Work terms solvent interactions

As already mentioned, the cavity term corresponds to the endoeigic process of separating the solvent molecules to provide a suitably sized and shaped enclosure for the solute, and measures the work required for such a purpose. This term is related to the tightness or structuredness of solvents as caused by intermolecular solvent/solvent interactions. The association of solvent molecules in the liquid state in order to accommodate the solute molecules can be quantified by means of the surface area and texture of the solute that are related with the m coefficient and by the cohesive pressure of the solvent given by fl. 

Other possible contributions to the work terms for outer-sphere reactions include the likelihood that the ion-solvent interactions will differ in the interfacial region from that in bulk solution resulting from the influence of the metal surface upon the local solvent structure. As noted in Sect. 2.4, this effect may be significant even for ions in the diffuse layer since the perturbation upon the solvent structure is liable to extend several layers out from the metal surfaces [19] (see also Sect. 4.6). 

To proceed further we recall that AG can be decomposed into an electrostatic contribution (AG ei) corresponding to solute-solvent interactions with the wave function already polarized by the solvent and into the polarization work (AG poi) needed to polarize the solute wave function from its optimum value in vacuo. Of course only the first term can be dissected into contributions originating from different spheres of the cavity. Table 8 shows this dissection for both the conformers obtained at HF/6-3l4G(d) level the largest contributions to the solvation energies are due to the ionized moieties of the zwitterion, which are more exposed to the solvent in the anti conformer than in its gauche counterpart. 

Equation (a), with set equal to ( >, is surprisingly successful in describing the effect of varying the double-layer structure upon the kinetics of electrochemical reactions at Hg electrodes, at least in the absence of specific adsorption of the supporting electrolyte (i.e., when the inner-layer region adjacent to the electrode contains only solvent molecules). However, this does not necessarily imply that average electrostatic interactions provide the sole contribution to the work terms, because contributions may arise from other sources that remain constant under these conditions. In particular, inner-sphere pathways commonly involve reaction sites within the outer Helmholtz plane. Consequently, the overall work terms consist of separate contributions from transporting the reactant from the bulk solution to this outer plane and from this plane to the reaction site within the inner layer. The latter will then be independent of and, therefore, influence only k j.. in Eq. (a). 

Additional alterations in the work terms with the electrode material for outer-sphere reactions may arise from discreteness-of-charge effects or from differences in the nature of the reactant-solvent interactions in the bulk solution and at the reaction plane. Thus metals that strongly chemisorb inner-layer solvent (e.g., HjO at Pt) also may alter the solvent structure in the vicinity of the outer plane, thereby influencing k bs variations in the stability of the outer-sphere precursor (and successor) states. Such an effect has been invoked to explain the substantial decreases (up to ca. 10 -fold) in the rate constants for some transition-metal aquo couples seen when changing the electrode materiaf from Hg to more hydrophilic metals such as Pt. Much milder substrate effects are observed for the electroreduction of more weakly solvated ammine complexes . 

Several groups have also made relevant contribution to the evolution of the original PCM. A related model based on conductor-like screening (COSMO) has been developed recently by Klamt and Schuiirmann [13]. Likewise, another approach to the PCM has been proposed in which the cavity surface is determined in terms of an electronic isodensity surface [14]. Olivares del Valle and coworkers [15] have focused their attention on aspects such as the inclusion of correlation effects in the PCM, or on the role of nonadditive effects in solute-solvent interactions. Pascual-Ahuir et al. [16] have paid most attention to the problem of the definition of the cavity surface. The work done in Barcelona has focussed mainly on the parametrization of the PCM to treating aqueous and nonaqueous solvents, as well as the application of the PCM to the study of biochemical systems [17, 18]. Finally, we and others have made new methodological developments to allow the implementation of the PCM in molecular dynamics or in Monte Carlo calculations [19]. 

The use of stodiastic tediniques in the study of local polymer motions entails certain approximations in the treatn nt of polymer/solvent interactions. The presence of a spatial delta function in Eq. (3) effectively impli that the solvent is being approximated as a cxHitinuum Thus the solvent strudure per se cannot have an effed on the simulated dynamics. Solvent propoties are usually induded in stochastic simulations only through the friction terms. In principle, the intramolecular potential should be modified by the solvent to produce a potential of mean force [48, 49]. In practice, this is usually not done. As discussed below, recent work has [ffoduced several examples which justify this practice in the absence of specific interadions between the polymer and solvent... 

---