Diffusion-controlled reactions in solution, spin statistics

1 Diffusion-Controlled Reactions in Solution, Spin Statistics 

Electronically excited states are highly reactive and so are many of their primary photoproducts. Therefore, intermolecular processes that occur on every encounter between reactant molecules in solution, also called diffusion-controlled reactions, are fairly common in photochemistry. 

Encounter frequencies between solute molecules are lower than collision frequencies in the gas phase, because the translational motion is hindered by the bulk of solvent molecules. Conversely, once reactants have encountered each other in solution, they 

The net rate of change of the encounter complex concentration cA..B is equal to its rate of formation minus its rate of decay (Equation 2.25). 

The concentration cA b will be small compared with cA and cB throughout the reaction and after a short induction period its rate of formation will be largely balanced by its rate of decay, that is, we may assume dcA..B/dt 0 (,steady-state approximation). Rearrangement yields cA..B kdcAcB/ k d + kB) and the rate of product formation, dcB/dt = kBcA..B, is given by Equation 2.26. 

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