bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deuterium fractionation factors for carbon–hydrogen bonds: Calculations using scaled quantum‐mechanical force constants
Authors: Williams Ian H. 
Published: January 2005

AbstractThe determination of uniform scaling factors for force constants calculated at the STO‐3G and 4‐31G levels of ab initio SCF MO theory is described; scaled 4‐31G force constants provide reasonable estimates of reduced partition function ratios for deuteriated/protiated molecules. Gas‐phase deuterium fractionation factors relative to methane calculated using scaled 4‐31G force constants are, however, consistently too high, whereas those obtained by the MP2/6‐31G* and scaled STO‐3G methods are inconsistent. Scaled 4‐31G deuterium fractionation factors relative to water for a range of structural moieties correlate linearly with experimental aqueous‐phase values, but are also consistently overestimated. Substitution at a hydrogen‐bearing carbon atom affects the deuterium fractionation factor through the combined effect of changes in reduced mass and changes in force constants other than for CH stretching.
The decay of 2‐phenyl‐1,3‐dioxolane‐2‐ylium salts in solution
Authors: Spange Stefan , Heublein Günther , Schütz Heidrun , Lukaszcyk Jan , Jedlinsky Zbigniew 
Published: January 2005

AbstractThe decay of 2‐phenyl‐1,3‐dioxolane‐2‐ylium salts in 1,2‐dichloroethane solution has been studied by means of 1H NMR and UV spectroscopy and conductimetry. The influence of the nature of the anion (SbCl, SbCl5Br−, AsF) and of the methyl substituents on the rate constants of decay is discussed.
Triple ion chelate structure of an equilibrium Michael adduct
Authors: Viteva L. , Stefanovsky Y. , Tsvetanov Chr. , Gorrichon L. 
Published: January 2005

AbstractThe equilibrium adduct from the Michael reaction between lithium dimethylphenylacetamide enolate and methyl cinnamate has been studied by means of 7Li, 13C and 1H NMR and IR spectroscopy as well as by electrical conductivity measurements in THF and diethyl ether. The 7Li NMR and the electrical conductivity measurements data are consistent with a triple ion structure of [A−M+A−]− M+ type, favoured in THF. All other spectral data support an intramolecular chelation in the triplet fragment.
Organophosphorus herbicides and plant growth regulators part 1. Synthesis and protonation behaviour of glyphosate and related compounds
Authors: Dhansay M. A. , Linder P. W. , Torrington R. G. , Modro T. A. 
Published: January 2005

AbstractIonization constants of N‐phosphonomethylglycine (glyphosate) and three structurally related plant‐growth regulators were determined by potentiometric titration. The effect of pD on the chemical shifts of the 1H and 13C nuclei of the methylene groups and of the 31P nucleus in these substrates was measured by NMR spectroscopy. The results allowed an assignment to be made of a specific site (carboxylic, phosphonic, ammonium) in the molecule to each of the determined pKa values.
Theoretical studies of hydride transfer reactions
Authors: Formosinho Sebastião J. 
Published: January 2005

AbstractThe intersecting‐state model (ISM) has been applied to the study of hydride transfer reactions between variously substituted and elaborated pyridinium ions in solution. Chemical bond order is conserved along the reaction coordinate, with a transition state bond order of n≠ = 0·5. This supports the view that these reactions have essentially a synchronous nature and occur by thermal activation over an energy barrier. Tunnelling of H− is negligible because the effective mass for the solvated species is high. Reaction energy and kinetic isotope effects are well accounted for by the model. For reactions in the vapour phase, ISM and the tunnel effect theory show that tunnelling becomes the dominant mechanism. The same is also valid for proton transfers in the vapour phase.
A study of a novel degenerate carbocation rearrangement of the 4,9‐dimethyl‐9‐barbaralyl cation by dynamic 13C and 1H NMR spectroscopy in superacid
Authors: Engdahl Carin , Ahlberg Per 
Published: January 2005

AbstractThe 4,9‐dimethyltricyclo [3.3.1.02,8]nona‐3,6‐dien‐9‐yl (4,9‐dimethyl‐9‐barbaralyl) cation (3) was generated from 4,9‐dimethyl‐9‐barbaralol (5) at −135°C in two different superacid mixtures [FSO3HSO2CIFSO2F2(1:6:1) and FSO3HSO2CIFSO2F2CHCl2 (2:10:10:1 by volume)]. Its 1H and 13CNMR spectra show a strong temperature dependence in the range −150 to −125°C. The changes in band shapes with temperature show that the following exchanges take place: 4‐methyl with 9‐methyl, C‐4 with C‐9, C‐1 with C‐3 and C‐2 with C‐8. C‐5, C‐6 and C‐7 are found not to exchange rapidly either with each other or with the other carbons in 3. The mechanism of this novel rearrangement is suggested to involve the bicyclic 2,7‐dimethylbicyclo [3.2.2] nona‐3,6,8‐trienyl cation and the secondary barbaralyl cation 4,6‐dimethyltricyclo [3.3.1.02,8] nona‐3,6‐dien‐9‐yl as intermediates rather than 7,8‐dimethyl bicyclo [3.2.2] nona‐3,6,8‐trienyl cation, which does not have a methyl group on the allyl cation moiety. Comparisons with rearrangement mechanisms for other barbaralyl cations were also made. The rate constant for the degenerate rearrangement of 3 is 160 s−1 at −140°C, which corresponds to ΔG≠s = 26 kJ mol−1 (6·3 kcal mol−1). At −125°C ion 3 rearranges non‐degenerately to the 1,4‐bishomotropylium cation 1,8‐dimethylbicyclo [4·3·0] nona, 2,4,7‐trienyl (4) with k = 3 × 10−4 s−1. A mechanism for this rearrangement and the synthesis and purification of the ion precursor 4 are also reported.
Stereochemistry and kinetics of thiophenoxide ion addition to methyl para‐substituted phenylpropiolate derivatives in methanol. I
Authors: Hamed Ezzat A. , Moussa Adel. M. , Youssef Abdel‐Hamid A. 
Published: January 2005

AbstractThe reactions of a series of methyl para‐substituted phenylpropiolates with thiophenoxide ion in methanol were studied and their rates measured. Methyl β‐thiophenyl esters were obtained as mixtures of Z and E isomers and their configurations were established by 1H NMR spectroscopy. Good Hammett correlations with ρ values of 1·57–1·97 were obtained, which suggest a carbanionic character of the transition state. A stepwise mechanism proceeding via an intermediate carbanion is postulated for this non‐sterospecific addition reaction.
Dephosphorylation by peroxyanions in micelles and microemulsions
Authors: Bunton Clifford A. , Mhala Marutirao M. , Moffatt John R. 
Published: January 2005

AbstractDephosphorylation of p‐nitrophenyl diphenylphosphate (pNPDPP) by OH− in aqueous cationic micelles of cetyltrimethylammonium hydroxide (CTAOH) is inhibited by tert‐butyl hydroperoxide (t‐BuO2H), which reduces the binding of OH−, and whose anion is a relatively ineffective nucleophile. Reaction is faster in microemulsions of CTACl, n‐octane and OH−, with t‐BuO2H as cosurfactant, than in otherwise similar microemulsions with t‐butyl alcohol as cosurfactant. Dephosphorylation by m‐chloroperoxybenzoate ion is slower in microemulsions of cetyltrimethylammonium mesylate or benzenesulfonate, with n‐octane and tert‐butyl alcohol, than in cationic micelles, but a wide range of concentrations can be examined. The reaction rates qualitatively follow the concentration of peroxy acid relative to surfactant, and inert anions slow the reaction. Dephosphorylation by peroxyphthalate dianion in micelles of CTACl fits a pseudophase kinetic model, except in very dilute surfactant where reaction is faster than predicted.
Homoallylic interaction of the double bond with substituents
Authors: Xue Liang , Lambert Joseph B. 
Published: January 2005

AbstractAb initio calculations were carried out on systems with a geometrically constrained relationship between a double bond and a homoallylic substituent X, as in YCCCCX. The axial–equatorial equilibria in 3‐substituted methylenecyclohexanes and 4‐substituted cyclohexenes are determined largely by the homoallylic interaction between X and the double bond. The homoallyic substituent was chosen to be OCH3, and the nature of the double bond was varied by changing Y (H, CH3, F, Cl, CN). NMR experiments previously had found very significant dependences of the axial–equatorial equilibrium on the nature of the X and Y substituents. Electrostatic calculations, based on Mulliken charges, reproduce the experimental (NMR) substituent effects on the axial–equatorial equilibrium and suffice to explain all the observations.
Evidence for formation of 2‐nitrobenzoylperoxy radical generated from 2‐nitrobenzoyl chloride and superoxide: Spin trapping of the peroxyradical intermediate and its oxidizing abilities
Authors: Kim Kyoung Soo , Lim Sang Chul , Hoshino M. , Kim Yong Hae 
Published: January 2005

Abstract2‐Nitrobenzoyl chlorde was reacted with superoxide in acetonitrile to form 1 2‐nitrobenzoylperoxy radical intermediate which showed a stronger oxidizing ability than that from superoxide, as confirmed by ESR spin trapping studies.
Contribution of activity coefficients to the nucleophilicity of solvent components in solvent mixtures. Influence of added acetone on selectivity values, kE/kW, in ethanol–water mixtures
Authors: Ta‐Shma Rachel , Rappoport Zvi 
Published: January 2005

AbstractLiterature data on the change in the selectivity values, kE/kw, for several solvolysing systems in ethanol (E)–water (W) on addition of acetone were analysed. It is suggested that acetone acts as a basic cosolvent for both ethanol and water and that the change in the activity coefficients on changing the solvent composition is mainly responsible for the observed effect.
Cross‐interaction constants as a measure of the transition state structure. Part 8. Mechanism of the reaction of 2‐phenylethyl benzenesulphonates with benzylamine in acetonitrile
Authors: Lee Ikchoon , Lee Won Heui , Lee Hai Whang 
Published: January 2005

AbstractThe kinetics of reactions between 2‐phenylethyl benzenesulphonates (2‐PEB) and benzylamines in acetonitrile at 65·0°C have been studied; the mechanism was examined on the basis of the sign and magnitude of cross‐interaction constants ρij and βij. In contrast to the reactions of 2‐PEB with anilines in methanol, participation of the aryl‐assisted pathway was negligible, with a strong indication that the reaction proceeds largely by an intermolecular SNi mechanism with a four‐centre transition state (TS). The effect of substituents on the TS variation was in accord with the predictions of the quantum mechanical model.
Cross‐interaction constants as a measure of the transition state structure. Part 9. The degree of bond formation in the SN2 transition state involving anionic nucleophiles
Authors: Lee Ikchoon , Koh Han Joong , Huh Chul , Lee Hai Whang 
Published: January 2005

AbstractThe second‐order rate constants of the reactions between benzenesulphonyl chlorides and anionic nucleophiles, benzoates and cinnamates, in methanol at 30·0°C are reported. A marked increase in rate found with a p‐nitro substituent in the substrate indicated the development of an electron‐rich centre on the S atom in the transition state. The two types of cross‐interaction constants, ρxy and λxy, suggested that bond formation in the SN2 transition state with anionic nucleophiles is greater than that for the corresponding reaction with aniline nucleophiles.
General‐acid, specific‐base and intramolecular catalysis of the dehydration step in nitrone formation from furfurals and phenylhydroxylamine
Authors: Fett Roseane , Simionatto Edésio L. , Yunes Rosendo A. 
Published: January 2005

AbstractNitrone formation from furfural and 5‐nitrofurfural with phenylhydroxylamine was studied in water at 25°C and ionic strength 1·0 (KCl). The reaction exhibits general acid catalysis (α = 0·52), specific base catalysis and a pH‐independent process. The results are consistent with a mechanism where dehydration of an N,N′‐dihydroxy addition intermediate, formed by a rapid pre‐equilibrium, is the only rate‐determining step over the pH range studied (1–11). In contrast, nitrone formation from p‐chlorobenzaldehyde and N‐methylhydroxylamine exhibits a clear change of rate‐determining step, from dehydration of the addition intermediate to attack of the nucleophile on the aldehyde, with increase in pH. The profile of log k2 (k2 = second‐order rate constant) vs pH indicates, by comparison with the corresponding profile for similar reaction, that phenylhydroxylamine exhibits unexpected nucleophilic behaviour in this reaction. The pH‐indpendent dehydration process, which has a higher rate constant than that predicted from the Brønsted line for the water catalysis, appears to involve intramolecular catalysis rather than bifunctional catalysis by water.
Rearrangement and cyclization in the ionization of the 4‐chloro‐3‐methylbutanoyl cation
Authors: Fărcaşiu Dan , Miller Glen , Sharma Shalini 
Published: January 2005

AbstractThe second ionization of the 4‐chloro‐3‐methylbutanoyl ion forms a primary alkyl acyl dication, as a tight ion pair. Methyl and hydrogen shifts occur to comparable extents indicating that the relative stability of the product (sec‐ or tert‐carbocation) does not influence the energy barrier for the shift. The product of methyl shift (1,3‐sec‐alkyl acyl dication) loses the proton closest to the counterion in the tight ion pair and forms the pent‐3‐enoyl cation. Protonation–deprotonation of the latter, followed by internal acylation, gives the protonated cyclopent‐2‐enone. The dication resulting from hydrogen shift loses a proton from C‐2 and gives the 3‐methylbut‐2‐enoyl cation.
Micellar catalysis of organic reactions. 28. A comparison of the catalytic effects of micelles containing bromide and fluoride counterions on some SNAr AND BAc2 reactions
Authors: Broxton Trevor J. , Christie John R. , Wright Sallyanne 
Published: January 2005

AbstractThe catalytic effects of micelles of cetyltrimethylammonium bromide (CTAB) and fluoride (CTAF) on several SNAr and BAC2 reactions were studied. Rate versus detergent concentration curves predicted by the pseudophase models PPIE (ion exchange) for CTAB studies and PPMA (mass action) for the CTAF studies were fitted to the experimental results to determine the best values of the substrate binding constant (Ks), the second‐order rate constant for reaction in the micelles (K) and the exchange constant KOH/X (PPIE) or binding constant (PPMA) for the counterions. Better catalysis by micelles of CTAF is shown to be due to the combined effects of a more favourable exchange constant in CTAF than in CTAB, slightly different rates of reaction within the two micelles and different values of β, the fraction of micellar head groups neutralized in CTAF and in CTAB.In both micelles the magnitude of catalysis depends on the charge produced in the intermediate complex and the presence of electron‐withdrawing groups to stabilize that charge. Strongest catalysis was found for a reaction leading to a monoanionic intermediate complex in which the charge was stabilized by two nitro groups, and the weakest catalysis was found for a reaction leading to a dianionic intermediate in which no electron‐withdrawing groups were present.
N‐hydroxy‐N‐arylacetamides. VI. Chemical and electrochemical oxidation of N‐hydroxy‐N‐arylacetamides
Authors: Lenk Werner , Riedl Michael 
Published: January 2005

AbstractThe electrochemical behaviour of seven N‐hydroxy‐N‐arylacetamides (RNOHCOCH3, R = phenyl, 4‐chlorophenyl, 3,4‐dichlorophenyl, 4‐ethoxyphenyl, 4‐biphenylyl, 2‐fluorenyl and 2‐phenanthryl) was investigated by cyclovoltammetry. Each compound showed two different oxidation potentials, attributed to a quasi‐reversible one‐electron transfer in the range between 0·55 and 0·63 V and an irreversible transfer of a second electron between 0·80 and 1·20 V. Since the one‐electron oxidation of N‐hydroxy‐N‐arylacetamides gives the corresponding nitroxides, the kinetics of the self‐reaction of these radicals and the concentration dependence of the product pattern were also studied.
Stereospecificity in anionic 1, 1‐addition to isocyanides. A re‐examination of the (H− + HNC) potential energy surface
Authors: Nguyen Minh Tho , Ha Tae‐Kyu , Hegarty Anthony F. 
Published: January 2005

AbstractThe potential energy surface for the addition of the hydride ion to hydrogen isocynide reported previously was re‐examined. An improved energy surface was explored at the MP2/6–31 + + G** level while relative energies were estimated at the MP4SDTQ/6–311 + + G** level and corrected for zero‐point vibrational contributions. The calculated results show the existence of transition structures for both cis and trans‐additions of H− to the carbon end of HNC even though the corresponding barrier heights are small. These transition structures did not exist at a lower level of theory (HF/4–31G) employed previously.The cis addition is calculated to be favoured over the trans addition by 10 kcal mol−1. The overall addition is thus strongly stereoselective, if not purely stereospecific. This result confirms our previous analysis of the stereoelectronic effect in the addition–elimination process involving imidoyl anions as intermediates. Recent experimental work on the addition of OH− to ArNC is discussed.
Comparison between some semi‐empirical and ab initio Hartree–Fock models for the description of amides (formamide revisited)
Authors: Louwen J. N. , Jenneskens L. W. 
Published: January 2005

AbstractA comparative study on the use of the MINDO/3, MNDO and AM1 semi‐empirical methods for the description of formamide and aliphatic amide systems is reported. For formamide, ab initio Hartree–Fock calculations are also reported using both the STO‐3G and 3–21G basis sets, as well as a basis set obtained by a minimal relaxation of the STO‐3G contraction scheme. It is shown that both the MNDO and the STO‐3G methods cannot properly reproduce the experimental results. In both cases this appears to be due to inadequacies of the basis sets used. Again in both cases only small improvements in the basis set necessary to allow for the diffuse character of the pπ orbitals lead to dramatic improvements in the calculated results. In the case of the STO‐3G basis set this is demonstrated by the fact that a small relaxation in the contraction leads to results even superior to those with the 3–21G set. The conclusions reached for amide systems can be extended to other systems where planarity around or inversion with respect to nitrogen is an issue. This is demonstrated for ammonia.
Absolute rate constants for the reactions of phenylhalocarbenes with alkynes
Authors: Moss Robert A. , Jang Eun G. , Ho Guo‐Jie 
Published: January 2005

AbstractAdditions of phenylchlorocarbene and phenylfluorocarbene to representative alkynes are rapid (106–107 l mol−1 s−1). The variation of the rate constants with alkyne structure parallels that observed with analogous alkene substrates. The addition of phenylchlorocarbene to 3‐hexyne is characterized by a very low activation energy (∼ 2 kcal mol−1) and a large, negative entropy of activation (∼ −20 e.u.).
Reactions of molecules with two equivalent functional groups. 2. Acetylation of the isomers of bis(hydroxymethyl)benzene
Authors: Constantinides Ioannis , Lourdes‐Guerra Maria , Macomber Roger S. 
Published: January 2005

AbstractThe acetylation (with acetic anhydride–pyridine in tetrahydrofuran) of the ortho, meta and para isomers of bis(hydroxymethyl)benzene (1) was studied. Third‐order rate constants k1 (formation of monoacetate 2 from 1) and k2 (formation of diacetate 3 from 2) for all three isomers were determined, as were the rate constants for acetylation of benzyl alcohol and its ortho‐ and para‐ethyl derivatives under the same conditions. The compositions of the final product mixtures (comprising 1, 2 and 3) were ascertained as a function of the initial ratio of acetic anhydride to 1. Comparisons were made with predictions based on a previously published independent functional groups model, which assumes that k2/k1 = 0·50. All three isomers gave product mixtures whose compositions were in generally good agreement with predictions based on the model. For the para and meta isomers the ratios k2/k1 were determined experimentally to be 0·548 and 0·521, respectively. The k2/k1 ratio for the ortho isomer was found to be 0·605, and its k1 and k2 values were lower than those for the para and meta isomers. Possible explanations for the anomalous behavior of the ortho isomer are discussed.
Negligible kinetic solvent interactions for the rate of photobromination of toluenes where the bromine atom is a principal chain carrier
Authors: Kim Sung Soo , Lee Chang Soo , Kim Chong Chan , Kim Hee Jin 
Published: January 2005

AbstractThe bromine atom is a major chain carrier in the photobromination of toluenes by N‐bromosuccinimide. Only an uncomplexed bromine atom can abstract hydrogen atoms from toluenes. The nature of the polar transition state is entirely different from that of solvolytic reactions and the solvent polarity therefore cannot infuence the rates of the bromination.
Geometry of 2‐benzopyrylium and pyrylium cations calculated by semi‐empirical methods. Crystal and molecular structure of 1,3‐dimethyl‐6,7‐dimethoxy‐2‐benzopyrylium perchlorate
Authors: Shcherbakova Irina V. , Yudilevich Iosif A. , Kuznetsov Evgenii V. , Yufit Dmitrii S. , Struchkov Yurii T. 
Published: January 2005

AbstractX‐ray data on the crystal and molecular structure of 1,3‐dimethyl‐6,7‐ dimethoxy‐2‐benzopyrylium perchlorate show a good correlation with the geometry of the same cation calculated by the MINDO/3 and MNDO methods with total optimization of all geometrical parameters. The geometry of substituted pyrylium cations was determined by the MNDO and AM1 methods. On the basis of the data obtained, some effects of benzo [c] annelation are displayed for the 2‐benzopyrylium cation in comparison with the pyrylium cation.
Effect of hexadecyltrimethylammonium bromide micelles on the hydrolysis of substituted benzoate esters
Authors: Correia Valdir R. , Cuccovia Iolanda M. , Chaimovich Hernan 
Published: January 2005

AbstractThe alkaline hydrolysis of nine para‐substituted phenyl esters of para‐substituted benzoates was studied in the presence and absence of hexadecyltrimethylammonium bromide (CTAB) micelles. The second‐order rate constant for alkaline hydrolysis in CTAB micelles (K2m) was calculated using a pseudo‐phase ion‐exchange model. the substituent effect on the second‐order rate constant in the aqueous phase (k2w) and on k2m was analysed using Hammett's σ ρ plots. The calculated ρ values in micelles were 0.8 unit. larger than those in water. The effective low dielectric constant in the micellar surface may be responsible for the general effect of increasing the sensitivity of reactions to the electronic effect of substituents in micelles.
Interactions between tetracyanoethylene and rauwolfia alkaloids
Authors: Cuervo L. , Muñtoz M. A. , Guardado P. , Carmona C. , Hidalgo J. , Balon M. 
Published: January 2005

AbstractInteractions 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

AbstractThe 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.