The present chapter is included in Part E with the other optical spectroscopy methods; however, the development of two-dimensional infrared (2D-IR) spectroscopy is strongly based on two-dimensional NMR, and it is easier to understand after reading the relevant sections in Part J, which the reader is strongly encouraged to do first.
HISTORICAL REVIEW AND INTRODUCTION TO BIOLOGICAL PROBLEMS
O. Hann proposed coherent spectroscopy – the use of radiation fields with well-defined phase properties – to extract information about atoms and molecules. The “spin echo” experiment in nuclear magnetic resonance was the first demonstration of the possibilities of coherent spectroscopy.
R. P. Feynman, F. L. Vernon Jr., and R. W. Hellwarth published a landmark paper pointing out that if coherent light fields were ever created, it would be possible to use these same methods on optical transitions. The invention of the laser in 1960 was followed quickly by a demonstration of the “photon echo” – the optical version of the “spin echo.”
R. M. Hochstrasser and collaborators proposed 2D-IR spectroscopy, in analogy with two-dimensional NMR, for the determination of time-evolving structures. The spins associated with the different nuclei in NMR are replaced in the IR experiments by a network of vibrational modes whose coupling can be used to determine molecular structure and dynamics. Structures of dipeptides, tripeptides, and pentapeptides were determined by 2D-IR spectroscopy. The most exciting aspect of 2D-IR spectroscopy, however, is the combination of its sensitivity to structure with time resolution.
Conventional FTIR spectroscopy can monitor protein secondary structure features on the nanosecond–second timescale. Pulsed IR methods can probe vibrations on timescales down to the femtosecond.
LINEAR AND MULTIDIMENSIONAL SPECTROSCOPY
Linear spectroscopy, such as visible or IR absorption, Raman scattering, and one-dimensional NMR, produces one-dimensional projections of electronic and nuclear interactions onto a single frequency (or time) axis. For simple molecules, direct information can be obtained about energy levels and absorption cross-sections. The situation is very different for complex molecules, with strongly overlapping levels. Here, the microscopic information is highly averaged, and is often totally buried under broad, featureless line shapes, whose precise interpretation often remains a mystery.