Solvent shells

The reason for this enliancement is intuitively obvious once the two reactants have met, they temporarily are trapped in a connnon solvent shell and fomi a short-lived so-called encounter complex. During the lifetime of the encounter complex they can undergo multiple collisions, which give them a much bigger chance to react before they separate again, than in the gas phase. So this effect is due to the microscopic solvent structure in the vicinity of the reactant pair. Its description in the framework of equilibrium statistical mechanics requires the specification of an appropriate interaction potential. 

Assuming that additive pair potentials are sufficient to describe the inter-particle interactions in solution, the local equilibrium solvent shell structure can be described using the pair correlation fiinction g r, r2). If the potential only depends on inter-particle distance, reduces to the radial distribution fiinction g(r) = g... 

In a microscopic equilibrium description the pressure-dependent local solvent shell structure enters tlirough... 

There are several effects present in the region where the molecule meets the solvent shell. The first is referred to as a cavitation energy, which is the energy required to push aside the solvent molecules, thus making a cavity in... 

This solvent shell is in the way, holding back the nucleophile from doing what it is supposed to do (go attack something). For the nucleophile to do its job, the nucleophile must first shed this solvent shell. This is always the case when you dissolve a nucleophile in a polar solvent, except when you use a polar aprotic solvent. 

This indicates that the polarity of a medium is a long-range property that goes much further than the first solvation shell and therefore involves the two adjacent bulk media properties. This result is, however, valid for compounds the solvation of which is not determined by specific interactions with the first solvent shell, but rather by long-range forces like dipole interactions. The solvation of DEPNA was determined by molecular dynamics too and similar conclusions were drawn [82]. 

We found that smaller shells of dynamically-restrained water molecules yielded active sites that are in broad structural terms similar to those obtained using much larger solvent shells of unrestrained molecules, without the artifacts and distortions that were apparent when restraining potentials were not used.48 This trend suggests that shells of restrained water would be an efficient model for bulk solvent in both active-site MD simulations and unconstrained-protein dynamics.49 Moreover, we found that the value of... 

Although entropy cannot be strictly localized, some contributing factors to the solvent entropy change induced by the solute are localized in the first solvent shell, and contributions to the entropy of mixing that are proportional to the number of solvent molecules in the first solvation shell might sometimes... 

In the development of solvation models, Cramer and Tmhalar have made several noteworthy contributions [8-11]. Most of the implicit solvation models do not include the effect of first solvation shell on the solute properties. This can be satisfactorily treated by finding the best effective radii within implicit models. In addition to the first-solvent-shell effects, dispersion interactions and hydrogen bonding are also important in obtaining realistic information on the solvent effect of chemical systems. 

Some recent papers permit an exciting outlook on the degree of sophistication of experimental techniques and on the kind of data which may be available soon. In the field of NMR spectroscopy, a publication by Hertz and Raedle 172> deals with the hydration shell of the fluoride ion. From nuclear magnetic relaxation rates of 19F in 1M aqueous solutions of KF at room temperature, the authors were able to show that the orientation of the water molecules in the vicinity of fluoride ions is such that the two protons are non-equivalent. A geometry is proposed for the water coordination in the inner solvent shell of F corresponding to an almost linear H-bond and to an OF distance of approximately 2.76 A, at least under the conditions chosen. 

Calculations on dynamics of solvation shells are still in their infancy. However, very recent papers on this subject, show that in most examples we cannot expect a realistic picture of solvent shells from a purely static approach. Most probably, molecular dynamics calculations and Monte Carlo methods will produce a variety of interesting data and will improve our knowledge on solvation of ions substantially. 

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

The occurrence of significant interpenetration of solvent shells has several kinetic implications clearly all r-depen-dent contributions will be affected, especially the exponential-... 

Marcus theory assumes that these solvent shells vibrate harmonically and with identical frequency so that the potential energies of both components in a redox couple can be represented by identical but mutually shifted parabolae. Only electrons from the Fermi level in the electrode and from the ground state of the redox system in solution participate in the redox process. 

---