bluuebunny/crossref_metadata_2025_split huggingface dataset:
Non‐hierarchical classification of organic solvents using characteristic vector analysis of physical and empirical solvent parameters
Authors: Zalewski Romuald I. , Kokocińska Henryka , Reichardt Christian 
Published: January 2005

AbstractCharacteristic vector analysis of a set of six physical and empirical parameters of 103 commonly used organic solvents (bp, ϵr, μ, nD, E, and δ) gives four vectors describing 95% of the total data variability. Non‐hierarchical cluster analysis, applied to our results, leads to ten separate classes of organic solvents.
Laser flash photolysis of a chromium carbene complex: Pentacarbonyl(1,3‐dimethyl‐4‐imidazolin‐2‐ylidene)chromium(0)
Authors: Oishi Shigero , Tokumaru Katsumi 
Published: January 2005

AbstractPhotoreactions of Cr(CO)5(Im) (Im = 1,3‐dimethyl‐4‐imidazolin‐2‐ylidene) have been investigated by laser flash and continuous photolysis. Quantum yields for the disappearance of Cr(CO)5(Im) in benzene under Ar or CO were very low, although transients observed by laser flash photolysis reacted rapidly with CO. This suggests that photodissociation of the carbene ligand is very minor. The laser flash photolysis at 337 nm in benzene (B) gave a transient (λmax = 630 nm), which converted to the second transient (λmax = 455 nm) within ∼100 ns. The kinetic studies of these transients revealed that the first one is a ‘free’ coordinatively unsaturated species (Cr(CO)4(Im)) and the second one carries a solvent molecule as a ligand (Cr(CO)4(Im)(B)). From the comparison of Cr(CO)4(Im) and Cr(CO)5, apparently lower reactivity of Cr(CO)5 was found to be due to the strong coordination of a solvent molecule. The Im ligand neutralizes the deficit of electron density on Cr. A simple synthesis of Cr(CO)5(Im) was also described.
ESCA studies of carbanionic compounds: Butyllithium in heptane solution and dibutylmercury in the gas phase
Authors: Moberg R. , Yuan Zhong Cai , Holmberg S. , Bökman F. , Ahlberg P. , Bohman O. , Siegbahn H. 
Published: January 2005

AbstractTwo organometallic reagents, butyllithium in heptane solution and dibutylmercury in the gas phase, have been studied by means of core electron spectroscopy. The property of particular interest was the charge polarization as reflected by the ESCA shifts. In the butyllithium compound the C1s binding energy is shifted to a lower value by 1·8 eV for C‐1 (situated closest to the Li+ ion) relative to the heptane solvent carbon. This is due to the anionic character of the C‐1. The interpretation of the experiments was supported by comparing the results with ab initio calculations made on geometry‐optimized butyllithium, butyl anion and butane.
Analysis of solvent effect on SN2 reactions by different theoretical models
Authors: Alemán Carles , Maseras Feliu , Lledós Agustí , Duran Miquel , Bertrán Juan 
Published: January 2005

AbstractThe solvent effect on two SN2 reactions was evaluated by discrete, continuum and discrete–continuum models. The potential energy profiles were found to change dramatically on introduction of the solvent effect. The double‐well shape which characterizes the F− + CH3F → FCH3 + F− reaction in the gas phase becomes unimodal when the solvent is introduced, in good agreement with experimental data. The solvation parameters are found to intervene in the reaction coordinate. Recent Monte Carlo and molecular dynamics calculations are discussed.
Comparative kinetic study of solvent effects in the reactions of 1,2‐dinitrobenzene with butylamine and piperidine
Authors: Chiacchiera S. M. , Cattana R. I. , Singh J. O. , Anunziata J. D. , Silber J. J. 
Published: January 2005

AbstractThe kinetics of the aromatic nucleophilic substitution (SNAr) reactions of 1,2‐dinitrobenzene (1,2‐DNB) with butylamine (BA) and piperidine (PIP) were investigated as a function of the amine concentration and temperature, in chloroform, ethyl acetate, tetrahydrofuran (THF), acetonitrile (ACN), dimethylformamide (DMF), dimethyl sulphoxide (DMSO). benzene, toluene, chlorobenzene and diisopropyl ether.In the set of solvents consisting of ethyl acetate, THF, ACN, DMF and DMSO, neither reaction is catalysed (kA=k1). The sequence and range of reactivity for BA and PIP are similar in these solvents. These results indicate that reactions in which nitro in the leaving group behave differently fromSNAr reactions with other leaving groups, such as halogens or alkoxy groups, since an intramolecular hydrogen bond may be expected between the leaving nitro group and the ammonium H of the nucleophiles. The correlations of the rate coefficients obtained with Taft and Kamlet's solvatochromic method support these conclusions.On the other hand, these reactions show mild acceleration with relatively non‐polar solvents such as the aromatics and diisopropyl ether. The donor properties of these solvents and experiments with solvent mixtures suggest the formation of electron donor–acceptor complexes between them and 1,2‐DNB. Hence the preferential solvation of 1,2‐DNB by the donor solvent accounts for the mechanism observed.
Rate‐equilibrium LFER characterization of transition states: The interpretation of α
Authors: Lewis Edward S. 
Published: January 2005

AbstractThe effect of substituents on the rate of a reaction and the effect of the same substituents on the equilibrium can often be related by a linear free energy relation (LFER): log k− = α log K= + constant, where k+ and K= are the rate constant and the equilibrium constant, respectively. This review, concentrating on group transfers, adds to many studies describing the use of α to describe the transition state. Although the use of α to describe transition states is general, group transfers constitute a simple class allowing a fairly complete description yet illustrating two often neglected contributions. Group transfers can be described by the Marcus equation relating rate to an average identity rate and the equilibrium constant; a major contributor to the slope, α, of the rate‐equilibrium LFER is the variation of identity rates with substituent, rather than reflecting product‐like character. Substituent effect LFERs are predominantly attributable to interaction of charges with the substituent. However, α is not an exact measure of the charge on the substituent‐containing group, because in a transition state, but often not in a reaction product, there are more remote centers of charge which exert a smaller attenuated effect. A simple treatment of this attenuation for group transfers is proposed. The possibility of application of these ideas to proton transfer reactions and the interpretation of the Brønsted α (or β) is proposed.
Hydration of 2,2‐dichloro‐1‐arylethanones in water and tetrahydrofuran–water mixtures
Authors: Wendhausen Renato , Zampirón Eduardo , Vianna José F. , Zucco César , Rezende Marcos Caroli , Nome Faruk 
Published: January 2005

AbstractThe hydration of 2,2‐dichloro‐1‐arylethanones was studied in water and tetrahydrofuran (THF)–water mixtures and the equilibrium constants in water were estimated for four ethanones. The process is subject to specific acid and to general base catalysis. The proton inventory technique was applied to the study of the reaction in L2O and THF–L2O solutions (L = H or D). In pure water and water‐rich mixtures, a very large number of protons participate in the open transition state, this number decreasing gradually with increasing water content in the mixture, until the minimum value of 2 is reached, corresponding to a cyclic, closed transition‐state structure.
Consecutive radical ion pairs in the cycloreversion of a dimethylindene dimer: A case of cooperative spin polarization effects
Authors: Roth Heinz D. , Hutton Richard S. 
Published: January 2005

AbstractNuclear spin polarization effects observed during the electron transfer‐induced cleavage of the trans head‐to‐head dimer of 3,3‐dimethylindene are incompatible with the intermediacy of a single dimer radical cation. Instead, they are compatible with the ‘cooperative’ involvement of a localized, doubly linked and a delocalized, singly linked dimer radical cation. The mechanistic consequences for the cycloaddition and cycloreversion of radical cations are discussed.
Stereochemical studies. XXXIV. Quantitative description of ring puckering via torsional angles. The case of six‐membered rings
Authors: Zefirov N. S. , Palyulin V. A. , Dashevskaya E. E. 
Published: January 2005

AbstractThe quantitative description of ring puckering suggested by the authors is compared with that of Cremer and Pople. The applicability of both methods is discussed for the case of six‐membered rings with the use of model calculations simulating various ring distortions and with the analysis and comparison of puckering parameters computed on the basis of x‐ray data for 40 six‐membered rings in different cyclic structures. The 2N times reduction of the field of variation of puckering parameters for the N‐membered ring is suggested and the algorithm for the renumbering of the ring atoms for this purpose is described for six‐membered rings.
Electron distribution in the molecule of nitrones
Authors: Exner Otto 
Published: January 2005

AbstractVector analysis of dipole moments of two series of nitrones revealed a mesomeric dipole moment compatible with a relatively great contribution of a zwitterionic structure and possible some contribution from a hypervalent structure. The latter was also suggested from a detailed comparison of bond lengths among several classes of compounds, involving the CNO grouping. Since both the CN and NO bonds seem to possess an appreciable double bond character, it is suggested that the N atom might be allocated more then eight valence electrons.
Photochemical electrocyclization of the allyl radical into the cyclopropyl radical
Authors: Holtzhauer K. , Cometta‐Morini C. , Oth J. F. M. 
Published: January 2005

AbstractFollowing the observation that allyl radicals trapped in an argon matrix can be photolytically converted into cyclopropyl radicals (λ = 410 nm, 18 K), the IR spectrum of the cyclopropyl radical was recorded for the first time and interpreted. Bicyclopropane and cyclopropane are formed when the photolysed argon matrix is warmed from 18 to 35 K. The identification of these new species unequivocally proves the presence of cyclopropyl radicals in the photolysed matrix. This radical is shown to be a σ‐type (Cs symmetry) and not a π‐type (C2v symmetry) radical; of the 18 normal frequencies of the Cs cyclopropyl radical, all active in the IR, 16 were observed and were assigned to their corresponding normal modes. For this assignment advantage was taken of ab initio frequency computations reported in the literature and performed by the authors.
Contrasting kinetic behaviour of allyl and crotyl alcohols towards N‐bromosuccinimide in aqueous methanol
Authors: Karunakaran C. , Ganapathy K. 
Published: January 2005

AbstractThe reactions of N‐bromosuccinimide (NBS) with allyl and crotyl alcohols in aqueous methanol are first order in NBS. Allyl alcohol displays a zero‐order dependence whereas crotyl alcohol exhibits a Michaelis–Menten‐type dependence on substrate concentration. The NBS–allyl alcohol reaction is inhibited by H+ whereas the NBS–crotyl alcohol reaction is independent of [H+]. However, both are fast in the absence of acid. The rates are unaffected by the addition of neutral salt and succinimide. The reaction of NBS with crotyl alcohol exhibits a positive dielectric effect whereas allyl alcohol displays no such effect. Inhibition by HgII is observed in the crotyl alcohol reaction but not with allyl alcohol. 3‐Bromo‐2‐methoxypropan‐1‐ol is the product of the allyl alcohol reaction but a mixture of 2‐bromo‐3‐methoxybutan‐1‐ol and 2‐bromobutane‐1,3‐diol in the crotyl alcohol reaction. In the allyl alcohol reaction the formation of solvated bromonium ion and of HOBr is suggested as rate‐limiting steps. In the crotyl alcohol reaction, rate determining breakdown of an NBS–crotyl alcohol complex, formed in a fast pre‐equilibrium, is proposed.
Photostimulated reactions of haloarenes with benzeneselenate ions by the SRN1 mechanism. Competition between electron transfer and fragmentation of radical anion intermediates
Authors: Pen̆én̆ory Alicia B. , Rossi Roberto A. 
Published: January 2005

AbstractThe photostimulated reaction of p‐iodoanisole (p‐IAn) and 2‐bromopyridine with PhSe− ions in liquid ammonia gave Ph2Se, ArSePh and Ar2Se (Ar = p‐anisyl, 2‐pyridyl) by the SRN1 mechanism. These products are formed due to the competition between two steps of the proposed mechanism, the fragmentation of the radical anion intermediate formed in the coupling of aryl radicals with the PhSe− ion, and the electron transfer from the radical anion to an electron acceptor. These photostimulated reactions were carried out in different solvents in order to study their influence on the rates of the two competing reactions. They were also studied at high concentrations of p‐haloanisoles in acetonitrile (ACN), and it was found that the straightforward substitution product AnSePh was formed only at a 2.5 M concentration of p‐IAn, 7·8 M p‐BrAn and 15·6 M p‐ClAn. The last compound did not react with the PhSe− ion at concentrations as high as 15·6 M, but the substitution reaction took place in the presence of 0·025 M p‐IAn (entrainment reaction). The relative reactivity of PhSe− vs PhS− toward 2‐quinolyl radicals was studied and it was found that PhSe− reacts 2·7 times faster than PhS− in ACN.
Mechanism of α‐substitution reactions of acrylic derivatives
Authors: Hill Jon S. , Isaacs Neil S. 
Published: January 2005

AbstractReactions between acrylic esters, nitriles and similar compounds on the one hand and aldehydes and ketones on the other, catalysed by tertiary amines, lead to α‐(hydroxyalkyl)acrylic compounds. Evidence on the basis of kinetics, volumes of activation and of reaction, and kinetic isotope effects is presented to deduce a mechanism in which amine first undergoes Michael addition, the resulting enolate ion adds to the carbonyl compound in a rate‐determining step and finally base is eliminated. An analysis of the solvent effect on rates is also presented.
Destabilized carbenium ions: α‐Carbamoyl‐α,α‐dimethylmethyl cations
Authors: Wolf Rainer , Grützmacher Hans‐Friedrich 
Published: January 2005

AbstractThe formation and unimolecular reactions of the especially electron‐deficient α‐carbamoyl carbenium ions [(CH3)2CCONH2]+ (a1) and [(CH3)2CCON(CH3)2]+ (a2) have been studied by mass spectrometric techniques. The ions a are formed by electron impact‐induced dissociation from the relevant isobutyric amides α‐substituted by I, Br and Cl, respectively, but the carbonyl protonated methacrylamides b1 and b2 are cogenerated. A similar mixture of ions a and b is obtained by CI(CH4) of the corresponding methacyrlamides. The metastable ions decompose by the loss of CO exhibiting in particular a large kinetic energy release (KER) and by the elimination of NH3 and HN(CH3)2, respectively. It is shown that isomeric ions of structures a and b interconvert prior to decomposition and that the energy barrier separating the isomeric ions is small. Hence the kinetic stability of the tertiary α‐carbamoyl‐α,α‐dimethylmethylcations is reduced compared with other types of α‐acyl carbenium ions. This effect can be attributed to the increased proton affinity of the amide group.
Structural studies of halogen adducts of diorganyl chalcogenides in solutions by 1H, 13C, 77Se and 125Te NMR
Authors: Nakanishi Warō , Hayashi Satoko , Tukada Hideyuki , Iwamura Hiizu 
Published: January 2005

Abstract1H, 13C, 77Se and 125Te NMR spectra were measured for several sulphides, selenides, tellurides and their halogen adducts to establish the criteria for distinguishing trigonal bipyramidal (TB) adducts from molecular complexes (MCs). TB formation caused large downfield shifts for methyl protons and methyl, ipso and para carbons of methyl and phenyl selenides and tellurides. Large downfield shifts (> 100 ppm) were also observed for selenium and tellurium by the formation of TB from the parent selenides or tellurides. The plots of the 125Te vs 77Se chemical shifts for a set of corresponding compounds gave a straight line with a slope of 1·74, showing that the linear relationship holds well for tellurides and selenides and their TB adducts examined in this work. Upfield shifts of the ipso carbons were characteristic of MC formation. The para carbon signals of MC appeared downfield relative to those of the parent selenides, although the shift values were not so large. The 77Se signals for MC shifted slightly downfield (⩽ 10 ppm). The dissociation constants for iodine adducts of dimethyl selenide and selenoanisole were determined. The MC structure of bromine adducts of sulphides is well demonstrated on the basis of the criteria set out. 1H and 13C NMR spectra of ethylbenzene and ethers were also recorded in the presence of iodine or bromine. However, their chemical shifts were the same as those in the absence of iodine or bromine, within experimental errors.
Structural studies of halogen adducts of some cyclic selenides and tellurides by 1H, 13C, 77Se and 125Te NMR. Evidence for the formation of molecular complexes of selenoxanthone and selenanthrene with bromine
Authors: Nakanishi Warō , Yamamoto Yasuhiro , Hayashi Satoko , Tukada Hideyuki , Iwamura Hiizu 
Published: January 2005

Abstract1H, 13C, 77Se and 125Te NMR spectra were recorded for selenoxanthone (1a), telluroxanthone (2a), selenanthrene (3a), related compounds and their halogen adducts to elucidate the structure of the adducts in solutions. Although selenides react with bromine to give trigonal bipyramidal (TB) hypervalent complexes in general, 1a reacted with bromine to yield a molecular complex (MC). Its chlorine adduct was found to be a TB. The effective electronegativity of selenium in 1a may be modulated by conjugation with the strongly electron‐withdrawing carbonyl group. In the case of 2a, its halogen adducts were TBs. The structure of the bromine adduct of 3a was also demonstrated to be an MC from NMR data.
Ion association and reactivity of the alkali metal salts of alkanoic acids in dipolar aprotic solvents
Authors: Crescenzi Manuela , Galli Carlo , Mandolini Luigi 
Published: January 2005

AbstractThe effect of added alkali metal perchlorates on the rate of lactonization of ω‐bromoalkanoate ions was investigated in 99% dimethyl sulphoxide (four‐membered ring formation) and in dimethylformamide (DMF) (four‐ and sixteen‐membered ring formation). In all cases the effect of the added metal salts is rate depressing, as a result of the lower reactivity of metal ion‐associated species relative to the free ions. Ion‐pairing association constants were determined, but the reactivity of ion pairs were so low as to elude direct measurement in most cases. Evidence was also obtained for the formation of ion triplets with Li+ in DMF. The relevance of the present results in connection with the ‘caesium effect’ is briefly discussed.
Role of 2‐carboxyl substituent in the tautomerization between two equivalent enol forms of 3‐hydroxyphenalenone
Authors: Matsumiya Shigeki , Izuoka Akira , Sugawara Tadashi , Nakamura Nobuo 
Published: January 2005

AbstractThe interconversion between two equivalent enol forms of 3‐hydroxyphenalenone took place predominantly through a diketo form with a free energy of activation of ca 14 kcal mol−1 in DMSO solution determined by 125·8 MHz13C NMR measurement. On the other hand, the corresponding interconversion in 2‐carboxy‐3‐hydroxyphenalenone was not frozen on the time scale of 13C NMR spectroscopy down to −60°C. This acceleration in the interconversion rate of the latter may be interpreted by a double proton switching between the hydrogen‐bonded 2‐carboxyl and 3‐hydroxyenone moieties.
Synthesis and chemistry of azolenines. Part 19. Basicities of some 2H‐ and 3H‐pyrroles and the remarkably large pKa′ of 2,2,3,5‐tetramethyl‐2H‐pyrrole
Authors: Lui Kon‐Hung , Sammes Michael P. 
Published: January 2005

AbstractpKa′ Values are reported for two 2H‐pyrroles and their 3H‐pyrrole isomers. The latter have basicities of the same order as similarly substituted pyridines, whereas the former are 2–2·5 pK units more basic, 2,2,3,5‐tetramethyl‐2H‐pyrrole having a pKa′ of 8·40, close to that for 2‐aminoimidazole. Possible reasons are given.
Frequency‐dependent branching ratio in the infrared multiphoton photochemistry of diethyl carbonates
Authors: Farneth William E. , Thomsen Marcus W. , Beck Thomas L. 
Published: January 2005

AbstractUnimolecular decomposition of 0·1 Torr of diethyl carbonate, diethyl‐d5 carbonate and diethyl‐d10 carbonate induced by irradiation with a CO2 TEA laser was investigated. In all cases only ethylene, ethanol and CO2 were obtained. In the case of diethyl‐d5 carbonate, both ethylene‐d4 and ethylene‐d0 were observed together with the corresponding isotopic variants of ethanol. The chemical branching ratio was determined as a function of irradiation frequency and laser fluence by analysis of the ethylene‐d0/ethylene‐d4 ratio. RRKM theory applied to the branching ratio data requires very different average energies for two sets of fluence and frequency conditions that give identical total yields.
Borohydride reduction of alkyl phenyl ketones within a reversed‐phase liquid chomatographic column
Authors: Jaeger David A. , Clennan Malgorzata Wegrzyn 
Published: January 2005

AbstractThe reductions of propiophenone and octanophenone to 1‐phenylpropan‐1‐ol and 1‐phenyloctan‐1‐ol, respectively, with sodium borohydride and tetrabutylammonium borohydride were performed on a reverse‐phase highperformance liquid chromatographic (HPLC) column of macroporous 10‐μm poly(styrene–divinylbenzene) undr HLPC conditions. In these reactions a lower concentration of tetrabutylammonium borohydride than of sodium borohydride was needed to effect the same extent of reduction, and modest substrate selectivity was obtained.
A valence and charge criterion for reactivity of π electron systems
Authors: Jug Karl , Köster Andreas M. 
Published: January 2005

AbstractA new criterion for the reactivity of π electron systems is proposed. The method incorporates and modifies ideas of Coulson and Fukui et al. about free valence. The proposed reactivity index includes covalent and lonic components and is suitable for elctrophilic, radical and nucleophillic attack. The method is tested for a few simple π systems and differences from Coulson and Fukui et al. are discussed. Finally, it is applied to the electrophilic attack of a series of substituted benzenes and to nucleophilic attack of a series of subsituted benzoic acid esters.
Electronic absorption spectra of aliphatic diamine radical cations. Conformation‐dependent charge delocalization
Authors: Gȩbicki Jerzy , Marcinek Andrzej , Stradowski Czesław 
Published: January 2005

AbstractElectronic absorption spectra of radical cations generated by steady‐state and pulse radiolysis from a series of α,ω‐diaminoalkanes (R2N(CH2)nNR2; n = 1–6, R = CH3 or C2H5), 1,4‐dimethylpiperazine and 1,4‐diazabicyclo [2.2.2] octane (DABCO) were measured. In some cases spectral evidence was obtained for an interaction between the nitrogen atoms leading to charge delocalization in the radical cation. A delayed formation of radical cations was associated with a conformational change necessary for achieving the most stable structure having a favourable conformation for charge delocalization.