Stability and Solvation

To sum up, in addition to the electronic stabilization and solvation, classical steric congestion caused by either the cation or anion moieties effectively controls the ease of ionic dissociation of the carbon-carbon a bond in hydrocarbons. 

Methods of Investigation Spectroscopic Methods Correlation Analysis Cation Stability and Solvation Intrinsic Cation Stability Cation Solvation. ... 

R. Parthasarathi, V. Subramanian, and N. Sathyamurthy, Structure, stability and solvation shell of HsO and H5O2 clusters (results to be published). 

These imbalances can be interpreted in terms of a transition state (II) in which delocalization of the negative charge into X and Y and concomitant solvation (equation 2) have made little progress compared to the degree of proton transfer. The low intrinsic rate constants for the formation of strongly resonance-stabilized carbanions thus seem to be a consequence of a transition state that is of high energy compared to the product (III), because the transition state is not able to benefit from the resonance stabilization and solvation of III. 

Unfortunately, the extreme sensitivity to environmental effects makes detailed interpretation of fluorescence changes difficult, particularly when several tyrosine and tryptophan residues are present. For example, Pjura et al (1993) reported that large changes in fluorescence and phosphorescence caused by perturbation of a tryptophanyl residue were not always correlated with the three-dimensional structure, stability and solvation properties of mutants of T4 lysozyme. Increases in polar relaxation about the excited state of tryptophan could result from only small increases in local dynamics or solvent exposure. 

Ionic liquids (ILs) have attracted considerable attention as environmentally benign reaction media. By the proper variation of their anionic and cationic parts, their physical properties, such as vapour pressure, thermal stability, and solvation strength, can easily be fine-tuned [215]. Although, the ILs are widely used as environmentally benign solvents in high-pressure catalysis, there are only a few examples of their application in TH reactions of carbonyls. Berthold et al. [216] first reported the application of ILs as reaction media for TH of ketones and their a,p-unsaturated derivatives. Hermecz et al. reported the reduction of chalchone and ot,p-unsaturated carbonyl derivatives by using [ClRh(PPh3)3] and [Rh(cod)Cl]2... 

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. 

The substituent stabilization effects calculated for the methyl cation and the methyl anion refer to the gas phase, where no solvation effects are present, and therefore are substantially larger, in terms of eneigy, than would be the case in solution, where solvation contributes to stabilization and attenuates the substituent effects. 

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... 

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... 

Carbonium ions are likewise known to be stabilized by solvation in strongly acidic media (Olah and Pittman, 1966), and consequently the reduction potential of carbonium ions, the ease of formation of a carbonium ion by the oxidation of a substrate and the products from these reactions would be expected to depend on the acidity of the electrolysis... 

The oxidation potential of carbanions, ox> or the reduction potential of carbocations, red> could be a practical scale of stability as defined by (3). These potentials can be measured by voltammetry, although the scale is subject to assumptions regarding elimination of the diffusional potential and solvation effects. 

The basic hydrolysis of A - and A -phospholenium bromides gives mixtures of (96) and (97), the ratio depending upon the conditions used. Ylide (98) formation can compete with phosphorane generation, particularly in media where anions are stabilized by solvation. Under these... 

It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg + ion disrupts the hydration region only when a > 0-60. 

Phenolic compounds may enhance the rate of decomposition of aromatic ether, because the phenoxy radical may be stabilized by solvation (18) or hydrogen bonding (19) with phenolic compounds and may result in the subsequent hydrogen transfer reaction from hydrogen donating solvent or phenols (20). 

Generally, these equilibrium constants are very solvent dependent. In particular, the values of KEA are expected to be relatively high in protic solvents because halide anions can be stabilized through solvation in such media [143], Also, on the other hand, Kx values will be likewise affected with changes in solvent polarity [50], Low values of Kx in polar and protic media have direct implications on the degree of polymerization control because of the decreased amounts of deactivator (Mt"+1X/L), as a result of halide anion dissociation from the metal center. 

Aside from the interaction of the radical with the solvent there are several factors that can make the ethane unstable or stabilize the resulting free radical. They are the stabilization of the radical by resonance, steric strain in the ethane, and dipole-dipole repulsion in the ethane. Steric inhibition of both resonance and solvation in the radical favor association to the dimer. 

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