzene by a cyclohexane ring on Z–E isomerism. The relative stabilities of the Z‐ and E‐isomers were compared by estimating the free energy differences from thermal equilibration experiments and the heats of formation by AM1 calculation. In the Z‐isomers, the possible existence of weakly attractive interaction between the C(4) = 0 and C(6)–H groups, which form parts of a stereochemically favourable, conjugated α,β‐unsaturated system, is suggested as a contributory factor to the observed configurational preference. Assignment of configurations was based mainly on NMR and is supported by IR and UV spectra. NMR spectra also revealed the conformation of the cyclohexane ring and the cyclohexyl proton signals were assigned by double resonance and by high‐field NMR in two solvents where solubility allows.
Diastereofacial selectivity in 1,3‐dipolar cycloadditions to cyclobutens. 8. HF/3–21G transition structures of the reactions of formonitrile oxide with CIS‐3,4‐dichlorocyclobutene and norbornene
Authors: Bagatti Marisa , Rastelli Augusto , Burdisso Marina , Gandolfi Remo
Published: January 2005

A transition structure for the syn and anti attack of formonitrile oxide on both cis‐3,4‐dichlorocyclobutene and norbornene was obtained using HF/3–21G method. These calculations correctly predict dominance of the syn attack in the reaction of dichlorocyclobutene and 100% syn selectivity, with respect to the methano bridge, in the reaction of norbornene. Analysis of the activation energy shows that the inherent facial bias of dichlorocyclobutene is reflected, at the transition state, in a complex way in the deformation energy of both dichlorocyclobutene and formonitrile oxide and in the interaction energy between them. With norbornene the out‐of‐plane deformation energy of the olefinic hydrogens clearly emerges as the major factor in controlling facial selectivity.
Kinetics and mechanism of the oxidation of substituted benzaldehydes by ethyl N‐chlorocarbamate
Authors: Varshney Seema , Kothari Seema , Banerji Kalyan K.
Published: January 2005

The kinetics of the oxidation of a number of para‐ and meta‐monosubstituted benzaldehydes by ethyl N‐chlorocarbamate (ECC) were studied in aqueous acetic acid solution in the presence of perchloric acid. The main product of the oxidation is the corresponding benzoic acid. The reaction is first order with respect to the aldehyde, ECC and hydrogen ions. The oxidation of benzaldehyde exhibits a substantial primary kinetic isotope effect (kH/kD = 5·20 ± 0·10). Addition of ethyl carbamate has no effect on the rate. [EtOC(OH)NHCI]+ is postulated as the reactive oxidizing species. The rates of oxidation of para‐substituted benzaldehydes exhibit an excellent correlation with Taft's σ1 and σ values whereas those of the meta‐substituted compounds compounds correlate best with σ1 and σ values. The reaction constants are large and negative. A mechanism involving transfer of a hydride ion from the aldehyde to the oxidant, in the rate‐determining step, is proposed.
Interaction of dichlorocarbene with imines
Authors: Kostikov R. R. , Khlebnikov A. F. , Bespalov V. Ya.
Published: January 2005

The interaction of dichlorocarbene with imines and ketene imines was studied by means of the MNDO method. The results indicate that the lack of ylide cyclization products in the reactions of dichlorocarbene with ketene imines is due to a high reactivity of ketene imine ylides in intermolecular reactions, rather than stereoelectronic factors preventing cyclization. It was shown that the dichlorocarbene reaction with N‐benzylideneaniline resulting in gem‐dichloraziridine proceeds through transient formation of ylide.