Dealing with the Solvent

Chemical reactions generally take place in condensed media. I have had very little to say so far about the presence or otherwise of a solvent, and many of our calculations refer to isolated molecules in the gas phase at 0 K. 

Over the years, many models have been tried that aim to take account of the solvent. I will just discuss four basic types. 

In Chapter 2,1 gave you a brief introduction to molecular dynamics. The idea is quite simple we study the time evolution of our system according to classical mechanics. To do this, we calculate the force on each particle (by differentiating the potential) and then numerically solve Newton s second law 

The calculation is advanced by a suitable timestep, typically a femtosecond, and statistical data is collected for comparison with experiment. 

---

In their classic review on Continuous Distributions of the Solvent , Tomasi and Persico (1994) identify four groups of approaches to dealing with the solvent. First, there are methods based on the elaboration of physical functions this includes approaches based on the virial equation of state and methods based on perturbation theory with particularly simple reference systems. For many years... 

The various factors that contribute to ion solvation were discussed in Section 2.2.1. In this section, we deal with the solvent effects on chemical reactions more quantitatively [5, 22]. To do this, we introduce two quantities, the Gibbs energy of transfer and the transfer activity coefficient. 

The first important question deals with the solvent used in the reaction. Is the solvent water or does it contain water If the answer is no, solvation and separation of both cations and anions is very slow and generally very unfavorable. Water facilitates ionization, but ionization is very slow in a solvent that does not contain water in such solvents, bimolecular processes may dominate. If there is no water present, assume that the reaction proceeds by a bimolecular mechanism or E2. If water is present in the solvent, ionization is possible. 

---