Protolytic properties of lignin in binary mixtures of water with aprotic solvents

Acid dissociation constants for three main types of phenol structural fragments of lignin in binary mixtures of water with N,N-dymethylformamide, dimethyl sulfoxide, and 1,4-dioxane were determined by spectrophotometric potentiometric titration. The effect of solvent on lignin protolytic properties was examined on the basis of the Kamlet-Taft principle of linear relationship of free solvation energies. It was found that nonspecific interactions with solvent make the dominating contribution to the effect of the medium.     

Thermosolvatochromism of betaine dyes revisited: theoretical calculations of the concentrations of alcohol-water hydrogen-bonded species and application to solvation in aqueous alcohols

Solvatochromic data of 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (RB) in aqueous methanol, 1-propanol, 2-propanol, and 2-methyl-2-propanol at 25 degrees C were recalculated by employing a recently introduced model that explicitly considers the presence of 1:1 alcohol-water hydrogen-bonded species, ROH-W, in bulk solution and their exchange equilibria with water and alcohol in the probe solvation microsphere. The thermosolvatochromic behavior of RB in aqueous ethanol was measured in the temperature range from 10 to 60 degrees C; the results thus obtained were treated according to the same model. All calculations require reliable values of Kdissoc, the dissociation constant of the ROH-W species. This was previously calculated from the dependence of the density of the binary solvent mixture on its composition. Through the use of iteration, the volume of the hydrogen-bonded species, VROH-W, and Kdissoc are obtained simultaneously from the same set of experimental data. This approach may be potentially problematic because Kdissoc and VROH-W are highly correlated. Therefore, we introduced the following approach: (i) VROH-W was obtained from ab initio calculations, (ii) these volumes were corrected for the nonideal behavior of the binary solvent mixtures at different temperatures, (iii) corrected VROH-W values were employed as a constant in the equation used to calculate Kdissoc (from density vs binary solvent mixture composition). VROH-W calculated by the COSMO-RS solvation model fitted the density data better than those calculated by the IEFPCM model. In all aqueous alcohols, solvation by ROH-W is favored over that by the two precursor solvents. In aqueous ethanol, a temperature increase resulted in a gradual desolvation of RB, due to a decrease in the hydrogen-bonding of both components of the mixture. The microscopic polarities of ROH-W are much closer to those of the precursor alcohols.     

Solvatochromic equilibria of cobalt(II) chloride in binary mixtures of dipolar aprotic solvents with water

The visible absorption spectra of CoCl₂ in binary mixtures of acetone, dmf, dma and dmso with H₂O show the existence of configuration equilibria between tetrahedral (T) and octahedral (O) species. Linear correlations between the values of log K (K = [O]/[T]) and the mole fraction of H₂O, _w, have been observed in all the systems under study; their slopes decrease linearly with an increase of the donor number, DN, of the organic component of the mixture. The formation of octahedral species is enthalpy favoured but entropy disfavoured. It was found that both H and S values change linearly with _w and the slopes of these relationships depend clearly on the DN values.     

Solvation of halide ions and the iodine molecule in binary mixtures of water + oxydipropionitrile application to extraction

In water-rich mixtures, the addition of oxydipropionitrile (ODPN) does not alter the solvation of halide ions, they are preferentially solvated by water. In ODPN-rich media, the addition of water to the poor solvent increases the solvation of halide ions by forming hydrogen bonds with water molecules. The iodine molecule is solvated mainly by ODPN and addition of water to the solvent has little influence. The triiodide complex, a highly polarisable species behaves in the same way as the iodine molecule. On demixing, the halide ions are solvated in a similar way in the water-rich phase and very differently in the solvent-rich phase. Advantage is taken of this property to extract cationic complexes quantitatively from their aqueous solutions with ODPN.     

Anion solvation II. Solvation of thiocyanate and halide anions in mixtures of protic and aprotic solvents

Infrared and Raman spectra of tetraalkylammonium and lithium thiocyanates and bromides in protic-aprotic solvent mixtures have been recorded in the 2100–2300 cm^?1 region at room temperature. It is shown that the anion is hydrogen bonded to the protic solvent, whether free or linked to a cation. Models are given for the solvated ion-pair taking into account both anion and cation solvation.     

Oligopyridones: preparation and their folding behavior in polar aprotic solvents

The oligomerization of 5-bromo-2-hydroxypyridine through a CuI catalyzed one-pot condensation tactics was developed. A series of oligopyridones, from dipyridione to penta-pyridone, were therefore synthesized via the CuI-promoted C-N coupling reactions. The oligopyridones favor to have folded conformations in polar aprotic solvents. These were confirmed by using ¹H NMR, NOESY, and 1D-NOE experiments.     

Localization and moving of a proton inside hydrogen-bonded complexes in aprotic solvents

The results of the study of a number of molecular and ionic H-bonded complexes in freon solutions by ¹H NMR at 100–150 K are reported. It is shown that under these conditions the signals of OH(NH) protons belonging to various complexes, self-associates and free molecules are observed separately. The spin-spin coupling of the signals is frequently discernible. The fine structure makes it possible to distinguish between complexes with fast proton migration between two wells on the potential surface and those with the proton localized in one well (in particular, the central one). Several complexes with slow (in the NMR scale) proton migration have also been found.The results of the study of the non-catalytic proton exchange kinetics between various molecules containing OH and NH groups in dilute solutions in aprotic solvents are considered. The exchange between the RCOOH and ROH molecules goes on via the intermediate formation of a cyclic ionic pair with two equivalent H-bonds even in non-polar solvents such as cyclohexane. For exchange between two RCOOH or ROH molecules a synchronous transfer of two protons in a cyclic molecular complex is likely.     

Simple methods for the estimation of ionization constants of substituted pyridine N-oxides in polar aprotic solvents and water

A direct method for the determination of the pK _a values of acids conjugated to substituted pyridine N-oxides has been proposed which is based on the pH measurement of the solution of the basic salt. It has been experimentally shown that the method is reliable when applied to N-oxides of not too low basicity (pK _a >5). Correlation has been performed between the pK _a values in aqueous and aprotic media solutions which shows the great influence of the solvation effect on the acid-base equilibria. The good correlation between the pK _a values in aqueous and non-aqueous solutions enables the pK _a values in water to be estimated with sufficient accuracy, even in the cases when the experimental limitations make the determination impossible which is shown on the basis of selected examples.     

Catalysis in aprotic solvents. Effect of the polarity of the catalyst.

The catalytic constants for the $\text{n}$-butylaminolysis of sultones in aprotic-apolar solvents are linearly correlated with a combination of the hydrogen-bonding parameter and the polarity parameter of the catalyst.     

The solvation of L-serine in mixtures of water with some aprotic solvents at 298.15 K

The integral enthalpies of solution Δ_sol H ^m of L-serine in mixtures of water with acetonitrile, 1,4-dioxane, dimethylsulfoxide (DMSO), and acetone were measured by solution calorimetry at organic component concentrations up to 0.31 mole fractions. The standard enthalpies of solution (Δ_sol H°), transfer (Δ_tr H°), and solvation (Δ_solv H°) of L-serine from water into mixed solvents were calculated. The dependences of Δ_sol H°, Δ_solv H°, and Δ_tr H° on the composition of aqueous-organic solvents contained extrema. The calculated enthalpy coefficients of pair interactions of the amino acid with cosolvent molecules were positive and increased in the series acetonitrile, 1,4-dioxane, DMSO, acetone. The results obtained were interpreted from the point of view of various types of interactions in solutions and the influence of the nature of organic solvents on the thermochemical characteristics of solutions.     

IR-spectroscopic investigations on solute-solvent interactions Part 1: Solvation of triethylphosphine oxide in aprotic and CH-acidic solvents

A systematic study has been carried out on the solvent and concentration dependence of the P=O stretching vibrationv(PO) of the model compoundEt ₃PO in various aprotic and several CH-acidic solvents. Evidence has been found for the existence of dipole-dipole complexes in concentrated solutions ofEt ₃PO in solvents of low acceptor numbers. In dilute solutions, however, the P=O stretching band generally occurs as a single symmetrical peak which shows that only one kind of solvate species is present in solution. In CH-acidic solventsEt ₃PO is present in the form of hydrogen bonded complexes. Informations about the structure of these complexes have been obtained by means of spectrophotometric titrations. It has been shown that the wavenumbersv°(PO) obtained by extrapolation ofv(PO) values to zero concentration are linearly related to the acceptor numbers of the solvents, previously derived from³¹P-chemical shift measurements. The existence of this linear relationships proves that both parameters are linearly related to the strength of the intermolecular interactions and are virtually free from unspecific contributions such as magnetic anisotropy or vibrational coupling effects. The results of the present study show that the P=O stretching vibration ofEt ₃PO represents an ideal probe for the investigation of solvent effects and solvation mechanisms.

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Infrarotspektroskopische Untersuchungen über Substrat-Lösungsmittel-Wechselwirkungen. Teil 1: Solvatation von Triethylphosphinoxid in aprotischen und CH-aciden Lösungsmitteln

Zusammenfassung Es wurde die Konzentrations- und Lösungsmittelabhängigkeit der P=O-Valenzschwingungv(PO) der ModellverbindungEt ₃PO in verschiedenen aprotischen und CH-aciden Lösungsmitteln systematisch untersucht. In hinreichend verdünnten Lösungen tritt die P=O-Valenzschwingung als einzelne symmetrische Bande auf, was beweist, daß nur eine einzige Solvatform vorliegt. In Lösungsmitteln niedriger Akzeptorzahl konnte bei höheren Konzentrationen die Existenz von Dipol-Dipol-Komplexen nachgewiesen werden. In CH-aciden Lösungsmitteln liegtEt ₃PO in Form von Wasserstoffbrückenkomplexen vor. Aussagen über die Struktur dieser Komplexe konnten mit Hilfe spektrophotometrischer Titrationen erhalten werden. Die durch Extrapolation auf Konzentration null erhaltenen Wellenzahlenv° (PO) hängen linear von den aus den³¹P-chemischen Verschiebungen vonEt ₃PO bereits früher bestimmten Akzeptorzahlen der Lösungsmittel ab. Die Existenz dieser linearen Beziehung beweist, daß beide Parameter frei von störenden Einflußfaktoren (magnetische Anisotropie- und Schwingungskopp-lungseffekte) sind und tatsächlich linear mit der Stärke der intermolekularen Wechselwirkung variieren. Die Ergebnisse der vorliegenden Studie zeigen, daß die P=O-Valenzschwingung vonEt ₃PO eine hervorragende Sonde zur Untersuchung von Lösungsmitteleffekten und Solvatationsmechanismen darstellt.

     

Anion solvation I. Solvation of the thiocyanate anion in aprotic solvents by near infrared spectroscopy

Solutions of tetrabutylammonium and lithium thiocyanates have been examined in the infrared 2000–5500 cm^?1 region at room temperature. New bands have been observed which can be assigned to simultaneous transitions between the v (CN) vibrations of the anion and the v (CH) vibrations of the solvent CH₃ group. These transitions which are independent of the state of association (free ion, ionpairs, dimers, triple cation) provide evidence of anion solvation by aprotic solvents.     

Solvation of copper(II) sulfate in binary water/N,N-dimethylformamide mixtures: from the solution to the gas phase

The solvation of copper(II) sulfate in binary mixtures of water and N,N-dimethylformamide (DMF) is studied by a combined approach using electrochemical studies in solution and a mass spectrometric assay of the solvated ions formed from these solutions upon electrospray ionization (ESI). In the condensed phase, the limiting transference numbers (t(+/-)(o)) and the apparent ion association constants (K(A)'s) of CuSO(4) have been determined in water/DMF solutions at 20 degrees C. The t(+)(o) values decrease with increasing DMF content, demonstrating a gradual solvation of Cu(2+) by DMF molecules. The association constants indicate that aggregation becomes more pronounced as the DMF content increases. In order to achieve complementary insight, the intrinsic interactions among the ions and solvent molecules are investigated in gas-phase experiments of the CuSO(4)/water/DMF system using ESI mass spectrometry. Under the conditions used, the dications [Cu(DMF)(n)](2+) (n = 3-6), [Cu(2)(DMF)(n)SO(4)](2+) (n = 2-7), and [Cu(3)(DMF)(n)(SO(4))(2)](2+) (n = 2-7), and the monocations [Cu(OH)(DMF)(n)](+), [Cu(DMF)(n)(HSO(4))](+) (both, n = 1-3), and [Cu(DMF)(n)](+) (n = 1, 2), are formed as the leading copper-containing cations. Likewise, polynuclear copper clusters observed in the anion ESI spectra support partial aggregation occurring in solution. The gas-phase studies clearly support the conclusions that (i) DMF is a highly preferred ligand for CuII in comparison to water and that (ii) DMF supports ion association for which the mass spectrometric data suggest the formation of polynuclear copper clusters.     

Effect of polar aprotic solvents and organometallic catalysts on the reaction of alcohols with isocyanates

DMF and DMSO catalyse the reaction of butanol with PhNCO but inhibit that with aliphatic isocyanates, due to formation of an active 1:1 charge transfer complex with the aromatic isocyanate. Similarity was found in the mode of catalysis of the urethane reaction with these solvents to that with tert, amines. Various organometallic compounds were tested as catalysts for urethane formation with aliphatic isocyanates. Those that gave fast addition to the NCO group, such as tributyltin oxide, Zr(OBu)₄ and Zr(acac)₄, were the strongest catalysts. In the presence of organometallic catalysts, urethane formation was the sole reaction and trimerization of the isocyanate was suppressed.     

Substituent effect on a family of quinones in aprotic solvents: an experimental and theoretical approach

In this work a comparison between redox potentials, obtained by constructing current-potential plots from chronoamperometric measurements, and the parameter sigma(x), as proposed by Zuman in terms of the Hammett substituent parameters, was performed for several quinone compounds. This study shows the limitations of this approach and proves that methods based on quantum chemistry can be used to study the substituent effect in quinone systems. By using the Density Functional Theory, in the Kohn-Sham context with three exchange-correlation functionals, BLYP, B3LYP, and BHLYP, it was found that the electron affinity is good enough to give a useful relationship with experimental redox potentials of quinone systems. This conclusion is reached when the basis set functions involve diffuse functions, and also when the Hartree-Fock exchange energy is included in the exchange-correlation functional. The Fukui function, to describe preferential sites involved at initial stages of a system that bind an electron, is analyzed when electron donor and electron acceptor groups are present as substituents in quinone systems. The methods applied in this work are valid for any kind of quinone compound and will be used in further analysis of the electron reorganization in semiquinone species.     

Structural-thermodynamic characteristics and intermolecular interactions in mixtures of strongly associated solvents with aprotic amides

Thermodynamic characteristics of mixtures of aprotic amides with water and organic solvents with hydrogen bond networks are calculated. Within a model approach the specific and non-specific components of the total energy of the intermolecular interaction are determined, based on which the corresponding contributions to the enthalpies of component mixing are calculated. It is found that negative enthalpies of mixing in the mixtures under study are due to non-specific interactions rather than heterocomponent specific ones. It is shown that the difference in the structural-thermodynamic characteristics of aqueous and nonaqueous mixtures of aprotic amides is mainly caused by packing features of solutions and the behavior of hydrogen bond networks of water and organic solvents.     

Decomposition of hydrogen peroxide in protic and polar aprotic solvents on TS-1 heterogeneous catalyst

Kinetic parameters of H₂O₂ decomposition in methanol, propanol-1, propanol-2, acetone, and acetonitrile at 30–55°C on a TS-1 heterogeneous catalyst were determined. Recommendations are given on choice of solvents in oxidation of organic compounds with hydrogen peroxide.