Standard Enthalpy Of Hydration Research Articles

New QSPR molecular descriptors based on low-cost quantum chemistry computations using DFT/COSMO approach

This work presents a simple computational methodology to determine new theoretical molecular descriptor scales convenient for use in QSPR correlations and the analysis of solvation-related properties. On the basis of low-cost quantum chemical computations using DFT/COSMO approach, in particular that implemented in the ADF/COSMO-RS module of the Amsterdam Modeling Studio program package, optimized geometry and local screening charge density of a considered molecule are obtained and related to the proposed descriptors through an a priori simple reasoning. Values of four molecular descriptors, namely volume VCOSMO∗, hydrogen bond/Lewis acidity αCOSMO and basicity βCOSMO, and charge asymmetry of the nonpolar region of the molecule δCOSMO have been calculated and are presented for sets of 128 non-ionic organic molecules and 47 ions composing ionic liquids. Relation of the proposed scales to some others that are widely known or presented recently in the literature has been examined. Although the present methodology is completely independent of any experimental data, the theoretical descriptor scales proposed here have been found to correlate linearly quite well with the respective empirical scales (mostly R2 > 0.8, and for some R2 > 0.9). For non-ionic organics, just the direct proportionality provided adequate fits of the well-established prominent scales such as those of Abraham, Klamt, Kamlet-Taft, Catalan, Gutmann, and Laurence and showed only a few statistically justifiable outliers. For ions composing ILs, the linear fits of seven acidity or basicity scales presented previously with those proposed in this work were of similar quality. Examining the observed outliers, several descriptor values reported in the literature were identified as being in error. Good performance of the new descriptor scales has been demonstrated by employing them for LSER fitting of various solvation-related thermodynamic and kinetic properties such as standard vaporization enthalpy, standard hydration enthalpy, air–water partition coefficient, air-IL partition coefficient, and solvent effects on the activation Gibbs energy or rate constant of SN1 and SNAr reactions. The quality of these correlations appears to be comparable to that of the correlations currently available for these properties, regardless of the role, solute or solvent, the involved molecules played.

Association of rhodamine cations with tetraphenylborate-anion in aqueous solution: structural properties of ions and stability of associates

The ability of rhodamine cations to form cation-anionic associates with tetraphenylborate-anion in an aqueous solution has been studied. The thermodynamic values of the equilibrium association constants Kasт were determined spectrophotometrically. The values of a number of properties of dye molecules (surface area, effective volume, hydrophobicity index log P, standard enthalpy of hydration ΔНohydr) were calculated using the QSAR model. The possibility of dependence of the value of Kasт on these factors has been reviewed. It was shown that changes in lg Kasт in the series of rhodamine associates are consistent with changes in the parameters log P and ΔНohydr.

Protons in piezoelectric langatate; La3Ga5.5Ta0.5O14

This contribution reports the hydration and electrical transport properties of effectively acceptor doped single crystalline and polycrystalline langatate, La3Ga5.5Ta0.5O14. The electrical properties are investigated over wide ranges of pH2O, pD2O and pO2 in the temperature range 400 to 1000°C. Acceptor doped langatate is dominated by oxygen vacancies in dry atmospheres and at high temperatures, and by protonic defects in wet atmospheres and at lower temperatures. The corresponding standard hydration enthalpy and entropy are −90±5kJ/mol and −130±5J/molK, respectively. Further, all compositions display pure proton conductivity in wet atmospheres below 700°C with a proton mobility enthalpy in the range of 70–75kJ/mol, depending on doping level and crystallographic direction. Hence, protons are important for the physiochemical properties of langatate even at 1000°C, and could therefore influence the behavior of langatate-based resonator devices. The proton conductivity is slightly anisotropic, being higher in the X- and Y- than in the Z-direction. At high temperatures and under dry conditions, electron holes and oxide ions dominate the conductivity, and the enthalpy of mobility of vacancies is 140±5kJ/mol.

Thermodynamics of dehydration in analcime: Absorption calorimetry and equilibrium observations

Analcime forms in a wide range of geologic environments from alkaline lakes to primary igneous occurrences in Si-undersaturated lavas and pegmatites and is present in many radioactive waste repository settings. To evaluate its hydration state in these environments, calorimetric-hydration heat measurements, equilibrium observations, and thermodynamic modeling were performed. The enthalpy of hydration of analcime determined by isothermal adsorption calorimetry was found to be independent of degree of hydration and relatively insensitive to temperature. Equilibrium hydration states of analcime as a function of temperature and vapor pressure were assessed by isothermal thermogravimetry. In light of the lack of excess enthalpy of mixing in this solution, an ideal solution model was applied to the equilibrium observations along with previously determined heat capacities hydration to retrieve the standard Gibbs energy of hydration from water vapor (-47.29 ± 0.56 kJ/ mol H 2 O), standard enthalpy of hydration (-85.10 ± 1.03 kJ/mol H 2 O), and entropy of hydration (-126.81 ± 1.60 J/mol H 2 O·K) at 298.15 K, 1 bar. The standard enthalpy of hydration at 298.15 K, 1 bar regressed from the calorimetric data are consistent with these results (-86.4 ± 1.9 kJ/mol H 2 O). Calculations of the hydration state of analcime as a function of temperature and pressure indicate that analcime is essentially fully hydrated at temperature and pressure conditions attending its formation in geologic and experimental systems. Dehydration of analcime under water-undersaturated conditions (as during heating in a radioactive waste repository) may contribute significantly to thermal budgets in systems where this phase is important.

Evaluation of Hydration Enthalpies of Monatomic Cations by Considering Both Long-Range and Short-Range Interactions

The standard hydration enthalpy, ΔH°hyd, of a monatomic cation was calculated as the sum of (1) the enthalpy due to the long-range interaction between a hydrated ion and bulk water, ΔH°LR, (2) the enthalpy due to the short-range interaction between the ion and water molecules in the first hydration shell, ΔH°SR, and (3) the enthalpy due to the ligand field stabilization of an ion, ΔH°LF, which arises for a transition-metal ion. ΔH°LR was estimated on the basis of the Born theory assuming the radius of the hydrated ion as the interatomic distance between the ion and the oxygen atom of a water molecule in the first hydration shell, rM-O, determined experimentally. ΔH°SR was evaluated on the basis of the donor−acceptor interaction between an ion and a water molecule coordinating to the ion, which was evaluated by the molecular orbital calculation of a monohydrated cluster of an ion combined with the Mulliken population analysis. ΔH°LF was calculated on the basis of the crystal field theory. Hydration enthalpies of 48 monatomic cations thus calculated agreed well with those observed experimentally.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. IX. Enthalpies of hydration of 9-methyl-8-alkyladenines and 6,9-dimethyl-8-alkyladenines

Enthalpies of solution in water, ΔH sol o , and enthalpies of sublimation, ΔH sub o , were determined experimentally for a number of crystalline derivatives of adenine: 6,8,9-trimethyladenine; 6,9-dimethyl-8-ethyladenine; 6,9-dimethyl-8-propyladenine; 6,9-dimethyl-8-butyladenine; 8,9-dimethyl-adenine and 9-methyl-8-ethyladenine. Standard enthalpies of hydration, ΔH hydr o , derived from these data were calculated. The latter were discussed together with the values for variously alkylated adenines, determined previously. The data obtained show that the dependence of enthalpy of hydration on the number of methylene groups added upon substitution with 8-n-alkyl groups of 9-methyladenine and 6,9-dimethyladenine is nonlinear.

Crystal radius and enthalpy of hydration of the cyanide ion.

On the basis that the (electron density) crystal radius of the ion CN- is 1·77 A, estimates have been made of the standard enthalpy of hydration of CN-(g) plus gaseous alkali metal ions. The results have been used in the derivation of the total lattice potential energies U POT(MCN) of alkali metal cyanides and other related thermochemical quantities. The following estimates have been made (298·15 K):

A revised calculation of the standard chemical free enthalpy and the standard enthalpy of hydration of the hydrogen ion and of the surface potential of water at 25°C

A revised calculation of the standard chemical free enthalpy and the standard enthalpy of hydration of the hydrogen ion and of the surface potential of water at 25°C