Proton-coupled electron transfer
A Proton-coupled electron transfer (PCET) is a chemical reaction that involves the transfer of electrons and protons. The term was originally coined for single proton, single electron processes that are concerted, but the definition has relaxed to include many related processes. Reactions that indeed involve the concerted shift of a single electron and a single proton are often called Concerted Proton-Electron Transfer or CPET.
In PCET, the proton and the electron (i) start from different orbitals and (ii) are transferred to different orbitals. They transfer in a concerted elementary step. CPET contrast to step-wise mechanisms in which the electron and proton are transferred sequentially.
PCET is thought to be pervasive in redox reactions that appear to be net hydrogenations and dehydrogenations. Relevant examples include water oxidation in photosynthesis, nitrogen fixation and oxygen reduction in many pathways for respiration. Inorganic chemists often study simple reactions to test this mechanism, one example being the comproportionation of a Ru(II) aquo and a Ru(IV) oxo reactants: cis-[(bipy)2(py)RuIV(O)]2+ + cis-[(bipy)2(py)RuII(OH2)]2+ → 2cis-[(bipy)2(py)RuIII(OH)]2+ PCET is also often invoked in electrochemical reactions where reduction is coupled to protonation or where oxidation is coupled to deprotonation.
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. General sequential pathways are higher in energy than concerted pathways. 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 pKa. 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).
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