Solvation, the Onsager model

however, also possible to find elementary reactions with very similar rates in gas and solution phases. Thus, the thermally-activated unimolecular isomerization of some substituted cyclo-butenes in a solution (of dimethyl phtalate at temperatures around 275°C) has been found to proceed at, essentially, the same rate as in the gas phase. 

The well-known Maxwell-Boltzmann distribution for the velocity or momentum associated with the translational motion of a molecule is valid not only for free molecules but also for interacting molecules in a liquid phase (see Appendix A.2.1). The average kinetic energy of a molecule at temperature T is, accordingly, (3/2)ksT. For the molecules to react in a bimolecular reaction they should be brought into contact with each other. This happens by diffusion when the reactants are dispersed in a solution, which is a quite different process from the one in the gas phase. For fast reactions, the diffusion rate of reactant molecules may even be the limiting factor in the rate of reaction. 

When a molecule is placed in a solvent, the interaction between the molecule (the solute) and the solvent can, roughly, be divided into long-range effects and short-range effects, that is, effects related to the first few solvation shells . 

In an exact representation of the interaction between a solute and a solvent, i.e., solvation, the solvent molecules must be explicitly taken into account. That is, the solvent is described on a microscopic level, where the individual solvent molecules are considered explicitly. The interaction potential between solvent molecules and between solvent molecules and the solute can, in principle, be found by solving the electronic Schrodinger equation for a system consisting of all the involved molecules. Typically, in practice, a more empirical approach is followed where the interaction potential is described by parameterized energy functions. These potential energy functions (often referred to as force fields) are typically parameterized as pairwise atom-atom interactions. 

Alternatively, as a first approximation, the solvent can be described on a macroscopic level, where the solvent is considered as a continuous medium, i.e., the molecular structure of the solvent molecules is not considered. This description cannot, of course, 

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