Spectroscopy's New Era

ACS MEETING NEWS

The chemical world is full of chemical-exchange reactions—those where A and B rapidly interconvert between individual species and AB complexes (A + B AB). Many of them are too fast to study directly via traditional methods under thermal equilibrium conditions.

Now, an ultrafast infrared analog of a routine nuclear magnetic resonance (NMR) spectroscopy experiment has proven up to the task, according to back-to-back reports by Robin M. Hochstrasser of the University of Pennsylvania and Michael D. Fayer of Stanford University at this week's American Chemical Society national meeting in Washington, D.C.

"Their stunning measurements of chemical exchange, watching in real time as solute-solvent complexes flicker in and out of existence within picoseconds, marks the beginning of a new era in spectroscopy," commented Dana D. Dlott of the University of Illinois, Urbana-Champaign.

Analogous to two-dimensional NMR, 2-D infrared vibrational echo spectroscopy uses femtosecond infrared laser pulses to excite vibrations in a molecule. If the molecule forms a complex or if a complex breaks up while the vibration remains excited, the vibration's frequency will shift, giving rise to cross-peaks in the 2-D IR spectra. The growth and decay of these cross-peaks reveal the time it takes for such complexes to form and break apart.

At the ACS meeting, both Hochstrasser and Fayer described how they've used this 2-D IR technique to reveal the picosecond-scale dynamics of model chemical-exchange reactions. Fayer and graduate student Junrong Zheng studied a number of complexes to learn how chemical modifications influence dynamics. For example, they studied the real-time equilibrium dynamics of phenol complexation to benzene in a benzene-CCl₄ solvent mixture (Science 2005, 309, 1338). Hochstrasser and graduate student Yung Sam Kim measured the rates with which the hydrogen bond between methanol and acetonitrile breaks and reforms (Proc. Natl. Acad. Sci. USA 2005, 102, 11185). By repeating this experiment at different temperatures, they calculated the activation energy required to break this hydrogen bond.

Fayer argued that 2-D IR has "tremendous general utility," a sentiment shared by Hochstrasser, who predicted that the technique would find general use "to probe complex kinetic networks of chemical species that interconvert with picosecond rate constants." Hochstrasser hopes to soon use the technique to look directly at hydrogen-bonding dynamics of peptides and proteins in aqueous solution. Fayer is excited to use it to follow the kinetics of isomerization, electron transfer, and proton-transfer reactions under thermal equilibrium conditions.