Solvation trends

Lithium ester enolates are extremely important in polymer chemistry as initiators and active centers of the anionic polymerization of acrylic and methacrylic monomers in polar solvents. Thus, HF-SCF studies, comparable to those mentioned above, were undertaken on monomeric methyl isobutyrate (MIB) enolate210,211. The overall conclusions on the aggregation and solvation trends are exactly the same, the bent rj3-0,C mode being preferred over the rj1-O planar one by ca 3.3 kcalmol-1. While the dimeric MIB enolate solvated by four molecules of THF was found to be the enthalpically most stable aggregate, the prismatic S6 unsolvated MIB hexamer was computed as the preferred structure in non-polar solvents (Scheme 55)212. In the latter case, the supplementary oxygen of the ester acting as a side-chain ligand for the lithium seems to explain this remarkable stability. 

Important solvation trends are evident in considering the simple ions. A clear trend in hydration energies emerges, with Li > Na > > Rb. The smaller the ion, the greater the... 

It is possible to go beyond the SASA/PB approximation and develop better approximations to current implicit solvent representations with sophisticated statistical mechanical models based on distribution functions or integral equations (see Section V.A). An alternative intermediate approach consists in including a small number of explicit solvent molecules near the solute while the influence of the remain bulk solvent molecules is taken into account implicitly (see Section V.B). On the other hand, in some cases it is necessary to use a treatment that is markedly simpler than SASA/PB to carry out extensive conformational searches. In such situations, it possible to use empirical models that describe the entire solvation free energy on the basis of the SASA (see Section V.C). An even simpler class of approximations consists in using infonnation-based potentials constructed to mimic and reproduce the statistical trends observed in macromolecular structures (see Section V.D). Although the microscopic basis of these approximations is not yet formally linked to a statistical mechanical formulation of implicit solvent, full SASA models and empirical information-based potentials may be very effective for particular problems. 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

However, solubility, depending as it does on the rather small difference between solvation energy and lattice energy (both large quantities which themselves increase as cation size decreases) and on entropy effects, cannot be simply related to cation radius. No consistent trends are apparent in aqueous, or for that matter nonaqueous, solutions but an empirical distinction can often be made between the lighter cerium lanthanides and the heavier yttrium lanthanides. Thus oxalates, double sulfates and double nitrates of the former are rather less soluble and basic nitrates more soluble than those of the latter. The differences are by no means sharp, but classical separation procedures depended on them. 

The trends of variation of the activation parameters are correlated with the solvation mechanism and dielectric behavior of the medium. Thus, AH, AG and A5 for the acidic resin-catalyzed hydrolysis of isopropyl acetate were calculated using the Wynne-Jones and Eyr-... 

The chemoselectivity of the other alkenes of Table 1 is more variable. It appears that bulky substituents favour bromide over methanol attack of the bromonium ion, since dibromlde increases from 39 to 70 % on going from methyl to tert-butyl in the monosubstituted series. The same trend is observed in the disubstituted series with a contraction of the chemoselectivity span (37 to 43 % on going from methyl to teH-butyl) for the trans isomers. Since the solvated bromide ion can be viewed as a nucleophile larger than methanol, the influence of steric effects, important in determining the regioselectivity, does not seem very significant as regards the chemoselectivity. This result has been interpreted in terms of a different balance between polar and steric effects of the substituents on these two selectivities. 

The nonnuclear chemistry of Fr reduces to studies of coprecipitation in which Fr shows the behavior of the heavier alkali metal. Coprecipitation is followed by ion exchange to concentrate Fr Physical (mp, density, crystal parameters) and chemical (thermodynamics, solvation entropies) properties are theoretically derived or extrapolated from the trends exhibited by the other alkali metals. 

The effects of the anions (i.e., their specific adsorbabilities) increase in the order F < Cr < Br < I . This trend is due to the fact that the solvation energy decreases with increasing crystal radius as one goes from F to I , and the transfer of the ions to the inner Helmholtz plane is facilitated accordingly. The opposite picture is seen for surface-active cations (e.g., [N(C4H5)4]+) the descending branch of the ECC is depressed, and the PZC shifts in the positive direction. 

Discussing principles of protein adsorption may start from general trends observed for the adsorption of more simple flexible, highly solvated polymers, in particular, polyelectrolytes. 

How well can continuum solvation models distinguish changes in one or another of these solvent properties This is illustrated in Table 2, which compares solvation energies for three representative solutes in eight test solvents. Three of the test solvents are those shown in Table 1, one is water, and the other four were selected to provide useful comparisons on the basis of their solvent descriptors, which are shown in Table 3. Notice that all four solvents in Table 3 have no acidity, which makes them more suitable, in this respect, than 1-octanol or chloroform for modeling biomembranes. Table 2 shows that the SM5.2R model, with gas-phase geometries and semiempirical molecular orbital theory for the wave function, does very well indeed in reproducing all the trends in the data. 

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