Anharmonic Coupling in a Strong Intramolecular H-Bond System: Contributions to Static and Time-Resolved Vibrational...

Anharmonic coupling between the excited modes and the collective low-frequency modes is a key to controlling energy flow in molecules via vibrational mode-specific manipulation, a long-time goal of chemistry. Hydrogen-bonded (H-bond) systems are special examples, as extra channels of anharmonic coupling can be exerted through H-bonds, and these H-bond associated anharmonic couplings play a key role in energy, proton, and electron transfers. Here, we focus on an intramolecularly hydrogen-bonded compound, 10-hydroxybenzo[h]quinoline (HBQ), to understand the origin of the spectral progression contained in the ν_OH feature of the mid-IR absorption spectrum. Static and time-resolved vibrational spectroscopy along with anharmonic ab initio frequency analysis reveals that the distinct finger-like spectral progression is a manifestation of anharmonic coupling between the OH stretch and four low-frequency, H-bond modulating degrees of freedom. This study provides a model to understand how low-frequency modes influence OH modes with the aid of an H-bond.