To my extent of knowledge, the Marcus theory was specifically developed for reactions in which no atoms and only charges are exchanged between the reactants. Obviously, this does not apply for a proton transfer so you probably just can't use it.

Indeed, the reorganization energy plays an important role in determining the activation energy in Marcus’s theory. To determine the reorganization energy in the DFT, you can use the following approach:

Let “D p” be the proton donor, and “A” be the proton acceptor.

1) Find the equilibrium geometry of the system: "D p + A".

2) Find the equilibrium geometry of the system: "D + p A".

3) Calculate the single-point energy of the system with the equilibrium geometry “D p + A” using the vector from stage 2) as the initial wave function.

4) The energy difference between 3) and 1) stages will correspond to the reorganization energy of the “D p + A” system in the “D p + A -> D + p A” reaction.

Thank you very much Jürgen Weippert and Gert Van der Zwan for your answer.. However, from all my reading, I found that Marcus theory can be applied to any single elementary step reaction and not only electron transfer.

Roman G. Fedunov, you are quite right. However, Since the intrinsic barrier is the barrier at zero free energy change (∆Gr = 0), therefore it will depend on the difference in free energy of the encounter complex of A......HB and AH……B. However, the difference in energy between the separated AH + B and A + HB represent the free energy of proton transfer reaction. This is what I saw in some papers and books.

AH + B = AH……B = A......HB = A + HB

∆G‡ = (∆Gr)2/16∆G*o+ ∆G*o + ½ ∆Gr , ∆G*o : intrinsic barrier

This is my humble knowldge..

If anyone has better information, please let us know...

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Best

Gert Van der Zwan