Chapter 2

Dielec tr ic c o n t i n u u m so lva t ion models a n d COSMO

2.1 THE BASIC IDEA AND ITS DEVELOPMENT

In 1920, Max Born, a Nobel Prize winner, .published some work on the free energy of solvation of ions, AG~ ~ [21]. He conceived the idea of approximating the solvent surrounding the ion as a dielectric continuum. Defining a spherical boundary, between the ion and the continuum by an effective ion-radius, R ran, he got the simple result

AV~On t:S- 1 Q i~ eS 2Rio n (2.1)

Here, we introduce the convention of denoting the solvent by a suffix, and the solute by a superscript index..Hence, es stands for the dielectric constant of the solvent S, and Qmn is the total charge of the ion. Surely, Born was aware that this is a crude approximation, but his formula led to a qualitative understanding of the experimentally observed values of solvation energies. Indeed, it was later used to define ionic radii R i~ and thus it became in some way a self-fulfilling prophecy.

Born's idea of the dielectric continuum solvation approxima- tion became very popular, and many researchers worked on its further development. Hence a brief overview of the most important development steps will be given, but it is impossible to mention all the different modifications and all workers who have been contributing to this field. Readers who seek a broader overview are referred to some reviews on continuum solvation methods, e.g., by Cramer and Truhlar [22] or by Tomasi and Persico [23]. The goal of the history given here is to enable

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