Solvated solute, effective

Solutions of alkali metals in liquid ammonia are used in organic chemistry as reducing agents. The deep blue solutions effectively contain solvated electrons (p. 126), for example... 

The second term allows for solvation, which effectively increases the volume fraction of the particles to a larger value than that calculated on the basis of dry solute. Equation (9.18) shows how this can be quantified. 

It is evident that continuum models can be quite effective, for ionic solutes as well as for neutral ones. They also have the advantage of not being highly demanding in terms of computer resources. However a problem associated with these methods is posed by so-called first-solvation-shell effects 6 One aspect of this is the difficulty of properly accounting for specific types of solute-... 

Pratt and co-workers have proposed a quasichemical theory [118-122] in which the solvent is partitioned into inner-shell and outer-shell domains with the outer shell treated by a continuum electrostatic method. The cluster-continuum model, mixed discrete-continuum models, and the quasichemical theory are essentially three different names for the same approach to the problem [123], The quasichemical theory, the cluster-continuum model, other mixed discrete-continuum approaches, and the use of geometry-dependent atomic surface tensions provide different ways to account for the fact that the solvent does not retain its bulk properties right up to the solute-solvent boundary. Experience has shown that deviations from bulk behavior are mainly localized in the first solvation shell. Although these first-solvation-shell effects are sometimes classified into cavitation energy, dispersion, hydrophobic effects, hydrogen bonding, repulsion, and so forth, they clearly must also include the fact that the local dielectric constant (to the extent that such a quantity may even be defined) of the solvent is different near the solute than in the bulk (or near a different kind of solute or near a different part of the same solute). Furthermore... 

The energy and wavefunction of the solvated solute molecule are obtained by solving the effective Schrodinger equation ... 

In both cases we can introduce a similar picture in terms of an effective Hamiltonian giving rise to an effective Schrodinger equation for the solvated solute. Introducing the standard Born-Oppenheimer approximation, the solute electronic wavefunction ) will satisfy the following equation ... 

In practical implementation of QM/MM-ER, the procedures (PI) and (P2) would be sufficient to compute the free energies with substantial accuracy. As was demonstrated in the previous paper [60], delocalization of electron distribution in space significantly affects the energetics of solvation. The effect of the electron density fluctuation can be safely neglected when one computes the free energy differences between reactants and products in chemical reactions in solution since the cancellations of the effect will take place. 

If the solute is nonpolar, there is only weak van der Waals attraction with water, and water molecules arrange around the nonpolar solute such that they form the most extensive number of hydrogen bonds, with the ice clathrates (Part IV, Chap. 21) the extreme case. The ordering of water molecules is entropically unfavorable, since they lose orientational and translational freedom. This can be compensated for if the solvated solute molecules aggregate and the ordered water molecules are released from their surface into bulk water, a process which is entropically favorable and the main driving force for the hydrophobic effect [128 to 134]. 

Another class of interacting forces are the so-called chemical forces (Prausnitz 1969). In contrast to the physical forces these forces are counterbalanced. Typical examples are the covalent bonds, electron donor-acceptor interactions, acidic solute - basic solvent interactions. Association and solvatation are effects well-known to every chemist. 

A number of points are neglected in the Nagy scheme (a) Since the stoichiometry of the solvates is not taken into consideration, in the calculation of the solvate concentrations the solvent concentration features to the first power in the expression (as if the solvate complexes all had 1 1 compositions). This simplification makes the calculation of the distribution factors Pup uncertain, (b) It does not assume the presence of free , unsolvated solute, (c) Free solvent, which is not bound in solvates, features only in the calculation of the solvate concentrations its solvate-independent effect on the reaction rate is not assumed. 

---