Just as a round stone rolls faster on a steep slope than on a gentle one, a chemical process speeds up when it’s made more energetically favorable. At least, that’s what usually happens. But 60 years ago when Rudolph Marcus developed his pioneering theory for electron transfer, he found that in a certain region of parameter space, increasing the driving force—the drop in free energy between the initial and final states—should actually slow the transfer down.1
That surprising prediction—the so-called Marcus inverted region—was experimentally confirmed2 in 1984, and in 1992 Marcus was awarded the Nobel Prize in Chemistry for his theory (see Physics Today, January 1993, page 20). Today, Marcus theory is textbook material in chemical kinetics,3 and inverted regions in electron-transfer reactions are widely observed.
Electron transfer underlies all of oxidation–reduction chemistry, including corrosion, combustion, electrochemistry, and ionic bonding. In photovoltaic cells, the creation...
