Solvation solute-solvent electrostatic polarization

The solvation free energy is usually considered to consist of a solvent-solvent cavity term (Gcav), a solute-solvent van der Waals term (Gvsolute-solvent electrostatic polarization term (Ges) (Eq. 2.37). 

In a recent approach that was successfully tested for small hydrocarbons the solvation was treated semianalytically as a statistical continuum1831. The method treats the sum of the solvent-solvent cavity (Gcav) and the solute-solvent van der Waals (Gvdw) terms by determining the solvent-accessible surface1841, and the solute-solvent electrostatic polarization term (Ges) is calculated by a modified version of the generalized Bom equation183,851. 

GB/S A Generalized-Born/Surface-Area. A method for simulating solvation implicitly, developed by W.C. Still s group at Columbia University. The solute-solvent electrostatic polarization is computed using the Generalized-Born equation. Nonpolar solvation effects such as solvent-solvent cavity formation and solute-solvent van der Waals interactions are computed using atomic solvation parameters, which are based on the solvent accessible surface area. Both water and chloroform solvation can be emulated. 

This methodology proceeds from the division of the total molecular solvation energy into the solute-solvent electrostatic and inductive polarization terms, standard-state free energy of eavity creation in the solvent plus the solute-solvent dispersion interaction, and an empirical part of the nuclear motion free energy. The solvent polarization term is presented using the generalized Bom formula ... 

The physical picture in concentrated electrolytes is more apdy described by the theory of ionic association (18,19). It was pointed out that as the solutions become more concentrated, the opportunity to form ion pairs held by electrostatic attraction increases (18). This tendency increases for ions with smaller ionic radius and in the lower dielectric constant solvents used for lithium batteries. A significant amount of ion-pairing and triple-ion formation exists in the high concentration electrolytes used in batteries. The ions are solvated, causing solvent molecules to be highly oriented and polarized. In concentrated solutions the ions are close together and the attraction between them increases ion-pairing of the electrolyte. Solvation can tie up a considerable amount of solvent and increase the viscosity of concentrated solutions. 

H-bonding is an important, but not the sole, interatomic interaction. Thus, total energy is usually calculated as the sum of steric, electrostatic, H-bonding and other components of interatomic interactions. A similar situation holds with QSAR studies of any property (activity) where H-bond parameters are used in combination with other descriptors. For example, five molecular descriptors are applied in the solvation equation of Kamlet-Taft-Abraham excess of molecular refraction (Rj), which models dispersion force interactions arising from the polarizability of n- and n-electrons the solute polarity/polarizability (ir ) due to solute-solvent interactions between bond dipoles and induced dipoles overall or summation H-bond acidity (2a ) overall or summation H-bond basicity (2(3 ) and McGowan volume (VJ [53] ... 

Considering that, roughly speaking, the electrostatic component of the solvation free energy varies as the cube of the molecular dipole moment, it becomes obvious that the corrective term (13.1) should be taken into account in the determination of differential solvation properties of very polar solutes. In the computation of transfer free energies across an interface, it has been suggested that equation (13.1) be expressed as a function of the number density of one of the two media, so that the correction is zero in solvent 1 and zl,l lsl lll in solvent 2 [115]. 

Solvent permittivity — is an index of the ability of a solvent to attenuate the transmission of an electrostatic force. This quantity is also called the -> dielectric constant. -> permittivity decreases with field frequency. Static (related to infinite frequency) and optical op (related to optical frequencies) permittivities are used in numerous models evaluating the solvation of ions in polar solvents under both static and dynamic conditions. Usually the refractive index n is used instead of op (n2 = eop), as these quantities are available for the majority of solvents. The theory of permittivity was first proposed by Debye [i]. Systematic description of further development can be found in the monograph of Frohlich [ii]. Various aspects of application to reactions in polar media and solution properties, as well as tabulated values can be found in Fawcetts textbook [iii]. 

In conclusion, it can be said that the electrostatic theory of solvent effects is a most useful tool for explaining and predicting many reaction patterns in solution. However, in spite of some improvements, it still does not take into account a whole series of other solute/solvent interactions such as the mutual polarization of ions or dipoles, the specific solvation etc., and the fact that the microscopic relative permittivity around the reactants may be different to the macroscopic relative permittivity of the bulk solvent. The deviations between observations and theory, and the fact that the relative permittivity cannot be considered as the only parameter responsible for the changes in reaction rates in solution, has led to the creation of different semiempirical correlation equations, which correlate the kinetic parameters to empirical parameters of solvent polarity (see Chapter 7). 

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. 

---