bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactions between tetracyanoethylene and rauwolfia alkaloids
Authors: Cuervo L. , Muñtoz M. A. , Guardado P. , Carmona C. , Hidalgo J. , Balon M.
Published: January 2005

Interactions of the Rauwolfia alkaloids yohimbine, corynanthine, ajmalicine and reserpine with tetracyanoethylene were investigated by UV—visible spectroscopy. The results suggest the instantaneous formation of blue complexes whose thermodynamic and spectroscopic properties closely resemble those of 1 : 1 charge‐transfer complexes of tetracyanoethylene with indole derivatives. The complexes are stable when the piperidinic nitrogen atom of the alkaloids is protonated or methylated. In contrast, those of the free bases rapidly decompose to give the corresponding 3,4‐dehydro derivatives. The kinetics of these reactions have been studied and a mechanism is proposed.
Acid hydrolysis of 2‐substituted 3‐methyl‐tetrahydro‐1,3‐oxazines: Simple models for tertiary glycosylamines
Authors: Parkkinen Aija , Pihlaja Kalevi
Published: January 2005

The hydrolytic decomposition of 2‐phenyl‐ and 2‐isopropyl‐3‐methyltetrahydro‐1,3‐oxazines to 3‐methylaminopropan‐1‐ol and appropriate aldehydes was studied by 1H NMR and UV spectroscopy in acidic solutions. The time‐dependent spectra confirmed that the formation of the final products was preceded by an equilibration of three components, the two Schiff base intermediates and the starting material, of which the former produced a carbinolamine which in turn underwent heterolysis to the final products.
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

Aqueous‐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

Kinetic 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

Although 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

The 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

The 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

Gas‐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

Addition (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

Treatment 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

It 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

A 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

The 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

The 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

Carbon 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

Two 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

The 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

Three 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

Long‐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

A 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

The 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

The 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

The 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

The 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.
Solvent and secondary substrate isotope effects on the acid‐catalyzed ketonization of acetophenone enol in aqueous solution
Authors: Andraos J. , Kresge A. J. , Obraztsov P. A.
Published: January 2005

The enol of acetophenone was generated flash photolytically in aqueous solution by photosolvolysis of PhCBrCH2 and photohydration of PhCCH and PhCCD, and rates of its ketonization were measured in dilute perchloric acid solutions in H2O and D2O at 25°C. The rate constants so obtained provide the solvent isotope effects, kH+/kD+ = 5.02 ± 0.08, and the secondary isotope effect for deuterium substitution at the β‐position of the enol double bond, (kH/kD)β = 0.999 ± 0.014. Arguments are presented which show that these isotope effects requre a stepwise rather than a concerted mechanism for the ketonization reaction.
Reactions of molecules with two equivalent functional groups. 3. Nucleophilic substitution reactions of α, ω‐dibromoalkanes [Br(CH2)nBr, n = 3,4,5] with potassium cyanide
Authors: Constantinides Ioannis , Macomber Roger. S.
Published: January 2005

The substitution reactions of Br(CH2)nBr(1, n = 3–5) with KCN in methanol were studied in detail. Second‐order rate constants k1 [formation of the mononitrile (2) from 1] and k2 (formation of the dinitrile (3) from 2) were determined, as were the rates for the reaction of H(CH2)nBr (4, n = 3‐5) with KCN under the same conditions. The ratios k2/k1 ( = x) of the three homologs of 1 were found to be 1.15, 0.77 and 0.61 for n = 3, 4 and 5, respectively; a x value of 0.5 indicates that the functional groups behave independently. The second‐order rate constants k1 (statistically corrected) and k2 exhibit modest enhancements compared with model compound 4 when any of the following substituents are present: δ or ϵ‐bromo, γ‐, δ‐, or ϵ‐cyano. By contrast, a γ‐bromo substituent slightly retards the rate. These results indicate that the functional groups do influence each other to a modest degree by field effects which accelerate the reactions. Steric effects do not appear to play a significant role.
New kinetic methods. 2.. An indirect measurement of ester formation and hydrolysis rates
Authors: Chandler W. David , Yan Yongfei , Lee Donald G.
Published: January 2005

The use of an assistant reagent to monitor spectroscopically the concentration of compounds that lack chromophores is described. It is demonstrated, for example, that the concentration of an aliphatic alcohol (present during acidcatalyzed hydrolysis or esterification reactions) can be monitored continuously by addition of a small amount of chromium trioxide. From a knowledge of the rate law and the rate constants for chromic acid oxidations, − d [CrO3]/dt = k [alcohol] [CrO3], the concentration of alcohol can be determined at any time by monitoring the absorbance of chromic acid at 363 nm. The rate at which the concentration of the alcohol is changing can then be used to calculate rate constants for the corresponding esterification or hydrolysis reactions. Rate constants obtained in this way are compared with those previously obtained by use of direct methods, and the application of this approach to the study of kinetics under conditions not accessible by other methods is illustrated.
Quantum mechanical and molecular mechanical studies of the hydrolysis of methyl nitrate and the solvent effect
Authors: Wang Daxi , Xiao Heming , Li Shusen
Published: January 2005

The alkaline hydrolysis reaction of methyl nitrate was simulated by quantum mechanical and molecular mechanical methods. The gas‐phase reaction was calculated to proceed with no barrier when ab initio calculations at the STO‐3G level were utilized, and with a very small barrier (5.70 kJ mol−1) when the MINDO/3 method was applied. In solution, the solvation energy was estimated to be 70.33 kJ mol−1 by molecular mechanical calculations. Thus the activation energy (76.03 kJ mol−1) of alkaline hydrolysis of methyl nitrate was found to be in good agreement with the available experimental value (82.42 kJ mol−1). The results indicated that the substantial activation barrier in alkaline hydrolysis was essentially solvent‐induced.
Catalysis in nucleophilic aromatic substitution reactions. A reinvestigation of the reaction between 1‐fluoro‐2,4‐dinitrobenzene and n‐butylamine
Authors: Forlani Luciano , Bosi Michele
Published: January 2005

The reaction between 1‐fluoro‐2,4‐dinitrobenzene and n‐butylamine in toluene shows a two‐step plot of kobs, values vs the initial values of the concentration of the amine. The usual base‐catalysis mechanism for HF elimination from the zwitterionic intermediate hardly explains this kinetic behaviour and the kinetic effect of addition of salts (and of 2‐hydroxypyridine) to the reaction mixtures at different initial values of the concentration of n‐butylamine. In contrast, the kinetic behaviours are easily explained by the presence of substrate–amine (or catalyst) interactions on the pathway of the substitution reaction.
Potentiometric evaluation of the ion‐selective characteristics of 2‐borono‐1,3‐XYLYL crown ethers
Authors: Williams Gwyn , Tuladhar Sudersan M. , D'silva Claudius
Published: January 2005

Poly(vinyl chloride) (PVC) membranes impregnated with 2‐borono‐1,3‐xylyl crown ethers were evaluated in ion‐selective electrodes. The response pattern obtained with these crown ethers appears to be essentially independent of ring size but there is a linear correlation between selectivity and hydration enthalpy of the cation which is similar to that obtained for electrodes prepared only with the lipophilic anionic site KTCIPB. The similarities in correlation and mechanism between these two types of electrodes has been attributed to the presence of anionic sites in both cases dominating the observed selectivity. For the four ionophores considered the pH‐dependent ionization of the boronate group to the boronate anion is proposed to be responsible for the domination of anionic sites on the properties of the ionophores. The ionophores proved to be good K+ neutral carriers, showing responses to this ion in the range 10−1−10−4 M. The compounds tested showed a fast response to changes in K+ concentration which was stable to within < 0.2 mV h−1.
Modulation of the cation complexing properties in the ‘lower rim’ chemically modified calixarene series
Authors: Schwing‐weill M. J. , Arnaud‐neu F. , McKervey M. A.
Published: January 2005

The effect of chemical modifications on the lower rim of calix [n] arenes is analysed with respect to the cation binding ability of the receptor. Extraction data and stability constants of the complexes are discussed. Three main factors are investigated: the size of the calixarene, the conformation of the calixarene and the nature of the ligating group attached to the phenolic oxygen. The work concentrates on esters, ketones, amides, thioamides and carboxylic acids. Some data concern chemically modified tetrahomodioxacalix [4] arene esters.
Solvent effects in aromatic nucleophilic substitution reactions in non‐polar aprotic solvents. Inhibition by electron‐donor–acceptor (EDA) complexation of the substrate by aromatic solvents
Authors: Durantini E. , Zingaretti L. , Anunziata J. D. , Silber Juana J.
Published: January 2005

The reactions of n‐butylamine and piperidine with 1‐chloro‐2, 4‐dinitrobenzene and 4‐chloro‐3‐nitrotrifluoromethylbenzene were studied in n‐hexane, benzene and mesitylene. The reactions seem to follow a bimolecular SNAr mechanism for both substrates and amines. However, whereas in n‐hexane they are base catalysed, in the aromatic solvents only mild acceleration is observed. The different behaviour of these reactions in the aromatic solvents in comparison with hexane is explained by the specific solvent effect exerted by the aromatic compounds, which solvates the aromatic reactant preferentially in view of its electron‐donor properties. These conclusions were confirmed by kinetic studies in hexane‐aromatic solvent binary mixtures. It can be concluded that the presence of electron‐donor solvents may inhibit SNAr reactions when association constant of the solvent with the substrate is greater than the EDA constants between the reactants. These specific solvent effects could also explain why only mild acceleration is observed in aromatic solvents.
Interaction of formamide with stilbazolium betaines: Steric effects in amides
Authors: Catalan Javier , Pérez Pilar , Elguero José
Published: January 2005

The visible spectrum of di‐tert‐butylstilbazolium betaine (DTBSB) was recorded in eight simple amides. The appearance of a ‘coarse’ structure in the main absorption band is due to the shielding effect of the tert‐butyl groups, which hinder the approach of the solvent. Since this structure disappears in formamide, it is concluded that this solvent associates through its NH to the carbonyl group of DTBSB. For the other amides, there is a clear relationship between the steric effect, for both the N‐ and C‐substituents, of the solvent–solute hydrogen bond association and the structure of the visible band.
Comparison of MNDO, AM1 and PM3 rotational barriers in branched alkanes
Authors: Burke L. A. , Krishnan P. N. , Morris R. E. , Famini G. R.
Published: January 2005

The MNDO, AM1 and PM3 semi‐empirical methods were used to calculate the equilibrium points and transition‐state structures of the rotational barriers of ethane and all methyl‐substituted propanes and butanes. Force constant methods were used to ensure proper geometries. In general, AM1 barriers tend tobe lower than MNDO calculated ones, which are themselves generally lower than experiment. The PM3 values are closest to experiment, ranging from slightly above to slightly below the experimental values. The complexity of various possible rotational minima makes the development of empirical rules to predict barrier heights difficult.
Allosteric effects in a synthetic receptor for alkali metal ions and dinitrobenzene
Authors: Sijbesma Rint P. , Nolte Roeland J. M.
Published: January 2005

A novel host is described which contains three potential binding sites, viz. a cleft and two crown ether rings. The molecule can adopt three different conformations. One of these is able to form a complex with two alkali metal ions. The binding of potassium ions is shown to be a cooperative process. The association constant of the second ion is 128 times larger than that of the first. The compound also binds 1,3‐dinitrobenzene at the third site by π–π interactions. The complex with two potassium ions binds 1,3‐dinitrobezene more strongly than does the free compound by a factor of 2–6, depending on the solvent used. This allosteric effect is caused by the conformational change which is induced by the binding of the potassium ions.
Kinetics and mechanisms of oxidation of thiosemicarbazide, its metal complex and hydrazones by N‐chlorobenzamide in water methanol medium
Authors: Nayak Suresh P. , Gowda B. Thimme
Published: January 2005

The kinetics of oxidation of thiosemicarbazide (TSC) its zinc metal complex and hydrazones by N‐chlorobenzamide (NCB) were investigated in water methanol (1 : 1, v/v) medium in the presence of perchloric acid. The hydrazones studied were benzaldehyde, propionaldehyde, acetone and acetophenone thiosemicarbazones. The reactions show first‐order kinetics in [NCB], a fractional order dependence in [substrate] and an inverse fractional to inverse first order in [H+]. Addition of benzamide has no significant effect on the rates of oxidations. Variation in the ionic strength of the medium has little effect on the rates of reactions, but a decrease in the dielectric constant of the medium by increasing the methanol concentration in the solvent increases the rates. The rate‐limiting steps were identified in all cases and the rate coefficients of these steps and the related activation parameters were also evaluated. The consistency of the deduced rate laws was checked by recalculating the rate constants as the substrate and H+ concentrations were varied.
Structure of transition states in forbidden pericyclic reactions. The second‐order similarity approach
Authors: Ponec R. , Strnad M.
Published: January 2005

The recently proposed similarity approach to the Evans/Dewar concept of aromaticity of transition states was generalized by introducing the second‐order similarity index. This generalization is especially convenient for the characterization of antiaromatic transition states in forbidden reactions since, owing to the partial incusion of electron correlation in the second‐order index, the detailed specification of their structure in terms of the electron states of these biradicaloid species becomes possible. The results obtained are in complete agreement with the conclusions of independent quantum chemical analyses.
Significance of the cis–trans isomerization of early intermediates in the carotene biosynthetic pathway
Authors: Paneth Piotr , Madhavan S. , O'Leary Marion H.
Published: January 2005

Using semi‐empirical self‐consistent field molecular orbital (SCF‐MO) quantum chemical calculations, structures of putative intermediates of the desaturation pathway for both cis‐ and trans‐ carotene biosynthesis have been optimized. It was observed that the cis isomers of the early biosynthetic intermediates are more stable than corresponding trans isomers. Both desaturation and cyclization steps confer increased stability on these carotenes. The results also argue for phytofluene, rather than the earlier suggested phytoene or ζ‐carotene, as the energetically favored branch point for poly‐cis‐carotene biosynthesis.
Micelle‐induced change in the rate‐limiting step of substituted benzoate ester thiolysis
Authors: Correia Valdir Rosa , Cuccovia Iolanda Midea , Chaimovich Hernan
Published: January 2005

The effect of hexadecyltrimethylammonium bromide (CTAB) micelles on the acid dissociation constant of para‐substituted benzenethiols, α‐toluenethiol and n‐heptanethiol was determined. The effect of CTAB on the rate of thiolysis by the thiols was measured using para‐substituted p‐nitrophenyl benzoates. The effects of micelles were analysed using a pseudophase ion‐exchange (PIE) model. The Brønsted plot for ester thiolysis showed discontinuity at the pKa of the leaving group in water and was linear in micelles. Micelles increased the Hammett ρ value for thiolysis of esters by α‐toluenethiol from 2·08 to 2·68. The Hammett plot for the rection of benzenethiol with esters in water is linear with σ− and displays a ρ value of 0·87, whereas in micelles the plot is linear with σ and presents a ρ of 2·83. Taken together, these data indicate that micelles produce a change in the rate‐limiting step of thiolysis of substituted benzoate esters, leading exclusively to rate‐limiting thiolate attack. In micelles thiolysis may be concerted and the effect of the aggregate on the reaction mechanism can be ascribed to the interaction of the thiolate ions, and the transition states, with the head groups as well as a medium effect.
Zn2+ complexes with N‐(2‐pyridyl)cinnamides: Characterization and UV studies
Authors: Duran H. , Duran E. , Gorrichon L. , Perry M.
Published: January 2005

Zinc(II) complexes of N‐pyridylcinnamides (H, m‐OCH3, p‐OCH3 and p‐OH derivatives) were studied by spectrophotometric methods in aprotic media, and represent a chemical model inhibition by cinnamides of coniferyl alcohol dehydrogenase (CAD), a zinc enzyme involved in the lignification process. The complexation of N‐pyridylcinnamide and m‐methoxy‐N‐pyridylcinnamide with zinc ion is effected according to a 1 : 1 stoichiometry (ML), whereas a two‐step equilibrium (M + L ⇌ ML \documentclass{article}\pagestyle{empty}\begin{document}$ V_{\rm m} = 4 \cdot 45C_0 + 6 \cdot 44C_1 - 3 \cdot 33C_2 + 0 \cdot 46C_4 + 52 \cdot 9{\rm (}n = 300,r = 0 \cdot 999,s = 0 \cdot 81) $\end{document} ML2) is preferred with p‐methoxy and p‐hydroxy compounds. These ligands are mainly bonded through the carbonyl oxygen atom and the nitrogen of the pyridyl ring. Molar absorptivities for these complexes, not directly available, were calculated from analysis of the experiemental data. The UV complexation results are also supported by the stoichiometry of the complexes, which were synthesized and characterized.
Carbon and hydride migrations of cyclobutylhalocarbenes
Authors: Moss Robert A. , Ho Guo‐Jie
Published: January 2005

The photolysis of cyclobutylchlorodiazirine affords chlorocyclopentene and chloromethylenecyclobutane by direct rearrangements from the diazirine's excited state (65%) and by competitive formation and subsequent rearrangement of cyclobutylchlorocarbene. The involvement of excited diazirine in the analogous cyclobutylfluorodiazirine photolysis is much reduced (12%).
Study of initial‐ and transition‐state solvation in the solvolysis of tert‐butyl halides in alcohols from infinite dilution activity coefficients
Authors: Gonçlalves Raquel M. C. , Simões Ana M. N. , Albuqerque Lídia M. P. C. , Macedo Euǵania A.
Published: January 2005

The Gibbs energies of activation of the solvolytic reactions of tert‐butyl halides (chloride, bromide and iodide) in eight mono‐alcohols and eight di‐alcohols were dissected into contributions from the initial state and transition state. The unified approach proposed by Abraham was applied and the various factors making up the overall solvent effect in both states were identified and compared. To perform this study, the Gibbs energies of transfer of the initial state were calculated using the infinite dilution activity coefficient values of the solutes in the solvents, γ∞. These values were determined from the UNIFAC group‐contribution method, with the modified Flory–Huggins equation in the combinatorial term and with group interaction parameters from the Parameter Table exclusively based on experimental γ∞ data reported in the literature. It is concluded that solvent polarizability and polarity–HBD acidity and cavity effects during the activated process are mainly due to transition‐state contributions.
Theoretical studies of substituent effects on thermal eliminations of aryl ethyl carbonates andS‐arylO‐ethyl thiocarbonates
Authors: Kim Chang Kon , Lee Bon‐Su , Lee Ikchoon
Published: January 2005

Substituent effects on the thermal elimination reactions of carbonates and thiocarbonates were investigated MO theoretically using the AM1 method. For both substrates, an electron‐withdrawing group is found to facilitate the decomposition. A two‐step process is favoured by carbonates, whereas one‐ and two‐step processes can compete in the pyrolysis of thiocarbonates. The ability to transmit substituent effect is greater with oxygen (carbonates) than with sulphur (thicarbonates) in contrast to the experimental results of Taylor and co‐workers. Modelling studies with replacement of the phenyl ring with a vinyl group are found to be justified in this type of work.
Ab initio studies of the reaction between diazonium ions and water
Authors: Sapse Anne‐Marie , Jain Duli C.
Published: January 2005

Ab initio calculations, using the 6–31G* basis set, were performed in order to study the energetics of the electrophilic attack on water by such diazonium ions as methyl‐, ethyl‐ and fluroethyldiazonium. For the first case, an indication of a transition state appears at the Hartree–Fock level, whereas for the other two, the energy is continuously decreasing from reactants to products. When correlation energy was taken into account via single‐point MP2/6–31G* calculations, a deep ion–dipole energy minimum was observed for the methyldiazonium ion followed by a low transition state, whereas the other two did not change their energy pattern. On performing the MP2/6–31G* optimization for the methyl species, the Hartree–Fock results were confirmed, with a slight displacement of the transition state toward a shorter CO distance.
Homoaromaticity of decamethyl[5]pericyclyne
Authors: Schaad L. J. , Hess B. A. , Scott L. T.
Published: January 2005

It is shown that a theoretical computation of the Dewar resonance energy of decamethyl [5] pericyclyne should give the same value for the homoaromaticity of this compound as that calculated earlier from heat of hydrogenation measurements. Both methods show this quantity to be small, but the accuracy of the methods appears insufficient for more exact agreement.
Temprature dependence of the deuterium isotope effect in the deprotonation of some nitroalkanes by anionic bases
Authors: Amin Mohammed , Saunders William H.
Published: January 2005

The temperature dependences of rates and deuterium isotope effects were determined for the deprotonation of 1‐phenyl‐1‐nitroethane (1), its analogue 1‐d, 1‐phenyl‐2‐nitropropane (2) and its analogue 2‐d by methoxide in methanol, ethoxide in ethanol, hydroxide/methoxide in 50% (v/v) methanol–water and hydroxide in water. The isotope effects varied little with substrate or solvent–base combination. kH/kD at 30°C was between 6·0 and 7·1 in all cases. The temperature dependences varied considerably [AaH/AaD = 0·14–1·35 and EaD – EaH 0·95–2·29 kcal mol−1 (1 kcal = 4·184 kJ)]. AaH/AaD was lower with 1‐phenyl‐1‐nitroethane than with 1‐phenyl‐2‐nitropropane, but there was no consistent variation with solvent/base.
Microwave activation of the mutarotation of α‐D‐glucose: An example of an interinsic microwave effect
Authors: Pagnotta M. , Pooley C. L. F. , Gurland B. , Choi M.
Published: January 2005

The mutarotation reaction of α‐D‐glucose in ethanol–water solvent in a modified commercial microwave oven, when compared with control reactions carried out at identical temperatures, shows a non‐thermal microwave effect, evidenced both by a more rapid reaction and by a change in relative amounts of α‐ and β‐D‐glucose over time.
Adsorption at the liquid–liquid interface: An important factor in phase‐transfer catalysis
Authors: Lasek Wojciech , Mąkosza Mieczysław
Published: January 2005

A new rationalization of some phenomena in the phase‐transfer catalysed processes promoted by concentrated aqueous alkali solutions, such as alkylations and β‐eliminations, is proposed. It is predicted on the basis of adsorption theory and subsequently evidenced experimentally that the size of the interfacial area, controlled by the rate of stirring, affects the position of the extraction equilibrium of various anions with quaternary ammonium cations supplied by the phase‐transfer catalysts.
Vibrational spectra of C4H isomers in low‐temprature antimony pentafluoride matrices
Authors: Vančik H. , Gabelica V. , Sunko D. E. , Buzek P. , v. R. Schleyer P.
Published: January 2005

The IR spectra of isomeric bicyclobutonium (1), delocalized cyclopropylcarbinyl (2) and 1‐methylally (3) cations were recorded at 180 K in SbF5 matrices. Cations 1 and 2 generated from cyclopropylcarbinyl and cyclobutyl chloride, respectively, rearrange to 3 at temperatures above 230 K. The structures 1, 2 and 3 were confirmed by comparison of the recorded frequencies with the MP2/6–31G*‐calculated values. These results are in accord with prediction that ions 1 and 2 are rapidly equilibrating non‐classical structures.