Proton and proton-coupled electron transfer with paradigms towards single-molecule systems

Proton (PT), H-atom and proton coupled electron transfer (PCET) are ubiquitously encountered in chemical and biological processes. PT and H-atom transfer can belong to the partially or totally adiabatic limits representing 'weak' or 'strong' interactions between the donor and acceptor, reflected most conspicuously in large, i.e.≫1 and small, i.e. 1-2, kinetic deuterium isotope effects (KIE). In view of the short proton/H-atom transfer distances, the electronically adiabatic limit prevails in either case. The PCET notion applies from sequential PT and ET events to fully synchronous ET/PT as in H-atom transfer. We first overview these notions. We then address several classes of PT reactions not commonly addressed in analytical condensed matter PT/H-atom transfer theory. These include viscosity (relaxation) controlled PTand KIE<1 in protein systems. Other classes are PT in strongly hydrogen bonded systems such as excess proton conduction in aqueous solution or in biological or synthetic membranes, and PT in the 'inverted' free energy region where excited proton vibrational states and a maximum in the Brønsted relation are important. We finally invoke an approach to single-molecule PT and H-atom transfer where the PT/H-atom transferring molecules are enclosed between the substrate and tip in electrochemical (in situ) scanning tunnelling microscopy (STM) or between a pair of nanoscale electrodes. No data are presently available but could be within reach considering the recent success of in situ STM in single-molecule electron transfer. Elusive notions such as PT/H-atom transfer distances and distance dependent KIEs would become accessible based on this approach. Copyright © 2010 John Wiley & Sons, Ltd.