Dual-Frequency 2D IR on Interaction of Weak and Strong IR Modes

Dual-frequency 2D IR heterodyne photon-echo spectroscopy of C⋮N and CO stretching vibrational modes in 2-cyanocoumarin is reported. We have shown that the interaction among these modes provides convenient and useful structural constraints for molecules. Implementation of two pulse sequences, 4, 4, and 6 μm and 6, 6, and 4 μm, allowed the clear determination of contributions caused by vibrational relaxation. Positive correlation between C⋮N and CO frequency distributions was observed in 2-cyanocoumarin. Because C⋮N modes are highly localized and have frequencies in a spectral region with minimal water absorption, the C⋮N/CO interactions have a strong potential for use as structural reporters in proteins. In addition to CN/CO peaks, the cross-peaks responsible for the C⋮N/CC interaction are also observed in the 2D IR spectra, where CC is a coumarin ring stretching mode. We have demonstrated that 2D IR spectroscopy can utilize interactions of strong IR modes with weak local modes as structural reporters.