Alternative theoretical approaches refinement of theory

2-benzanthracene -I- carbon tetrabromide in viscous solvents (d) M.H. Hui and W.R. Ware, J. Amer. Chem. Soc. 98 (1976) 4718, on anthracene -I- dimethylaniline in cyclohexane (e) G.S. Beddard, S. Carlin, L. Harris, G. Porter and C.J. Tredwell, Photochem. Photobiol. 27 (1978) 443, on chlorophyll a-y nitrobenzene in ethanol. 

The Mathematical Theory of Diffusion and Diffusion-Controlled Reaction Rates 

To apply this relation we need to know the diffusion coefficients but for many systems of interest these have not been measured, and it would be very convenient to have expressions relating Dab to molecular sizes (which can usually be estimated) along with the viscosity of the solvent, so that values of Dab could be estimated. Such relations, of which the Stokes-Einstein equations are the best known, can be obtained by applying classical hydrodynamics on the assumption that the motions of molecules can be treated like those of macroscopic particles of simple shapes. Comparisons with the experimental values show how far these hydrodynamic values are reliable. The result is that, for many reactions in solution, fairly good estimates of diffusion coefficients and their temperature coefficients can be made, but that for special cases such as high-viscosity solvents it is important to use experimentally-determined values. 

The term diffusion (or more strictly translational diffusion ) is commonly used to denote the migration of molecules in a fluid, whether under the influence of a concentration 

This equation is the mathematical expression of Pick s law. It defines Da, the translational diffusion coefficient of molecules of type A, independent of concentration, for the particular solvent and temperature. More generally, to include systems where the concentration gradient may vary with time, it may be written  

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