bluuebunny/crossref_metadata_2025_split huggingface dataset:
Potentiometric and spectroscopic investigations of the aqueous phase acid–base chemistry of urazoles and substituted urazoles
Authors: Bausch Mark , Selmarten Don , Gostowski Rudy , Dobrowolski Piotr 
Published: January 2005

AbstractAqueous‐phase equilibrium acidity constants have been determined for urazole (1,2,4‐triazolidine‐3,5‐dione), several substituted urazoles and other selected imides. Aqueous phase pKa values for urazole (5.8), 1‐methylurazole (5.3), 4‐methylurazole (5.7) and 1,4‐dimethylurazole (5.3) indicate that (in water) the methyl group acidifies the urazole moiety when bonded to N‐1 but has little effect on acidity when bonded to N‐4. Aqueous phase pKa values for hydantoin (9.0) and 1‐methylhydantoin (9.1) suggest that a 1‐methyl substituent has little effect on the acidity of the imide proton present in hydantoin. These data, combined with aqueous phase pKa values for 1,2‐dimethylurazole (7.5), a substituted urazole lacking amide protons, indicate that an amide proton in urazole (i.e. the proton bound to either N‐1 or N‐2 in urazole) is ca 1.5 pKa units more acidic than the imide proton (i.e. the proton bound to N‐4). Changes in 13C NMR chemical shifts for the carbonyl carbon atoms present in variously substituted D2O phase urazoles, hydantoins, succinimide and their conjugate bases also confirm that amide NH bonds found in urazole and 1‐methylurazole are weaker, in a heterolytic acid—base sense, than imide NH bonds.
Cross‐interaction constants as a measure of the transition state structure. Part 15. Kinetic isotope effects in the SN2 reactions of 2‐phenylethyl derivatives with deuterated aniline nucleophiles
Authors: Lee Ikchoon , Koh Han Joong , Lee Hai Whang 
Published: January 2005

AbstractKinetic isotope effects (KIE) were determined for the reactions of 2‐phenylethyl and 1‐methyl‐2‐phenylethyl benzenesulphonates with deuterated aniline nucleophiles in acetonitrile at 65.0 °C. The results are in good agreement with the transition state (TS) structures proposed based on the sign and magnitude of the cross‐interaction constants, pxz, between the substituents in the nucleophile (X) and the leaving group (Z). In the reactions of 2‐phenylethyl derivatives, all three reaction pathways, kr, kt and kΔ, were found to be contributing competitively; the inverse secondary KIE observed with a stronger nucleophile changed into the primary KIE with a weaker nucleophile owing to the predominant contribution of the four‐centre TS in the kf path. For the reactions of the 1‐methyl‐2‐phenylethyl series, the kr path played a major role, the contribution from the front‐side nucleophilic attack, kf, being negligible. In both reaction series, the aryl participation was important for the p‐CH3O‐substituted substrate.
The grignard reaction of α‐diketones. Radical intermediates as the determinant of carbon‐ and oxygen‐attacked products
Authors: Maruyama Kazuhiro , Katagiri Toshimasa 
Published: January 2005

AbstractAlthough carbon (normal)‐ and oxygen (abnormal)‐attacked products are usually afforded in the Grignard reaction of α‐diketones, the relative ratio of the two are strongly dependent on the structure of the Grignard reagents. The product‐determining factors are discussed on the basis of the intermediate radicals formed from α‐diketones and Grignard reagents.
Intermolecular and intramolecular hydrogen bonding in 5‐pyridylmethylenehydantoins: IR and NMR study
Authors: Tan Sau‐Fun , Ang Kok‐Peng , How Gee‐Fung 
Published: January 2005

AbstractThe patterns of the NH stretching vibrations in the solid‐state IR spectra of a series of 5‐pyridylmethylenehydantoins revealed the presence of various modes of hydrogen bonding: intermolecular NH&4nldr; &4nldr;OC and intermolecular or intramolecular NH&4nldr; ‥‥N(py). These variations are related to the orientation of the pyridyl nitrogen and to stereochemistry. A distinction between intramolecularly and intermolecularly hydrogen‐bonded compounds was also provided by comparison of the 1H NMR spectra in (CD3)2SO and in CDCl3.
Conversion of flavanones into chalcones in alkaline medium. Kinetic and spectroscopic studies
Authors: Nudelman N. S. , Furlong J. J. P. 
Published: January 2005

AbstractThe conversion of flavanone and 7‐hydroxyflavanone in alkaline water and heavy water and of the same compounds and of 4′ ‐nitroflavanone in alkali‐methanolic media into the corresponding substituted chalcones was studied kinetically and spectroscopically. Treatment of kinetic data in this work and data for the reverse reaction determined previously allowed the estimation of the partial rate coefficients for each step and of the free‐energy changes for the three systems studied. To disentangle isotope effects, the conversion of [3‐D2]‐flavanone was also studied. The present results confirm a previously suggested mechanism for the spontaneous reaction and afford essential information that may contribute to a more detailed understanding of the mechanism of the enzyme‐catalysed reaction.
The nature of field effects and their fall‐off with distance: The acidity of substituted quinuclidinium and bicyclooctylammonium ions
Authors: Adcock W. , Anvia F. , Butt G. , Cook A. , Duggan P. , Grob C. A. , Marriott S. , Rowe J. , Taagera M. , Taft R. W. , Topsom R. W. 
Published: January 2005

AbstractGas‐phase acidities are reported for series of substituted quinuclidinium and bicyclo [2.2.2] octylammonium ions. Ab initio calculations are also reported for these equilibria at the STO‐3G and 3–21G (for the quinuclidines) bases and can be used to extend the results. pKa values obtained for the bicyclooctylammonium ions are compared with previously reported results for the quinuclidinium ions. The results, as a whole, are considered together with similar values obtained for other saturated systems, to look at the varying transmission of field effects with distance. The falloff observed between the two series in the gas phase is considerably less than expected from simple electrostatics. In contradication with a recent claim, it is also shown that field effects, as measured by the parameter σF, give a good measure of the effect of the substituents in each series compared with a combination of resonance and electronegativity effects.
Controlling factors determining addition and reduction products in the grignard reaction of substituted benzophenones, acetophenone and fluorenone with ‘EtMgBr’
Authors: Maruyama Kazuhiro , Katagiri Toshimasa 
Published: January 2005

AbstractAddition (Add.) vs reduction (Red.) products in the reactions of substituted benzophenones, acetophenone and fluorenone with ‘EtMgBr’ were compared in diethyl ether, 1,2‐dimethoxyethane and tetrahydrofuran. Plots of log (Add./Red.) vs σ‐values or the oxidation potentials of ketones showed characteristic upward curves depending on the polarity of the solvents. The results are discussed on the basis of both the combined effects of the stability of the monomeric radical ion pair and the dimeric radical ion pair derived from ketone and ‘EtMgBr’, and the well known Schlenk equilibrium of ‘EtMgBr’ in solutions.
SRN1 Reactions of nucleophiles with radical clocks: Rate constants for some radical–nucleophile reactions
Authors: Beckwith Athelstan L. J. , Palacios Sara M. 
Published: January 2005

AbstractTreatment of o‐3‐(but‐3‐enyloxy)benzene (1) with anions −SPh, −PPh2 and −PO(OEt)2 in ammonia, acetonitrile or dimethyl sulphoxide gives both direct substitution and cyclized products. The formation of the latter confirms the intermediacy of the o‐3‐(but‐3‐enyloxy)phenyl radical (2) and confirms that the reactions follow the SRN1 pathway. The kinetics of the coupling of the aryl radical 2 were determined by comparison with the known rate of cyclization of 2. Similar experiments with the neopentyl bromide, 6‐bromo‐5,5‐dimethylhex‐1‐ene, and its 3‐oxa derivative also gave cyclized and uncyclized products, thus confirming that the SRN1 mechanism applies to the reactions. Approximate rate constants for the coupling of a neopentyl radical with the nucleophiles −SPh and −PPh2 have thus been determined.
Applicability of excess acidity functions in low‐acidity media
Authors: García Begoña , Leal José M. 
Published: January 2005

AbstractIt is demonstrated that excess acidity functions do not correspond well to the pH scale in dilute solutions. Acidity constants, pK, of seven moderately weak bases were determined; they were calculated by means of excess acidity functions, since the corresponding ionizations occur at acidity levels where the pH scale is partly used. The excess acidity method permitted extrapolation of measurements made in concentrated acids down to dilute solutions only up to about 5% (w/w) perchloric acid.
The α‐bromocyclohexanone ring: The relationship between its conformation and the ultraviolet and infrared absorption of the keto group
Authors: Laing Michael 
Published: January 2005

AbstractA series of diterpenoids whose C‐ring was an α‐bromocyclohexanone showed a discontinuity in the relationship between the BrCCO torsion angle and the Br…︁O contact distance at about 50°, below which the Br…︁O distance remained effectively constant at about 3.0 Å. This effect is caused by the non‐compressibility of the van der Waals radii of the Br and O atoms. Analysis of the parameters for all compounds in the Cambridge Data File containing an α‐bromocyclohexanone ring confirmed this phenomenon. Once the Br…︁O distance approaches 3.0 Å, the BrCC and CCO angles are forced open to accommodate the Br…︁O compression strain as the torsion angle is further reduced. An approximate value of the Br…︁O distance, d in Å, can be estimated for any torsion angle, ø, by the empirical equation d2 = R – S cos ϕ + T cos2 ϕ where R = 11.73, S = 3.62 and T = 0.75. There is a linear relationship between the Br…︁O separation and both the infrared vibration frequency v and the ultraviolet absorption wavelength δmax of the CO group: the greater the distance, the lower is the frequency v and longer the wavelength δmax. Thus measurement of the ultraviolet and infrared spectra of a compound containing an α‐bromocyclohexanone system can yield information about the Br…︁O distance and hence the conformation of the six‐membered ring. There are also systematic trends in the effect on the spectroscopic properties of the keto group that accompany changing the halogen from fluorine through to iodine. When the halogen is axial, its electronegativity has a maximum influence on the CO vibration frequency by a ‘through‐bond’ inductive process. When the halogen is equatorial, it exerts a steric compression on the non‐bonded electrons in the 2p orbital of the oxygen atom. This enhances hybridization to the sp2 state and this simultaneously strengthens the CO bond and enlarges the n → π* energy gap. This ‘through‐space’ influence is simply a process of steric compression, dependent on the size of the halogen atom. It accounts well for all of the systematic trends in both infrared and ultraviolet frequencies of the CO group when the halogen is equatorial and eclipses the CO bond.
Kinetics and mechanism of the cleavage of phthalimide in buffers of tertiary and secondary amines. Evidence of intramolecular general acid–base catalysis in the reactions of phthalimide with secondary amines
Authors: Khan M. Niyaz , Ohayagha J. E. 
Published: January 2005

AbstractThe reaction rates of the cleavage of phthalimide (PTH) were studied at 30°C in buffer solutions of trimethylamine, triethylamine, triethanolamine, carbonate, piperidine, dimethylamine, morpholine, piperazine and N‐methylpiperazine at different pH values. The reactivity of carbonate, triethylamine and triethanolamine toward PTH could not be detected under the experimental conditions applied. However, triethylamine revealed nucleophilic reactivity toward non‐ionized PTH (SH). The steric requirements of triethylamine and triethanolamine are ascribed to the absence of nucleophilic reactivities of these amines toward SH. Secondary amines show nucleophilic reactivity toward both SH and ionized PTH (S−). The absence of nucleophilic reactivity of trimethylamine and the presence of nucleophilic reactivity of secondary amines toward S− are attributed to the occurrence of intramolecular general acid‐base catalysis due to the presence of a mobile proton at the nucleophilic site of the nucleophile. Intermolecular general base catalysis was observed in the reactions of SH with all secondary amines except piperidine and such catalysis could be detected in the reactions of S− with morpholine, piperazine and N‐methylpiperazine. Nucleophilic second‐order rate constants for the reactions of secondary amines with SH (k1) and S− (k2) reveal the Brønsted plots of slopes of ca 0.8 and 0.3, respectively. Similarly, the general base‐catalysed third‐order rate constants (k4) for the reactions of secondary amines with SH yield a Brønsted plot of slope 1.2. These observations suggest the occurrence of diffusion‐controlled trapping stepwise and pre‐association stepwise mechanisms in the aminolysis of non‐ionized and ionized phthalimide, respectively.
Gas‐phase thermolysis of aryl tert‐butyl disulphides
Authors: Martin Gonzalo , Ascanio Julian 
Published: January 2005

AbstractThe pyrolysis kinetics of RC6H4SStC4H9 (RH, p‐NO2, p‐Cl, p‐F) were determined at 390–450°C and 7–15 Torr in a stirred‐flow reactor using toluene as carrier gas. The reaction products were 95% isobutene, 5% isobutane and the corresponding RC6H4SSH disulphanes. The first‐order rate constants, k (s−1), based on isobutene production, followed the Arrhenius equations: phenyl tert‐butyl disulphide, k = 1013.49±0.31 exp (– 182 ± 4 kJ mol−1)(RT)−1; pnitrophenyl tert‐butyl disulphide, k = 1013.46 ± 0.32 [exp(– 185 ± 5 kJ mol−1)(RT)−1]; p‐chlorophenyl tert‐butyl disulphide, k = 1014.44 ± 0.66 [exp(– 196 ± 9 kJ mol−1)(RT)−1; p‐fluorophenyl tert‐butyl disulphide, k = 1010.80 ± 0.16 exp(– 144.5 ± 2 kJ mol −1)(RT)−1]; The observed reactivities, within the above temperature range, follow the order pF > H > pCl > pNO2. A four‐centre, cuadrupolar cyclic transition state mechanism is proposed for the formation of the isobutene and aryldisulphane products. The optimized ground‐state molecular geometries of the reactants were calculated by using the MINDO/3 procedure.
Semiempirical SCF‐MO calculations of kinetic isotope effects
Authors: Paneth Piotr 
Published: January 2005

AbstractCarbon and deuterium kinetic isotope effects (KIEs) on the isomerization of methyl isocyanide were calculated by using the geometries and force fields obtained with MNDO, AM1 and PM3 Hamiltonians. These theoretical KIE values were compared with the corresponding experimental values. None of the hamiltonians used proved to be superior to any of the others. The best agreement for the carbon isotope effect was obtained by using the PM3 method; however, the deuterium isotope effect obtained by this method gave the poorest fit to the experimental data. The opposite is true when the MNDO Hamiltonian was used.
Synthesis and NMR study of 5‐furylmethylenehydantoins and 5‐thienylmethylenehydantoins
Authors: Tan Sau‐Fun , How Gee‐Fung , Yeoh Phee‐Teik 
Published: January 2005

AbstractTwo series of 5‐furylmethylenehydantoins and 5‐thienylmethylenehydantoins were prepared and their 1H and 13C NMR spectra studied in comparison with those of compounds in the analogous 5‐aryl‐ and 5‐pyridyl‐ series. Differences in the effects of the aromatic, six‐ or five‐membered heteroaromatic rings are discussed. Spectral analysis enables Z/E configurations to be assigned with confidence and conformational preferences to be qualitatively deduced. Some interesting solvent effects were also observed.
Z/E, photoisomerization of 5‐arylmethylenehydantoins and 5‐pyridylmethylenehydantoins
Authors: Tan Sau‐Fun , Ang Kok‐Peng , How Gee‐Fung 
Published: January 2005

AbstractThe Z/E photoisomerization of a series of 5‐arylmethylenehydantoins and a series of 5‐pyridylmethylenehydantoins by direct irradiation in methanol or ethanol solvent was investigated. The isomeric ratios at the photostationary state are dependent on the excitation wavelength and are related to the relative absorption coefficients of the two isomers. Measurements of quantum yields show the absence of quenching effects by oxygen or azulene, suggesting that triplet states either are not involved or are too short‐lived for quenching to be observed. Preparative photoisomerization provides a route to the isomer which is not isolated or obtained only as a minor product by the usual method of synthesis.
Estimation of thermodynamic stability and charge distribution for unknown molecules: Nitro derivatives of cubane
Authors: Reed Lynne H. , Jayasuria Keerthi , Koovakkat Sunil K. , Allen Leland C. 
Published: January 2005

AbstractThree useful procedures for estimating the thermodynamic stability and charge distribution of moderately complex unknown species are illustrated by the successive nitration of cubane. (1) Five different, but interrelated, energy criteria are employed, which leads to mutually supportive conclusions that overcome deficiencies in any one of the single measures. (2) Mulliken charge and overlap populations can lead to correct bond strength trends if appropriate averaging over bond types is carried out. (3) Lewis—Langmuir atomic charges, an interpolation between the formal charges of Lewis dot structures and oxidation numbers which does not require use of computers, provides atomic charges similar to those from ab initio, wavefunctions. The simplicity of this scheme aids in identifying the chemical and topological origin of molecular charge distributions. Ab initio, calculations for the strain energies and heats of reaction for four different reaction sequences are reported, together with Mulliken atomic charges and overlap populations for the nitrocubanes. Trends in these measures suggest that hexa‐ and octa‐nitrocubane are thermodynamically stable species.
EPR/ENDOR study of the decay of trapped radicals in photopolymerized butane‐1,4‐diol diacrylate
Authors: Oliva Cesare , Selli Elena , Ponti Alessandro , Bellobono Ignazio Renato , Morelli Roberto 
Published: January 2005

AbstractLong‐lived trapped radicals produced during the photopolymerization of butane‐1, 4‐diol diacrylate (BDDA) were studied by EPR and ENDOR spectroscopy and their thermal decay was followed by EPR measurements at different temperatures (40–120 °C) for kinetic study. The EPR signal showed the superimposition of two different patterns, a three‐line and a single‐line spectrum. Both EPR patterns decayed following first‐order kinetics in the investigated temperature range. Activation parameters of the decay were obtained. The EPR patterns were attributed to the same radical species situated in fluid and in cross‐linked regions of the photopolymerized BDDA. Radicals of the latter kind undergo electron spin exchange strong enough to wash out the hyperfine splitting. The single‐line width is mainly determined by electron spin dipole–dipole interactions. The ENDOR response is only of the matrix kind, typical of radicals in a solid phase. The present model was also compared with recent literature reports.
Supramolecular sensitizer–relay assemblies. An evaluation of binding constants from non‐linear stern–volmer plots
Authors: Boßmann Stefan , Seiler Martin , Dürr Heinz 
Published: January 2005

AbstractA novel technique for determining the binding constants of host–guest complexes, viz. the bis‐cationic electron relays methylviologen and octylviologen in the macrocyclic host of novel (bisheteroleptic) crown ether‐ruthenium sensitizers, is described. This technique allows the separation of the quenching effects of bound and free electron relays. It can be used to calculate binding constants in non‐covalently linked supramolecular systems evaluating the electron transfer process between guest and host. High binding constants comparable to those in many natural enzymes and the photosynthetic reaction centre were found. As an explanation for the behaviour of the sensitizers bound to the bipyridinium cations it is suggested that photoelectron transfer affects the binding between the excited sensitizer and the bound acceptor.
Enthalpies of solution and intermolecular forces. tert‐butyl halides in hydroxylic solvents
Authors: Gonçalves Raquel M. C. , Albuquerque Lídia M. P. C. , Martins Filomena E. L. , Simões Ana M. N. , Ramos Joaquim J. Moura 
Published: January 2005

AbstractThe application of cavity theories of solution and linear solvation energy relationships to the interpretation of enthalpy of solution values of tert‐butyl halides in hydroxylic media, measured at 25 °C and infinite dilution, is deserted, in order to define the intermolecular forces acting between solutes and solvents. It is concluded that solvent dipolarity and solvent HBD acidity effects dominate the solution process. An empirical equation relating the enthalpies of solution with selected properties of both solvents and solutes is proposed. New experimental Δ Hs∞ values for tert‐butyl iodide in 13 alcohols are reported.
Protonated propylene oxide is stable towards isomerization in the gas phase
Authors: Lin Peiping , Kenttämaa Hilkka I. 
Published: January 2005

AbstractThe relative stabilities of gaseous protonated propylene oxide and its isomers, protonated propanal, oxetane and acetone, were reinvestigated in a Fourier transform ion cyclotron resonance mass spectrometer by using multiple‐stage tandem mass spectrometri experiments. The dependence of ion structure on internal energy was examined by generating the ions in proton transfer reactions with different exothermicities and then probing their structures by using energy‐resolved mass spectrometry (collision‐activated dissociation as a function of collision energy). In contrast to results obtained in several recent investigations, protonated propylene oxide was found to be distinct from its more stable isomers when generated with only a small amount of internal energy. When the exothermicity of the proton transfer reaction was higher than 6 kcal mol−1 (1 kcal = 4·184 kJ) or when the epoxide ion was subjected to multiple activating collisions, rapid isomerization to protonated propanal occurred. The energy required for opening of the epoxide ring estimated to be similar to that measured earlier for protonated cyclohexene oxide (5–10 kcal mol−1).
Substituent effect on the relative strength of some arylthiomethyl‐ and arylsulphonylmethylbenzoic acids
Authors: El‐Bardan Ali A. , El‐Mallah Nabila M. , Hamed Ezzat A. 
Published: January 2005

AbstractThe pK values of some arylthiomethylbenzoic acids and the corresponding sulphones were determined spectrophotometerically. The role of substituent and the position of the carboxylic group are discussed.
Charge‐transfer transition energies and the prediction of reactivity in electrophile–nucleophile reactions
Authors: Bethell Donald , Parker Vernon D. 
Published: January 2005

AbstractThe differences between charge‐transfer transition energies calculated for donor‐acceptor pairs at infinite separation and values determined experimentally for the charge‐transfer complex geometry vary according to the charge type of the pairs and within a group of fixed charge type. It is argued that these differences provide a guide to the slope of the state‐correlation curves for the product configuration in electrophile‐nucleophile reactions of the corresponding charge type and that the observed variation invalidates the use of the simple expression ΔE=fΛ‐β with constant f, derived from the curve‐crossing model, to estimate energy barriers to reaction.