Proton solvation in protic and aprotic solvents

Protonation pattern strongly affects the properties of molecular systems. To determine protonation equilibria, proton solvation free energy, which is a central quantity in solution chemistry, needs to be known. In this study, proton affinities (PAs), electrostatic energies of solvation, and pK_A values were computed in protic and aprotic solvents. The proton solvation energy in acetonitrile (MeCN), methanol (MeOH), water, and dimethyl sulfoxide (DMSO) was determined from computed and measured pK_A values for a specially selected set of organic compounds. pK_A values were computed with high accuracy using a combination of quantum chemical and electrostatic approaches. Quantum chemical density functional theory computations were performed evaluating PA in the gas‐phase. The electrostatic contributions of solvation were computed solving the Poisson equation. The computations yield proton solvation free energies with high accuracy, which are in MeCN, MeOH, water, and DMSO ?255.1, ?265.9, ?266.3, and ?266.4 kcal/mol, respectively, where the value for water is close to the consensus value of ?265.9 kcal/mol. The pK_A values of MeCN, MeOH, and DMSO in water correlates well with the corresponding proton solvation energies in these liquids, indicating that the solvated proton was attached to a single solvent molecule. © 2016 Wiley Periodicals, Inc.     

Spectroscopic studies of ionic solvation. XXI. A raman,infrared, and NMR study of sodium perchlorate solutions in nonaqueous solvents

Sodium perchlorate solutions in several nonaqueous solvents were examined by²³Na,³⁵Cl-NMR, infrared, and Raman spectroscopic techniques. The formation of contact ion pairs lowers the symmetry of ClO ₄ ^– ion from T_d to C_3v or C_2v provided that the interaction is fairly strong. This was manifested for NaClO₄ solutions in acetonitrile, tetrahydrofuran, and pyridine. Combination of the vibrational measurements with NMR shows that in the above three cases the anion-cation interactions are quite strong. Sodium-23 NMR studies confirm the above results and, being a more sensitive technique, also indicates weak cation-anion interaction in propylene carbonate, formic acid, acetone, methanol, ethanol, dimethyl sulfoxide, and water. In all solvents the²³Na resonance shifts upfield with increasing concentration of NaClO₄, indicating that the replacement of solvent by ClO ₄ ^– ion decreases electron density around the cation.     

Cellulose solutions in dipolar aprotic solvents

Preparation conditions and properties of cellulose solutions in DMAA containing 7–8% lithium chloride are considered. Investigation of the optical anisotropy and structure of cellulose solutions in this solvent confirms that the LS state is attained with an increase in the concentration of cellulose. On the basis of these cellulose solutions, fibers not ranking below viscose fibers in mechanical properties are prepared. The use of a mixed solution of 95% cellulose and 5% poly(amidobenzimidazole) in DMAA containing 7% lithium chloride makes it possible to prepare films and fibers whose strength is more than two times greater than the strength of cellulose hydrate fibers. The deformation-driven orientation and supramolecular structure of fibers and films are studied by spectroscopy and small-angle scattering of polarized light.     

Spectroscopic studies of ionic solvation. XX. Cesium-133 NMR study of cesium salts in different solvents

Cesium-133 chemical shifts were measured in a number of solvents as a function of salt concentration and of the counterion. Infinite-dilution chemical shifts (vs. aqueous Cs⁺ ion at infinite dilution) ranged from +59.8 ppm for nitromethane solutions to –29.4 ppm for pyridine. In general, the magnitude of the downfield chemical shift reflected the donor ability of the solvents. Ion-pair formation constants were calculated from the concentration dependences of¹³³Cs chemical shifts in several nonaqueous solvents.     

Spectroscopic measurements of ionic association in solutions of LiPF6

Ionic association in nonaqueous electrolytes containing LiPF6 was investigated with infrared absorption spectroscopy. The spectral intensity of the nondegenerate nu1 mode of the PF6- anion was found to be sensitive to ion pairing. Although the nu(1) mode of an isolated PF6- anion is only Raman active, coordination of Li+ to PF6- destroys the octahedral symmetry of the anion and results in nu1 becoming simultaneously IR and Raman active. When the dielectric constant of the solvent is increased, the IR-intensity of the nu1 band decreases because ion pairing is not favored in high dielectric solvents. Spectroscopic studies of solutions containing LiPF6 in diglyme show that ion pairing is also affected by specific cation-solvent interactions. The diglyme-containing solutions contain significantly fewer ion pairs than expected on the basis of the solvent's dielectric constant. It is thought that diglyme:LiPF6 solutions consist mostly of "spectroscopically free" PF6- anions because the sixfold coordination of Li+ by two diglyme molecules hinders Li+...PF6- ion pairing.     

Ionization condition of lithium ionic liquid electrolytes under the solvation effect of liquid and solid solvents

Ionization condition and ionic structures of the lithium ionic liquid electrolytes, LiTFSI/EMI-TFSI/(PEG or silica), were investigated through the measurements of ionic conductivity and diffusion coefficient. The size of the hydrodynamic lithium species (rLi) evaluated from the Stokes-Einstein equation was 0.90 nm before gelation with the PEG or silica. This reveals that the TFSI- anions from the solvent are coordinated on Li+ for solvation, forming, for example, Li(TFSI)4(3-) and Li(TFSI)2- in the electrolyte solution. By the dispersion of PEG for gelation, rLi increased up to 1.8 nm with the 10 wt % of PEG. This indicates that the lithium species is directly interacted with the oxygen sites on the polymer chains and the lithium species migrate, reflecting the polymer by hopping from site to site. In case of the silica dispersion, rLi decreased to 0.7 nm at 10 wt % silica. Although the silica surface with silanol groups fundamentally attracts Li+, the lithium does not migrate from site to site on the silica surface as in the gel of the polymer and follows random walk behavior in the network of the liquid-phase pathways in the two-phase gel. In the process, that solvated TFSI- anions are partially removed may be due to the attractive effect of H+, which was dissociated from the silanol group. It is concluded that the dispersed silica is effective to modify the hydrodynamic lithium species to be appropriate for charge transport as reducing the size and anionic charge of Li(TFSI)4(3-) by removing one or two TFSI- anions.     

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.     

Structure of dimethylammonium bromide in solutions in aprotic solvents

The structure of the dimethylammonium bromide molecule in solution has been studied on the basis of IR absorption spectra and low-temperature¹H NMR spectra. An analysis of the spectroscopic characteristics of the compounds shows that the most preferable of the possible structures is an ion pair with equivalent NH protons and two hydrogen bonds, which form a stable four-membered ring.Scientific-Research Institute of Physics, Leningrad University. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 2, pp. 98–102, March–April, 1991.     

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.     

Spectroscopic studies of ionic solvation. XVIII. Solvation of the lithium ion in acetone and acetone-nitromethane mixtures

Solvation of the lithium ion by acetone was studied in acetone-nitromethane solutions by Raman, infrared, and⁷Li and³⁵Cl NMR spectroscopic techniques. It was confirmed that the 390-cm^?1 IR acetone band is split by the lithium ion and that a 369-cm^?1 IR band, attributed by other authors to Li⁺-nitromethane vibration, is due to the vibration of acetone in the lithium inner solvation shell. The frequency of the Li⁺-nitromethane vibrational band is strongly anion dependent due to contact-ion-pair formation. Several different techniques indicate that Li⁺ is solvated by four acetone molecules, and approximate equilibrium-constant values for the stepwise solvation reaction were calculated. The influence of weak complexing agents on Li⁺ ClO ₄ ^? ion-pair formation was investigated.     

Solute-solvent interactions. VII. Proton magnetic resonance and infrared study of ion solvation in dipolar aprotic solvents

The solvation of a variety of ions by the dipolar aprotic solvents acetonitrile, sulfolane, and dimethylsulfoxide was studied through the influence of salts on the proton magnetic resonance chemical shifts of the solvents. In the case of acetonitrile the results were supplemented with infrared measurements, which showed that in general anions affect only the C–H and cations both the C–C and particularly the CN stretching frequencies of acetonitrile. The results are discussed in conjunction with transport and other data already in the literature. Current views on the structure of these solvents are summarized.From the Ph.D. thesis of this author, University of Pittsburgh, 1972.     

The solubility and solvation of salts in mixed nonaqueous solvents. 1. Potassium halides in mixed aprotic solvents

The solubilities of potassium fluoride, chloride, and bromide in acetonitrile, N,N-dimethylformamide, and dimethylsulfoxide and in binary mixtures of these solvents were determined at 25°C. The standard molar Gibbs free energies of solution, _solnG°, in the neat solvents were related to the polarizabilities and basicities of the anions and the dipole moments and acidities of the solvents. The values of _solnG° in the mixtures were fitted by expressions from the quasi-lattice quasi-chemical theory. The mean number of each kind of solvent in the nearest environment of the ions was obtained from these results.     

Study of association of alcohols in aprotic solvents by multinuclear NMR

Binary mixtures of alcohols(ethyl,n-butyl and n-amyl)with several aprotic sol-vents,such as acetone,dioxane,THF,DMSO and DMF have been studied systematically by~1H,~(13)C,~(15)N and ~(17)O NMR measurements.The concentration dependence of chemical shift ofthe solvent was used to evaluate equilibrium constants of the complexation of alcohols withthe solvents.A relationship between the proton shift of alcohol uncombined with the solventand its concentration was found,and the fraction of unassociated hydroxyl groups was thusquantitatively described.The effect of solvent on self association and complexation of thealcohol is discussed on the basis of the electron donicity of the solvents.     

Structural and thermodynamic characteristics of dilute solutions of alcohols in aprotic solvents

The enthalpies of H-bonds in ethanol and isopropanol complexes with aprotic solvents, extractants of aromatic hydrocarbons, were determined. The thermodynamic functions of viscous flow activation and the mean degrees of association in dilute solutions of the alcohols in N-methylpyrrolidone, dimethylsulfoxide, and tetramethylenesulfone were obtained. The roles played by the energy and steric effects in the solution of alcohols in the solvents were estimated.     

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.     

Plastic crystalline lithium salt with solid-state ionic conductivity and high lithium transport number

Plastic crystallinity of lithium salt, [LiB(OCH(2)CH(2)OCH(3))(4)] (1), and its solid-state ionic conductivity are disclosed. The addition of small amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to borate 1 led to the drastic increase of the ionic conductivity and lithium transport number of the electrolyte.     

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.     

Spectroscopic study of association of a hemicyanine dye in mixed aqueous binary solvents

Electronic absorption and steady state emission properties of a hemicyanine dye [4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide], have been studied in several pure solvents and two mixed binary solvents (water+ethanol and water+acetonitrile). In pure solvents the band-width of the absorption spectrum correlates well with the Stoke's shift. In mixed aqueous solvents two different molecular forms of the solute, viz. the monomer and the dimer of the solute exists in equilibrium. Thermodynamic parameters (e.g. the Delta G degrees and Delta H degrees ) characterizing the equilibrium have been determined. While the value of Delta G degrees changes very slightly with the composition of the binary mixture, the value of Delta H degrees has been observed to depend significantly with solvent composition.