Two Dimensional Infrared Spectroscopy

Two-dimensional infrared (2D IR) spectroscopy was developed as a tool to study transient molecular structure and dynamics in solution. As a vibrational spectroscopy, it directly interrogates the vibrations of chemical bonds and how the many vibrations of a system interact with one another. Inspired by the two-dimensional techniques first developed by NMR, 2D IR spectroscopy spreads a vibrational spectrum over two frequency axes. Cross peaks in the spectrum encode the couplings and orientation between vibrations. Modeling the spectra reveals structure in terms of connectivity, distance or orientation between chemical bonds. Since the measurement is made with a picosecond or faster “shutter speed,” it captures information on molecular structure in solution on a time scale fast compared to most dynamics. By progressively lengthening the “exposure time” one can follow time-dependent structural changes until the motion of molecules blurs the “picture.” Such molecular dynamics experiments do not just tell you how fast you got from one state to another, but mechanistically how the structure evolved along that path. Additionally, it characterizes distributions of molecular conformations or local solvent environments. This information is encoded in the 2D IR lineshapes, which can be analyzed statistically to describe the variance in structural parameters.

To enable the most revealing experiments, an important component of our work is the development of new 2D IR methods and technology. We also incorporate 2D IR as a sophisticated structural probe of ultrafast photoinduced chemical processes. Such transient 2D IR experiments provide a way of directly monitoring chemical and biological dynamics.

Two-dimensional Fourier Transform Spectroscopy in the Pump-probe Geometry L.P. DeFlores, R. A. Nicodemus and A. Tokmakoff, Opt. Lett., 32(20), 2966-2968 (2007).

Transient two-dimensional IR spectrometer for probing nanosecond temperature jump kinetics H.S. Chung, M. Khalil, A.W. Smith and A. Tokmakoff, Rev. Sci, Instr., 78(6) (2007).