Showing papers on "Solvation published in 1982"
TL;DR: In this paper, the Gibbs free energy of cavity formation is derived as a function of cavity radius up to 0.32 nm (the radius of the approach of water oxygens to the cavity center).
Abstract: Thermodynamic quantities related to the solvation of hydrophobic solutes in water can be approximated by the application of scaled-particle theory. The crucial quantity is the Gibbs free energy of cavity formation. A series of six molecular-dynamics simulations of water including repulsive cavities of various sizes has been carried out. Using perturbation statistical mechanics, the free energy has been derived as a function of cavity radius up to 0.32 nm (0.32 nm approach of water oxygens to the cavity centre). The free energy agrees well with predictions from scaled-particle theory and the experimental surface tension is predicted to within 5%. The radial distribution of water molecules with respect to the cavity has been determined for five cavity sizes; for one size (radius of approach of water oxygens of 0.3 nm) the orientational distribution and the residence-time distribution in the hydration shells has been determined.
234 citations
TL;DR: A rich variety of interactions and interaction potentials that depend on the nature of the surfaces and intervening liquids are revealed and new insights into liquid structure adjacent to surfaces and the interactions occur in complex systems are shed.
178 citations
TL;DR: In this paper, the dynamics of electron-induced solvation clusters in polar liquids were investigated and integrated with previous picosecond data, in order to outline the roles molecular dynamics and structure can play both in initiating electron localization at subpicosecond times and in governing the solvation dynamics to form e/sup -sub s/ in the picoseccond domain.
Abstract: New spectroscopic evidence concerning the dynamics of electron-induced solvation clusters in polar liquids is presented and integrated with previous picosecond data, in order to outline the roles molecular dynamics and structure can play both in initiating electron localization at subpicosecond times and in governing the solvation dynamics to form e/sup -//sub s/ in the picosecond domain. Particulr emphasis is placed on the picosecond time-resolved absorption spectroscopy of electrons in a wide range of alcohols and alcohol-alkane systems at 300/sup 0/K as a framework for the cluster model of electron solvation. While the configurationally relaxed final quantum state of e/sup -//sub s/ appears identical for e/sup -//sub s/ generated by different techniques, it is possible that the time evolution of the solvation cluster and the dynamics of electron populations between localized and continuum states could be influenced by the initial state of the system. Selected examples are discussed for alcohols, amines, and water, and comparisons are made for picosecond observations from different visible and IR spectroscopic techniques, NMR, and complementary nanosecond electron mobility data to demonstrate the overall consistency of a model in which only the dynamical, microscopic properties of the liquid determine these solvation events.
143 citations
TL;DR: In this paper, a ternary stationary phase model for RP-18 and RP-8 was proposed and the overall stationary phase formation was found to be dependent on the chain length of the bonded organic moiety and residual silanol activity.
115 citations
TL;DR: In this article, the ratios of the zwitterion form to the uncharged form of glycine were determined in mixtures of water with methanol, ethanol, t-butyl alcohol, dimethyl sulfoxide, acetonitrile, 1,4-dioxane, N,N-dimethylformamide, and tetrahydrofuran.
Abstract: The ratios of the zwitterion form to the uncharged form of glycine, KD=[NH3+CH2COO−]/[NH2CH2COOH], were determined in mixtures of water with methanol, ethanol, t-butyl alcohol, dimethyl sulfoxide, acetonitrile, 1,4-dioxane, N,N-dimethylformamide, and tetrahydrofuran. The value of In KD decreases and, therefore, ΔG298° increases, with the increase in the mole fractions of the organic solvents. The plots of ΔG° vs. reciprocal of the dielectric constants of the alcoholic mixtures are simply linear indifferent to the kind of alcohols, indicating that this relation obeys the Scatchard equation and that the free energy change of the process of the intramolecular protonation in glycine is controlled mainly by electrostatic interaction, especially by the entropy change associated with the preferential solvation around the charged amino and carboxyl groups.
111 citations
TL;DR: In this article, Monte Carlo statistical mechanics simulations have been carried out for dilute solutions of Na+ in water and tetrahydrofuran (THF) at 25 °C and 1 atm.
Abstract: Monte Carlo statistical mechanics simulations have been carried out for dilute solutions of Na+ in water and tetrahydrofuran (THF) at 25 °C and 1 atm. The intermolecular interactions were described by Lennard‐Jones and Coulomb terms in the TIPS format including the TIPS2 parameters for water–water interactions. In conjunction with previous simulation results for Na+ in methanol, the present study provides detailed insights into the nature of ionic solvation by dipolar protic and aprotic solvents. In agreement with x‐ray data, the coordination number of Na+ in water is six, identical to the value obtained for Na+ in methanol. However, the coordination number of Na+ in THF fluctuates between five and six. Consistently, the ion–solvent interaction is less exothermic in THF compared to the other two solvents. However, the heat of solution which is the sum of two large opposing contributions, viz., the ion–solvent energy and the solvent reorganization energy, does not differ much for the three solvents. In all...
98 citations
93 citations
TL;DR: The reversible character of the folding-unfolding process of a protein in solution can be taken as a proof that the final three-dimensional structure is a thermodynamically stable conformation, which is totally determined by the nature and sequence of the amino acids, their mutual interactions and the interactions with the surrounding solvent.
88 citations
TL;DR: In this paper, the ET(30)-parameter, derived from a negatively solvatochromic pyridinium-N-phenolate betaine dye as reference compound, is used to describe the solvation capability by virtue of empirical parameters of solvent polarity.
Abstract: Typical examples for different types of solvent effects on chemical reactivity are given, e. g. solvent effects on reaction rates, on the position of chemical equilibria, on competitive reaction mechanisms, on dichotomic reaction paths, on chemoselectivity, and on stereoselectivity. According to their solvent sensitivity, most organic reactions can be classified into dipolar transition-state reactions, isopolar transition-state reactions, and free-radical transition-state reactions, typical examples of which are given. Attempts to describe the solvation capability by virtue of empirical parameters of solvent polarity are mentioned. Particular attention is merited by the ET(30)-parameter, an empirical parameter derived from a negatively solvatochromic pyridinium-N-phenolate betaine dye as reference compound. Some examples of the application of this ET(30)-scale to solventsensitive chemical reactions are given.
87 citations
TL;DR: Fluorescence lifetimes (τf) of chlorophyll a (Chi a) have been measured by the single-photon counting technique over a wide range of concentrations in deoxygenated pyridine, diethyl ether, toluene and methanol.
Abstract: Fluorescence lifetimes (τf) of chlorophyll a (Chi a) have been measured by the single-photon-counting technique over a wide range of concentrations (˜10-7˜10-4M) in deoxygenated pyridine, diethyl ether, toluene and methanol. At pigment concentrations ˜1 μM, reabsorption of fluorescence induces significant artifacts on measured values of τf which are dependent on detection wavelength and the specific geometry of the experiment. There is a clear dependence of τf on the nature and degree of solvation, including both coordination of the central magnesium and hydrogen-bonding of the solvent (viz. alcohols) to the macrocycle. Quenching of the excited singlet state by molecular oxygen was measured quantitatively in ether, and a bimolecular rate constant markedly slower than the diffusion-controlled limit was obtained.
87 citations
TL;DR: In this article, an interamolecular conformational transition induced by pH change is shown for pectic acid in aqueous solution, and the transition from a more rigid, probably H-bonded, structure prevailing at low pH to a more extended one at around neutrality is accompanied by a ΔH value of about 500 cal/equiv and a ΔS value of 1.6 cal/ eqiv K in water at 25°C.
Abstract: On the basis of measurements of enthalpy of dissociation and of dilution, an interamolecular conformational transition induced by pH change is shown for pectic acid in aqueous solution. Additional evidence is given by potentiometic, viscometric, and chiroptical results. The transition from a more rigid, probably H-bonded, structure prevailing at low pH to a more extended one at around neutrality is accompanied by a ΔH value of about 500 cal/equiv and a ΔS value of 1.6 cal/equiv K in water at 25°C. The addition of salts increases the stability of the rigid conformation without changing the general features of the phenomenon. Dilatometric measurements suggest that the transition is accompanied by practically no change in the overall solvation of the polymer chain.
TL;DR: In this article, an analysis of the Raman band due to C-C stretching in acetone yields solvation numbers for the Li+ ion of the order of 3, and the dependence on salt concentration of all four species has been determined.
Abstract: Vibrational spectra and 7Li, 13C and 35C1 n.m.r. spectra have been obtained for solutions of LiC104 in acetone for salt concentrations from 0 05 to 6 m. Infrared spectra give qualitative indications of ion association. Analysis of the Raman band due to C-C stretching in acetone yields solvation numbers for the Li+ ion of the order of 3. Component band analysis of the CIO4- symmetric stretching vibrational band and the various n.m.r. spectra lead to the identification of solvent- separated ion pairs, contact ion pairs and ion aggregates, in addition to free solvated ions. The dependence on salt concentration of all four species has been determined. The association quotient for the association equilibrium.
TL;DR: It is concluded that the propionic residues of bilirubin and its dimethyl ester are tied to the skeleton via bound solvent molecules.
Abstract: The conformation of bilirubin and its dimethyl ester in dimethyl sulphoxide (DMSO) was investigated by n.m.r. spectroscopy. The chemical shifts of the pyrrole NH and Lactam protons of bilirubin and its dimethyl ester in DMSO indicate a strong interaction with the solvent. Inter-proton distances were calculated from nuclear Overhauser effects (NOE), selective and non-selective relaxation times (T1) and rotational correlation times taken from 13C relaxation times. The interproton distances indicate that the conformation of the skeleton of bilirubin and its dimethyl ester in DMSO is similar to that of bilirubin and mesobilirubin in the crystalline state and in chloroform solutions, except for a possible slight twist of the pyrrolenone rings about the methine bonds, which may be a consequence of solvation of the NH groups by DMSO. Unlike in chloroform solutions, no direct hydrogen-bonding occurs between the carboxylic acid and the lactam groups of bilirubin in DMSO, as shown by the absence of an NOE between these groups. The fast exchange of the pyrrole NH protons with 2H shows that no hydrogen-bonding occurs between these protons and the propionic residues, in line with their solvation by DMSO. From the above results, and from the slowness of the internal motion of the propionic residues of bilirubin and its dimethyl ester, it is concluded that these residues are tied to the skeleton via bound solvent molecules.
TL;DR: From studies of electron solvation in reverse micelles by pulse radiolysis, this article concluded that the hydrated electron can be used as a probe to give information on the water pool in reverse mouselles because the probability of electron capture and the spectroscopic properties of the Hydrated electron depend on the amount of free water in the micellar pool.
TL;DR: In this paper, the thermodynamic quantities associated with the solvation of argon in water and aqueous mixtures are reinterpreted on the basis of two contributions: the first is related to the hydrogen-bonding connectivity of water and is assumed to be approximately represented by corresponding thermodynamic quantity in solvents such as hydrazine and ethylene glycol; following a proposal by Lumry and Frank [R. S. Frank, Proc. 6th Int. Biophys. Congr.(1978), vol 7, p. 554; R. Y. Bra
Abstract: The thermodynamic quantities ΔG°, ΔH°, ΔS° and ΔC°p associated with the solvation of argon in water and aqueous mixtures are reinterpreted on the basis of two contributions. The first is related to the hydrogen-bonding connectivity of water and is assumed to be approximately represented by corresponding thermodynamic quantities in solvents such as hydrazine and ethylene glycol; following a proposal by Lumry and Frank [R. Lumry and H. S. Frank, Proc. 6th Int. Biophys. Congr.(1978), vol. 7, p. 554; R. Lumry, in Bioenergetics and Thermodynamics; Model Systems, ed. Y. Braibanti (Reidel, Dordrecht, 1980), p. 405] this contribution determines the free energy of hydrophobic hydration and is dominated by a positive enthalpy. The second contribution, which is responsible for marked enthalpy–entropy compensation and large heat-capacity effects in argon hydration, is assigned to the characteristic fluctuation behaviour of liquid water. This assignment is substantiated by comparisons of the bulk properties of water and various other liquids, and a model is suggested to rationalize the uncommon thermodynamic properties of water, aqueous mixtures and solutions of hydrophobic solutes. The phenomenological representation proposed is an overlay of a randomly connected H-bond network and a local fluctuation process defined in terms of a minimum cooperative unit. This process, labelled “geometric relaxation”, is pictured as a cooperative H-bond rearrangement involving a water molecule coordinated by four neighbours. The limiting microstates of the cooperative units are “short-bond” forms (with short, stiff, near-linear bonds) and “long-bond” forms (with long, weak, bent hydrogen bonds). The first is dominated by the low enthalpy of short hydrogen bonds and the second by the high entropy resulting from motions of the water molecules on flexible hydrogen bonds into the free volume not available to the “short-bond” form.
TL;DR: In this paper, the authors proposed a method to solve the problem of the problem: this paper ] of "uniformity" of the distribution of data points in the data set.
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TL;DR: In this paper, the stability of the complexes appears to depend primarily on the relative sizes of the metal ions and of the internal cavity of each macrocycle, and is influenced by solvation effects.
Abstract: The complexation in anhydrous proplene carbonate of the lanthanide ions by various macrocylic polyethers featuring from four to ten oxygen atoms has been investigated by a competitive potentiometric technique with lead(II) or thallium(I) as auxilliary ions. The stability of the complexes appears to depend primarily on the relative sizes of the metal ions and of the internal cavity of each macrocycle. It depends also on the rigidity of the ligands and is influenced by solvation effects. The small ligands 12-crown-4 (1) and 15-crown-5 (2) exhibit a similar behavior: they form 1:2 lanthanide complexes, the stability of which decreases with decreasing ionic radius despite the higher charge density of the metal ions. The presence of an electron-withdrawing phenyl group, as in 4-tert-butylbenzo-15-crown-5 (3), leads to a strong reduction of the stability constants. A maximum stability of 1:1 complexes of 3 is found at Nd(III) while a marked minimum is observed at Gd(III) in the complexation curve of dibenzo-30-crown-10 (5). Divalent samarium and ytterbium are more strongly coordinated than the corresponding trivalent ions by the crown ethers 1-5. The larger divalent ions fit better into the internal cavity of 5 and they form stable 1:2 sandwich complexes with 1 and 3. Themore » properties of the complexes described in the present work are completely different from those reported so far in the case of noncylic ligands.« less
TL;DR: In this paper, the distribution of oxygen and deuterium atoms from the water molecules in the first hydration sphere of the neodymium ion was probed directly and unambiguously by neutron diffraction.
Abstract: Much information on ionic solvation in electrolyte solutions has been inferred from macroscopic thermodynamic and transport properties and from spectroscopy. These ion-water interactions can now be probed directly and unambiguously by neutron diffraction. Such measurements have been done with neodymium trichloride solutions in heavy water that are identical in every respect except the isotopic state of the neodymium ions; these experiments yield in a straightforward manner the distribution of oxygen and deuterium atoms from the water molecules in the first hydration sphere of the neodymium ion. Each ion is surrounded by 8.6 oxygen atoms at a distance of 2.48 angstroms and 16.7 deuterium atoms at 3.13 angstroms indicating a well-defined first hydration sphere of water molecules, the deuterium atoms pointing away from the cation.
TL;DR: In this article, the Stokes radii for tetra-alkyl(aryl)onium cations were calculated using a structural model and it was shown that these radii are virtually independent of the nature of the organic solvent.
Abstract: When considering the dimensions of tetra-alkyl(aryl)onium ions it is important to take into consideration the medium in which these ions are situated, since the behaviour of the organic chains of these ions depends on their surroundings. The Stokes radii for such large ions in organic solvents are considered to be the real dimensions of these ions in their unsolvated form. The calculated Stokes radii for tetra-alkylammonium cations, beginning with the Pr4N+ ion, and for some other large tetra-alkyl(aryl)onium ions are virtually independent of the nature of the organic solvent. However, these radii differ from the experimental values in H2O and D2O, where the specific hydrophobic hydration phenomenon associated with the formation of clathrate-like structures occurs. The virtual independence from organic solvent of the solvation radii of large tetra-alkyl(aryl)onium ions is a striking demonstration that these ions are unsolvated in organic solvents. The radii of NH+4 and Me4N+ ions were calculated using a structural model.
TL;DR: In this paper, the conformational equilibria of four β-maltose conformers have been studied theoretically in 12 solvents and the stability of the conformers in dilute solution has been compared by using the continuum reaction field method.
Abstract: The conformational equilibria of four β-maltose conformers have been studied theoretically in 12 solvents. The stability of the conformers in dilute solution has been compared by using the continuum reaction field method. The solvation energy consists of electrostatic, dispersion, and cavity terms, which have been determined from the calculated properties of β-maltose and physicochemical properties of solvents. The calculated population of four conformers significantly depends on the solvent (e.g., in dioxane, M1:M2:M3:M4 = 53.3: 20.3:17.7:8.7; in dimethyl sulfoxide, 40.1:21.8:22.8:15.3; and in water, 25.7:17.5:26.3:30.5) and was found to correlate with experimentally observed data. The results obtained indicate that the conformation adopted by β-maltose in the crystalline form is not the one preferred in solution. The roles of the individual contributions to the solvation energy have been analyzed. Based on the determined abundance of conformers, averaged residual optical activity and vicinal carbon–proton coupling constants have been calculated and discussed from the point of view of the solution behavior of β-maltose.
TL;DR: In this article, Voronoi polyhedra are employed to generate a coherent description of the immediate solvent sheath of complex solutes in water solution, which is then used to describe the solutes' behavior.
Abstract: Voronoi polyhedra are employed to generate a coherent description of the immediate solvent sheath of complex solutes in water solution.
TL;DR: In this paper, the extended Hildebrand solubility approach was used to obtain fits of the experimental curves for testosterone and testosterone propionate in the various binary solvents and to express these in the form of regression equations.
TL;DR: In this article, the electrostatic entropy of an ion, ΔS°e, or the contribution to ΔSI, II, have been shown to be quantitative measures of the structure-making and structure-breaking effects of ions of the alkali halide series in water and in non-aqueous solvents.
Abstract: The electrostatic entropy of solvation of an ion, ΔS°e, or the contribution to ΔS°e from the co-ordination sphere of the ion, ΔSI, II, have been shown to be quantitative measures of the structure-making and structure-breaking effects of ions of the alkali halide series in water and in non-aqueous solvents. Both entropy criteria indicate that in water the ions Li+, Na+, Ag+ and F– are net structure-makers, the ions Rb+, Cs+, Cl–, Br–, I– and ClO–4 are structure-breakers, and K+ is a borderline case. In the non-aqueous solvents formamide, methanol, N-methylformamide, dimethylformamide, dimethylsulphoxide and acetonitrile, all the above ions are structure-makers with the exceptions of the weak structure-breaking ion ClO–4 in formamide and the borderline cases of ClO–4 in methanol and I– in formamide.It is shown that the ΔS°e or ΔS°I, II values may be used to assign single-ion B- or B′-coefficients and that for water and several non-aqueous solvents there are good linear correlations between the entropy values and the single-ion coefficients. There are also good linear correlations between the entropy values and single-ion text-decoration:overlineV° values when the latter are based on text-decoration:overlineV°(H+, aq, 1 mol dm–3)=–5.4 cm3 mol–1 and when values of text-decoration:overlineV° in non-aqueous solvents are assigned by the correspondence method. It is further shown that the general conclusions reached do not depend on any particular choice of ionic radii, although the Goldschmidt–Pauling set is preferred, and it is suggested that the derived ΔS°e and ΔS°I, II values are close to ‘absolute’ values and hence provide an ‘absolute’ measure of ion–solvent interactions.
TL;DR: In this article, an expression for the effects of solvation and hydrogen bonding on the hyperfine splitting constants of a free radical has been derived by obtaining π-electron spin densities of the radical in solution by perturbation theory.
Abstract: An expression for the effects of solvation and hydrogen bonding on the hyperfine splitting constants of a free radical has been derived by obtaining ..pi..-electron spin densities of the radical in solution by perturbation theory. When no hydrogen bonding occurs between the radical and a solvent molecule, the splitting constant is approximately proportional to the Block and Walker parameter of theta(epsilon/sub r/) identical with 3 epsilon/sub r/ (ln epsilon/sub r/)/(epsilon/sub r/ ln epsilon/sub r/ - epsilon/sub r/ + 1) - 6/(ln epsilon/sub r/) - 2, where epsilon/sub r/ is the relative permittivity of the solvent. The expression is successfully applied to the di-tert-butyl nitroxide radical, the 1-methyl-4-(methoxycarbonyl)pyridinyl radical, and other free radicals. The effects of hydrogen bonding are discussed.
TL;DR: In this paper, a doubly linked diporphyrins, comprising magnesium and free base subunits (Mg-H/sub 2/), were made to assess the sensitivity of their light-driven electron transfer (ET) reactions to solvation and structural changes.
Abstract: Picosecond absorbance measurements on doubly linked diporphyrins, comprising magnesium and free base subunits (Mg-H/sub 2/), were made to assess the sensitivity of their light-driven electron transfer (ET) reactions to solvation and structural changes. Solvents vary the lifetime of the ET product (Mg/sup +/.-H/sub 2//sup -/.) in the 0.2 to 1.3-ns range and have no observable effect on the rate of formation of the ET product from the initial excited state (k > 10/sup 11/s/sup -1/). The lifetime of the Mg/sup +/.-H/sub 2//sup -/.ET product thus can be manipulated to approach the 2-4 ns lifetime of the primary photoproducts in photosystem II of green plants. Kinetic results on a noncofacial diporphyrin (slipped structure) show that the relative orientation of the macrocycles plays a measurable role in controlling ET reactions within diporphyrins. The results are discussed in terms of excited-state couplings in Mg-H/sub 2/ diporphyrins. 7 figures, 1 table.
TL;DR: In this article, the density functional theories of solvation forces in charged fluids are extended to treat electrolytes consisting of finitesized ions and neutral solvent particles, and the resulting forces display pronounced oscillations whose magnitude is a strong function of the bulk density of the neutral species.