Rehm–Weller equation

Empirical correlation found between the observed second-order rate constant, k q, for an intermolecular electron-transfer reaction and the Gibbs energy of the photoinduced electron transfer process within the encounter complex (Δ ET G o):
k q = k d 1 + k d K d Z exp ( Δ G ‡ R T ) + exp ( Δ ET G o R T )
with k d and k −d the rate constant for the formation and separation, respectively, of the encounter (precursor) complex, K d = k d k −d, Z the universal collision frequency factor, R the gas constant, T the absolute temperature and Δ G ‡ the activation Gibbs energy of the forward electron transfer reaction.
In the original formulation of this equation the value k d K d Z = 0.25 in acetonitrile was used.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 413