Nonelectrostatic contribution

In the previous section, various methods employed to calculate the electrostatic contributions to the free energy of solvation have been presented. However, it is important to provide some ideas about the calculation of nonelectrostatic contributions. These factors are essential for solutes, which are neither charged nor polar. The cavity and van der Waals terms can be combined and represented as [5]... 

The difference of the standard free enthalpies of two different solvates of a certain metal ion may be considered as due to the superimposition of electrostatic and nonelectrostatic contribution 124X... 

The most important nonelectrostatic contributions to derive from the... 

In this expression, the dipole dipole interactions are included in the electrostatic term rather than in the van der Waals interactions as in Eq. (9.43). Of the four contributions, the electrostatic energy can be derived directly from the charge distribution. As discussed in section 9.2, information on the nonelectrostatic terms can be deduced indirectly from the charge density. The polarizability a, which occurs in the expressions for the Debye and dispersion terms of Eqs. (9.41) and (9.42), can be expressed as a functional of the density (Matsuzawa and Dixon 1994), and also obtained from the quadrupole moments of the experimental charge density distribution (see section 12.3.2). However, most frequently, empirical atom-atom pair potential functions like Eqs. (9.45) and (9.46) are used in the calculation of the nonelectrostatic contributions to the intermolecular interactions. 

Among the alternative definitions of the dispersion-repulsion energy, we mention the quantum mechanical approach presented in ref. [12], which has the merit of including this part of the nonelectrostatic contribution in the molecular Hamiltonian (like the electrostatic term), so that the solvent affects not only the free energy but also the electronic distribution. In this approach, however, the dispersion part is highly expensive except for very small systems, and it is not routinely used in any computational package the analytical derivatives of quantum mechanical /disp rep can be derived but they have not been implemented until now. 

To determine the coupling work between solute and solvent, it is convenient to decompose AGsol into separate, more manageable terms, which typically involve the separation between electrostatic and nonelectrostatic contributions. The former accounts for the work required to assemble the charge distribution of the solute in solution, while the latter is typically used to account for dispersion and repulsion interactions between solute and solvent molecules, as well as for cavitation, i.e. the work required to create the cavity that accommodates the solute. 

X 10 M" s. Although the work-term correction decreases the value of k j(calc), the effect is small at the ionic strength used (the effect increases if nonelectrostatic contributions are included). If the adiabaticity assumption is dropped and it is assumed instead that k,... 

The broad applicability often claimed lor Eq. (j) of 12.3.7.2, therefore, needs to be tempered with an awareness of its often serious limitations . Large breakdown in the applicability of such simple relationships may result from several factors, such as nonelectrostatic contributions to the work terms, differences in between corresponding homogeneous and heterogeneous reactions, and specific solvation effects. Further measurements of electrochemical-rate parameters with due regard for double-layer effects are needed to resolve this question. 

FIG. 14 Osmotic coefficient p for the divalent (left) and trivalent (right) systems from Figure 13, separated into the nonelectrostatic contribution coming from virial and ideal gas (heavy dots on solid lines) and negative electrostatic contribution (crosses on dotted lines). Again, the lines are fits that merely serve to guide the eye. 

Even at low density the nonelectrostatic contribution to the osmotic coefficient is larger than 1, particularly for the trivalent system. This must originate from the strong repulsive hard-core interactions between condensed ions and the rod, since at those densities interionic repulsions can no longer play a role. This strong repulsion is compensated by the electrostatic attraction of the condensed ions. However, since the latter also acts upon the ions that do not touch the rod, the total pressure drops below the ideal gas contribution. 

The nonelectrostatic contribution to the solvation energy consists of two parts the energy of creating a cavity in solvent and the energy of nonpolar interactions, or van der Waals energy, Uv dw- From theoretical considerations, the free energy of creating a cavity in a solvent should depend on the surface area (S) and on the volume (V) of a solute [68] ... 

The interactions between CO molecules and a graphite surface were examined in Ref. 138 for a variety of potentials models. Total energy minimizations were performed using the nonelectrostatic contribution from Ref. 17 (i.e., the ab initio data [23] obtained from N2-N2 interactions), together with a different four-point charge representation of the electrostatics the dispersion terms are corrected for surface-mediated effects [49, 225] as in... 

The various continuum solvation models may differ in many respects and several classifications have been proposed (Tomasi and Persico [69], Cramer and Truhlar [79]). They may differ on (a) how the size and shape of the cavity is defined (b) how the nonelectrostatic contributions are eomputed (c) how the reaction field is determined (d) how the solute M is described, classically or quantum mechanically. 

Mozgawa K, Mennucci B, Frediani L. Solvation at surfaces and interfaces a quantum-mechanical/continuum approach including nonelectrostatic contributions. J Phys Chem C. 2014 118(9) 4715-4725. http //dx.doi.org/10.1021/jp4117276. 

PAMPS and its random copolymer containing 18-crown-6 (PAMPS -co-crown), are used to further study the nonelectrostatic contribution to desorption force [56]. The primary structures of polymers are shown in Scheme 30.5. As shown in Fig. 30.12, the typical force curves of PAMPS with a plateau are obtained from amino-modified quartz in the buffer of water. The long plateau suggests that the desorption process of the PAMPS chain from the substrate is smooth and that it adopts a train-like conformation at the interface and the desorption force remains about 120 pN. The desorption-adsorption process is in equilibrium in the experimental time scale, which is confirmed by the constant desorption force when changing the stretching velocity. The desorption force of PAMPS from the amino-modified quartz has been... 

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