Complete resolution of the ionization equilibria of 5-deoxypyridoxal in water-dioxane mixtures

We have used a potentiometric method to determine the thermodynamic equilibrium constants for the macroscopic ionization processes of 5-deoxypyridoxal (DPL) in water-dioxane mixtures (0-70% weight fraction in dioxane) at temperatures ranging from 10°C to 50°C. These data, together with previously published equilibrium constants for the tautomerism and hydration processes, have allowed us to resolve the complete microconstant system. We have also calculated the microscopic ionization equilibrium constants under all the experimental conditions. The changes of standard thermodynamic function for the macroscopic and microscopic ionization processes were obtained in various water-dioxane mixtures at 25°C. The values of a given microscopic pK with different solvents and temperatures fit very well to an equation which relates this magnitude with the thermodynamic parameters, the solvation of the components of the reaction, and a solvent parameter. We have obtained an interesting linear correlation between the thermodynamic parameters corresponding to all the microscopic ionizations of DPL and the net change of the solvation during the process: enthalpies correlate linearly for all the microscopic ionizations, while entropies do so for the phenols and pyridinium ions separately.     

Thermodynamics of the solution equilibria of 3-hydroxypyridine and pyridoxine in water-dioxane mixtures

The thermodynamics of tautomeric and microscopic ionization equilibrium constants of 3-hydroxypyridine and pyridoxine have been determined in waterdioxane mixtures (0–70% weight fraction in dioxane) ranging from 10°C to 50°C. Generally, for both types of processes, the entropic contributions to the Gibbs energy predominate and this tendency increase concomitantly with the dioxane content in the media. The thermodynamic parameters are consistent with the corresponding values obtained from macroscopic ionization processes. An equation for the equilibrium constant K as a function of temperature and a solvent characteristic parameter is proposed to predict the K values throughout the whole range of the temperature and solvent compositions studied.     

Thermodynamics properties of pyridoxal 5 phosphate

The ionization constants and heats of ionization of pyridoxal 5 phosphate groups in the pH range 2–10 have been determined by potentiometric and calorimetric techniques at 20, 25, 30, and 37°C in 0.1M KCl.Presented at the sixth Italian meeting on Calorimetry and Thermal Analysis (AICAT) held in Naples, December 4–7, 1984.     

Solvation and hydrophobic hydration of different types of alkylamines inN,N-dimethylformamide and water mixtures

Enthalpies of solution of twelve amines of different type have been determined at 25°C in mixtures of N,N-dimethylformamide and water over the whole composition range. The enthalpies of transfer from water to the mixtures deviate substantially from a linear dependence on the mole fraction of water. These deviations appear to contain additive contributions of the different alkyl groups. By application of a simple hydration model the enthalpic effect of hydrophobic hydration has been calculated for each amine. For alkylamines this is determined by the number and size of the alkyl groups present in the molecule. The contribution of each alkyl group is the same in primary, secondary and tertiary amines. Results for the different alkyl groups show a close relationship with values for alcohols obtained previously. Differences between alcohols and amines can be attributed to differences in the hydrophobic hydration of the parts of the solute molecules which are adjacent to the polar group. The influence of the polar group does not seem to extend beyond the second carbon atom.     

Thermodynamics of molecular interactions in ethyl iodide+toluene mixtures

The excess Gibb's free energy of mixing, G^E, for ethyl iodide+toluene at 25°C have been obtained from the measured vapor pressuure data. The H^E and G^E values for ethyl iodide+toluene are positive throughout the ethyl iodide concentration range and G^E>H^E. The results have been analyzed in terms of Flory and ideal associated model theory of nonelectrolyte solutions. It has been observed that the ideal associated model approach which assumes the presence of AN and A₂B molecular species describes well (within±10 J-mol^–1 in the worst case) the general dependence of H^E on X_A (mole fraction of ethyl iodide) over the whole composition range for ethyl iodide+toluene mixtures. The equilibrium constants for A+A AB and 2A+BA₂B reactions along with the enthalpies of formation of AB and A₂B molecular species have been calculated.     

Hydrophobic hydration of some different types of quaternary ammonium bromides in mixtures of water andN,N-dimethylformamide

Enthalpies of solution of methyltributylammonium bromide, tetraethanolammonium bromide, and three azoniaspiroalkane bromides in binary solvent mixtures of water and N,N-dimethylformamide have been measured calorimetrically at 298.15^oK over the whole mole fraction range. The results are interpreted in terms of a simple hydration model with two parameters and compared with those of the corresponding tetraalkylammonium bromides. Enthalpic studies with different types of quaternary ammonium bromides are useful in reexamining the assumptions and limitations of the hydration model.     

Thermodynamics of Solution of Some Alkyl Acetates in Water

Summary.  The temperature dependence of the solubility of methyl acetate (MeOAc), ethyl acetate (EtOAc), 1-propyl acetate (1-PrOAc), 1-butyl acetate (1-BuOAc), 2-methyl-1-propyl acetate (iso-BuOAc), 2-butyl acetate (sec-BuOAc), and 2-methyl-2-propyl acetate (tert-BuOAc) in water in the temperature range from 298.2 to 318.2 K was determined. The experimental solubility data together with some literature values are presented as a function of temperature and given in analytical form. The solubilities of the investigated compounds at 298.2 K were correlated with the number of carbon atoms in the solute molecule. The standard thermodynamic functions for solubility, i.e. Gibbs free energy, enthalpy, heat capacity, and entropy, were calculated. The Gibbs free energy is positive and increases linearly with the number of carbon atoms, whereas the enthalpy and entropy are negative. The standard thermodynamic functions were converted to thermodynamic quantities for hydration. The Gibbs free energies of hydration of alkyl acetates, which are negative, are compared with the Gibbs free energy of hydration of some n-alkanes, 1-alcohols, and 1-alkylamines. The standard thermodynamic functions of hydration were analyzed using a modified version of the theoretical approach developed by Lee and Graziano [1]. Received October 16, 2000. Accepted May 14, 2001     

Theoretical thermodynamic study of the adenine–thymine tautomeric equilibrium: Electronic structure calculations and steered molecular dynamic simulations

Free energy of the tautomeric equilibrium A‐T ? A*‐T* between the canonical and noncanonical DNA base equilibrium in aqueous solution was theoretically determined by applying electronic structure methods (at the M06‐2X‐PCM/6‐311++G(d,p) level) and steered molecular dynamic simulations. Concerted and stepwise mechanisms were considered for the double proton transfer in an effort to explain the anomalous behavior of this system where an unfavorable process without a transition state can be observed depending on the level of calculation used. Of the different mechanisms used in the simulations, the stepwise mechanism, in which the first step implies the transference of a proton from thymine to adenine, and a second step with the transference of a different proton from adenine to thymine, was the only one that showed two transition states and a reaction intermediate. However, a concerted and stepwise mechanism has similar kinetic and thermodynamic behavior, with similar reaction and activation energies. Simple proton transfer was more favorable for the transference of the hydrogen from the adenine to the thymine. The inclusion of an aqueous medium in this study only slightly modified these energies, but the barrier energy was higher when the solvent was described as a discrete medium. Transition states and intermediate structures were analyzed at molecular dynamic level.     

Thermochemistry of interactions of Na+ with benzo-15-crown-5 ether in acetonitrile-water mixtures at 298.15 K

Enthalpies of dissolution of benzo-15-crown-5 ether (B15C5) in mixtures of acetonitrile with water and in solutions of NaI and NaBPh₄ (I=0.05 mol dm^–3) in these mixtures were measured at 298.15 K. From the obtained results and appropriate literature data, the thermodynamic functions of B15C5/Na⁺ complex formation in acetonitrile-water mixtures were determined. The enthalpies of transfer of the complex B15C5/Na⁺ from pure acetonitrile to the examined mixtures were calculated and are discussed.     

Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents: Part VII. Enthalpy of solution of 15-crown-5 and benzo-15-crown-5 in the mixtures of water with acetone at 298.15 K

Enthalpy of solution of crown ethers (15-crown-5 and benzo-15-crown-5) in water-acetone mixtures have been measured within the whole range of mole fraction at 298.15 K. The obtained data have been compared with those of the solution enthalpy of both crown ethers in the mixtures of water with dimethyl sulfoxide. The replacement of SO group with CO in the molecule of the organic solvent brings about an increase in the exothermic effect of the solution of 15-crown-5 and benzo-15-crown-5 ethers, especially in the mixtures with a medium water content. The observed effect is connected with the preferential solvation of the molecules of both crown ethers by acetone molecules in the water-acetone mixtures. The process of preferential solvation of 15-crown-5 and benzo-15-crown-5 ethers does not take place in the water-dimethyl sulfoxide mixture.     

The effect of properties of water-organic solvent mixtures on the solvation enthalpy of 12-crown-4, 15-crown-5, 18-crown-6 and benzo-15-crown-5 ethers at 298.15 K

Crown ethers are preferential solvated by organic solvents in the mixtures of water with formamide, N-methylformamide, acetonitrile, acetone and propan-1-ol. In these mixed solvents the energetic effect of the preferential solvation depends quantitatively on the structural and energetic properties of mixtures. The energetic properties of the mixtures of water with hydrophobic solvents (N,N-dimethylformamide, dimethylsulfoxide, N,N-dimethylacetamide, hexamethylphosphortriamide) counteract the preferential solvation of the crown ether molecules. The effect of the hydrophobic and acid-base properties of the mixture of water with organic solvent on the solvation of 12-crown-4, 15-crown-5, 18-crown-6 and benzo-15-crown-5 ethers was discussed. The solvation enthalpy of one -CH₂CH₂O- group in water, N,N-dimethylformamide and hexamethylphosphortriamide is equal to −24.21, −16.04 and −15.91 kJ/mol, respectively. The condensed benzene ring with 15-crown-5 ether molecule brings about an increase in the exothermic effect of solvation of the crown ether in the mixtures of water with organic solvent.     

Thermodynamics of phenylated polyphenylene between 0 and 340 K

In an adiabatic vacuum calorimeter, the temperature dependence of the heat capacity C _p of phenylated polyphenylene and initial comonomer 1,4-bis(2,4,5-triphenylcyclopentadienone-3-yl)benzene was studied between 6 and 340 K with an uncertainty of about 0.2%. In a calorimeter with a static bomb and an isothermal shield their energies of combustion DUcomb were measured. From the experimental data, the thermodynamic functions C _p ⁰ (T), H ⁰(T)-H ⁰(0), S ⁰(T)-S⁰(0), G ⁰(T)-H ⁰(0) were calculated from 0 to 340 K, and standard enthalpies of combustion ΔH _comb ⁰ and thermodynamic parameters of formation-enthalpies ΔH _f ⁰, entropies ΔH _f ⁰, Gibbs functions ΔG _f ⁰ - of the substances studied were estimated at T=298.15 K at standard pressure. The results were used to calculate the thermodynamic characteristics (ΔH _f ⁰ ,ΔS _f ⁰, ΔG _f ⁰) of phenylated polyphenylene synthesis in the range from 0 to 340 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of the components of aldoses in a tautomeric equilibrium mixture as their trimethylsilyl ethers by capillary gas chromatography

Equilibrium tautomers of aldopentoses (arabinose, ribose, xylose and lyxose) and aldohexones (allose, altrose, gulose, idose and talose) have been analyzed as their trimethyl-silyl ethers by capillary GC. Pentoses were separated on SE-54 and hexoses on OV-225. The four components of the tautomer equilibrium mixture have been identified using GC/MS and NMR data.     

Enthalpies of solution of tetra-n-butylammonium bromide in binary mixtures of water,formamide, N-methylformamide,and N,N-dimethylformamide

The enthalpies of solution of tetra-n-butylammonium bromide have been measured in mixtures of formamide (F) with water, N-methylformamide (NMF) with water, N,N-dimethylformamide (DMF) with water, F with NMF, DMF with F, and NMF with DMF at 25°C in the whole mole-fraction range. The enthalpies of solution vs composition profiles show a maximum value in the DMF-H₂O and in the DMF-F systems. In the F-NMF and NMF-DMF mixturesn-Bu₄NBr displays a nearly ideal behavior, whereas in the other solvent systems the excess enthalpy of solution deviates substantially from zero.     

Excess permittivities,excess refractive indices and NMR spectroscopic studies of solutions of 1,2-dibromoethane in aromatic hydrocarbons

Excess permittivities and excess refractive indices data for solutions of 1,2-dibromoethane (DBE) in benzene, toluene, o-xylene, m-xylene and p-xylene over the whole composition range at 25°C have been obtained. The analysis of excess dielectric properties suggests the existence of weak electron donor-acceptor type interactions between 1,2-dibromoethane (DBE) and aromatic hydrocarbons. The equilibrium constants and the dipole moments of the DBE-aromatic hydrocarbon complexes in solutions have been evaluated. Strneght of interaction increases with electron donating power of aromatic hydrocarbons. NMR spectroscopic studies made on these binary liquid systems also provide evidence for the existence of weak electron donor-acceptor type specific interactions between DBE and aromatic hydrocarbons.     

Thermodynamics of Cadmium Chloride in 2-Butanone + Water Mixtures (5, 10, and 15 Mass%) from Electromotive Force Measurements

The emf (electromotive force) of the cell: CdHg_x (two phase) | CdCl₂ (m) | AgCl | Ag in 2-butanone + water mixtures (containing 5, 10, and 15 mass% 2-butanone) was measured at varying temperature (293.15, 298.15, 303.15, 308.15, and 313.15 K) and in the CdCl₂ molality range from 0.002 to 0.02 mol-kg^–1. At each temperature the standard emf of the cell (E_m^o) was determined using potentiometric data and literature values for the stability constants of chlorocadmium complexes. The E_m^o values were used to calculate the standard thermodynamic quantities for the cell reaction, the stoichiometric mean molal activity coefficients of CdCl₂, and the thermodynamic functions for CdCl₂ transfer from water to 2-butanone + water mixtures. The transfer process is a forced one and results in an entropy decrease. The transfer functions were compared to those obtained for the same electrolyte in acetone + water mixtures, as well as to those for HBr in ketone + water mixtures. Medium effects upon CdCl₂ were calculated and discussed for the examined mixtures (2-butanone + water).     

Activities and thermodynamic excess properties of aqueous 2,5-dimethylpyridine mixtures near the critical demixing point: Comparison with 2,6-dimethylpyridine-water mixtures

Activities of 2,5-dimethylpyridine in aqueous solution have been measured at eleven temperatures, two below and nine above the lower critical temperature of the mixture, by the dynamic method of Randall and Weber. Gibbs free energy, enthalpy and entropy are derived to obtain excess quantities and all the results are discussed in connection with the liquid-liquid phase separation, the critical situation and the association properties of the molecules. A comparison is also made with the aqueous 2,6-dimethylpyridine system. A new version of the phase diagram is proposed.     

Thermodynamics of the dissociations of glycine in 50 mass % aqueous monoglyme from 5 to 55°C

Electromotive-force measurements on cells without liquid junction have been used to determine the pK ₁ and pK ₂ values of glycine in 50 mass % aqueous monoglyme at 11 temperatures from 5 to 55°C. The change in the first dissociation constant is given as a function of the thermodynamic temperatureT by the equation pK ₁=–2058.6/T+15.421–0.019169T, whereas that for the second dissociation constant is given by the equation pK ₂=1200.5/T+6.7211–0.0042897T. At 25°C, the pK ₁ is 2.806 in the mixed solvent, as compared with 2.350 in water; hence, protonated glycine becomes a weaker acid in the mixed solvent. The pK ₂ is 9.453 in the mixed solvent, whereas that in water is 9.780, suggesting that the second dissociation process becomes stronger in terms of acidity. The thermodynamic quantities G ^o, H ^o, S ^o, and C _p ^o have been calculated, and the results have been discussed with respect to preferential solvation and also compared with similar data for the same two processes in 50 mass % methanol.