Ultrafast vibrational dynamics and anharmonic couplings of hydrogen-bonded dimers in solution

Hydrogen-bonded molecular pairs and dimers represent important structural motifs in nature. They display highly complex vibrational spectra reflecting their equilibrium structure and structural dynamics. Femtosecond nonlinear vibrational spectroscopy provides direct insight into vibrational dynamics and the underlying intra- and intermolecular couplings. In this article, we discuss recent experimental and theoretical results on the ultrafast vibrational response of carboxylic acid and 7-azaindole dimers. Distinct femto- and picosecond time scales are found for the vibrational dephasing of OH/NH stretching excitations and low-frequency hydrogen bond motions, respectively. Anharmonic couplings giving rise to Fermi resonances are determined quantitatively and separated from couplings between stretching and low-frequency modes. In 7-azaindole dimers, the prominent role of Fermi resonances for the NH stretching lifetime is demonstrated.