  The aqueous ferrous-ferric system provides a classic example of an
electron-transfer process in solution. There has been a long standing argument
spanning more than three decades around the importance of nuclear tunnelling in
this system, with estimates based on Wolynes theory suggesting a quantum
correction factor of 65, while estimates based on a related spin-boson model
suggest a smaller factor of 7-36. Recently, we have shown that Wolynes theory
can break down for systems with multiple transition states leading to an
overestimation of the rate, and we suggest that a liquid system such as the one
investigated here may be particularly prone to this. We re-investigate this old
yet interesting system with the first application of the recently developed
golden-rule quantum transition-state theory (GR-QTST). We find that GR-QTST can
be applied to this complex system without apparent difficulties and that it
gives a prediction for the quantum rate 6 times smaller than that from Wolynes
theory. The fact that these theories give different results suggests that
although it is well known that the system can be treated using linear response
and therefore resembles a spin-boson model in the classical limit, this
approximation is questionable in the quantum case. It also intriguingly
suggests the possibility that the previous predictions were overestimating the
rate due to a break down of Wolynes theory.
