Essentials of Computational Chemistry

E
11.2 ELECTROSTATIC INTERACTIONS WITH A CONTINUUM 393
gas
phase
aqueous
solution
NH 3 + CH 3Cl
Reaction coordinate
d H
H3N C Cl
+ d− H H
H 3NCH 3 + + Cl −
Figure 11.6 Menschutkin reaction of ammonia and chloromethane. In the gas phase nucleophilic
displacement fails to take place, while in water solvation of the anions allows the reaction to proceed
of this situation is the organic Menschutkin reaction, where nucleophilic displacement of a
halide by an amine generates a halide anion and an alkylammonium cation, as shown in
Figure 11.6. In the gas phase, such separation of opposite charges is so unfavorable that no
stationary point exists for the separated ions. In highly polar solvents, on the other hand, the
solvation free energy of the ions is sufficiently high that not only are the products stationary
points, but the reaction is exergonic. This is a situation where invoking a free-energy cycle
is not particularly useful, although the direct computation of the lower leg (i.e., the solvated
process) by one or another solvation model is a perfectly valid option.
Besides affecting equilibria and kinetics on single energy surfaces, differential solvation
effects on distinct electronic states can cause significant changes in UV-Vis absorption
spectra. Such so-called solvatochromic effects are discussed in more detail in Chapter 14.
The various effects of solvation discussed above may in principle be modeled in different
ways. For the remainder of this chapter, we will focus on the utility of continuum solvation
models in this regard. Having identified the importance and utility of the free energy
of solvation, we will pay special attention to prediction of this quantity as a measure of
quantitative accuracy.
11.2 Electrostatic Interactions with a Continuum
When a solute is immersed in a solvent, its charge distribution interacts with that of the
solvent. In a continuum model, rather than representing the charge distribution of the solvent