bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cross interaction constants as a measure of the transition state structure. 13. Steric effects of the N,N‐dimethyl group on the transition state structure in aminolysis of alkyl benzenesulphonates
Authors: Lee Ikchoon , Rhyu Keun Woo , Lee Hai Whang , Shim Chang Sub , Oh Hyuck Keun 
Published: January 2005

AbstractKinetic studies on the reactions of methyl (MBS) and ethyl benzenesulphonates (EBS) with N,N‐dimethylanilines (DMA) in methanol and acetonitrile are reported. The cross interaction constants ρXZ and βXZ, between the substituents in the nucleophile (X) and the leaving group (Z) indicated that the transition states (TS) are looser than those for the reactions with anilines, but the relative tightness between the two substrates was the same; the TS was tighter for EBS despite the increase in steric effect leading to looser TSs for MBS and EBS alike. The TS variation between two different reaction series expected from the simple Hammett and Brønsted coefficients, ρX, ρZ, βX and βZ, was incompatible with that predicted by the cross interaction constants, demonstrating again the unreliability of the simple parameters.
Microenvironmental effects in solid‐state reactions. Dispersive kinetics of conformation‐dependent charge delocalization in aliphatic diamine radical cations
Authors: Marcinek Andrzej , Gȩbicki Jerzy , Płonka Andrzej 
Published: January 2005

AbstractThe kinetics of conformation‐dependent charge delocalization in radical cations generated from a series of α,ω‐diaminoalkanes, (CH3)2N(CH2)nN(CH3)2, (n = 1–3) in a 3‐methylpentane glassy solution at 90 K are discussed in terms of dispersive kinetics with the use of the time‐dependent rate constant in the form k(t) = Btα − 1. It was found that the activation energy distribution function became broader as the number of methylene groups separating the two nitrogen atoms increased, and that this process was accompanied by a substantial increase in the mean value of the activation energy.
Mechanisms of acylation of 2,3‐dihydrobenzoxazol‐2‐one and 2H‐1,4‐benzoxazin‐3(4H)‐one
Authors: Cotelle Nicole , Cotelle Philippe , Capelle Sylvie , Lesieur Daniel 
Published: January 2005

AbstractThe kinetics of the acylation of 2,3‐dihydrobenzoxazol‐2‐one with dodecanoic acid in polyphosphoric acid were investigated. Two competitive reactions occurred at the 3‐ and 6‐positions. The acylation reactions in polyphosphoric acid of 2,3‐dihydrobenzoxazol‐2‐one and 2H‐1,4‐benzoxazin‐3(4H)‐one give 6‐ and 7‐acyl derivatives. 4‐Acetyl‐2H‐1,4‐benzoxazin‐3(4H)‐one rearranged in polyphosphoric acid exclusively to the 7‐acetyl derivative. Two mechanisms are proposed to explain the different reaction sites for the acylation of 2H‐1,4‐benzoxazin‐3(4H)‐one.
Reactivity and reaction patterns of alkyl alkoxybenzene radical cations. Mechanistic pathways of the reactions between 2,5‐DI‐tert‐Butyl‐1, 4‐Dimethoxybenzene and perfluorodiacyl peroxides
Authors: Jiang Xi‐Kui , Zhao Cheng‐Xue , Gong Yue‐Fa 
Published: January 2005

AbstractIn the reactions of 2,5‐di‐tert‐butyl‐1,4‐dimethoxybezene (1) with different oxidants, the radical cation 1+. is always detectable by EPR. However, the observed reactivity of 1+. depends greatly on the oxidation systems employed. In S2OCu2+HOAc and Ce4+HOAc systems (HOAc = acetic acid), 1+. appears to have long lifetimes and does not undergo fragmentation spontaneously. In contrast, in (RFCO2)2CF2CICFCI2 (F113) systems, the readily formed 1+. is short‐lived, and large amounts of de‐tert‐butylation products have been isolated. Experimental results imply that the CC bond cleavage involved in de‐tert‐butylation could be a consequence of an attack by perfluoroacyloxy radical on 1+. in their original solvent cage. The fact that addition of methanol to the reaction mixture leads to the formation of a large amount of tBuOCH3 (46%) and other evidence suggest that the tert‐butyl group leaves as a carbocation. On the basis of these results, we conclude that the reactions of 1 with (RFCO2)2 are initiated by electron transfer and followed by a fast coupling of various radical species, namely, 1+. with RFCO2 or with RF in the solvent cage, to form σ‐complexes which collapse or react with nucleophiles to yield the final products.
1,5‐Sigmatropic shifts of bromine over a cyclopentadiene ring
Authors: Minkin Vladimir I. , Mikhailov Igor E. , Dushenko Galina A. , Yudilevich Joseph A. , Minyaev Ruslan M. , Zschunke Adolf , Mögge Klemens 
Published: January 2005

AbstractThe intrinsic mechanism of circumambulatory rearrangements of 5‐bromo‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene, 5‐bromo‐1,2,3,4 5‐pentamethoxycarbonylcyclopentadien and 5‐bromo‐ 1,2,3,4,5‐pentaphenylcyclopentadiene due to sigmatropic shifts of bromine over the cyclopentadiene ring was proved, using the dynamic 13C and 1H NMR technique, to be governed by successive intramolecular 1,5‐sigmatropic shifts. Semi‐empirical AM1 and MINDO/3 calculations of reactions paths performed for fluoro‐, chloro‐ and bromocyclopentadienes are in accord with the conclusion of a preference for a 1,5‐ over a 1,3‐shift reaction path of halogen migration over a cyclopentadiene system. Intramolecular 1,5‐sigmatropic shifts of chlorine in 5‐chloro‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene with the free energy barrier of ΔG = 26.1 kcal mol−1 were studied using 1H NMR spectroscopy.
Specific micellar rate effects on unimolecular decarboxylation and cyclization
Authors: Cerichelli Giorgio , Mancini Giovanna , Luchetti Luciana , Savelli Gianfranco , Bunton Clifford A. 
Published: January 2005

AbstractDecarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion and cyclization of o‐3‐halopropyloxyloxyphenoxide ion (PhY7) (Y = Br, I) are accelerated by aqueous cationic and zwitterionic micelles. For cationic micelles the rate enhancements (kM′kW′) increase with increasing bulk of the surfactant head groups but are largest for decarboxylation. There are good linear free energy relationships between micellar effects on these reactions and the plots of log kM′ for cyclization against log kM′ for decarboxylation have slopes of 0.46 and 0.64 for Y = Br and I, respectively. Surfactants that have twin tail or tri‐N‐alkylbenzyloxy head groups do not fit the relationship. Despite mechanistic differences between decarboxylation and cyclization (an intramolecular SN2 reaction), similar factors control micellar effects on these reactions. In the absence of surfactant there is only a qualitative relationship between free energies of activation of cyclization and decarboxylation in different solvents.
Evaluation of solvent effects on the dissociation of aliphatic carboxylic acids in aqueous N,N‐dimethylformamide mixtures according to the scaled particle theory
Authors: Gonzalez A. Gustavo , Rosales D. , Gomez Ariza J. L. , Fernandez Sanz J. 
Published: January 2005

AbstractThe standard Gibbs energies of transfer, ΔGt° from water to water–N,N‐dimethylformamide mixtures for the dissociation process of several organic acids (formic, acetic, propionic, n‐butyric, isobutyric, n‐valeric, isovaleric, 2‐methylbutyric and trimethylacetic acids) obtained from the pKa* values are discussed in terms of the extended Scaled Particle Theory from quantum mechanical calculations in order to estimate the cavity and electrostatic contributions. The residual energy term was correlated with the hydrogen bond acceptor density and the Kamlet–Taft β parameters. The main contribution to ΔGt° was due to dipolar and specific interactions.
Internal rotation of CH3 groups and entropy difference between diastereoisomers: A 13C NMR study via relaxation time measurements
Authors: Brunel Y. , Coulombeau C. , Lavaitte S. , Reisse J. 
Published: January 2005

AbstractOverall and internal rotation can be studied by means of relaxation time measurements on carbon‐13. Diffusion constants associated with overall rotation and internal rotation can be used to quantify the intermolecular and intramolecular motional constraints. During the course of the study, a very careful examination of monomethyl and dimethylnorbornanes was performed and the results were compared with data obtained by neutron inelastic scattering. This study led to a clear conclusion concerning the real usefulness of relaxation time measurements as a very good alternative method for the determination of entropy contributions, which are usually calculated on the basis of vibrational studies.
Reactivity in radical abstraction reactions: Application of the curve crossing model
Authors: Pross Addy , Yamataka Hiroshi , Nagase Shigeru 
Published: January 2005

AbstractThe curve crossing model was applied to a series of hydrogen abstraction reactions from a family of alkanes, RH (R = methyl, ethyl, isopropyl, tert‐butyl) by alkyl, hydrogen and chlorine radicals. The analysis was based on quantitative data obtained from an ab initio MO study. Schematic reaction profiles for the reaction of RH with alkyl and hydrogen radicals are built up from just two configurations: reactant, DA, and product D3* A. For the Cl atom reaction, however, a significant contribution of D+ A−, a charge‐transfer configuration, is also shown to be present. A simple explanation for differences in the intrinsic barrier for the identity radical abstraction reaction based on the initial gap size between DA and D3* A configurations is provided. The influence of the D+ A− configuration on the nature of the transition state of the Cl atom reaction and its intrinsic barrier is described. It is the D+ A− configuration that is responsible for the polar character often observed in radical abstraction and addition reactions.
AB initio MO study of the halogen cation basicities of some organic bases
Authors: Alcamí M. , Mó O. , Yáñtez M. , Abboud José‐Luis M. 
Published: January 2005

AbstractHartree—Fock calculations were performed to investigate the structure and relative stabilities of complexes between halogen cations and first‐ and second‐row bases. It is shown by means of both a qualitative perturbation molecular orbital treatment and a topological analysis of the electronic charge density that second‐row bases present enhanced halogen cation basicities compared with first‐row bases. In this respect the results predict that although the fluorine cation basicity of water is smaller than its proton affinity, the fluorine cation basicities of SH2, thioether, phosphine and trimethylphosphine are considerably higher than their proton basicities. Similarly, phosphine and trimethylphosphine should have chlorine cation basicities greater than their proton affinities, in contrast with ammonia and trimethylamine. The results also show that fluorine and chlorine cation basicities are more sensitive to methyl substitution than proton affinities.
Kinetics and mechanism of the alcoholyses of ethyl fluorosulphate
Authors: Cafferata Lázaro F. R. , Svartman Esther L. 
Published: January 2005

AbstractThe alcoholyses of ethyl fluorosulphate in methyl, ethyl, n‐propyl, isopropyl, n‐butyl, isobutyl, tert‐butyl and n‐pentyl alcohols in the initial concentrations and temperature ranges of (1–10) 10−3 M and − 15.00 to 22.00 °C, respectively, were studied. The reactions follow pseudo‐first‐order kinetic laws up to at least ca 75 per cent ester conversions. At 10°C the experimental rate constant for ethanolysis is similar to that of ethyl triflate but much smaller than for its hydrolysis. The reactivity of the ester is ca 104–105 times higher than that of alkyl halides and benzenesulphonates. In alcohol–benzene and alcohol–acetone mixtures the ΔH≠ and ΔS≠ activation parameters are only slightly affected by the media composition. The effect of the ‘lyate ion’ on the kinetics of these reactions were investigated and the effects of changes in the dielectric constant of the media were evaluated. It is concluded that the alcoholyses of the ester fulfil the requirements for predominant SN2‐type mechanism, where the reaction ground states are more polar than the corresponding transition states.
Gas‐phase acidity of some α‐keto aldoximes: Experiment and theory
Authors: Bouchoux G. , Jaudon P. , Decouzon M. , Gal J.‐F. , Maria P.‐C. 
Published: January 2005

AbstractExperimental gas‐phase acidities of 2‐oxopropanaloximes, XCH2COCHNOH (X  H, CH3S, CH3SO, CH3SO2; compounds 1–4), were determined by Fourier transform ion cyclotron resonance (FT‐ICR) spectrometry. The values are δ Gacid° = 1401, 1381, 1360 and 1351 kJ mol−1 for 1, 2, 3, and 4, respectively. Molecular orbital calculations using the semi‐empirical AM1 method provided information on the geometry and relative energy of neutrals species 1–4 and their conjugate bases, together with charge distributions and entropies of deprotonation. It is demonstrated that the proton abstraction occurs preferentially at the oxime function; the formation of an enolate as a conjugate base is unfavourable by 70–140 kJ mol−1. The large variation of the gas‐phase acidities for 1–4 is explained in terms of the field/inductive empirical substituent constant σF. The variation of solution acidities appears to be comparatively strongly attenuated. This attenuation is attributed mainly to charge delocalization in the anion, which was confirmed by charge density calculations.
Kinetic modelling of the low‐temperature photo‐oxidation of hexafluoropropene
Authors: Faucitano A. , Buttafava A. , Comincioli V. , Marchionni G. , Pasquale R. J. De 
Published: January 2005

AbstractBy kinetic simulation, the significant features of the mechanism of the low‐temperature photo‐oxidation of hexafluoropropene were elucidated and the rate constants for the major elementary reactions of the intermediate fluorinated peroxy and alkoxy radicals were determined through a best‐fit procedure. Comparison with analogue reactions in non‐fluorinated systems showed a significant increase in reactivity for the self‐reaction and double bond addition by peroxy radicals and β‐scission by alkoxy radicals, which are discussed in terms of fluorine substituent effects.
Two reintroduced solvatochromic indicators for hydrogen bond donation and acceptance
Authors: Migron Y. , Marcus Y. 
Published: January 2005

AbstractThe behaviour of cis‐dicyanodi‐1‐10‐phenanthrolinoiron(II) and acetylacetonato‐N, N,N′,N′‐tetramethylethylene‐diaminocopper(II) perchlorate as solvatochromic indicators for the ability of solvents to donate a hydrogen atom or a pair of electrons, respectively, towards the formation of a hydrogen bond is examined by multivariable linear regressions and principal component analysis.
Crystallographic studies of intra‐ and inter‐ molecular interactions. Part VI. Crystal and molecular structure of N,N′‐dimethyl‐N′‐phenylsulphonylformamidine. Equalization of CN bond lengths in the amidine fragment as a result of substituent effects due to push–pull
Authors: Anulewicz Romana , Krygowski Tadeusz Marek , Raczyńska Ewa D. , Laurence Christian 
Published: January 2005

AbstractThe crystal and molecular structure of N,N‐dimethyl‐N′‐phenylsulphonylformamidine is solved by direct methods and refined to R = 0.041 for 935 reflections, Equalization of CN bond lengths in the amidine fragment is discussed in terms of a push—pull effect operating between the N,N‐dimethyl and SO2Ph groups. A significant role of the substituent at the functional carbon on the degree of π‐electron delocalization on the NCN fragment is interpreted by use of the HOSE model.
The schiff base between pyridoxal‐5′‐phosphate (PLP) and hexylamine. Formation of the unprotonated form of the imine by reaction of the unprotonated PLP and free amine
Authors: Blázquez Manuel , Pineda Teresa , Sevilla Jose M. , Domínguez Manuel , García‐Blanco Francisco 
Published: January 2005

AbstractElectrochemical and spectrophotometric studies of the pyridoxal‐5′ ‐phosphate hexylamine Schiff base (PHSB) in strongly basic media were carried out. The equilibrium constant and rate constants of the formation and hydrolysis of the unprotonated imine from the unprotonated pyridoxal‐5′ ‐phosphate and free amine were determined by linearscan cyclic voltammetry. The acid‐base dissociation constant of the imine group was obtained by spectrophotometric measurements. Absorption bands for the free imine and the conjugate acid are characterized by a log‐normal distribution. These studies are for general application to the quantitative characterization of non‐enzymatic model compounds.
Short communication. Solvolysis of some benzylic and non‐benzylic tertiary p‐nitrobenzoates. A new YBnOPNB scale
Authors: Liu Kwang‐Ting , Chen Hung‐I , Chin Chien‐Pu 
Published: January 2005

AbstractA new Y scale for the correlation of solvolytic reactivities for benzylic ρ‐nitrobenzoates was established based on rate data for 2‐phenyl‐2‐adamantyl ρ‐nitrobenzoate, and from which the solvent assistance in the solvolysis of tert‐cumyl ρ‐nitrobenzoate is suggested.
Methyl substituent effects on the dissociation energies of SiX bonds
Authors: Luo Yu‐Ran , Pacey Philip D. 
Published: January 2005

AbstractEmpirical expressions are obtained for bond dissociation energies (BDEs) of alkylsilanes and their derivatives. It is pointed out that the sensitivity of the BDEs of SiX bonds to methyl substitution depends on the electronegativity of the substituent X. When X is an atom or group with low or moderate electronegativity, the SiX BDE is insensitive to methyl substitution, but when X is a halogen atom or OH, the SiX BDEs increase with increasing methyl substitution.
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

AbstractA 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

AbstractA 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

AbstractCationic 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

AbstractThe 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.
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

AbstractThe 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

AbstractWith 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.