text: Although it is relatively simple to demonstrate that the electron and proton begin and end in different orbitals, it is more difficult to prove that they do not move sequentially.  The main evidence that PCET exists is that a number of reactions occur faster than expected for the sequential pathways.  In the initial electron transfer (ET) mechanism, the initial redox event has a minimum thermodynamics barrier associate with the first step. Similarly, the initial proton transfer (PT) mechanism has a minimum barrier associated with the protons initial pK. Variations on these minimum barriers are also considered.  The important finding is that there are a number of reactions with rates greater than these minimum barriers would permit. This suggests a third mechanism lower in energy; the concerted PCET has been offered as this third mechanism.  This assertion has also been supported by the observation of unusually large kinetic isotope effects (KIE).
  A typical method for establishing PCET pathway is to show that the individual ET and PT pathways operate at higher activation energy than the concerted pathway.