2023-07-09 06:37:19
Manuel Ruiz-Lopez, what was the impact, at the time of its release, of this publication??
Let us first recall that the majority of theoretical chemists of the time were focused on the study of small isolated molecules. The computing means available did not indeed make it possible to approach the electronic calculation of complex systems. Thus, at first, the publication of this model mainly attracted the attention of the few theoreticians who were interested in the effects of the solvent, hitherto taken into account in a very rudimentary way (for example, by explicitly considering a single molecule of solvent interacting with the solute). But it should also be noted that the article was written in French (with a summary in English), a common practice in the newspaper Journal of Theoretical Chemistry, where one might also publish in German and even in Latin! This fact undoubtedly limited the immediate impact of the 1973 publication. This will be a second article, published in 1976 (this time in English in Chemical Physics) and describing a more elaborate version of the model, which will give it the decisive impetus and encourage other groups to develop similar dielectric models.
How is the solvent introduced into the dielectric model and what is its influence on the result of the calculations?
Conceptually, the model is very simple. The solvent is represented by a continuous dielectric medium surrounding the solute. The latter is supposed to be in a “cavity” created in the middle. In the presence of electric charges carried by the solute, the dielectric medium becomes polarized. This creates an electrostatic potential which will interact with electrons and nuclei. The mathematical expression of the potential is obtained by solving the classical electrostatic equations. This potential is then added to the Hamiltonian of the solute whose wave function can be calculated by the usual methods of quantum chemistry. Of course, the introduction of the external potential due to the solvent results in the polarization in turn of the charges of the solute, which leads to a system of non-linear equations which must be solved iteratively (self-consistent reaction-field equations). The complexity of the mathematical machinery of the problem depends to a large extent on the shape chosen for the cavity. In the first model of 1973, a spherical or ellipsoidal shape was considered, which leads to an analytical solution of the potential consisting of a sum of multipolar contributions. The case of a cavity with a general shape that best fits the structure of the molecule will be developed later, and uses numerical calculations. The great merit of this model has been to make possible the rationalization of solvent effects observed experimentally from relatively simple theoretical calculations of quantum chemistry. These effects sometimes modify in a very drastic way the structure and the spectroscopic properties of the solutes, as well as their stability. They therefore exert a major influence on chemical balances and molecular reactivity.
Has this model evolved since its birth? Is it still in use and what is its future?
The model has obviously evolved a lot since its creation. There are several approaches today, depending on how the dielectric equations are solved and the non-electrostatic terms estimated. They are available in most quantum chemistry programs and allow routine studies in solution. The model therefore continues to be used daily by a large number of researchers. In Nancy, we have gradually moved towards more elaborate solvent models, which combine the techniques of quantum chemistry and statistical mechanics. This evolution is necessary to describe the solvation phenomena from a dynamic point of view, while taking into account the microscopic structure of the solvent. That said, dielectric approaches, despite their obvious limitations, remain extremely useful because the additional computational cost compared to isolated molecules is quite low. They can therefore be used to easily explore the problems of interest before implementing, if necessary, more sophisticated techniques. Regarding their future, the main challenge will probably be their extension to the study of unconventional media, such as solvents compatible with green chemistry (deep eutectic solvents, ionic liquids, supercritical fluids, etc.) or interfaces. Some efforts in this direction have already been made, but the road is still long.
© Manuel Ruiz-Lopez
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