Research

Two-dimensional infrared (2D IR) spectroscopy is a tool we use to study transient molecular structure and dynamics in solution. As a vibrational spectroscopy, it directly interrogates the vibrations of chemical bonds and how the vibrations interact with one another. 2D IR spectroscopy spreads a vibrational spectrum over two frequency axes, allowing you to reveal structural and kinetic correlations. Cross peaks in the spectrum encode the couplings and orientation between vibrations. Modeling this spectrum reveals a 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.

Selected References

“Coherent 2D IR Spectroscopy: Molecular structure and dynamics in solution,” M. Khalil, N. Demirdöven and A. Tokmakoff, Journal of Physical Chemistry A 107, 5258-5279 (2003).

“Two-dimensional Fourier transform spectroscopy in the pump-probe geometry,” L. P. DeFlores, R. A. Nicodemus, and A. Tokmakoff, Optics Letters 32 2966-2968 (2007).

“A Fast-Scanning Fourier Transform 2D IR Interferometer,” S. T. Roberts, J. J. Loparo, K. Ramasesha, and A. Tokmakoff, Optics Communications 284 1062–1066 (2011).

“A Source for Ultrafast Continuum Infrared and Terahertz Radiation,” P. B. Petersen and A. Tokmakoff, Optics Letters 35 1962-1964 (2010).