Electron transfer and dynamic infrared-band coalescence: it looks like dynamic NMR spectroscopy, but a billion times faster.

Broadening and coalescence of infrared bands can occur due to chemical exchange processes occurring on very fast, femtosecond-to-picosecond timescales. One such fast process of recent investigation is intramolecular electron transfer in transition-metal complexes with strong communication between electron-donor and -acceptor sites. The observation of partial coalescence of metal-carbonyl stretching bands in hexanuclear ruthenium mixed-valence complexes due to electron-transfer rates on the order of 10(11)-10(12) s(-1) is chronicled here. Several important advances have been made with the aid of dynamic infrared-band coalescence in these complexes, including the observation of dynamic solvent relaxation effects on electron-transfer rates, the determination of the equilibrium constant between charge-transfer isomers, and a reconsideration of the theory of electron transfer and delocalization in bridged, near-delocalized electron-transfer systems.