Intermolecular nuclear spin—spin coupling and scalar relaxation. A quantum-mechanical and statistical-mechanical study for the aqueous fluoride ion

Coupling between¹⁹F and magnetic nuclei (¹H,¹⁷O) in water molecules in the first hydration sphere of F^? are calculated using Monte Carlo simulations. The simulations are based on intermolecular potentials from the literature and on variations of coupling constants with geometry obtained by coupled Hartree-Fock calculations. Average coupling constants are ≈20–40 Hz. Intermolecular scalar relaxation in aqueous solutions is discussed.     

Hydration of methylamine and methylammonium ion: structural and thermodynamic properties from the data of the integral equation method in the RISM approximation

The structural and thermodynamic properties of hydration of methylamine and methyl-ammonium ion were investigated by the integral equations method in the RISM approximation. According to calculations, the average number of water molecules in the first hydration shell of CH₃ group is 14.4 for aqueous methylamine and 12.7 for aqueous methylammonium solution. The first hydration shells of the NH₂ group of methylamine and the NH₃ ⁺ group of methylammonium ion contain 6.9 and 5.6 water molecules, respectively. The average number of H-bonds formed by the NH₂ group is 2.4 and that formed by the NH₃ ⁺ group is 3. The results obtained show no H-bonding between the nitrogen atom of NH^{3 +} group of methylammonium and water molecules. The hydrogen atom of water participating in the hydrogen bonding with the nitrogen atom of methylamine now is a constituent of the NH₃ ⁺ group of methylammonium ion. The hydration free energies and the ionization constant calculated within the framework of the RISM theory are in good agreement with experimental data.     

Uncatalyzed partial oxidation of p-xylene in sub- and supercritical water

In sub- and supercritical water, partial oxidation of p-xylene was performed in a batch reactor without a catalyst at 240-500^oC, 220-300 bar. The loaded amount of hydrogen peroxide was set to 0-100% of the theoretically required oxygen amount. Conversion of p-xylene was reached over 99% within 15-20 min. In sub- and supercritical water, we propose two parallel pathways and major products that consist of p-tolualdehyde, p-toluic acid, terephthalic acid, toluene and benzaldehyde. Yields of major products in subcritical conditions were higher than in a supercritical conditions.     

Structural parameters of close surroundings of Sr2+ and Ba2+ ions in aqueous solutions of their salts

Published data on various techniques of studying structural characteristics of close surroundings of strontium(II) and barium(II) ions in aqueous solutions of their salts under standard conditions (coordination numbers, interparticle distances, parameters of the second coordination sphere, and ionic association types) have been generalized. It has been concluded that the Sr²⁺ ion coordinates eight water molecules in the first coordination sphere, and the Ba²⁺ ion, nine water molecules, at average distances of 0.262 and 0.283 nm, respectively. Both ions form the second coordination sphere at average distances of 0.493 and 0.500 nm, respectively. There is a well-pronounced trend to the formation of ion pairs.     

Tetraalkylammonium Ions in Aqueous and Non-aqueous Solutions

Property data for tetraalkylammonium cations, [H(CH₂) _n ]₄N⁺, are reviewed. They pertain to the isolated cations and their transfer from the gas phase into aqueous solutions. Various properties of these cations in aqueous and non-aqueous solutions and data for their transfer between these are also reviewed. Emphasis is placed on the dependence of data on the length n of the alkyl chains rather than on the absolute values. Most of the data are available only for the first four members of the series. The properties of the isolated ions increase linearly with the chain length. Molar enthalpies of formation of the gaseous and aqueous cations, and absolute standard molar enthalpies of hydration, are derived. Standard molar entropies of dissolution of several salts in water are obtained from their solubilities and enthalpies of solution. The molar entropies of the crystalline iodides of the first four members of the series then give the standard partial molar entropies of the aqueous cations and their molar entropies of hydration. The standard partial molar volumes in aqueous and non-aqueous solutions are quite linear with n and in non-aqueous solutions the molar volume hardly depends on the nature of the solvent. On transfer from water to non-aqueous solvents the volume of Me₄N⁺ suffers some shrinkage, that of Et₄N⁺ appears to be unaffected, but from Pr₄N⁺ onwards an increasing expansion takes place. This unexpected result is tentatively explained by hydrophobic intra-molecular association of pairs of alkyl chains in aqueous solutions, resulting in a tightening of the structure. The transfer of the R₄N⁺ cations from water into non-aqueous solvents is governed by a large positive entropy change, outweighing the smaller positive enthalpy change. The transport properties of the aqueous R₄N⁺ cations are non-linear with n. A major impediment to movement is thus the sticking of the water molecules to the ice-like hydrophobic hydration sheaths of the larger cations. The number of water molecules affected by the hydrophobic cations is open to widely differing estimates resulting from various approaches, and constitute an open issue.     

State of the hydroxide ion in water and aqueous solutions

Conclusion Analysis of these experimental facts leads to the conclusion that in water and aqueous solutions of alkali metal hydroxides it is extremely probable that the hydroxide ion exists in the form H₃O₂ ^–. The marked displacement of the extrapolated chemical shift of the proton of the H₃O₂ ^– ion towards weak fields and the displacement of the frequency of the bending vibrations of the OH bond towards higher frequencies for hydroxide solutions indicate strong hydrogen bonding between the OH^– ion and the H₂O molecule. The comparatively low heat of hydration of the OH^– ion (111 cal/mole) compared with the heat of hydration of the H₊ ion (276 cal/mole) cannot, as has been shown, serve as proof that there is no strong electrostatic bond between the OH^– ion and a water molecule. All the heat of hydration is used up in the formation of this bond; this can be regarded as additional confirmation of the hydrophobic nature of the ion produced. The experimental data on the absolute value of the chemical shift of the proton of the H₃O₂ ^– ion indicate the important role played by the excited state of the proton in this complex. This conclusion agrees with the spectroscopic data.M. V. Lomonosov Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 12, No. 6, pp. 969–974, November–December, 1971.     

Reconsideration on Hydration of Sodium Ion: From Micro-Hydration to Bulk Hydration

Micro hydration structures of the sodium ion, [Na(H₂O) _n ]+, n = 1–12, were probed by density functional theory (DFT) at B3LYP/aug-cc-pVDZ level in both gaseous and aqueous phase. The predicted equilibrium sodium–oxygen distance of 0.240 nm at the present level of theory. The four-, five- and six-coordinated cluster can transform from each other at the ambient condition. The analysis of the successive water binding energy and natural charge population (NBO) on Na⁺ clearly shows that the influence of Na⁺ on the surrounding water molecules goes beyond the first hydration shell with the hydration number of 6. The Car-Parrinello molecular dynamic simulation shows that only the first hydration sphere can be found, and the hydration number of Na⁺ is 5.2 and the hydration distance (r_Na–O) is 0.235 nm. All our simulations mentioned in the present paper show an excellent agreement with the diffraction result from X-ray scattering study.     

Structure of the nearest surrounding of the Li+ ion in aqueous solutions of its salts

Structural characteristics of hydration of lithium ion in aqueous solutions of its salts are studied by X-ray diffraction. The results are summarized and correlated with relevant published data obtained by various methods. The structural parameters of the nearest surrounding of the Li⁺ ion, such as the coordination number, interparticle distances, and types of ionic association, are discussed.     

Structural parameters of hydration of Be<Superscript>2+</Superscript> and Mg<Superscript>2+</Superscript> ions in aqueous solutions of their salts

Published data on structural characteristics of hydration of beryllium and magnesium ions in aqueous solutions of their salts under standard conditions, obtained by various methods, as well as authors’ X-ray data are reviewed. Structural parameters of the immediate environment of Be²⁺ and Mg²⁺, specifically coordination numbers, interparticle distances, and types of ionic association, are discussed. It is noted that Be²⁺ coordinates four water molecules at an average distance of 0.167 nm and Mg²⁺ coordinates six water molecules at an average distance of 0.210 nm. In aqueous solutions of their salts, both Be²⁺ and Mg²⁺ form the second coordination spheres.     

Self-diffusion coefficients and structure of the trivalent transplutonium ion curium and gadolinium in aqueous solution

Self-diffusion coefficients D of the trivalent aquo ion Cm³⁺ have been determined in aqueous Nd(ClO₄)₃?HClO₄ solutions (pH 2.5) at 25°C, by the open-end capillary method (O. E. C. M.). The variation of D versus the square root of the concentration of inactive solution is an exponential form in the studied range of concentration. The limiting value D₀ at zero ionic strength is 6.0·10^?6 cm²·s^?1. The curve \(D = f(\sqrt c )\) relating to Cm³⁺ can be compared to those of²⁴¹Am³⁺ and¹⁵³Gd³⁺ obtained under similar conditions. We find a similar ionic structure of Cm³⁺ with Am³⁺ and Gd³⁺. They have the same hydration as a tripositive of 5f and 4f ions in the absence of hydrolysis, complexing, or pairing at pH 2.5. The present study contributes to show the analogy of the solvation structure of trivalent actinide ions in aqueous solution at pH 2.5 with that of the trivalent lanthanide ions as a help for predicting the thermodynamic properties.     

Thermodynamic linkage of ion binding and hydration in myofibrillar protein solutions determined from multinuclear spin relaxation studies

The mechanisms for the anionic and cationic interactions with myofibrillar proteins in aqueous solutions were investigated by nuclear magnetic resonance over a wide range of salt concentration. Markedly nonlinear dependeces of the ¹⁷O and ²³Na NMR transverse relaxation rates on salt concentration were analyzed with a thermodynamic linkage model of salt-dependent solubility and hydration (ligand-induced association model), according to Wyman's theory of linked functions. Nonlinear regression analysis of both ¹⁷O and ²³Na NMR data suggested cooperative, reversible binding of hydrated ions to myofibrillar proteins. Both ions and water were found to exchange fast, on the NMR timescale, between the binding sites of the myofibrillar proteins and the aqueous solution. At sodium chloride concentrations higher than about 0.1 grams salt/gram water, ion activities have marked effects upon the NMR relaxation rates of both ions and water. A salt activity model allowed quantitative fitting of the NMR data at high salt concentrations. The effect of neglecting the ion activity in solutions of myofibrillar proteins was also estimated and compared with the ligand-induced, cooperative association model for myofibrillar proteins. The comparison between the ¹⁷O and ²³Na results strongly suggests that water is exchanged as the hydrated ion species between the myofibrillar protein binding sites and the bulk, aqueous solution.     

Adiabatic compressibility and structural features of non-electrolyte aqueous solutions

Structural characteristics of the hydration complexes of non-electrolytes such as the hydration numbers h, molar adiabatic compressibility of hydration complexes β _h V _h , the molar volume of water in the hydration sphere V _1h , the solute molar volume without hydration environments V _2h and others are determined using the data on the ultrasonic velocity, the density and heat capacity of aqueous solutions of urea, urotropine, acetonitrile, and a number of amides of N-acetyl amino acids. A theoretical model of solvation is also applied. A comparison of the environments of hydrated urotropine molecules with those of urea and acetonitrile molecules in an aqueous medium shows a considerable hydrophobic interaction of urotropine with a solvent.     

Extraction of ruthenium(III) by dihexyl sulfoxide from hydrochloric solutions

The extraction of ruthenium(III) by dihexyl sulfoxide (DHSO) from hydrochloric solutions is studied. Ruthenium(III) is first extracted by a hydration/solvation mechanism followed by the incorporation of extractant molecules into the inner coordination sphere of the ruthenium(III) ion. The composition of the extracted compound is suggested proceeding from the resuls of electronic, ¹H NMR spectroscopy, IR spectroscopy, and elemental analysis.     

Volumetric properties, structural effects, and hydration of amino acids in aqueous salt solutions

The dependence of the standard partial volumes of glycine, α-alanine, and serine on the ionic strength of aqueous sodium chloride and sulfate solutions is modeled by the extended Masson equation: {ie534-1}. The error of less than 0.2 cm³/mol is a result of using five values of the A and B parameters: the two values of A are determined by the type of salt and the three values of B by the type of amino acid. A new variation of the additive-group approach is proposed for {ie534-2}. The partial volumes of the CH₃ group (α-alanine) and the CH₂ group (serine) are found not to depend on the salt concentration. The partial volume of the CH₂ group of glycine grows with concentration. The structural characteristics of the hydrated complexes of the NH ₃ ⁺ and COO^? groups are calculated: the hydration numbers, the molar volumes of water inside and outside the hydration sphere, and the intrinsic volume of NH ₃ ⁺ in COO^? in solution. Given the same ionic strength, the aqueous sodium sulfate solution produces a somewhat stronger dehydration of the charged groups.     

Structures of the nearest surroundings of the K<Superscript>+</Superscript>, Rb<Superscript>+</Superscript>, and Cs<Superscript>+</Superscript> ions in aqueous solutions of their salts

Published data on structural characteristics of hydration of K⁺, Rb⁺, and Cs⁺ ions in aqueous solutions of their salts under standard conditions, including authors’ X-ray diffraction data, are summarized and correlated. The structural parameters of the nearest surrounding of the K⁺, Rb⁺, and Cs⁺ ions, such as the coordination numbers, interparticle distances, and types of ionic association, are discussed. It is noted that, because of weak tendency of these cations to hydration, the parameters of their coordination spheres strongly depend on the concentration and chemical nature of counterions.     

Influence of the size of fixed-rigidity spheres on the structural and energy characteristics of hydrophobic hydration

Monte Carlo simulations of molecular configurations of aqueous solutions of spherical particles with a special potential of solute—water interaction were carried out. The influence of the particle size on the properties of hydration shells was investigated. Two regimes of hydrophobic hydration with a crossover point at 0.4 nm were found. Hydration of smaller particles causes insignificant changes in the properties of water. Particles larger than 0.4 nm break the liquid water structure. Breaking effects are more pronounced in the first hydration shell of particles. Dewetting of hard sphere surfaces predicted by the LCW phenomenological theory has peculiarities in the case of hydration of fixed-rigidity spheres. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1777–1786, September, 2008.     

Ion association in lithium metaborate solution: a Raman and ab initio insight

Ion association and hydration clusters in aqueous lithium borate solution are extremely important to understand some extraordinary properties of lithium borates. In the present work, polyborate distribution in aqueous LiBO₂ solution was investigated through Raman and thermodynamics equilibrium analysis. Geometry and stability of hydrated clusters LiB(OH)₄(H₂O)_n up to n = 8 were calculated at the B3LYP/aug-cc-pVDZ level. Three different types of ion association, namely, contact ion pairs (CIP), solvent-shared ion pairs (SIP) and solvent separated ion pairs (SSIP) were obtained; characteristics of all of these stable configurations were determined, and the most stable hydrated clusters were chosen. Then the mechanisms of ion aggregation and crystal nuclei formation in the LiB(OH)₄ solution were proposed. The tight four-hydrated sphere of Li⁺ makes it difficult for the dehydrated form of its first hydration sphere to from a CIP, which is the passible reason that lithium borate always has a large super-saturation degree.     

Hydration of simple amides. FTIR spectra of HDO and theoretical studies

The hydration of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), acetamide (A), N-methylacetamide (NMA), and N,N-dimethylacetamide (DMA) has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure has been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To facilitate the interpretation of obtained spectral results, DFT calculations of aqueous amide clusters were performed. Molecular dynamics (MD) simulation for the cis and trans forms of NMA was also carried out for the SPC model of water. Infrared spectra reveal that only two to three water molecules from the surrounding of the amides are statistically affected, from among ca. 30 molecules present in the first hydration sphere. The structural-energetic characteristic of these solute-affected water molecules differs only slightly from that in the bulk and corresponds to the clathrate-like hydrogen-bonded cage typical for hydrophobic hydration, with the possible exception of F. MD simulations confirm such organization of water molecules in the first hydration sphere of NMA and indicate a practical lack of orientation and energetic effects beyond this sphere. The geometry of hydrogen-bonded water molecules in the first hydration sphere is very similar to that in the bulk phase, but MD simulations have affirmed subtle differences recognized by the spectral method and enabled their understanding. The spectral data and simulations results are highly compatible. In the case of F, NMF, and A, there is a visible spectral effect of water interactions with N-H groups, which have destabilizing influence on the amides hydration shell. There is no spectral sign of such interaction for NMA as the solute. The energetic stability of water H-bonds in the amide hydration sphere and in the bulk fulfills the order: NMA > DMA > A > NMF > bulk > DMF > F. Microscopic parameters of water organization around the amides obtained from the spectra, which have been used in the hydration model based on volumetric data, confirm the more hydrophobic character of the first three amides in this sequence. The increased stability of the hydration sphere of NMA relative to DMA and of NMF relative to DMF seems to have its origin in different geometries, and so the stability, of water cages containing the amides.     

Structural Features of Ion Hydration in Sodium Nitrate and Thiosulfate

The features of formation of hydration spheres around electrolyte ions in aqueous solutions of sodium nitrate and thiosulfate in a wide concentration range (from 2 to 42 wt %) at temperatures from 278.15 to 318.15 K were determined from the isoentropy compressibility data. The structural characteristics of the solute hydration complexes were determined. The hydration numbers decrease with increasing concentration and are independent of temperature. Na2S2O3 has the highest hydration number at infinite dilution (h ⁰) and is characterized by the lowest molar isoentropy compressibility of water in the hydration spheres of the ions (S,1hV1h). Sodium thiosulfate, compared to sodium nitrate, interacts with water stronger, and its aqueous solutions show a greater degree of ordering.     

Structure of the nearest surrounding of the Na<Superscript>+</Superscript> ion in aqueous solutions of its salts

Published data obtained by various research methods on structural characteristics of sodium ion hydration in aqueous solutions of its salts and authors, X-ray diffraction data have been generalized. Structural parameters of the nearest surrounding of Na⁺ ion, such as its coordination number, interparticle distances, and types of ion association, have been discussed. It has been noted that the coordination number of the cation changes from four to six upon dilution of the solutions.