Two-Dimensional Spectroscopy Isolates Infrared Rovibrational Patterns

The rovibrational spectra of freely rotating gas phase molecules are often plagued by spectral congestion due to the high density of rotational peaks. The congestion is especially severe at higher infrared frequencies due to the large numbers of overlapping overtones and combination bands that form polyads. As a result, rovibrational peaks in the near-infrared region of the spectrum are seldom assigned. This work describes how two-dimensional (2D) rovibrational spectroscopy can use the coupling between vibrational modes to isolate rovibrational bands that would otherwise remain overlapped and congested. Multidimensional spectroscopic techniques that make use of the large number of cross-peaks that form rich 2D rovibrational patterns are explored. Propyne is used to demonstrate 2D methods for identifying the frequencies and symmetries of coupled vibrations and for assigning rotational quantum numbers, even in regions that are heavily congested.