- Manuel Ruiz-Lopez, what was the impact of this release at the time of publication?
First, let us recall that the majority of theoretical chemists of the time focused on the study of small isolated molecules. True, the available computing resources did not make it possible to approach the electronic calculation of complex systems. Therefore, the publication of this model initially attracted the attention of the few theoreticians interested in the effects of the solvent, which until now had only been taken into account in a very rudimentary manner (e.g. by explicitly considering a single solvent molecule in the interaction). with the solute). But it should also be noted that the article was written in French (with an abstract in English), a common practice in the journal Theoretica Chimica Acta, where one could also publish in German and even in Latin! This fact undoubtedly limited the immediate impact of the 1973 publication. It will be a second paper published in 1976 (this time in English in Chemical Physics) describing a more detailed version of the model, which will give it decisive impetus and inspire other groups to develop similar dielectric models.
- How is the solvent introduced into the dielectric model and what influence does it have 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 said to be in a “cavity” created in the middle. In the presence of the electrical charges carried by the solute, the dielectric medium becomes polarized. This creates an electrostatic potential that interacts 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 solute Hamiltonian, whose wave function can be calculated using standard methods of quantum chemistry. Of course, the introduction of the external potential due to the solvent in turn leads to a polarization of the solute charges, leading to a system of nonlinear equations that have to be solved iteratively (self-consistent reaction field equations). . The complexity of the mathematical machinery of the problem depends to a large extent on the cavity shape chosen. In the first model from 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 is developed later and uses numerical calculations. The great merit of this model is that it allows the rationalization of experimentally observed solvent effects using relatively simple theoretical calculations of quantum chemistry. These effects sometimes drastically change the structure and spectroscopic properties of the solutes, as well as their stability. They therefore have a major impact on chemical equilibrium and molecular reactivity.
- Has this model evolved since its birth? Is it still used and what does its future hold?
The model has obviously evolved a lot since its inception. Today there are several approaches depending on how the dielectric equations are solved and the non-electrostatic terms are estimated. They are available in most quantum chemistry programs and allow routine studies in solution. Therefore, the model continues to be used daily by a wide range of researchers. At Nancy we have progressively moved to more sophisticated solvent models that combine the techniques of quantum chemistry and statistical mechanics. This development is necessary to describe the solvation phenomena from a dynamic point of view, taking into account the microscopic structure of the solvent. However, despite their obvious limitations, dielectric approaches remain extremely useful because the computational overhead is quite small compared to isolated molecules. They can therefore be used to simply study the issues of interest before implementing more complex techniques if needed. As for their future, the biggest challenge will probably be to extend them to the study of unconventional media, such as green chemistry compatible solvents (deep eutectic solvents, ionic liquids, supercritical liquids, etc.) or interfaces. Some efforts have already been made in this direction, but there is still a long way to go.
© Manuel Ruiz Lopez