bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theoretical study of pyrazole elimination of N‐alkyl pyrazoles
Authors: Perez Jorge D. , Phagouapé Leonardo M. , Davico Gustavo E. 
Published: January 2005

AbstractThe gas‐phase pyrazole elimination of N‐alkyl pyrazoles has been studied using MNDO semi‐empirical molecular orbital (MO) theory with complete geometry optimization of all stationary points. We found that the activation energies (Ea) of the concerted processes are around 80 kcal/mol, 25 Kcal/mol higher than experimental values. But the differences in Ea between compounds with different substituents are in good agreement with the experimental ones.
Hydrogen‐bonding. Part 4. An analysis of solute hydrogen‐bond basicity, in terms of complexation constants (log K), using F1 and F2 factors, the principal components of different kinds of basicity
Authors: Abraham Michael H. , Grellier Priscilla L. , Prior David V. , Morris Jeffrey J. , Taylor Peter J. , Maria Pierre‐Charles , Gal Jean‐Francois 
Published: January 2005

AbstractThe principal components factors F1 and F2 in the equation

have been used to obtain S1 and S2 values for sets of hydrogen‐bond bases against 32 reference acid/solvent systems. The constants S1 and S2 define an angle θ = tan−1 S2/S1 that is a measure of the electrostatic:covalent bonding ratio in the hydrogen‐bond complex. It is shown that θ can vary from 53 (4‐fluorophenol in CH2Cl2)to 86 degrees (Ph2NH in CCl4) depending on the reference acid and solvent. This variation in θ can lead to family dependent behaviour in plots of log K for bases against a given reference acid system vs log K for bases against another reference acid system, and precludes the construction of any general scale of hydrogen‐bond basicity using log K values. Amongst a quite wide range of reference acid/solvent systems θ varies only from 64 to 73 degrees, and for bases against these reference systems a ‘reasonably general’ scale could be set up. Such a scale could be extended to bases against reference acid/solvent systems outside the 64–73 degree range provided that certain classes of base (e.g. pyridines, alkylamines) were excluded from the additional reference acid/solvent systems.
Photostimulated reactions of neopentyl halides with nucleophiles by the SRN1 mechanism
Authors: Bornancini Esteban R. N. , Palacios Sara M. , Peñéñory Alicia B. , Rossi Roberto A. 
Published: January 2005

AbstractThe photostimulated reaction of neopentyl halides with different nucleophiles by the SRN1 mechanism of nucleophilic substitution has been studied. Neopentyl halides do not react with carbon nucleophiles, diethylphosphite, diphenylphosphonite and azide ions, but they react with arsenide and selenide ions. The photostimulated reaction of neopentyl bromide with diphenylarsenide ions gave only the straightforward substitution product neopentyldiphenylarsine. On the other hand, the photostimulated reaction of bromobenzene with dineopentyl arsenide ions gave three arsines: dineopentylphenylarsine, neopentyldiphenylarsine and triphenylarsine. Neopentyl chloride reacts under irradiation with diphenylphosphide ions giving good yields of the substitution product.
Theoretical studies on the hydrolysis of urea in acid solution
Authors: Lee Ikchoon , Kim Chang Kon , Lee Byung Choon 
Published: January 2005

AbstractMechanisms of the hydrolysis of urea have been investigated using the MNDO and AMI methods. All geometries were fully optimized and the transition states were characterized by calculating force constants. The results showed that: (i) The unimolecular decomposition process via the direct intramolecular proton transfer is preferred to both the A1 and the bimolecular nucleophilic attack by water, in agreement with the experimental results of Shaw et al. in the low acidity medium. (ii) The diprotonated form of urea exists as an equilibrium species, which undergoes the A2 type hydrolysis more favorably than the monoprotonated form, as Moodie et al. found in the intermediate acidity medium. (iii) The A2 hydrolysis of the monoprotonated form is very similar to those of acetamide and methyl carbamate. (iv) As the number of the solvate water molecules increases, the activation barrier for the A2 process of the monoprotonated form increases while that for the unimolecular decomposition of the free base form decreases, indicating a possibility of the barrier height reversal in the bulk solvent in favor of the latter process, thus accommodating all the experimentally found trends in the urea hydrolysis. The A1 mechanisms involving six‐membered ring type intermediates can be ruled out as untenable since no such equilibrium species was obtained by both the MNDO and AM1 calculations.
Experimental determination of carbon vs. oxygen regioselectivity in reactions of gas‐phase enolate ions
Authors: Brickhouse Mark D. , Squires Robert R. 
Published: January 2005

AbstractA gas‐phase experimental investigation of the competition between carbon and oxygen alkylation of a series of cyclic and acyclic enolate ions is described. Perfluoropropylene is shown to react in a characteristic way with oxyanions and carbanions to produce distinctive ionic products. The relative yields of these products formed in reactions with ambident enolate ions provides a measure of their intrinsic carbon vs. oxygen regioselectivity. The results for a series of enolates derived from aldehydes, ketones, esters, amides and related compounds show a wide range of reactivity which is a function of the nature of the central substituent. Most aldehyde and ketone enolates react mainly through oxygen, while enolates with σ‐acceptor of π‐donor type central substituents react mainly thorugh carbon. Ring‐size in cyclic ketone enolates also influences C vs. O regioselectivity, i.e. small‐ring enolates react mainly through carbon, while larger ring enolates (C7–C9) react preferentially through oxygen. The enolate reactivity patterns can be generally accounted for by the keto–enol energy differences for the parent carbonyl compounds, although some exceptions are evident. The origins of the highly variable, kinetically determined regioselectivities are discussed.
Group transfers 4. Arylselenide aniondiaryl diselenide exchange
Authors: Jacobson Barry M. , Kook Alan M. , Lewis Edward S. 
Published: January 2005

AbstractBarriers for group transfers between nucleophiles have been postulated to be lowered when the transferring group can carry a considerable negative charge. Furthermore, anions readily subject to one electron oxidation appear to lead to lower barriers than do those of high oxidation potential. These suggestions are pursued here on the identity reaction ArSe− + ArSeSeAr → ArSeSeAr + ArSe−. Indeed the reaction is very fast, as shown by the appearance of only a single peak in the 77Se‐NMR in an acetonitrile solution containing both ArSeNa and ArSeSeAr. The rate constant can be only very roughly estimated at low temperatures and dilute solutions, and is likely diffusion controlled for Ar = phenyl and p‐methoxyphenyl. A stable intermediate (ArSe)3−, analogous to Br3−, is indicated, but quantitative stability could not be determined, from either the NMR or the UV spectra. Some properties of 77Se‐NMR are discussed.
Binding of pyridoxal‐5′‐phosphate and pyridoxamine‐5′‐phosphate: Electrochemical characterization
Authors: Eraso F. , Pineda T. , Blazquez M. , Garcia‐Blanco F. , Dominguez M. 
Published: January 2005

AbstractThe interaction between pyridoxal‐5′‐phosphate (PLP) and pyridoxamine‐5′‐phosphate (PMP) was studied by d.c. and differential pulse polarography. Two reduction waves (or two peaks) were observed. The first wave corresponds to the reduction of the adduct and the second to the reduction of free PLP. The behaviour was similar in analogous Schiff bases. The effects of pH, PMP concentration and the capillary characteristics were studied. The apparent formation constant as a function of the pH was calculated. Thermodynamic parameters were estimated at pH 7 and 0 · 1 M ionic strength. The polarographic and kinetic results show that the overall electrode process appears to be irreversible in a basic medium.
Dipole moments and electron distribution of acyl chlorides and acyl bromides
Authors: Exner Otto , Hnyk Drahomír , Všetečka Václav 
Published: January 2005

AbstractDipole moments of para‐substituted benzoyl chlorides and bromides, partly measured in benzene and partly recalculated from elderly literature, were analysed in terms of bond moments and additional contributions expressing the conjugation. Although the gross moments are similar, their decomposition yields different pictures for acyl chlorides and bromides. Only in the latter can significant electron transfer from the halogen be observed. On the other hand, conjugation with the benzene nucleus is strong in both aromatic acyl chlorides and bromides, but stronger in chlorides. There is agreement with the evidence from some other physical quantities but not from all.
Amines as leaving groups in nucleophilic aromatic substitution reactions. II. Hydrolysis of N‐(2,4,6‐trinitrophenyl)amines
Authors: De Vargas Elba B. , De Rossi Rita H. 
Published: January 2005

AbstractThe hydrolysis reactions of N‐(2,4,6‐trinitrophenyl)piperidine (2) and N‐(2,4,6‐trinitrophenyl)‐morpholine (3) were studied. Two kinetic processes well separated in time are observed in both reactions. The fastest process, which is reversible, leads to the formation of a species of λmax 260 and 410 nm and is attributed to the formation of a σ complex of stoichiometry 1 : 2 due to the addition of a second HO− to the σ complex of 1 : 1 stoichiometry. The slowest process leads quantitatively to picrate ion. The equilibrium constants for the formation of the σ complexes of 1:1 and 1:2 stoichiometries and the rate of formation and decomposition of the latter complex were determined. The kinetic data for the slow process lead to the conclusion that the picrate ion is formed from the attack of HO− on the two σ complexes, confirming previous findings. There are some differences in the calculated rates for 2 and 3 which may be an indication that the elimination of the amine is partially rate determining.
Micellar catalysis of organic reactions. 24. Comparison of SNAr reactions in hydroxy‐functionalized micelles and in the presence of cyclodextrins
Authors: Broxton Trevor J. , Christie John R. , Chung Roland P.‐T. 
Published: January 2005

AbstractThe basic hydrolysis of 2,4‐dinitrochlorobenzene (DNCB) and 2,4‐dinitrofluorobenzene (DNFB) was studied in the presence of β‐cyclodextrin (CDOH) and in the presence of hydroxy‐functionalized micelles containing either a primary hydroxy group [hexadecyl‐2‐hydroxyethyldimethylammonium bromide (CHEDAB)] or a secondary hydroxy group [headecyl‐2‐hydroxypropyldimethylammonium bromide (CHPDAB) and 2‐hydroxyhexadecyltrimethylammonium bromide (2‐OHCTAB)].In all systems a biphasic reaction was observed. The first phase consisted of a competition between the additive (either micelle or cyclodextrin) and hydroxide ion for the aromatic substrate, and the second phase consisted of the hydrolysis of the trapped aryl micellar or cyclodextryl ether.The percentage of trapping of the aromatic substrate by the cyclodextrin was similar to that found for reactions in the hydroxy‐functionalized micelles (CHPDAB and 2‐OHCTAB) which contained secondary hydroxy groups. The relative rates of reaction for DNFB and for DNCB, i.e. F/Cl rate ratios, in the presence of CDOH were similar to those obtained in the presence of 2‐OHCTAB but less than that obtained in the presence of CHEDAB. These results support the assumption that in CDOH the secondary hydroxy groups of C‐2 or C‐3 are involved in covalent bond formation with the aromatic substrate rather than the primary hydroxy group of C‐6. All the reactions studied proceed much more slowly in the presence of CDOH than in the presence of the hydroxy‐functionalized micelles. This may reflect a catalytic effect of the positively charged surface present in the micelles but not in the cyclodextrin.
Cation radical pericyclic reactions: Cyclopropanation
Authors: Bauld Nathan L. , Stufflebeme Gary W. , Lorenz Kurt T. 
Published: January 2005

AbstractTriarylaminium salts smoothly catalyze the cyclopropanation of dienes, styrenes and tetrasubstituted alkenes by ethyl diazoacetate. The reactions are regioselective and, in the case of additions to conjugated dienes, cyclopropane‐periselective. A cation radical chain mechanism involving carbene transfer from ethyl diazoacetate to a substrate cation radical is proposed.
Self‐catalysis and molecular complexes in aromatic nucleophilic substitution reactions: Reaction between 1,3,5‐trinitrobenzene and 1,8‐diazabicyclo [5.4.0] undec‐7‐ene in toluene
Authors: Forlani Luciano , Cimarelli Cristina 
Published: January 2005

AbstractThe formation of a σ‐like complex by reaction between 1,3,5‐trinitrobenzene and 1,8‐diazabicyclo [5.4.0]‐undec‐7‐ene was investigated in toluene at various temperatures. The kinetic data showed autocatalytic behaviour. Inspection of the reaction mixtures at zero reaction time indicated the presence of an equilibrium preceding the attack of the nucleophile, affording a molecular complex (substrate–nucleophile) which is responsible for the observed kinetic features. The present and the previous data led to the conclusion that the catalytic behaviours usually observed in SNAr reactions (in apolar solvents with amines as nucleophiles) are better explained by the presence of substrate‐catalyst interactions than by catalysis on departure of the proton and leaving group from the zwitterionic intermediate.
Oxidation of substituted ethanols by sodium N‐bromobenzenesulphonamide: A kinetic study
Authors: Puttaswamy  , Mahadevappa D. S. 
Published: January 2005

AbstractThe kinetics of the oxidation of seven substituted ethanols by sodium N‐bromobenzenesulphonamide or bromamine‐B (BAB) in the presence of HCl was studied at 45 °C. The rate shows a first‐order dependence on [BAB]0 and is fractional in [alcohol]0, [H+] and [Cl−]. Ionic strength variations, addition of benzenesulphonamide reaction product and variation of dielectric constant of the medium have no effect on the rate. The solvent isotope effect k/k ≈ 0·97. The rates do not correlate satisfactorily with Taft's substituent constants. Activation parameters ΔH≠, ΔS≠, ΔG≠ and log A were calculated by studying the reaction at different temperatures (308–323 K). It was found that ΔH≠ and ΔS≠ are linearly related and an isokinetic relationship is observed with isokinetic temperature β = 393 K, indicating enthalpy as a controlling factor.
Probing of transient photoenol formation from ortho‐methyl‐substituted phenyl ketones by hydrogen–tritium exchange
Authors: Gȩbicki Jerzy , Reimschüssel Władysław , Zurawińska Beata 
Published: January 2005

AbstractStudies on the photochemically induced hydrogen–tritium exchange in o‐methyl‐substituted phenyl ketones are reported. It is shown that the tritium exchange can be beneficial for probing transient photoenol formation, particularly when photochemical side‐reactions are occurring.
Maximally inhibited elimination of kinetics of (2‐bromoethyl)benzene and 1‐bromo‐3‐phenylpropane in the gas phase. Anchimeric assistance of the phenyl group
Authors: Chuchani Gabriel , Martin Ignacio 
Published: January 2005

AbstractThe gas‐phase elimination kinetics of (2‐bromoethyl)benzene and 1‐bromo‐3‐phenylpropane were studied in a static system and seasoned vessels over the temperature range 390–450 °C and the pressure range 32–104 Torr. The reactions, under maximum inhibition of 4‐fold pressure of the free‐radical suppressor cyclohexene and/or propene, are homogeneous, unimolecular and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius equations: for (2‐bromoethyl)benzene, log k1 (s−1) = (13·04 ± 0·10) − (210·8 ± 1·3) kJ mol−1 (2·303RT)−1, and for 1‐bromo‐3‐phenylpropane, log k1 (s−1) = (14·09 ± 0·27) − (227·7 ± 3·6) kJ mol−1 (2·303RT)−1. The phenyl group of (2‐bromoethyl)benzene appears to provide anchimeric assistance in the HBr elimination of this compound. However, neighbouring C6H5 participation at the 3‐position in 1‐bromo‐3‐phenylpropane for a C‐4 conformation is apparently absent. The mechanisms of these reactions are discussed.
MNDOC‐CI calculations for organic photoreactions. I. The α‐cleavage reaction of carbonyl compounds
Authors: Reinsch Manfred , Klessinger Martin 
Published: January 2005

AbstractMNDOC‐CI results are reported for the photochemical α‐cleavage reaction of carbonyl compounds. Potential energy surfaces for some excited singlet and triplet states and cross‐sections along the bitopic reaction path show that the activation energy of the α‐cleavage reaction is determined by the relative energetic position of the lowest two triplet states. The reactivity of aliphatic ketones depends only on the degree of branching at the α‐carbon, because the triplet barrier is due to an (avoided) crossing of two surfaces corresponding to an nπ* and ππ* excitation respectively, which correlates with the stability of the leaving alkyl radical. For conjugated carbonyl compounds, on the other hand, the position of the crossing point is independent of the 3nπ* excitation energy, but if the 3ππ* surface is below the 3nπ* surface all the way along the reaction coordinate, the reaction is determined by the barrier on the 3ππ* surface and any additional stabilization of the 3ππ* state increases the barrier.
Kinetic and spectroscopy of 2‐naphthylphenylcarbene
Authors: Maloney Vincent , Platz Matthew S. 
Published: January 2005

Abstract2‐Naphthylphenylcarbene (2‐NPC) was studied by low‐temperature EPR spectroscopy and by solution‐phase laser flash photolysis. 2‐NPC was found to have a triplet ground state with two distinct rotomeric forms. The triplet carbene was found to react with methanol, 1,1‐diphenylethylene, styrene, carbon tetrachloride, isoprene, toluene and 2‐methyltetrhydrofuran.
15N NMR of nitranions: Opposite shift effects of σ and π charge
Authors: Barchiesi Emma , Bradamante Silvia 
Published: January 2005

AbstractNitranions of pyrrole, aromatic and heteroaromatic amines, carboxamides and pyridyl carbanions are generated by deprotonation either of NH or CH acids. The positive or negative displacements undergone by the 15N nucleus relative to the neutral precursor depend on whether a σ or π charge is increased. They are associated with the symmetry of the nitrogen orbital in which the electron pair is either generated or that to which it is delocalized.
Topological evidence for an NN bond in cis‐1,2‐dinitrosoethene: The remarkable structure of the di‐N‐oxide of 1,2‐diazacyclobutadiene
Authors: Sarasola C. , Choi S. C. , Ugalde J. M. , Boyd Russell J. 
Published: January 2005

AbstractAnalysis of the topological properties of the electron density distribution of cis‐1,2‐dinitrosoethene (I) shows that there is a normal covalent, albeit very long, single bond between the nitrogen atoms in the molecule, which is in fact the di‐N‐oxide of 1,2‐diazacyclobutadiene (II). Despite its extraordinary length, the NN bond is shown to be consistent with the strong correlation between the electron density at the bond critical points and the calculated bond lengths of NN bonds. The geometry and electronic structure exhibit a number of remarkable features.
Kinetics and mechanism of the oxidation of organic sulphides by N‐chloroacetamide
Authors: Agarwal Anupma , Bhatt Pallavi , Banerji Kalyan K. 
Published: January 2005

AbstractThe kinetics of the oxidation of a number of monosubstituted aryl methyl, alkyl phenyl, dialkyl and diphenyl sulphides by N‐chloroacetamide (NCA) to yield the corresponding sulphoxides were studied. The reaction is first order with respect to the sulphide, NCA and hydrogen ions. There is no effect of added acetamide. Protonated NCA has been postulated as the reactive oxidizing species. The rates of oxidation of meta‐ and para‐substituted phenyl methyl sulphides were correlated with Taft's and Swain's dual substituent parameter equations. For the para‐substituted compounds, the best correlation is obtained with σI and σ; the meta‐substituted compounds correlate best with σI and σR° values. The reaction constants are negative. The oxidation rates of the ortho‐substituted sulphides yield an excellent correlation in a triparametric equation involving Taft's σI and σ values and Charton's steric parameter, V. The oxidation of alkyl phenyl sulphides is susceptible to both polar and steric effects of the alkyl group. A mechanism involving the formation of a halogenosulphonium cation in the rate‐determining step is proposed.
Photochemistry of pyridinium salts 1. Intramolecular charge transfer in N‐amino‐substituted pyridinium cations
Authors: Feygelman Vladimir M. , Tymyanskii Yakov R. , Makarova Nadezhda I. , Knyazhanskii Michail I. , Druzhinin Sergei I. , Uzhinov Boris M. 
Published: January 2005

AbstractThe photophysics and photochemistry of N‐amino‐substituted pyridinium salts were investigated. Electron transfer from the amino group to the pyridinium ring takes place in the singlet excited state. The fluorescence quenching and photoreactivity of these compounds is conditioned by this process. Irradiation of N‐aminopyridinium salts in solution leads to novel fused heterocycles or to triphenylpyridine, depending on the concentration of the initial compound. Free radical intermediates are involved in the photoreaction.
Relationship between substituent electronegativity and field parameter in the aliphatic series. Introduction of charge separation parameter (λ)
Authors: Inamoto Naoki , Masuda Shozo , Niwa Jun 
Published: January 2005

AbstractThe problem of non‐proportionality between the substituent field parameter (σF) and substituent electronegativity (Δι or σχ) was clarified by the introduction of the ‘charge‐separation parameter’ (λ), which is the component for the direction of the HX axis of the distance between positive and negative poles of the dipole in the HX molecule. Thus σF was correlated with Δι or σχ by the equation σF = aΔιλ (or σF = a′σχλ). The above proposal was further supported by performing an energy decomposition analysis for the isodesmic reaction in the isolated molecule system (\documentclass{article}\pagestyle{empty}\begin{document}${\rm X}\raise1pt\hbox{---} {\rm H} \cdots {\rm H}\mathop {\rm N}\limits^ + {\rm H}_3 $\end{document} or XH…︁HCO) at shorter distances (2·0–4·5 Å). It was further shown that field parameters such as σi, F and σF(theor), and the pKa values in the series of aliphatic acids, and spectral data (UPS and 13C‐SCS) were also well correlated with Διλ or σχλ. It is concluded that λ and Δι or σχ are essential parameters for interpreting the electrostatic field effect in aliphatic substituent effects, and that the transmission due to the σ‐inductive effect is considered to be not as significant as the field effect after a few bonds away from the substituent.