Term: Rehm–Weller equation
https://doi.org/10.1351/goldbook.RT07472

Definition:
Empirical correlation found between the observed second-order rate constant, kq, for an intermolecular electron-transfer reaction and the Gibbs energy of the photoinduced electron transfer process within the encounter complex (ΔET.Go): \[k_{\text{q}} = \frac{k_{\text{d}}}{1+ \frac {k_{\text{d}}}{K_{\text{d}}\, \text{Z}}\left [ \text{exp}\left ( \frac{\Delta G^{\ddagger }}{RT} \right ) + \text{exp}\left ( \frac{\Delta_{\text{ET}} G^{o }}{RT} \right ) \right ]}\] with kd and k-d the rate constant for the formation and separation, respectively, of the encounter (precursor) complex, Kd = kd/k-d, Z the universal collision frequency factor, R the gas constant, T the absolute temperature and ΔG‡ the activation Gibbs energy of the forward electron transfer reaction.

Note: In the original formulation of this equation the value kd/(Kd.Z) = 0.25 in acetonitrile was used.

Related Terms:
1) rate constant (http://doi.org/10.1351/goldbook.O04322).
2) intermolecular (http://doi.org/10.1351/goldbook.I03098).
3) photoinduced electron transfer (http://doi.org/10.1351/goldbook.P04617).
4) encounter complex (http://doi.org/10.1351/goldbook.E02087).
5) encounter (http://doi.org/10.1351/goldbook.E02086).
6) collision frequency (http://doi.org/10.1351/goldbook.C01166).
7) gas constant (http://doi.org/10.1351/goldbook.G02579).
8) activation (http://doi.org/10.1351/goldbook.A00093).
9) electron transfer (http://doi.org/10.1351/goldbook.E02011).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 413 (https://doi.org/10.1351/pac200779030293)

Citation: 'Rehm–Weller equation' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.RT07472

License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.

Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.