Short-range solvation

B1.20.3.1 MEASURING SHORT-RANGE SOLVATION AND HYDRATION FORCES... 

A major difficulty with this analysis, however, is that the assumption AS t % 0 requires that the solvation environment of the transition state is unaffected by its proximity to the electrode surface (Sect. 3.4). Stated equivalently, it is often expected that the temperature-dependent work terms required to extract kscorr from k ob contain large components from short-range solvation and other factors in addition to the usual "electrostatic doublelayer effects (Sect. 2.4 and 4.6). As noted in Sect. 2.3, the situation is somewhat more straightforward for surface-attached reactants since then the effects of work terms at least partly disappear. This question underscores the inevitable difficulties involved in extracting quantitative information on electron-transfer barriers from rate measurements. 

This species is essentially an electron stabilised by the surrounding water molecules. It has been the subject of detailed theoretical studies(35), but can be considered as an electron in a spherical potential well consisting of solvent molecules. Specific short-range solvation effects are thought to be important as well as long-range polarization forces. 

Concerning the best way of treating the solvent molecules, there is still plenty to do before this question can be answered. The supermolecule approach is quite appropriate for describing specific or short-range solvation effects. On the other hand, long-range effects require a large number of solvent molecules and the use of sophisticated ab initio methods to properly describe the solvent-solvent and solute-solvent interactions becomes prohibitive. Hybrid methods (QM/MM), where part of the solvent structure is represented by a MM force field, have not been extensively tested. In... 

Specific (short-range) solvation ° Hydrogen bonds... 

Thus, the pressure coefficient of the kinetic rate constant can be factorized into two terms, one involving the solvent s isothermal compressibility, and the other containing short-ranged solvation contributions to the species i in solutions relative to their ideal gas contributions, V (SR) - where all these finite quantities can be explicitly written in terms of experimentally obtained volumetric properties (vide supra Section 8.2). 

The attempt to model vibrational spectra of species in aqueous solutions involves added complications compared to gas-phase spectra. First, one must account for the short-range solvation effects (i.e., H-bonding). Doing so requires that the model size be increased significantly to add enough water molecules to form at least one complete solvation sphere. In addition, a higher level of theory should be used to accurately account for the H-bonds. 

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