bluuebunny/crossref_metadata_2025_split huggingface dataset:
Enhancement or suppression of CIDNP intensities by a second magnetic nucleus: Interplay between g factor difference, external magnetic field, and hyperfine coupling constants
Authors: Hwang Kuo Chu , Roth Heinz D. , Turro Nicholas J. , Welsh Kevin M. 
Published: January 2005

AbstractThe enhancement factors of the 1H polarization induced in radical pairs generated by α‐cleavage of 12CO‐ and CO‐labelled ketones deviate from the ratios expected on the basis of the relative abundances of the respective carbon isotopomers. For dibenzyl ketone, the ratio of absolute CIDNP intensities of the 1H(13C) doublet (90% 13CO) to the 1H(12C) singlet (10% 12CO) was found to be >9:1 at 58·3 kG and <9:1 at 21 kG. Similar deviations were observed for other 13C labelled ketones, viz. deoxybenzoin (13CO) and acetophenone (13CO or 13CH3). This novel enhancement or suppression effect on the CIDNP enhancement can be ascribed to the presence of a second hyperfine coupled nucleus in the intermediate radical pair. The individual contributions of the g factor difference (Δg), the external magnetic field strength (Ho), and the hfc constants of the observed (aA) and the interacting nucleus (ax) in the enhancement‐suppression are exemplified. The experimental results are consistent with theoretical calculations of the mutual effect of Δg, H0, aA and ax on the CIDNP enhancement factor of the observed nucleus.
O‐versus S‐protonation in diaryl sulfoxides: A semi‐empirical mo investigation: Comparison with solution studies in superacids
Authors: Laali Kenneth K. , Houser John J. 
Published: January 2005

AbstractThe protonation of diphenyl sulfoxide and several substituted diphenyl sulfoxides was probed by the MNDO‐PM3 method to gain a theoretical insight into the experimentally observed preference for stable diphenyl sulfoxonium ion formation (O‐protonation) in 1:1 FSO3H—SbF5 (Magic Acid)—SO2. In agreement with solution studies, O‐ protonation is uniformly favored (by ca 17 kcal mol −1) over S‐protonation. The differences in the heats of formation of protonated and unprotonated diphenyl sulfoxides are increased by electron‐withdrawing substituents (F, CF3) and decreased by electron‐donating groups (Me, OMe). Variations in the SO bond length and bond order in the onium ions are compatible with simple resonance arguments and the oxonium/sulfonium character of both the O‐ and S‐protonated onium ions. Surprisingly, the O‐ and S‐protonated dibenzothiophene S‐oxides are predicted to have identical energetics as compared with the corresponding protonated diphenyl sulfoxides. On structure optimization dibenzothiophene S‐oxide itself, if the initial geometry is somewhat twisted, rearranges by ring expansion to give a new heterocycle with lower energy. A rotational barrier study on the parent O‐protonated diphenyl sulfoxide showed two minima, separated by 1.3 kcal mol−1, at HOSC dihedral angles of 60° and 240°. The two conformations correspond to syn and anti orientations of the OH proton relative to the aromatic rings, and support the lowtemperature solution observations of the presence of two distinct sulfoxonium ions in solution. The rotational barriers for diphenyl sulfoxonium cation are compared with those of O‐protonated dimethyl sulfoxide.
Stabilization of molecular association: An AM1/PM3 study of tetrahalomethane–arene molecular complexes
Authors: Mager Thomas , Bertagnolli Helmut 
Published: January 2005

AbstractAn AM1/PM3 study of several tetrahalomethane–arene molecular complexes is presented. The results indicate the existence of weak attractive non‐dispersion interactions in some of these complexes, which can be rationalized in terms of multipole—multipole and multipole—induced dipole interactions. No evidence was found for the occurrence of charge transfer in the ground states of these complexes.
Quantitative aspects of chirality. I. Method of dissymmetry function
Authors: Kuz'min V. E. , Stel'makh I. B. , Bekker M. B. , Pozigun D. V. 
Published: January 2005

AbstractTo study molecular features connected with chirality, a procedure for the quantitative estimation of the chirality level of compounds of different classes is needed. A procedure for estimating the molecular asymmetry level relative to mirror‐reflection axes of symmetry, S1, S2, S4 and S6, has been developed. The geometrical mean of these parameters is the disymmetry function (DF). To calculate the DF, the molecule must be fixed in the coordinate system, transferred to the main axes of inertia.
Photochemical reactions of 2,2′‐furil. Solvent dependent photoreduction via the lowest excited singlet and triplet states
Authors: Inoue Hiroyasu , Sakurai Tadamitsu , Hoshi Toshihiko , Ono Isao , Okubo Jun 
Published: January 2005

Abstract2,2′‐Furil (di‐2‐furyl diketone) undergoes photoreduction to give furoin on irradiation at 366 nm in triethylamine‐containing benzene and ethers. On the other hand, in ethanol the photoirradiation leads to the formation of an enediol type of compound, 1,2‐di(2‐furan)ethene‐1,2‐diol. The results of quenching and sensitization both for the photoreactions and the phosphorescence demonstrate that furoin and the enediol are formed via the lowest excited singlet and the triplet states of 2,2′‐furil, respectively.
Using theoretical descriptors in quantitative structure activity relationships: Some physicochemical properties
Authors: Famini George R. , Penski Carl A. , Wilson Leland. Y. 
Published: January 2005

AbstractThe application of computational techniques to biology, chemistry and physics is growing rapidly. Quantitative structure–activity relationships (QSAR) have been used widely to relate biological activities and physicochemical properties to molecular structural features. A difficulty in this approach has been non‐uniformity of parameter sets resulting in the inability to examine contributions across properties and data sets. Linear solvation energy relationships (LSER) developed by Kamlet and Taft successfully utilize a single set of parameters to correlate a wide range of biological, chemical and physical properties. The empirical LSER solvatochromic parameters have been replaced with theoretically determined parameters to permit greater ease in a priori property prediction. These TLSER descriptors have given good correlations and interpretations for some biological activities. This paper discusses the application of these descriptors to six physicochemical properties involving equilibria, kinetics and spectra. The results show good correlation and physical interpretation.
O‐17 NMR studies on α‐diazoketones
Authors: Cerioni Giovanni , Culeddu Nicola , Saba Antonio 
Published: January 2005

AbstractThe O‐17 NMR spectra of 13 α′‐alkyl‐ and aryl‐substituted α‐diazoketones were measured. The results obtained show a clear discrimination between them. A good correlation in a series of para‐substituted α‐diazoacetophenones between O‐17 chemical shifts and both α+p and dual substituent parameters indicated a greater contribution of resonance than inductive effects.
Survey of the DNA binding properties of natural and synthetic polyamino compounds
Authors: Stewart Kent D. , Gray Thomas A. 
Published: January 2005

AbstractUsing a flourescence‐detected ethidium displacement assay, the calf thymus DNA complexation properties of 27 mono‐, di‐, tri‐, tetra‐ and hexacationic polyamines were determined. The DNA‐binding affinity of these polyamine compounds increased with increasing cationic charge on the polyamine. Although most of the compounds exhibited no base pair binding selectivity, two of the tricationic polyamines possessing additional neutral amine groups exhibited approximately tenfold GC binding selectivities.
Potassium fluoride for the promotion of β‐cyclodextrin‐induced regioselective PO(3′) cleavage of adenosine 2′, 3′‐cyclic phosphate
Authors: Sawata Shinya , Komiyama Makoto 
Published: January 2005

AbstractBoth the regioselectivity and the reaction rate for the β‐cyclodextrin (β‐CyD)‐induced P–O(3′) cleavage of adenosine 2′, 3′‐cyclic phosphate (A > p) to adenosine 2′‐phosphate are greatly promoted by KF. The promoting activity of KF is much larger than that of KCl. KF amplifies the difference in the chemical circumstance between the P–O(2′) and the P–O(3′) bonds of A > p, induced by β‐CyD on complex formation with A > p. The higher activity of KF is ascribed to absence of complex formation of F− ion with β‐CyD, which results in deeper penetration of A > p into the cavity of β‐CyD.
Re‐evaluation of 1,4‐Non‐bonded interactions in molecular mechanics
Authors: Hirano Tsuneo , Miyajima Takeshi , Imai Keisuke , Fujiyoshi‐yoneda Teruyo , Ōsawa Eiji 
Published: January 2005

AbstractAnalysis of the sum of two‐centre terms obtained from semi‐empirical molecular orbital calculations into atom‐pair interactions revealed a unique behaviour of 1,4‐non‐bonded atom‐pair interactions compared with 1,5‐ and longer range interactions. Explicit inclusion of a special 1,4‐non‐bonded interaction term in an MM2‐like molecular mechanics scheme gave a test force field with good performance.
Solvent effect on the hammett reaction constant for the electroreduction of substituted benzophenones
Authors: Jaworski Jan S. , Malik Marcin , Kalinowski Marek K. 
Published: January 2005

AbstractSolvent effects on the Hammett ρ value for the cathodic reduction of substituted benzophenones were determined. The electrochemistry of a series of 11 compounds was studied in acetonitrile, acetone, dimethyl sulphoxide, propylene carbonate, N,N‐dimethylformamide, N,N‐dimethylacetamide, N,N‐diethylformamide and hexamethylphosphoric triamide. The ρ values for the reversible one‐electron transfer are described by the Lewis acid‐base model ρ = − 0.006AN + 0.003DN + 0.391, where AN = solvent acceptor number and DN = solvent donor number.
Dynamic behaviour of [2] metacyclo [2] (3,4) thiophenophane
Authors: Takeshita Michinori , Tsuge Akihiko , Tashiro Masashi 
Published: January 2005

AbstractThe conformational analysis of dimethyl [2] metacyclo [2] (3,4)thiophenophane (1), which is the thiophene analogue of [2.2] orthometacyclophane, is described. The comparison between the dynamic NMR spectra of 1 and the computer‐simulated spectra led to the energy barrier between syn and anti conformers of 1. This is the first conformational analysis of a [2,2] orthometa‐type cyclophane.
Binding of lithium ion isotopes with redox‐active macrocyclic and macrobicyclic crown ethers
Authors: Chen Zhihong , Echegoyen Luis 
Published: January 2005

AbstractThree anthraquinone‐containing crown ether macrocycles and one macrobicycle were reduced coulometrically to their corresponding anion radicals under vacuum. The resulting anion radical solutions in CH2CL2 were stirred with an excess of a solid 6LiClO4−7LiClO4 mixture of known isotropic composition in order to form the appropriate Li+ complexes. After equilibration, the solution was separated from the excess solid and the [6Li+]/[7Li+] ratio was determined by atomic absorption spectrometry. The values were converted into the corresponding separation factors (α) after division by the original composition ratio of the solid mixture. The values were in the range 1.04–1.18. Except for one compound, 2, all of the reduced systems studied exhibited a much larger α value than the 1.057 reported for 12. crown‐4 at 0° C in a liquid–liquid extraction system.
Solution conformations and hydrolytic stability of 2′ ‐ and 3′ ‐substituted 2′,3′‐dideoxyribonucleosides, including some potential inhibitors of human immunodeficiency virus
Authors: Oksman Pentti , Hakala Harri , Zavgorodny Sergei , Polianski Michael , Azhayev Alex , van Aerschot Arthur , Herdewijn Piet , Lönnberg Harri 
Published: January 2005

AbstractSugar ring conformations of a number of 2′ ‐ and 3′ ‐substituted 2′,3′‐dideoxyribonucleosides were determined in 2H2O by 1H NMR spectroscopy. First‐order rate constants for the cleavage of their N‐glycosidic bond in aqueous acid were measured. The dependence of sugar ring conformation and hydrolytic stability on the polar nature of the 2′/3′‐substitutent is discussed.
Effects controlling the conformational selectivity and association parameters of H‐bonded assemblies between di‐ and triaminotriazines and bemegride
Authors: Willner Itamar , Rosengaus Jacqueline , Eichen Yoav 
Published: January 2005

Abstract2,4,6‐Tris(aminocyclohexyl)triazine (1) and 2,4‐bis(aminocyclohexyl)‐6‐methoxytriazine (2) are present in solution in two and three equilibrating conformations, respectively. The activation barrier for interconversion of the conformers 1a ⇌ 1b is 14·55 ± 0·2 kcal mol−1 (1 kcal = 4·184 kJ) and the activation barrier for interconversion of the conformers 2a ⇌ 2b ⇌ 2c is 14·2 ± 0·2 kcal mol−1. The conformational analyses of 1 and 2 were also followed by molecular mechanics calculations. Compounds 1 and 2 form H‐bonded intermolecular complexes with bemegride (3). Association of 3 proceeds by selection of a specific conformation of 1 and 2, i.e. 1b and 2a, respectively. The association constants of 3–1b and 3–2a are to Ka = 915 and 450 l mol−1, respectively. Molecular mechanics calculations for the H‐bonded intermolecular assemblies support the experimental observations of the selective conformational association.
Ambident reactivity of aryloxide ions towards 1,3,5‐trinitrobenzene, low‐temperature characterization of the elusive Oxygen‐bonded σ‐complexes by 1H and 13C NMR spectroscopy
Authors: Buncel Erwin , Manderville Richard A. 
Published: January 2005

AbstractThe ambident reactivity of phenoxide ion towards 1,3,5‐trinitrobenzene (TNB) was re‐examined by means of a novel reaction system(CD3CN–glyme‐d10) which allows the investigation of species formed at low temperatures (−40°C), contrasting with previous studies in dimethyl sulfoxide (DMSO) at ambient temperature. The new method coupled with 400 MHz NMR spectroscopy has allowed the definitive observation of both O‐ and C‐bonded phenoxide σ‐complex adducts for the first time, confirming the formation of the former through kinetic control and of the latter through thermodynamic control. The corresponding O‐bonded σ‐adduct in the TNB–mesitoxide system (whereC‐bonded σ‐adduct formation is not possible but where there is competing nitro group displacement) has also been characterized by 1H and 13C NBR. Another O‐bonded aryloxide adduct characterized is that from the reaction of TNB with3,5‐di‐tert‐butylphenoxide; in this system there is also competing NO2 displacement. Stereoelectronic factors in the O‐bonded σ‐adducts and aryl ethers are
discussed.
Molecular rearrangements in homogeneous gas‐phase elimination reactions. Pyrolysis kinetics of alkyl methanesulphonates
Authors: Chuchani Gabriel , Martin Ignacio , Dominguez Rosa M. , Rotinov Alexandra , Pekerarm Sara , Gomez Matilde 
Published: January 2005

AbstractThe rates of elimination of several alkyl methanesulphonates were determined in a seasoned, static reaction vessel over the temperature range 300–420°C and the pressure range 28–163 Torr. The reactions are homogeneous, unimolecular and follow a first‐order rate law. The overall rate coefficients are given by the following equations: for isobutyl methanesulphonate, log k1 (s−1) = (12·51 ± 0·38) − (177·0 ± 2·1) kJ mol−1 (2·303RT)−1; for 2‐phenyl‐1‐propyl methanesulphonate, log k1(s−1) = (12·62 ± 0·04) − (176·2 ± 0·5) kJ mol−1 (2·303RT)−1; for neopentyl methane‐sulphonate, log k1(s−1) = (13·35 ± 0·42) − (198·2 ± 5·2) kJ mol−1 (2·303RT)−1; and for 3‐chloro‐2,2‐dimethyl‐1‐propyl methanesulphonate, log k1(s−1) = (13·87 ± 0·42) − (218·2 ± 5·4) kJ mol−1 (2·303RT)−1. Rearrangements in these methanesulphonate pyrolyses may proceed via an intimate ion‐pair type of mechanism. Consequently, the results appear to confirm that intramolecular migration through autosolvation is possible in gas‐phase elimination reactions of certain types of organic molecules.
Conformational interconversion of [1.1] ferrocenophane: A molecular mechanics study
Authors: RudzińAski Jerzy M. , ŌSawa Eiji 
Published: January 2005

AbstractA molecular mechanics scheme developed for bridged ferrocene derivatives was applied to study the conformational energy surface of [1.1] ferrocenophane (1). The mechanism of the intramolecular interconversion of 1 is shown to be different from the mechanism of the syn–syn exchange proposed earlier for this ‘molecular acrobat’.
Multinuclear NMR and ab initio MO studies of 7‐methyl‐7H‐pyrrolo [2,3‐b]pyridine and related compounds
Authors: Minakata Satoshi , Itoh Shinobu , Komatsu Mitsuo , Ohshiro Yoshiki , Yokomichi Yasunori 
Published: January 2005

AbstractThe contribution of the polarized resonance structures to 7‐methyl‐7H‐pyrrolo[2,3,‐b]pyridine and 4‐methyl‐4H‐pyrrolo [3,2‐b] pyridine, which have recently attracted much attention in physico‐chemical studies, was considered based on multinuclear (1H, 13C and 15N) NMR spectroscopy and MO calculation. Comparison of the chemical shifts of the compounds with those of other relevant compounds and the effects of concentration and solvents observed by multinuclear NMR suggested that the contribution of the non‐polarized structures to both compounds predominates over that of the polarized structures. This result was also supported by ab initio MO calculations using the 6–31G basis set.
Mechanism of SYN addition of molecular fluorine to ethylene. An ab initio MO study
Authors: Iwaoka Tomoyasu , Kaneko Chikara , Shigihara Atsushi , Ichikawa Hiroshi 
Published: January 2005

AbstractThe whole process for the syn addition of molecular fluorine to ethylene was analysed at the MP2/6–31 + G level with IRC calculation. The analysis indicates that fluorine approaches the CC double bond vertically to form a perpendicular complex as the intermediate, which then reorientates to a rhombic‐type complex as the transition state to give the final syn addition product. This shows that the square‐type complex previously proposed is not involved in any stage of the reaction.
Substituent effects on the kinetic solvent isotope effect in solvolyses of arenesulphonyl chlorides
Authors: Koo In Sun , Lee Ikchoon , Oh Jieun , Yang Kiyull , Bentley T. W. 
Published: January 2005

AbstractThe Hammett type plot of kinetic solvent isotope effect (KSIE = kSOH/kSOD), log(KSIE) vs σ, can be a useful mechanistic tool for solvoltic reactions. The slopes of such straight line plots for para‐substituted benzenesulphonyl chlorides in methanol (0·15), water (0·05) and methanol–water (0·05) are interpreted in terms of different reaction channels (general base‐catalysed and SN2), in contrast to simple Hammett plots, log kSOH (or log kSOD) vs σ, invariably exhibiting non‐linear concave curves.
Crystal and moecular structure of N‐nitro‐N‐methyl‐p‐nitroaniline: Analysis of substituent effects on the ring geometry and estimation of the Hammett substituent constant for the N‐methylnitramino group
Authors: Anulewicz Romana , Krygowski Tadeusz Marek , Gawinecki Ryszard , Rasała Danuta 
Published: January 2005

AbstractThe crystal and molecular structure of N‐nitro‐N‐methyl‐p‐nitroaniline was solved with good precision: R = 0·058 and average estimated standard deviation for bond lengths = 0·003 Å. Analysis of the geometry reveals that the NO2 attached to the N atom is strongly conjugated. Application of the HOSE model to its geometry and to geometries of 12 other para‐substituted nitrobenzenes yields good linear regressions between canonical structure weights and Hammett σ values by interpolation, σ for the N(Me)NO2 group is estimated to 0·36.
Hydrogen bonding of aromatic amines in hydroxylic solvents. Part 1. Absorption spectroscopy of substituted anilines and hydrogen bond‐induced rehybridization of trivalent nitrogen atoms.
Authors: Nakagaki Ryoichi , Aoyama Iwao , Shimizu Kazuhisa , Akagi Masao 
Published: January 2005

AbstractThe longest wavelength absorption band of substituted anilines containing electron‐accepting groups is characterized as an intramolecular charge‐transfer (ICT) transition. Dual absorption peaks observed for the ICT transition of some para‐substituted anilines in a strongly hydrogen‐bonding solvent, hexafluoropropan‐2‐ol (HFP) can be attributed to two inequivalently hydrogen‐bonded species with differently hybridized aniline nitrogens in HFP. The hydrogen bond‐induced rehybridization causes appreciable changes in the ICT absorption spectra of highly polar aromatic amines. Rehybridization of aromatic amino nitrogens depends on the OH acidity in the solvent molecule and the basicity of the substituted anilines. The geometry of trivalent nitrogens in aryl amines is critically dependent upon the ionization potential of the amino group, the electron affinity of the electron‐accepting substituent and the substitution site of the electron‐withdrawing group with respect to the amino moiety.
The Marcus cross relation for hydrogen‐transfer reactions
Authors: SüHnel JüRgen 
Published: January 2005

AbstractThe Marcus cross relation provides a tool to calculate cross rates from the corresponding identity rates and the equilibrium constant. Assumptions made in deriving the Marcus cross relation and conclusions which can be drawn from agreement or disagreement between experimental and calculated cross rates are discussed. For a small variation in identity rate constants, agreement between experimental and calculated cross rates are discussed. For a small variation in identity rate constants, agreement between experimental and calculated cross rates simply means that the rate constant of a hypothetical thermoneutral cross reaction has a value within the range covered by the identity rates. Whether or not this value corresponds to the mean value of logarithmic identity rate constants can only be checked for a large variation in identity rates. Therefore, a newly defined index M is used to relate the difference in experimental and calculated logarithmic rate constants to the variation in the logarithmic identity rates. Provided that the Marcus cross relation is not obeyed, this indicates non‐additivity of kinetic parameters. Applying this analysis to protontransfer reactions between different transition metal acids and between 9‐alkylflourenes and (9‐alkylfluorenyl)lithium shows that claims about agreement or disagreement of experimental rate constants with the rate constants calculated by means of the Marcus cross relation have to be revised in part.
Hydrogen‐bond basicity of nitriles
Authors: Berthelot Michel , Helbert Maryvonne , Laurence Christian , Questel Jean‐Yves Le 
Published: January 2005

AbstractA thermodynamic hydrogen‐bond basicity scale, pKHB, and a spectroscopic hydrogen‐bond basicity scale, Δν(OH), were measured which permitted the construction of the solute hydrogen‐bond basicity scale, β, for 71 nitriles embracing a wide range of structures from trichloroacetonitrile to cyanamides. Field, resonance, and polarizability contributions of the X substituents to the hydrogen‐bond basicity of XCN compounds were established. Steric effects do not contribute to the hydrogen‐bond basicity of nitriles.
Interaction of 3,4‐diphenyl‐1,2,5‐thiadiazole 1,1‐dioxide with proton donor solvents
Authors: Mirífico M. V. , Caram J. A. , Vasini E. J. , Sicre J. E. 
Published: January 2005

AbstractThe UV spectra of 3,4‐diphenyl‐1,2,5‐thiadiazole 1,1‐dioxide (I) and that of its thiadiazoline and thiadiazolidine derivatives were measured in several aprotic and protic solvents. Strong specific interactions of I with protic solvents are observed and the formation of stable carbinolamine type derivatives of I with methanol or ethanol is proposed. Spectroscopic data (UV, 1H and 13C NMR) and electrochemical evidence for their formation are given and a new thiadiazoline derivative of I (3‐ethoxy‐2‐methyl‐3,4‐diphenyl‐1,2,5‐thiadiazoline 1,1‐dioxide) was synthesized. The equilibrium constant for the reaction of I with ethanol is reported.