Asymmetric solvation

Ellison, G.B. Engelking, PC. Lineberger, W.C. J. Am. Chem. Soc., 1978, 100, 2556. Retention of configuration has never been observed with simple carbanions. Cram has obtained retention with carbanions stabilized by resonance. However, these carbanions are known to be planar or nearly planar, and retention was caused by asymmetric solvation of the planar carbanions (see p. 764). 

Another example is the m-dinitrobenzene anion radical in aqueous solution,11,12 where the effect is due to asymmetric solvation (one nitro group solvated, the other not), an effect very similar to that with dinitrodurene anions. In this case the mean lifetime of one solvation state was 0.8 ps at 291 K and 4.5 ps at 282 K. Still more examples are mentioned in the reviews by Atkins in the early 1970s.13... 

Stokes s law and the equations developed from it apply to spherical particles only, but the dispersed units in systems of actual interest often fail to meet this shape requirement. Equation (12) is sometimes used in these cases anyway. The lack of compliance of the system to the model is acknowledged by labeling the mass, calculated by Equation (12), as the mass of an equivalent sphere. As the name implies, this is a fictitious particle with the same density as the unsolvated particle that settles with the same velocity as the experimental system. If the actual settling particle is an unsolvated polyhedron, the equivalent sphere may be a fairly good model for it, and the mass of the equivalent sphere may be a reasonable approximation to the actual mass of the particle. The approximation clearly becomes poorer if the particle is asymmetrical, solvated, or both. Characterization of dispersed particles by their mass as equivalent spheres at least has the advantage of requiring only one experimental observation, the sedimentation rate, of the system. We see in sections below that the equivalent sphere calculations still play a useful role, even in systems for which supplementary diffusion studies have also been conducted. 

Asymmetric Solvation from Many-Body Interactions... 

They were able to identify several different structures for each value of For the structures of the lowest total energy for zz < 3 the water molecules are bound preferentially to one of the Na atoms by O atoms, so that Na2 becomes asymmetrically solvated. For these structures, the Na -Na (H20) structure grows with n resulting in one Na ion and a solvated Na ion. 

Stereochemistry develops in nucleophilic solvents where carbocations are shortlived intermediates. Any observation of stereospecificity excludes the intervention of achiral carbocations. Chirality may be conferred on otherwise planar carbocations by their environment. Ion pairing is the best known example of asymmetric solvation (Section 4). Incorporation of a carbocation in micelles has recently been recognized as another way of controlling its stereochemistry (Section 5). 

Details of the transition state are not clear, but retention of configuration at both centers (Si and C) is noted. Whether discharging of the incipient carbanion is beginning in the transition state, or the formed carbanion is asymmetrically solvated (cf. Cram and Streitweiser (47d)) so that discharge preserves its original configuration is unknown. The reaction is first order in silicone, and pseudo-first-order over-all. 

In this limit, the origin of the recrossings can be understood in terms of solvent configurations, rather than solvent dynamics. For trajectories of type Fig. 4b, there is nonequilibrium asymmetric solvation... 

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