Probing Fingerprint-Combination Band Mode Coupling through Multidimensional Spectroscopy*

Vibrational fingerprints and combination bands are a direct measure of the interbond couplings that control the chemical properties of molecules. Coherent multidimensional spectroscopy (CMDS) shows promise to probe coupling between a variety of vibrational and electronic states. However, most studies are constrained to small spectral ranges of ca. 400 cm^-1. Here, we employ a novel femtosecond laser system to probe vibrational coupling in acetonitrile, a prototypical molecule for expanding the methods of CMDS, over the 2750 – 6100 cm^-1 range. Through doubly vibrationally enhanced (DOVE) CMDS, we resolve coupling in acetonitrile between a fingerprint mode and combination bands whose transition dipole moments vary over two orders of magnitude. We observe spectral features which suggest coupling between ν(CC) and 2ν(CH), whose energies differ by nearly 5000 cm^-1. Vibrational perturbation theory results suggest the observed coupling arises due to more complex interactions than simple mechanical or electrical anharmonicities. The results discussed here show a promising application for DOVE as an explicit probe of vibrational coupling of a molecule throughout the infrared, specifically between modes of large and small infrared transition dipole moments, such as those of fingerprint and combination bands.