Application of the Bjerrum Association Model to Electrolyte Solutions. II. Enthalpies of Dilution in Water and Nonaqueous Solvents

The association theory based on the Bjerrum model, which had been developed for the treatment of apparent and partial molar volumes of electrolyte solutions, is extended to apparent molar relative enthalpies L _2,. Experimental enthalpies of dilution for tetrabutylammonium bromide in acetonitrile, propylene carbonate, and -butyrolactone and for lithium perchlorate and sodium thiocyanate in acetonitrile were obtained and analyzed with this model. Literature data for various electrolytes in water, acetonitrile, and n-propanol were also reanalyzed. Through the Bjerrum equations, enthalpies of dilution can be extrapolated to infinite dilution and reliable L _2, obtained for associated electrolytes. The model can be used to estimate the association constant K _A of the electrolyte and these K _A are compared with literature values (generally obtained from conductivity). Considering the difference in the concentration ranges investigated in L _2, and measurements, K _A extracted from L _2, generally fall within an expected range of deviation from the ones obtained by conductivity, provided that no specific interactions are present in solution.     

Application of the Bjerrum Association Model to Electrolyte Solutions. IV. Apparent Molar Heat Capacities and Compressibilities in Water and Aprotic Solvents

The Bjerrum association model, which has already been applied successfully to volumes and enthalpies of dilution of electrolyte solutions, has now been extended to apparent molar heat capacities and compressibilities of these systems. The proposed method of calculation, which takes into account the relaxation effect observed in second derivatives of the excess Gibbs free energy, can be used to extrapolate to infinite dilution the experimental data for systems showing a wide range of association constants in acetonitrile, propylene carbonate, and water. The concentration dependence of the thermodynamic properties can be reproduced quantitatively by the addition of one or two virial coefficients. Literature data for C _P,2, and K _S,2, of electrolytes in aprotic solvents were refitted with this equation. For dissociated or slightly associated systems (K _A < 10), the standarY ₂ ^o d infinite dilution quantities () are in excellent agreement with literature values. For systems with high K _A, Y ₂ ^o obtained by the model are systematically lower than those reported in the literature. This is not surprising, since the traditional method of extrapolation using the Debye–Hückel limiting law or the Pitzer equation does not take association into account. A computer software that performs the calculations for the application of the Bjerrum model to thermodynamic properties has been designed and is presented in the appendix.     

Formulation and Preliminary in vivo Testing of Rufloxacin-Cyclodextrin Ophthalmic Solutions

Aim of the present work was to investigate the effect of somecyclodextrins (CDs) on the solubility and ocular bioavailability of rufloxacin base (RUF), with theultimate goal of developing an ophthalmic formulation. Phase solubility studies of RUF inpH 7.4 buffer were carried out in the presence of -cyclodextrin (-CD),hydroxypropyl--cyclodextrin (HP--CD) and -cyclodextrin(-CD). The effect of hydroxypropyl methylcellulose (HPMC) on RUF solubility was evaluated after heating the solutionscontaining HP--CD at 120 °C.A significant enhancement of RUF solubility was achieved by associatingthe drug with CDs, particularly HP--CD. This CD formed with RUF a less stablecomplex than that formed by -CD, but did not suffer the solubility limitations ofthe parent CD, and showed a higher solubilizing capacity than -CD. Addition of 0.25%(w/v) HPMC to solutions containing HP--CD increased the solubilizing effect of this CD,thus allowing reduction of the amount necessary for solubilization of 0.3% (w/v) RUF.Preliminary pharmacokinetic data in rabbits indicated that theocular bioavailability of 0.3% (w/v) RUF solubilized by HP--CD was higher when compared witha 0.3% (w/v) RUF suspension used as reference.     

Dielectric constants of acetonitrile, γ-butyrolactone,propylene carbonate,and 1,2-dimethoxyethane as a function of pressure and temperature

The dielectric constants of 1,2-dimethoxyethane, acetonitrile, -butyrolactone, and propylene carbonate were determined from capacitance measurements extrapolated to infinite frequency; ln are reported as a function of pressure up to 80 MPa at 15, 25, 35, 45°C and as a function of temperature in the range 10 to 50°C at 0.10133 MPa. The variation of ln with temperature or pressure can be expressed by a second order polynomial expression. The isothermal compressibilities of the solvents were determined at 25°C from sound velocities, densities, and heat capacities. A simple correlation can be established between ln /P and for most aprotic solvent.     

A New Relation Between the Maxima Conductivities of Nonaqueous Concentrated Electrolytes and Chemical Hardness of Solvents and Salts

For nonaqueous electrolytes, using the HSAB principle, we tried to correlate the conductivity maxima _MAX, vs. only two intrinsic parameters: chemical hardness of the solvent and that of the salt. Thus, not only the nature of the solvent but also that of the salt were taken into account. We were able to predict for a given solvent the variation of _MAX as a function of the hardness of the salt and that of the solvent: _MAX = K(1 – ||/_SOLVENT) with || = |_SOLVENT – _SALT| and K a constant in S-cm^–1 independent of the salt, but not of the solvent.     

Theoretical study of the potential energy curves of the diatomic radicals MeIIX. III. Application to MgCl,CaF and CaCl radicals and some preliminary remarks

The series of calculations of the potential energy curves of the diatomic radicals Me_IIX (Me_II = second group metal, X = halogen), has been extended to MgCl, CaF and CaCl. The calculations have been performed according to a stepwise procedure, outlined in previous works. The presently available results allow comparisons for the first members of the series.     

Reactions with β-cyanoethylhydrazine,III. A new approach for the synthesis of substituted 3,5-diaminopyrazole and 1,2,4-triazoles

The reactivity of -cyanoethylhydrazine toward enaminonitrile and aroyl isothiocyanates is reported. A variety of 3,5-diaminopyrazole and ³-1,2,4-triazolin-5-thione derivatives could be prepared.

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Reaktionen mit -Canyethylhydrazin, 3. Mitt.: Ein neuer Weg zur Synthese von substituierten 3,5-Diaminopyrazolen und 1,2,4-Triazolen

Zusammenfassung Es wird über die Reaktivität von -Cyanethylhydrazin gegenüber Enaminonitril und Aroylisothiocyanaten berichtet. Es konnte eine Reihe von 3,5-Diaminopyrazol-und ³-1,2,4-Triazolin-Derivaten hergestellt werden.

     

Effect of Gamma-Cyclodextrin, Methanol and pH on the Parameters of Reversed-Phase Thin-Layer Chromatography of Some Antisense Nucleotides

The binding of twelve 8-substituted-2'-deoxyadenosineand seventeen 5-substituted-2'-deoxyuridinederivatives with gamma-cyclodextrin (-CD) was studiedby reversed-phase thin-layer chromatography in acidicand alkaline environments. The relative strengths ofthe binding were calculated and compared with thosedetermined in an ion-free environment. The bindingstrength of the antisense nucleosides dependedconsiderably on both the chemical structure of thenucleosides and on the pH of the environment.     

Theoretical Insights into the Formation, Structure, and Energetics of Some Cyclodextrin Complexes

To investigate the physicochemical aspects relevant for the formation of various cyclodextrin inclusion complexes and to search for corresponding general structure–complex-stability relationships, stability data of 1 : 1 complexes for 179, 310, and 51 guest molecules with unsubstituted -, -, and -cyclodextrin were collected. Statistical analysis using structure-based parameters such as molecular size, hydrophobicity, rotatable bonds, electronic properties, and the presence or absence of more than 150 various functional or structural moieties were performed. The complexation thermodynamics could be well described within the framework of our recently introduced molecular size-based model for nonassociative liquids. With increasing guest size, 1 : 1 complex stability, as measured by ln K or G⁰, increases linearly up to a size limit characteristic for each CD, and the corresponding slopes and intercepts are in agreement with those predicted by the model. For larger structures, values level off and are scattered around an average value depending on shape, goodness of fit, and possibly lipophilicity and some specific effects (e.g. such as those caused by presence of phenol functionality). The complexation between -cyclodextrin and certain large steroidal guest molecules, especially a brain-targeted estradiol chemical delivery systems (E₂-CDS) that is under clinical development, was investigated in details based on fully relaxed semiempirical AM1 quantum chemical calculations. A deformation index (DI) of the CD ring computed using these fully optimized host-guest geometries could be used to characterize the conformational change of the guest.     

Triterpene Glycosides from Plants of the Astragalus Genus. III. Structure of Cyclounifolioside C from Astragalus unifoliolatus

The known compound cyclocantogenin (1) and a new cycloartane glycoside cyclounifolioside C (2), which has the structure 24R-cycloartan-3,6,16,24,25-pentaol 3-O--D-glucopyranoside, were isolated from roots of Astragalus unifoliolatus Bunge. The structures of the isolated compounds are established using hydrolysis and spectral data.     

Synthesis of γ-zirconium phosphate by the fluoro-complex method and the intercalation behavior of some α-diimines

The ammonium form of -zirconium phosphate has been synthesized by the direct precipitation method from fluorozirconate solution in the presence of ammonium dihydrogen phosphate. The hydrogen form Zr(HPO₄)₂ · 2H₂O, was obtained by acid treatment of the ammonium form -Zirconium phosphate , Zr(HPO₄)₂ · H₂O, with a relatively large particle size resulted from fluorozirconate solution in the presence of concentrated orthophosphoric acid. The intercalation behavior of such -diimines as 2,2-bipyridine (bpy) and 1,10-phenanthroline (phen) toward -zirconium phosphate was investigated, and it was found that about 0.2 mol of bpy and 0.5 mol of phen have been incorporated respectively per one mol of the host, with the expansion of interlayer distances. Further incorporation of Cu(H) ions into the interlayer space of these intercalates was possible. The bpy intercalate took up more Cu(II) ions than -zirconium phosphate, indicating that effective pillars are constructed between layers and the ion exchange of Cu(II) ions is facilitated thereby.     

Linear silver clusters in aqueous solution (clusterites): radiation-induced chemical synthesis and properties

The formation and properties of the products of the radiation-induced chemical reduction of silver ions in aqueous solutions containing sodium polyacrylate were investigated. The absorption spectra of these species exhibit a band at 290 mn and a band in the visible region. The latter shifts from 470 nm to -800 nm with time or after the addition of silver ions. The species obtained (clusterites) arc linear silver clusters bound to the carboxyl groups of the ion exchanger. The red shift of the visible band is due to the increase in their size. The mechanism of the formation and the structure of the clusterites are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1429–1435, June, 1996.     

Influence of cyclodextrins on nitrosation of drugs

The World Health Organization issued a nitrosation procedure (NAP Test) which allows to carry out nitrosation under standard conditions. It has proved that the in vitro reaction rates of the fast nitrosatable drugs piperazine, cimetidine and ethambutol are not influenced by -, - and -cyclodextrin. On the contrary, -, -cyclodextrin and heptakis-2,6-di-O-methyl--cyclodextrin enhance the nitrosation of the slower nitrosatable 1-ephedrine and fencamfamine significantly. This possible reaction must be considered if nitrosatable drugs are formulated with cyclodextrins to be administered to human beings.     

Inclusion Complexes of Dyes and Cyclodextrins: Modeling Supermolecules by Rigorous Quantum Mechanics

The results of structure optimization and molecular dynamics simulationof host-guest -cyclodextrin-pinacyanol dye inclusion complexesare obtained by applying a density functional based tight-binding code.The results attempt to correlate UV/Vis and circular dichroism spectraldata with calculated aggregate structures of the sandwich dimer, withthe monomers twisted slightly against each other. The sense of twist ispredetermined by the chirality of the complexing host. The UV/Vis-spectralare interpreted using the exciton model. Within this model, each excitedstate of the monomer generates two excitonic states in the dimer. Theinteraction between the two monomers results then in a Davydov splittingof the two dimer states. The opposite signs of the two dimer states can beattributed to the twist of the monomers when they interact.     

Application of interactive image analysis to light scattering patterns of some polymer systems

In this paper we describe a system for interactive picture analysis useful for extraction of various types of physical information from both real- and Fourier-space images. The possibility of hardware and software operations are illustrated, with examples of applications such as analysis of light scattering patterns from phase separated systems, semicrystalline polymer films with significant supermolecular morphology and from less regular polymer systems with a pronounced speckle effect. The analysis concentrates on both spatial and time dependent scattering processes and measurements of intensity correlation of scattered light. Although the present examples of application are limited to light scattering patterns of polymer systems, the technique of interactive picture analysis is useful for quantitative extraction of information from real-and Fourierspace images of arbitrary structure.     

Investigation of gas chromatographic interaction mechanism on permethylated cyclodextrins by molecular modelling

Summary The capillary gas chromatographic retention behavior of -pinene and tricyclene has been investigated on stationary phases of different polarities. On all but one of the columns employed, tricyclene eluted before -pinene; only permethylated -cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated -cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Nonspecific interactions played an especially important role for these kinds of substances.     

Photoredox properties of Kojic acid and its derivatives complexed to Iron(III)

Summary Iron(III) complexes of the formcis-[Fe(SB(X - K)] in whichSB ^2– are open-chain tetradentateSchiff base N₂O₂-ligands ofacacen,benacen orsalen type, andX - K ^– are bidentate anions of kojic acid (5-hydroxy-2-hydroxymethyl-4H-pyran-4-one) or other -pyranones undergo photoredox transformations when being irradiated in methanol into intraligand orLMCT bands. The quantum yields of the photoredox reactions depend on the peripheral constitution of the N₂O₂ ligands, the substituentsA andB of the ligandsX - K ^–, and the wavelength of irradiation _irr. The proposed mechanism involves the population of photoredox reactiveLMCT states by photophysical deactivation steps, primary photoredox formation ofX - K radicals and Fe(II), and subsequent dark redox processes giving back the anionsX - K ^– and the final products Fe(II) and formaldehyde formed in the molar ratio 2:1.

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Photoredoxeigenschaften von Fe(III)-Komplexen von Kojic Acid und ihren Derivaten

Zusammenfassung Eisen(III)-Komplexe der Formcis-[Fe(SB)(X - K)] mitSB ^2–=offenkettige tetradentateSchiff-Basen-N₂O₂-Liganden undX - K ^–=bidentate Anionen von 5-Hydroxy 2-hydroxymethyl-4H-pyran-4-on oder andere -Pyranone unterliegen bei Bestrahlung in Methanol Photoredoxumwandlungen, was durch Intraligand- oderLMCT-Banden belegt wird. Die Quantenausbeuten der Reaktionen hängen von der Konstitution der N₂O₂-Liganden, den SubstituentenA undB und dem LigandenX - K ^– sowie der Anregungswellenlänge _irr ab. Der vorgeschlagene Mechanismus beinhaltet die Population photoredoxreaktiverLMCT-Zustände durch photophysikalische Deaktivierungsschritte, die Bildung vonX - K*-Radikalen und Fe(II) und-Über Dunkelprozesse — die Rückbildung der AnionenX - K ^– sowie die Reaktion zu den Endprodukten Fe(II) und Formaldehyd im molaren Verhältnis 2:1.

     

Asymmetric michael addition of aromatic thiols to 2-cyclohexenone and maleic acid esters via formation of crystalline cyclodextrin complexes

The asymmetric Michael addition of aromatic thiols to 2-cyclohexenone and maleic acid esters has been carried out by utilizing their crystalline cyclodextrin complexes suspended in water. The best chiral induction, 30% enantiomeric excess (ee), was achieved in combinations of 2-cyclohexenone and octyl maleate with the crystalline -cyclodextrin complex of benzenethiol (method A) to afford (S)-3-phenylthiocyclohexanone and (S)-octyl-2-phenylthiosuccinate, respectively, whereas the reaction of benzenethiol with 2-cyclohexenone included in -cyclodextrin (method B) inversely induced the chiral recognition to give the (R)-adduct with 4–9% ee.     

Synthesis and thermal decomposition of 3-aroyl-4-aryl-2-pyrazolines

Summary 3-Aroyl-4-aryl-2-pyrazolines (21–40) have been synthesized by the reaction of ,-unsaturated ketones (1–20) with diazomethane. These 2-pyrazolines gave -methyl-,-unsaturated ketones (41–46) on thermal denitrogenation.Dedicated to Prof. Dr.F. Sauter on the occasion of his 65^th birthday     

Molecular orbital studies on the conformation of GABA (γ-aminobutyric acid)

In distinction to Extended Hückel Theory which predicts as the most stable conformation of free zwitterionic GABA a totally extended form, PCILO and SCF ab initio studies show that the intrinsically preferred conformation of the isolated molecule is a highly folded one, resulting from strong interactions between the two charged ends. Computations are also carried out for hydrated GABA in the supermolecule approach allowing moreover for the flexibility of binding of some of the water molecules of the first hydration shell. They predict the coexistence in solution of a large number of conformations showing different degrees of folding (or extension), a result confirmed by recent NMR studies. This and a number of similar results show that we have to adapt our thinking on the role of conformations in pharmacological activity to this situation, which was frequently obscured by the more abundant results of X-ray crystallography yielding a single conformation.