Cage effect limiting conditions

FcAb depends not only on k and / dAB but also on co- Central to the results from the steady-state and laser-pulsed determinations of the cage effect. Equation 13.21 predicts that the tme cage effect (Fcab) and that obtained from photoproduct distributions (Fc) will be the same only when special conditions are met. In principle, there are two practical limiting conditions for Fc. ... 

The density of molecules is substantially higher in liquids than in the gas phase. However, for reactions carried out in solution under relatively dilute conditions, the concentrations of reactants are not appreciably different from in tlie gas phase. Since reactant molecules A and B must undergo collision in solution in order to react, many of the same principles developed for gas-phase reactions also apply in solution. However, the presence of solvent molecules leads to important differences between reactions in solution and in the gas phase. In solution, the rate of diffusion often limits the rate of approach of molecule A to within a sufficient distance to B for reaction to occur. Once an encounter pair AB is formed, however, the solvent may act as a cage, effectively holding them in close proximity, thereby increasing the probability of reaction. 

---