Thermodynamics and kinetics of proton-coupled electron transfer: stepwise vs. concerted pathways

Reactions that involve transfer of an electron and a proton can proceed by stepwise pathways involving initial electron transfer (ET) or initial proton transfer (PT), or by a concerted pathway without an intermediate. The concerted mechanism is termed proton-coupled electron transfer (PCET). Understanding such reactions requires knowledge of the thermodynamics of the possible ET, PT, and PCET steps. Many reactions have a large thermochemical bias favoring the PCET pathway. This bias is often sufficient to rule out stepwise mechanisms. The DeltaG degrees for ET, PT, or PCET has a strong influence on the rate of that step. Using the terminology of Marcus theory, PT and PCET reactions at C-H bonds have higher intrinsic barriers than such reactions at O-H or N-?H bonds. The intrinsic barriers to ET and PCET are often similar when there is a small intrinsic barrier to PT. Reactions with a thermochemical bias toward PCET and with similar intrinsic barriers for all the pathways are most likely to occur by concerted PCET.