Stability of solvates

This process was shown by voltammetry to be reversible, with the standard potentials equal to -3.44 V vs Ag/0.1 M AgCl04 in HMPA (25 °C) and -2.90 V vs Ag/ 0.1 M AgCl04 in methylamine (-50 °C) [46]. The stability of solvated electrons in these solvents has been attributed to the capture of electrons into the cavity of the solvent. 

The stability of solvated electrons in some polar solvents (liquid ammonia, HMPA, and amines) can be explained either by a solvent cavity in which the electron is trapped or by the presence of an expanded orbital occupied by the electron this orbital could... 

Solvated forms have to be treated as different chemical entities, and therefore, the rules given for polymorphism cannot be applied to relate two different solvates even though polymorphism within a certain solvate is often observed (and then these rules can be applied). One distinguishing feature of solvated forms is that their relative stability usually depends on the solvent composition. Equilibrium studies are an effective means of investigating the stability of solvates in various solvent mixtures and at different temperatures. 

Similarly to the use of other physico-chemical parameters in characterizing the solvent effect, models based on an electrostatic approach have been produced for the description of the solvent dependence of the absorption spectrum these models describe the shifts of the absorption bands in terms of the relative permittivity and refractive index of the solvent [Oo 54, Ba 54], These have fairly limited validity, however, since specific coordinative interactions cannot be neglected when considering the factors that determine the stabilities of solvates. 

The stabilities of solvates in phosphorus oxychloride and phenylphosphonic dichloride follow the same order ... 

Equations 4.1-4.4 explicitly ignore the solvent, even though the discussion in the last chapter indicates that the solvent can have a dramatic influence on the magnitude of binding forces. We do not need the solvent explicitly written as part of these equations because AG for the association reflects the stability of solvated H and G relative to solvated H G and released solvent. As such, binding constants and related thermodynamic quantities should always be tabulated as being measured in a particular solvent, as well as at a particular temperature. 

M. Gutowski and P. Skurski,/. Phys. Chem. B, 101, 9143-9146 (1997). Dispersion Stabilization of Solvated Electrons and Dipole-Bound Anions. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

An understanding of a wide variety of phenomena concerning conformational stabilities and molecule-molecule association (protein-protein, protein-ligand, and protein-nucleic acid) requires consideration of solvation effects. In particular, a quantitative assessment of the relative contribution of hydrophobic and electrostatic interactions in macromolecular recognition is a problem of central importance in biology. 

Stabilization of the syn conformer in the gas phase is explained rather intuitively in terms of the extra stabilization due to increased interactions between the H atom in the OH group and the O atom in C=0 group. As one can see in Figure 5, the extra stabilization in the anti confonner in aqueous solution arises from the solvation energy, especially at the carbonyl oxygen site. 

The central role of the concept of polarity in chemistry arises from the electrical nature of matter. In the context of solution chemistry, solvent polarity is the ability of a solvent to stabilize (by solvation) charges or dipoles. " We have already seen that the physical quantities e (dielectric constant) and p (dipole moment) are quantitative measures of properties that must be related to the qualitative concept of... 

Continuum models of solvation treat the solute microscopically, and the surrounding solvent macroscopically, according to the above principles. The simplest treatment is the Onsager (1936) model, where aspirin in solution would be modelled according to Figure 15.4. The solute is embedded in a spherical cavity, whose radius can be estimated by calculating the molecular volume. A dipole in the solute molecule induces polarization in the solvent continuum, which in turn interacts with the solute dipole, leading to stabilization. 

The Self-Consistent Reaction Field (SCRF) model considers the solvent as a uniform polarizable medium with a dielectric constant of s, with the solute M placed in a suitable shaped hole in the medium. Creation of a cavity in the medium costs energy, i.e. this is a destabilization, while dispersion interactions between the solvent and solute add a stabilization (this is roughly the van der Waals energy between solvent and solute). The electric charge distribution of M will furthermore polarize the medium (induce charge moments), which in turn acts back on the molecule, thereby producing an electrostatic stabilization. The solvation (free) energy may thus be written as... 

A theoretical ab initio study of the gas-phase basicities of methyldiazoles (90JA1303) included a discussion of the 4(5)-methylimidazole tautomer-ism. The RHF/4-31G calculations led to the conclusion that the 4-methyl tautomeric form 14a (R = Me, R = R = H) is 5.2 kJ moP more stable than its 5-methyl counterpart 14b. It was emphasized that this result is to be considered as basic-set dependent. However, a recent theoretical study [94JST(T)45] showed that, starting from the RHF/6-31G level, all the more accurate approximations indicate a higher intrinsic stability for the 4-methyl tautomer. At the MP2/6-31G level, the total energy of the 4-methyl tautomer is 0.7 kJ mol lower than that of the 5-methyl tautomer. Inclusion of solvation effects can, thus, strongly affect the position of the tautomeric equilibrium 14a 14b. Recently, a systematic theoretical study... 

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