Structural and thermodynamic properties and intermolecular interactions in aqueous and acetonitrile solutions of aprotic amides

The structural and thermodynamic properties are calculated for mixtures of aprotic amides with water and acetonitrile. The simulation approach is used to identify the specific and nonspecific components of the total energy of intermolecular interactions, which are used to calculate the corresponding contributions to the enthalpy of mixing. The negative enthalpies of mixing in the aqueous mixtures are found to be caused not by heterocomponent specific interactions, but by nonspecific ones. The difference in the structural and thermodynamic properties of the aqueous and nonaqueous mixtures of aprotic amides is shown to be largely due to the behavior of the hydrogen bond network of water and the packing of the resulting solutions.     

Thermodynamic characteristics of self-associated aminoalcohols

Thermodynamic characteristics of aminoalcohols self-associated by hydrogen bonds have been calculated. Specific and nonspecific components of the total energy of intermolecular interactions have been determined within the model approach. Main goal of this work is the search of thermodynamic characteristics of the liquid-phase systems effectively reflecting both features of intermolecular interactions, and structural changes of individual aminoalcohols. It is established that the most part of the studied aminoalcohols belongs to solvents with H-bonds networks in which there is an intensifying of nonspecific interactions with the temperature rise, and to solvents with chained self-association where the contribution of these interactions practically do not depend on temperature. The aminoalcohols obtained by the substitution of nitrogen atom protons of monoethanolamine by alkyl radicals belong to group of solvents similar aprotic ones at which nonspecific interactions are weakened with the temperature rise. The reasons of it have been discussed by comparison of the data with results for diols, oxyethylated glycols, monoalcohols, and aprotic amides obtained by us earlier.     

The acid-base equilibrium constants of some azine compounds in various aqueous-organic solvent mixtures

The acid dissociation constants of the protonated form of some azine compounds (acridine, acridine orange and neutral red, BH+) were determined pH-metrically at 25 degrees C and at the constant ionic strength I = 0.1 mol l(-1) (KNO3) in pure water as well as in various aqueous mixtures having different proportions (w/w%) of organic solvents. The organic solvents used are methanol, ethanol (as amphiprotic solvents), N,N-dimethylformamide, dimethylsulfoxide (as dipolar aprotic solvents) and acetonitrile (as a low basic solvent). The results obtained indicated that the pKa values decrease as the content of the organic solvent in the medium is increased. It is deduced that, the major effect responsible for this behaviour is the differences in stabilization of the free base (B) by dispersion forces and of the proton by its interaction with solvent and water molecules in aqueous-organic solvent mixtures (ion-solvent interaction). Moreover, it is concluded that the ability of the solvent to accept hydrogen bond from the protonated form (BH+) contributes significantly to the deprotonation process of the compounds.     

Thermodynamic and structural characteristics of aqueous diamine solutions

Thermodynamic characteristics are calculated for aqueous diamine solutions prepared by substituting an amino group for the hydroxyl group of amino alcohols. Patterns are revealed in the change of the structural properties of the mixtures. The correlation between the entropy and enthalpy characteristics of the water–diamine systems and the excess packing coefficients suggests that the universal interactions determine the structural and energy properties of aqueous solutions of the studied diamines. The form of the concentration dependences of the structural and thermodynamic characteristics in the studied systems is found to be symbatic with the data for the mixtures of water with aprotic amides. The reasons for this are discussed by comparing the results with our previously published data for aqueous solutions of aprotic amides.     

Thermodynamic Characteristics of Resolvation of Bromide Ions in Water–Dimethyl Sulfoxide Mixtures

Real and chemical thermodynamic characteristics of resolvation of bromide ions in water–dimethyl sulfoxide mixtures and a surface potential of dimethyl sulfoxide are presented and analyzed. The data are obtained by the method of Volta potential differences. The real thermodynamic characteristics of the bromide ion transport are positive (as those of the chloride ion studied earlier). This is due to structural rearrangement of the surface layer at the solution/gas interface when passing from water to water–organic substance mixtures. According to an analysis of the chemical energy of the bromide ion resolvation, anions that are capable of forming hydrogen bonds with proton-donor solvents are weakly solvated in aprotic solvents.     

Structural and thermodynamic characteristics of amide solvents

The structural and thermodynamic characteristics of amide solvents are calculated with different types of molecular self-assembly through hydrogen bonding. Under a model-based approach, the specific and nonspecific components of the total energy of intermolecular interactions are identified for primary, secondary, and tertiary amides of carboxylic acids. It is found that similarly to water, primary amides have a network of hydrogen bonds and belong to the class of liquids characterized by an increase in nonspecific interactions with temperature. In secondary amides with the chain self-assembly, the contribution of these interactions is practically independent of temperature, and in tertiary amides it decreases with an increase in temperature. The molar values of the specific and nonspecific components are used to analyze the intermolecular interactions and the structural properties of amides with different degrees of N-substitution.     

Effect of organic co-solvents on the solvation enthalpies of amino acids and dipeptides in mixed aqueous solutions

Transition enthalpies (Δ_tr H ^o) of substances from water to binary solutions were calculated at 298.15 K on the basis of standard dissolution enthalpies (Δ_sol H ^o) for six amino acids and five dipeptides in the mixtures of water with organic solvents of various chemical natures. The enthalpy pair coefficients of interaction h _xy for biomolecules with organic component of mixture were estimated within the formalism of the McMillan-Mayer theory. The change in the character of the interaction of the components of solution was demonstrated in dependence on the physicochemical properties of the solvent and nature of the side radical of the dissolved bioorganic substance. Quantitative estimation of the type of interaction of the substance with the solvent was performed on the basis of correlation ratios relating the enthalpy characteristics of bioorganic substances to properties of organic cosolvents. It was shown that in the solutions under study, the effects of both the specific (mainly electron donor) and non-specific solvation of amino acids and peptides are observed.     

Solvation of Aniline in Mixtures of Water with N,N-Dimethylformamide and Acetonitrile

Thermal effects of aniline solution in water-N,N-dimethylformamide (DMF) and water-acetonitrile mixtures were measured at 25°C. In almost the whole range of compositions of the mixed solvents, the thermal effects are more positive in aqueous acetonitrile than in aqueous DMF. Particular attention was given to binary solvents with a very low content of the organic cosolvent. In the mixture with the mole fraction of DMF of 10^{- 3}, the enthalpy of aniline solution is higher than in water by 5%, and in the mixture with the mole fraction of acetonitrile of 4 × 10^{- 4}, even by 15%. Features of specific solvation of aniline and an aliphatic amine (n-BuNH₂) in the water-DMF mixture were discussed taking into account the acid-base properties of the mixtures. The coefficients of pair interactions aniline-organic solvent in water and aniline-water in the organic solvent were calculated using the McMillan-Mayer theory. These coefficients correlate with the enthalpies of hydration of aprotic solvent molecules.     

Thermochemical characteristics of transfer of copper(II) and nickel(II) nitrates from water to water-formamide mixtures

The enthalpies of mixing of aqueous solutions of copper(II) and nickel(II) nitrates with water-formamide mixtures were measured calorimetrically at 298.15 K in the entire composition range. The standard enthalpies of transfer of the electrolytes from water to water-formamide mixtures were calculated from these data. The shape of the isotherms of transfer of the electrolytes is discussed on the basis of the calculated enthalpies of transfer of individual ions and contributions to these quantities made by universal, chemical, and electrostatic interactions of ions with the solvents.     

Thermochemistry of Benzene Solution in Alcohols, Aprotic Solvents, and Mixtures of Aprotic Solvents with Methanol

The standard enthalpies of solution of benzene at 25°C in alcohols (methanol, 1-propanol, 1-pentanol, 1-decanol), aprotic solvents (1,4-dioxane, acetone, acetonitrile, dimethyl sulfoxide, dimethylformamide, propylene carbonate), and mixtures of methanol with these aprotic solvents were determined. Multiple regression analysis revealed the role of specific and nonspecific interactions in solvation of benzene in these solvents.     

New Screening Protocol for Effective Green Solvents Selection of Benzamide,Salicylamide and Ethenzamide

New protocol for screening efficient and environmentally friendly solvents was proposed and experimentally verified. The guidance for solvent selection comes from computed solubility via COSMO-RS approach. Furthermore, solute-solvent affinities computed using advanced quantum chemistry level were used as a rationale for observed solvents ranking. The screening protocol pointed out that 4-formylomorpholine (4FM) is an attractive solubilizer compared to commonly used aprotic solvents such as DMSO and DMF. This was tested experimentally by measuring the solubility of the title compounds in aqueous binary mixtures in the temperature range between 298.15 K and 313.15 K. Additional measurements were also performed for aqueous binary mixtures of DMSO and DMF. It has been found that the solubility of studied aromatic amides is very high and quite similar in all three aprotic solvents. For most aqueous binary mixtures, a significant decrease in solubility with a decrease in the organic fraction is observed, indicating that all systems can be regarded as efficient solvent-anti-solvent pairs. In the case of salicylamide dissolved in aqueous-4FM binary mixtures, a strong synergistic effect has been found leading to the highest solubility for 0.6 mole fraction of 4-FM.     

Enthalpy characteristics of sodium phenylalaninate solvation in mixtures of water with ethanol at 298 K

The heat effects of mixing for aqueous solutions of sodium phenylalaninate (NaPheAla) with mixtures of water and ethanol are measured by calorimetry at 298.15 K. The standard enthalpies of transfer of a stoichiometric mixture of ions, and of an organic ion of α-PheAla^? from water to water-alcohol mixtures are calculated. The enthalpy coefficients of binary and ternary ion-alcohol interactions are calculated using the McMillan-Mayer theory. The obtained enthalpy characteristics are compared with previously studied systems containing synthetic β- and natural α-alanine.     

Enzyme-catalysed hydrolysis of L-amino acid esters in a low water organic solvent studied by isothermal calorimetry

Isothermal calorimetry and UV-visible spectrophotometry have been used to study the thermochemistry of the enzyme-catalyzed hydrolysis of hydrophobic L-amino acid esters in organic solvents with low water content at 298 K. The p-nitrophenyl esters of Z-L-tyrosine and Z-L-phenylalanine were used as model hydrophobic substrates. Acetonitrile was used as a model organic solvent. A special preparation protocol of the reactants in the calorimetric vessel was applied in order to determine the heat effects accompanying the enzyme-catalyzed hydrolysis reaction in organic mixtures with low water content and the Tris buffer ionization enthalpies over the whole range of water content in acetonitrile. It was found that the molar enthalpy of the hydrolysis of p-nitrophenyl esters and buffer ionization enthalpy depend significantly and similarly on the water content in acetonitrile. However, the reaction enthalpy corrected for the buffer ionization enthalpy does not depend on the water content in organic solvent mixtures. An explanation of the effect of the selected organic solvent on the thermochemical parameters was provided on the basis of the IR spectroscopic data for the hydrogen bond network of water in acetonitrile. The results obtained show that the state of water in organic solvents is an important factor that determines the reaction enthalpy as well as buffer ionization enthalpy.     

Thermochemistry of mixing imidazole-based ionic liquids with water and organic solvents

The results of the calorimetric studies of the enthalpies of mixing of some ionic liquids, 1-butyl-3-methylimidazolium chloride and acetate, with water, pentanol, dimethyl sulfoxide and dimethylformamide are presented. Protic solvents interact with the ionic liquid anion, wheras aprotic, with the cation. The mixing enthalpy depends on the dissociation degree of the ionic liquid.     

A TD-DFT study on the hydrogen bonding of three esculetin complexes in electronically excited states: strengthening and weakening

Time-dependent density functional theory (TD-DFT) method was used to study the excited-state hydrogen bonding of three esculetin complexes formed with aprotic solvents. The geometric structures, molecular orbitals (MOs), electronic spectra and the infrared (IR) spectra of the three doubly hydrogen-bonded complexes formed by esculetin and aprotic solvents dimethylsulfoxide (DMSO), tetrahyrofuran (THF) and acetonitrile (ACN) in both ground state S(0) and the first singlet excited state S(1) were calculated by the combined DFT and TD-DFT methods with the COSMO solvation model. Two intermolecular hydrogen bonds can be formed between esculetin and the aprotic solvent in each hydrogen-bonded complex. Based on the calculated bond lengths of the hydrogen bonds and the groups involved in the formation of the intermolecular hydrogen bonds in different electronic states, it is demonstrated that one of the two hydrogen bonds formed in each hydrogen-bonded complex is strengthened while the other one is weakened upon photoexcitation. Furthermore, it is found that the strength of the intermolecular hydrogen bonds formed in the three complexes becomes weaker as the solvents change from DMSO, via THF, to ACN, which is suggested to be due to the decrease of the hydrogen bond accepting (HBA) ability of the solvents. The spectral shifts of the calculated IR spectra further confirm the strengthening and weakening of the intermolecular hydrogen bonds upon the electronic excitation. The variations of the intermolecular hydrogen bond strengths in both S(0) and S(1) states are proposed to be the main reasons for the gradual spectral shifts in the absorption and fluorescence spectra both theoretically and experimentally.     

Synthesis and transformations of 6,6,7,7-tetramethyl-2-quinuclidone

6,6,7,7-Tetramethyl-2-quinuclidone was synthesized, and its differences from the usual amides were shown. It enters into three types of chemical reactions: 1) the N-CO bond is broken under the influence of protic nucleophilic agents (water, alcohols, amines, hydroxylamine, and hydrazines), and nucleophilic agents are acylated by the (2,2,6,6-tetramethyl-4-piperidyl)-acetic acid residue; 2) the N-C(CH₃)₂ bond is broken by reaction with nucleophilic agents in aprotic media (phenyllithium in ether, PCl₅ in benzene, acetone cyanohydrin, and LiAlH₄ in ether) to form 4-substituted 6,6-dimethyl-2-piperidones; 3) reactions with the preservation of the quinuclidine ring occur on treatment with electrophilic reagents (hydrogen chloride and methyl iodide) in aprotic solvents and during reduction.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 378–384, March, 1971.     

Enthalpy-entropy compensation reveals solvent reorganization as a driving force for supramolecular encapsulation in water

A chiral self-assembled M4L6 host assembly has been shown to be a suitable host for the supramolecular encapsulation of a series of guests in polar solvents, ranging from simple organic ammonium cations to more complex organometallic species. This molecular recognition process creates highly selective reactivity within the host cavity. In order to understand the factors driving the molecular recognition process, the standard thermodynamic parameters for encapsulation were determined for a series of protiated and fluorinated iridium guests in a variety of polar solvents using van't Hoff analysis. The encapsulation process for these guests exhibited enthalpy-entropy compensation effects. In solvents such as water and methanol, error analysis suggests a chemical origin for this behavior. In contrast, error analysis of this compensation behavior in polar aprotic solvents such as dimethyl sulfoxide reveals that this correlation is due to an artifact inherent in the intrinsic correlation between the enthalpy and entropy terms in the van't Hoff analysis. Guest encapsulation in polar protic solvents such as water appears to be driven by initial desolvation of the guest with concomitant rearrangement of the hydrogen bond networks in solution. This behavior shares common characteristics with other synthetic and natural host-guest and molecular recognition processes in aqueous solution, ranging from simple crown ether to complex enzyme-ligand interactions.     

Determination of solvation and specific interaction enthalpies of adamantane derivatives in aprotic solvents

Solution and solvation enthalpies at infinite dilution of 1-bromoadamantane, 1-adamantanol, and 2-adamantanone are reported at 298.15 K in a set of 14 aprotic solvents. The specific interaction enthalpies of the solid solutes are calculated using a methodology recently published by other authors. 1-Adamantanol’s specific interaction enthalpies show a good correlation with the Kamlet–Taft β scale.     

Enthalpy of cooperative hydrogen bonding in complexes of tertiary amines with aliphatic alcohols: Calorimetric study

The work is devoted to the investigation of thermodynamics of specific interaction of the tertiary aliphatic and aromatic amines with associated solvents as which aliphatic alcohols were taken. Solution enthalpies of aliphatic alcohols in amines (tri-n-propylamine, 2-methylpyridine, 3-methylpyridine, N-methylimidazole) as well as amines in alcohols were measured at infinite dilution. The enthalpies of specific interaction (H-bonding) in systems studied were determined based on experimental data. The enthalpies of specific interaction of amines in aliphatic alcohols significantly lower than the enthalpies of hydrogen bonding in complexes amine–alcohol of 1:1 composition determined in base media due to the reorganization of aliphatic alcohols as solvents. The determination of solvent reorganization contribution makes possible to define the hydrogen bonding enthalpies of amines with clusters of alcohols. Obtained enthalpies of hydrogen bonding in multi-particle complexes are sensitive to the influence of cooperative effect. It was shown, that hydrogen bond cooperativity factors in multi-particle complexes of alcohols with amines are approximately equal for all alcohols when pyridines and N-methylimidazole as solutes are used. At the same time, H-bonding cooperativity factors in complexes of trialkylamines with associative species of alcohols decrease with increasing of alkyl radical length in alcohol and amine molecules.This work shows that the thermodynamic functions of specific interaction of solutes with associated solvents cannot be described using the H-bond parameters for the complexes of 1:1 composition.     

Specific and non-specific interactions of hydropseudohalic acids with water and organic solvents

Partition coefficients P of the HNCS, HNCO and HN₃ hydropseudohalic acids between a number of organic solvents and water were determined. It has been found that log P increases with pKa of the acid and with the basicity of the solvent, but the effect of pKa on P is the smaller the more basic is the solvent. The relationships have been explained in terms of hydrogen bond formation between undissociated acid and solvent molecules. H-bonding between the pseudohalic acids and organic solvents has been confirmed by IR spectra on the example of HN₃ in benzene. Association constants for H-bonding between the three acids and water, benzene, dibutyl ether and tri-n-butyl phosphate were determined from partition data. It has been found that H-bonding increases with the strength of the acid, whereas the contribution to partition from non-specific interactions with water and organic solvents depends on the molecular surface area of the acid molecule.