Term: Marcus equation (for electron transfer)
https://doi.org/10.1351/goldbook.M03702

Definition:
Relation between the rate of outer-sphere electron transfer and the thermodynamics of this process. Essentially, the rate constant within the encounter complex (or the rate constant of intramolecular transfer) is given by the Eyring equation: \[k_{\mathrm{ET}}=\frac{\kappa _{\mathrm{ET}}\ k\ T}{h}\ \exp (- \frac{\Delta G^{\ddagger }}{R\ T})\] where k is the Boltzmann constant, h the Planck constant, R the gas constant and κ ET the so-called electronic transmission factor (κ ET ∼ 1 for adiabatic and 

Note: Whereas the classical Marcus equation has been found to be quite adequate in the normal region, it is now generally accepted that in the inverted region a more elaborate formulation, taking into account explicitly the Franck–Condon factor due to quantum mechanical vibration modes, should be employed.

Related Terms:
1) outer-sphere electron transfer (http://doi.org/10.1351/goldbook.O04351).
2) rate constant (http://doi.org/10.1351/goldbook.O04322).
3) encounter complex (http://doi.org/10.1351/goldbook.E02087).
4) intramolecular (http://doi.org/10.1351/goldbook.I03130).
5) boltzmann constant (http://doi.org/10.1351/goldbook.B00695).
6) planck constant (http://doi.org/10.1351/goldbook.P04685).
7) gas constant (http://doi.org/10.1351/goldbook.G02579).
8) transmission factor (http://doi.org/10.1351/goldbook.T06482).
9) adiabatic (http://doi.org/10.1351/goldbook.A00141).
10) diabatic electron transfer (http://doi.org/10.1351/goldbook.D01659).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 368 (https://doi.org/10.1351/pac200779030293)

Citation: 'Marcus equation (for electron transfer)' 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.M03702

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