bluuebunny/crossref_metadata_2025_split huggingface dataset:
Zero‐field splitting versus interelectronic distance in triplet electron spin resonance spectra of localized dinitrenes
Authors: Minato Masaki , Lahti Paul M. 
Published: January 2005

AbstractTriplet zero‐field splitting parameters were obtained in 2‐methyltetrahydrofuran glass at 77 K for phenylene‐1,4‐dinitrene (1), biphenyl‐4,4′‐dinitrene (2), (E)‐1,2‐bis(4′‐nitrenophenyl)ethene (3), 1,4‐bis(4′‐nitrenophenyl)buta‐1,3‐diene (4) and 1,8‐bis(4′‐nitrenophenyl)octa‐1,3,5,7‐tetraene (5). The results were (1) |D/hc| = 0·169 cm−1 |E/hc| = 0·004 cm−1, (2) |D/hc| = 0·189 cm−1, |E/hc| = 0·00 cm−1, (3) |D/hc| = 0·122 cm−1, |E/hc| = 0·00 cm−1, (4) |D/hc| = 0·0865 cm−1, |E/hc| = 0·00 cm−1 and (5) |D/hc| = 0·0442 cm−1, |E/hc| = 0·00 cm−1. All these biradicals are ground‐state singlets. Based on the observed decrease in triplet signal intensities as temperature decreases. The substantial magnitudes of |D/hc| for 3–5, despite the large distance between localized nitrene electrons, is much more than can be explained by a simple dipolar interaction between localized electrons, and is attributed at least partly to spin polarization effects on the π‐electron clouds of these systems.
Micellar catalysis of organic reactions. Part 34. Nucleophilic aromatic substitution reactions in micelles with bulky head groups
Authors: Broxton Trevor J. , Christie John R. , Theodoridis Dorothy 
Published: January 2005

AbstractObserved second‐order rate constants for the hydroxydechlorination of 1‐chloro‐2,4‐dinitrobenzene (2), 2‐chloro‐3,5‐dinitrobenzoate ions (3) and 4‐chloro‐3,5‐dinitrobenzoate ions (4) in micelles of cetyltrialkylammonium bromide (C16H33NR3Br, where R = Me, Et, n‐Pr and n‐Bu) are reported. For substrate 2, the observed catalysis increased as the size of the micellar head group was increased. This was shown to be primarily due to an increase in the rate of reaction in the micellar pseudo‐phase (k). For substrates 3 and 4, the observed catalysis decreased as the size of the micellar head group was increased. Much smaller changes in k were observed in these reactions, which lead to a dianionic intermediate which is more sensitive to polarity effects at the micelle surface than is the monoanionic intermediate formed from 2. These results support the contention that as the size of the micellar head group is increased, water is squeezed away from the micelle surface, resulting in a less polar reaction environment.
Attractive interactions between negative charges and polarizable aryl parts of host–guest systems
Authors: Schneider Hans‐Jörg , Werner Frank , Blatter Thomas 
Published: January 2005

AbstractNMR measurements of associations between simple organic host and guest molecules bearing alternatively negatively charged groups and electroneutral π‐moieties show in aqueous solutions weak but distinct attractions reaching approximately 2 kJ mol−1 and X−/arene unit. A similar value is observed for the complex between a calixarene with 4‐sulfonato groups and toluene, for which the NMR shifts indicate an orientation with the guest methyl substituent upwards. The conformational fit of the selected complexes is checked by molecular mechanics simulations.
Structure–odour relations for bitter almond odorants
Authors: Zakarya Driss , Yahiaoui Mohamed , Fkih‐Tetouani Souäd 
Published: January 2005

AbstractStructure–bitter almond odour relations are established for a set of 65 organic compounds (40 having the bitter almond character) belonging to benzene, pyrrole, thiophene, acyclic and cyclic compounds. Compounds are described using components of autocorrelation method, and the odour is described with a binary variable. Data were analysed using principal component analysis followed by a linear discriminant analysis. The obtained model gives satisfactory classification and prediction of the training set and the test set, respectively. In addition, obtained structure–odour relations were translated to structural elements (and rules) necessary for a stimulus to have bitter almond character.
Classification of the solubility behaviour of β‐cyclodextrin in aqueous–CO‐solvent mixtures
Authors: Coleman Anthony W. , Munoz Marc , Chatjigakis Alexis K. , Cardot Philippe 
Published: January 2005

AbstractThe solubility behaviour of β‐cyclodextrin in aqueous mixtures was studied by determining the interactions between the co‐solvent and water. Three clear classes of co‐solvent are evident. In the first, including ethanol, tetrahydrofuran and acetone, there is formation of clathrate hydrates by the co‐solvent. In these systems the solubilization is a maximum at the mole fraction corresponding to the minimum in the partial excess molar volume. The second class, including such solvents as dimethyl sulphoxide and pyridine, forms strong molecular complexes with water, and here a valley and a plateau region occur in the solubility values. In the third class, including formamide and urea, a steady increase in the solubility is observed. For these highly polar solvents no maxima are observed in intrinsic solvent properties such as the partial excess molar volume.
Acidities of 4‐substituted benzoic, bicyclo[2.2.2]oct‐1‐yl and bicyclo[2.2.2]oct‐2‐enyl carboxylic acids
Authors: Koppel I. A. , Mishima M. , Stock L. M. , Taft R. W. , Topsom R. D. 
Published: January 2005

AbstractExperimental gas‐phase acidities are reported for a series of 4‐substituted bicyclo [2.2.2] oct‐1‐yl carboxylic acids and for a limited number of the corresponding bicyclo [2.2.2] oct‐2‐enyl carboxylic acids. Similar acidities are also reported for additional 4‐substituted benzoic acids, allowing a comparison of field and resonance effects between the three series. Ab initio molecular orbital calculations for these series of acids confirm the conclusion that the aromatic acids display direct field and resonance effects, whereas the acidities in the aliphatic series acids are largely determined by direct field effects.
Magnetic coupling between two phenoxyl radicals attached to the phenyl rings of CIS‐ and trans‐stilbenes
Authors: Mitsumori Teruyuki , Koga Noboru , Iwamura Hiizu 
Published: January 2005

AbstractMagnetic coupling between two sterically protected phenoxyl radicals through the cis‐ and trans‐stilbene chromophores was studied by means of their EPR fine structures. While the zero‐field splitting parameters D, which are governed by the magnitude of dipolar coupling, were dependent on the geometrical isomerism, the sign of exchange coupling was independent of it and dictated by the topology of the π‐conjugated systems: ferromagnetic in o,m‐isomers and antiferromagnetic in m,m'‐isomers.
Estimation of marcus λ for p‐phenylenediamines from the optical spectrum of a dimeric derivative
Authors: Nelsen Stephen F. , Yunta Maria J. R. 
Published: January 2005

AbstractThe radical cation of N,N′‐bis‐(4‐dimethylaminophenyl)pyridazine (3˙+) has a near‐IR band with a transition energy of 29·7 ± 0·5 kcal mol−1 (1 kcal = 4·184 kJ) in acetonitrile. This band is assigned to a Hush‐type charge‐transfer band, and the transition energy to the Marcus λ value for electron transfer. Such a large λ is inconsistent with previous estimates of λ for intermolecular electron transfer between mono‐p‐phenylenediamines and their radical cations. It agrees well with estimates of λ based on AM1 semi‐empirical MO calculations, which gives the enthalpy contribution to λin at about 21 kcal mol−1 for 3·+. It is suggested that the traditional method of estimating λin by summing bond displacements weighted by force constants produces values which are significantly too low for systems such as phenylenediamines, where pyramidalization changes are important. The results suggest that λ for tetramethyl‐p‐phenylenediamine self‐electron transfer (10/·+) has been significantly underestimated, and that this prototype organic electron transfer reaction is not as strongly diabatic as was previously concluded.
Single‐parameter versus dual‐parameter correlation for radical reactions. Addition of bromine atoms to α‐methylstyrenes
Authors: Jiang Xi‐kui , Liu Wayne Wei‐zhong , Wu Shi‐hui 
Published: January 2005

AbstractA rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α‐methylstyrenes (1‐Y, with Y = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O2and dibutyl peroxyoxalate at 30°C. All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2. Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual‐parameter equation and not by a single‐parameter equation. Among various combinations of σ* and σx, the (σJJ* + σmb) combination yields the best correlation.
Estimation of ionization constants of azo dyes and related aromatic amines: Environmental implication
Authors: Hilal S. H. , Carreira L. A. , Baughman G. L. , Karickhoff S. W. , Melton C. M. 
Published: January 2005

AbstractIonization constants for 214 dye molecules were calculated from molecular structures using the chemical reactivity models developed in SPARC (SPARC Performs Automated Reasoning in Chemistry). These models used fundamental chemical structure theory to predict chemical reactivities for a wide range of organic molecules from molecular structure. The energy differences between the protonated state and the unprotonated state for a molecule of interest are factored into mechanistic components including the electrostatic and resonance contributions and any additional contributions to these energy differences. The RMS deviation was found to be less than 0·62 pKa units, which is similar to the experimental error.
Estimation of the OH ⃛O interaction energy in intramolecular hydrogen bonds: A comparative study
Authors: Dziembowska T. , Szczodrowska B. , Krygowski T. M. , Grabowski S. J. 
Published: January 2005

AbstractAn empirical formula relating directly H ⃛O interatomic distance to H ⃛O interaction energy is used to estimate this energy in intramolecular OH ⃛O hydrogen bonds calculated for systems of known precise geometry. The values obtained were correlated with spectroscopic characteristics of the H‐bond (Δv for OH bands in IR spectra and NMR chemical shifts, δ, for protons) and CNDO and INDO estimations of H‐bond energy. The regressions obtained had good or very good correlation coefficients.
Enolimine and geminaldiamine forms in the reaction of pyridoxal phosphate with ethylenediamine. An electrochemical and spectroscopic contribution
Authors: Hidalgo Carmen , Sevilla Jose M. , Pineda Teresa , Blazquez Manuel 
Published: January 2005

AbstractThe enol–keto tautomerism of the Schiff bases formed by pyridoxal 5'‐phosphate (PLP) with ethylenediamine (Etd) and ethylamine (Et) was studied by electrochemical and spectrophotometric methods. The spectroscopic results revealed differences between the two reactions, i.e. the enolimine/ketoenamine ratio observed in PLP–Etd mixture is higher than that of PLP–Et. The differences observed in the electroreduction mechanism and stability of the Schiff bases formed provide additional evidence for an unusually high concentration of enolimine in PLP–Etd in buffered aqueous solutions.The results are consistent with a cyclic structure such as that previously proposed by Robitaille et al. [J. Am. Chem. Soc. 111, 3034–3047 (1989)] on the basis of spectroscopic data. Protonation of the terminal amino group of the Etd moiety in the Schiff base involves the formation of the cyclic species. The low basicity of ethylenediamine favours the formation of similar concentrations of the enol and keto tautomers of the Schiff base at neutral pH. This behaviour, also observed in the Schiff base formed by PLP and polylysine, may be involved in coenzyme–protein linkages.
Relative thermodynamic stabilities of the isomeric propenylbenzenes
Authors: Taskinen Esko , Lindholm Nina 
Published: January 2005

AbstractThe relative thermodynamic stabilities of 2‐propenylbenzene (allylbenzene), and the E and Z forms of 1‐propenylbenzene were determined over the temperature range 50–170°C by chemical equilibration in DMSO solution with t‐BuOK as catalyst. The values of the thermodynamic parameters ΔG⊖, ΔH⊖ and ΔS⊖ at 298·15 K for each isomerization reaction between the title compounds were evaluated.
Solvolysis of 3‐substituted 4‐homoadamantyl methanesulphonates. Can the β‐substituent effect distinguish between classical and non‐classical ion intermediates?
Authors: Okazaki Takao , Kitagawa Toshikazu , Takeuchi Ken'Ichi 
Published: January 2005

AbstractThe rates of the solvolysis of 3‐R‐4‐homoadamantyl methanesulphonates (mesylates) (3) were determined in 80% aqueous ethanol. The relative first‐order rate constants at 25 °C were 1·0 (R = H), 2·29 (R = Ph), 3·26 (R = p‐anisyl), 73·6 (R = Me) and 209 (R = Et). The methanolysis of 3 gave rearranged methyl ethers and rearranged olefins as major products together with small amounts (0·9–3·4%) of unrearranged products. The order of the accelerating effect suggests that the transition states involve significant σ‐participation, despite the fact that 3 (R = H) solvolyses via a classical ion intermediate. The logarithms of the solvolysis rate constants of 3 showed linear correlations with those of 1‐R‐2‐adamantyl tosylates (1) and 1‐R‐exo‐2‐norbornyl tosylates (2), indicating that the linear free‐energy relationship between the β‐substituent effects on the solvolysis rate is not a definite measure to distinguish between classical and non‐classical intermediates.
Nucleophilic substitution reaction of benzyl bromide with N, N‐Dimethylaniline: Significance of equilibrium cross‐interaction constant
Authors: Lee Ikchoon , Park Yong Kyun , Huh Chul , Lee Hai Whang 
Published: January 2005

AbstractKinetic studies on the reversible reactions of benzyl bromides with N, N‐dimethylanilines were carried out for both the forward (kf) and reverse (kr) directions. The equilibrium constants, K were calculated using the ratio kf/kr and the equilibrium cross‐interaction constant, ρeXY was determined. The ρeXY value was shown to represent a maximum intensity of interactions between substituents X and Y through covalent bonds within a molecule. The normalized values of ρX (ρnuc) and ρXY indicate that in the transition state the fractional development or loss of polar and resonance interactions in the benzyl system are imbalanced or non‐perfectly synchronized. In the forward reaction the fractional loss of resonance interaction becomes enhanced, whereas in the reverse reaction the fractional development of resonance interaction lags behind the corresponding changes of polar interactions by ca 45%.
Directional hydrogen bonding in the MM3 force field. I
Authors: Lii Jenn‐Huel , Allinger Norman L. 
Published: January 2005

AbstractThe MM3 molecular mechanics program calculates a fair representation of hydrogen bonding interactions, but to improve the MM3 hydrogen bond potential, a directional term has been added to the hydrogen bonding function. The resulting total function was reoptimized. Comparisons of the hydrogen bonding potential functions from ab initio, the original MM3, the current MM3(92) force field and the reoptimized MM3 force field MM3(94) for a variety of C,N,O systems are described.
Carbonyl oxygen as a hydrogen‐bond super‐base: The amidates
Authors: Chardin Aurelie , Berthelot Michel , Laurence Christian , Morris David G. 
Published: January 2005

AbstractAn infrared spectroscopic study of the 1 : 1 hydrogen‐bond association of amidates with both methanol and 4‐fluorophenol showed that the site of complexation is the oxygen of the amidate function. However the formamidate HCON2Me3 forms a second 1 : 1 complex on the nitrogen of the amidate. The formation constants of the hydrogen‐bond complexes of the amidates with the reference hydrogen‐bond donor 4‐FC6H4OH indicate that the amidates are stronger hydrogen‐bond bases than are amides and amide vinylogues. As such, the amidates constitute the strongest carbonyl bases hitherto investigated on the hydrogen‐bond basicity scale.
Rechecking of the equilibrium gas‐phase basicity scale for low‐basicity compounds using fourier transform ion cyclotron resonance spectrometry
Authors: Koppel Ilmar A. , Anvia Frederick , Taft Robert W. 
Published: January 2005

AbstractFourier transform ion cyclotron resonance (FT‐ICR) spectrometry was used to study and recheck the proton transfer equilibria for a large number of low‐basicity compounds, particularly those which are less basic than H2O. The data obtained were used for the construction of a continuous scale of the relative gas‐phase basicities between H2S and SO2F2. The present results are compared with the results obtained by McMahon and co‐workers using ICR spectrometry and high‐pressure mass spectrometric (HPMS) techniques. Satisfactory agreement is found with the existing ICR spectrometric data. The results, however, show unexplained variances with earlier and recent HPMS results that also not internally self‐consistent. The substituent effects for some families of low‐basicity compounds (nitriles, carbonyl compounds, ethers and some others) are discussed.
Distinguishability of liquid environments in the presence of brownian noise
Authors: Grunwald Ernest , Steel Colin 
Published: January 2005

AbstractThis paper extends previous work on the dynamic averaging of distinct liquid environments by cage exchange to include potential energy fluctuations at the cage centers due to Brownian motions of molecules in the cage walls. After a brief review of the effect of cage exchange on the Schrödinger wave train associated with the electronic ground state of the caged molecule, typical magnitudes and time‐scales for the Brownian potential energy fluctuations are estimated. Then a zig‐zag model for the resulting noise in the wave trains is developed, and applied to analyze distinguishability in the presence of noisy cage exchange. When distinguishability survives the noise, the distinct caged species are fully fledged environmental isomers. Applications of these concepts are discussed.
Model solvent systems for QSAR. Part IV. The hydrogen bond acceptor behaviour of heterocycles
Authors: Leahy David E. , Morris Jeffrey J. , Taylor Peter J. , Wait Alan R. 
Published: January 2005

AbstractAn LSER analysis based on the partitioning of 15 proton acceptor heterocycles has succeeded in extracting Σβ values, but only at the cost of demonstrating solvent dependence for some of them. As noted by Abraham, the division lies between protic and aprotic organic phases. His observation that pyridine and quinoline are less effective acceptors when surrounded by solvent than in 1 : 1 association was confirmed, and possible reasons for this are discussed. Two other such cases are N‐methylimidazole and pyridazine, both of which give lower Σβ values in octanol than in PGDP. For both, Σβ in PGDP is what would be expected on the basis of log Kβ. The value for pyridazine in octanol suggests that, here, the ‘α‐effect’ is no longer operative; it is possible that this result can be generalized to other such heterocycles. Elsewhere, the most remarkable finding is that, where there are two proton acceptor sites in one heterocyclic ring, Σβ is the simple unattenuated sum of the separate βf values. If this result is general, it leads to a very simple way of estimating Σβ for heterocycles by calculation where data are unavailable. Evidence was also found, in certain cases, for hydrogen bonding to the π‐donor heteroatom or the aromatic ring. The QSAR implications of these results are discussed.
Control of the thermal cis to trans isomerizations of azobenzene and thioindigo derivatives by the formation of supramolecular H‐bonded assemblies
Authors: Rosengaus Jacqueline , Willner Itamar 
Published: January 2005

Abstract2,3‐Bis(aminocyclohexyl)−6‐methoxy‐1,3,5‐triazine (1a) forms intermolecular H‐bonded complexes with 3,3′‐diacetyl‐cis‐azoenzene (4) and 6,6′‐diethoxy‐cis‐thioindigo (5b), (association constants K = 4.9 × 104 and 3.5 × 105 1 mol−1, respectively). The thermal cis→trans isomerization of 4b and 5b to 3,3′‐diacetyl‐trans‐azobenzene (4a), and 6,6′‐diethoxy‐trans‐thioindigo (5a), is inhibited in the intermolecular complex 1a–4b and 1a–5b. Molecular mechanics calculations support the formation of the intermolecular H‐bonded complexes between 1a and 4b or 5b.
Oil‐in‐water microemulsions based on chemodegradable surfactants as reaction media
Authors: Bieniecki Albert , Wilk Kazimiera A. 
Published: January 2005

AbstractThe reaction of 2‐(p‐nitrophenyl)ethyl bromide with hydroxide ion was studied in oil‐in‐water (o/w) microemulsions at 50°C. The octane‐in‐water microemulsion systems were stabilized by chemodegradable cyclic acetal‐type cationic surfactants as [(2‐alky‐1,3‐dioxolan‐4‐yl)methyl]trimethylammonium bromides Ia–c (alkyl: a = n‐C9H19; b = C11H23; c = C13H27) and butan‐1‐ol as co‐surfactant. The rate constants were also determined in the corresponding aqueous micelles and in water. The hydrolytic lability of Ia–c in the microemulsion systems was achieved at 50°C by acid hydrolysis of the 1,3‐dioxolane ring. Systematic synthetic studies of the dehydrobromination were performed in order to develop a preparative approach for the product separation without unfavourable process such as foaming and emulsification.
A remarkably bent allene. X‐Ray crystal structure and ab initio calculations
Authors: Weber Edwin , Seichter Wilhelm , Hess Bernd , Will Georg , Dasting Hans‐Jürgen 
Published: January 2005

AbstractX‐ray data on the crystal and molecular structure of bis(biphenyl‐2,2′‐diyl)allene, reveal a remarkably strong deviation from linearity (170·1°) of the allene unit of the molecule. Ab initio calculations involving dimers of molecules yield good correlation with this geometry, showing that the non‐linearity is due to packing effects in the crystal.
Are kekulene, coronene, and corannulene tetraanion superaromatic? Theoretical examination using hardness indices
Authors: Zhou Zhongxiang 
Published: January 2005

AbstractThe superaromaticity concept is examined and developed, taking into account what is known about aromaticity. Three new hardness indices indices are defined, and shown to be excellent parameters for characterizing superaromaticity. High superaromaticity indicates significant global annulenoid conjugation relative to local benzenoid conjugation in circularly annealed benzenoid molecules. Kekulene, coronene and corannulene tetraanion all are predicted to be superaromatic. The prediction for the corannulene tetraanion strongly supports the “annulene‐within‐an‐annulene” structure derived from NMR data.
Several mechanisms in the elimination kinetics of ω‐chlorocarboxylic acids in the gas phase
Authors: Chuchani Gabriel , Martin Ignacio , Rotinov Alexandra , Dominguez Rosa M. , Milagros Perez I. 
Published: January 2005

AbstractThe kinetics of the gas‐phase pyrolysis of ω‐chlorocarboxylic acids were examined in a seasoned static reaction vessel and in the presence of at least twice the amount of the free radical inhibitor cyclohexene or toluene. In conformity with the available experimental data on rate determination, these reactions proved to be unimolecular and obeyed a first‐order rate law. The presence of the primary chlorine leaving group in Cl(CH2)nCOOH (n = 1–4) showed a change in mechanism from intramolecular displacement of the Cl leaving group by the acidic hydrogen of the COOH to anchimeric assistance of the carbonyl COOH to the CCl bond polarization in the transition state. This mechanistic consideration is nearly the same for the series of 2‐, 3‐, and 4‐chlorobutyric acids. The chlorine atom at the 2‐position of acetic, propionic and butyric acids is dehydrochlorinated through a prevailing reaction path involving a polar five‐membered cyclic transition state.