On mechanism behind Marcus theory Regarding the mechanism behind Marcus theory, I was wondering if reaction occurs through tunneling when it does not have sufficient activation energy? Wouldn't expecting reactions to occur this way be like expecting random combustion to happen in the arctic?
The reorganisation is just the extension/contraction of of bonds and most importantly solvent motion /reorientation to equalise the energy at which point the electron tunnels and relaxation occurs into the product. Reorganisation happens before the reaction, it is, as it were, how the 'hill' is climbed to reach the transition state thats all. (This is the insight that Marcus had). Consequently it is the rate limiting process, even though it may take less than a picosecond in some reactions.  After reaction, relaxation (reorganisation) also occurs so that the solvent reaches equilibrium with the new charges distribution.

Figure source; R. Marcus, Biochimica et Biophysica Acta., 811, 1985, 265 - 322)