Two-dimensional infrared study of the CD and CO stretching vibrations in strongly hydrogen-bonded complexes

We report a study of the CD and CO stretching modes of formyl deuterated formic acid (DCOOH) hydrogen bonded to nitrogenous bases. These modes are spectators to the hydrogen bond itself but are sensitive to the strength of that interaction. Using 2D IR spectroscopy, we show that in strongly hydrogen bonded complexes the CD stretching vibration becomes anharmonically coupled to low-order combination states leading to strong state mixing resulting in Fermi resonance transitions in the 2D IR spectra. In contrast, the CO vibrations show multiple peaks as a result of conformational heterogeneity associated with internal rotation about the hydrogen bond. For both modes, the population relaxation rates tend to be quite short, ∼1 ps or less. Having characterized the vibrational spectroscopy of these spectator vibrations, we conclude that they are quite sensitive to details of the hydrogen bonding interactions though their utility is limited by spectral complexity and short population lifetimes.