Solvation and Its Effects on Rates

The complexity of reactions in solution has already been described briefly. However, many unimolecular reactions have rates in solutions that are approximately equal to those in the gas phase. The population of the transition state depends on the number of critical vibrational states populated, which is a function of temperature rather than the environment of the reacting molecule. The localization of the required energy in a vibrational mode for a bond to be broken is often somewhat independent of the environment of the molecule. 

Generally, reacting molecules must come together and collide, form a transition state and react, and allow the products to be removed by diffusion from the reaction zone. In viscous media, the collision frequency of the reactants may Hmit the rate of formation of the transition state as a result of 

Consider a process in which two solvated reactant molecules A and B must come together to form a transition state. This process can be considered as requiring close proximity of A and B (sometimes called a collision complex) followed by the formation of the actual critical configuration in space, which is the reactive transition state. This process can be shown as follows. 

Although diffusion controlled reactions constitute a difficult special case, a general comparison of the behavior of gas phase reactions with those taking place in solution needs to be made. A problem with doing this is that few reactions that occur in the gas phase can be studied in solution under conditions that are otherwise the same with respect to temperature, concentration, etc. In some cases, even the products of the reaction may be different. The majority of studies on solvent effects have dealt with investigating the differences in kinetics of a reaction when different solvents are used rather than comparing the rates of gas phase reactions with those taking place in a solvent. 

Let us consider the reaction between A and B that takes place in the gas phase and in some solvent to form the same products. We will write the process in the two phases as follows. 

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