The universal vibrational dynamics of water bound to tertiary amines: more than just Fermi resonance

Amines with three alkyl substituents are shown to be strongly microsolvated by water molecules, unless the steric hindrance of the alkyl groups overcompensates the increase in basicity of the N atom by alkylation. The hydrogen bond interaction of the first water molecule is so strong that the softened OH vibration shares its intensity with up to three largely dark states involving quanta of intramolecular bending or stretching and intermolecular stretching vibration. A combination of FTIR, Raman, isotope and chemical substitution spectroscopy in supersonic jet expansions establishes the existence, character and extent of the underlying anharmonic coupling. The observed resonance pattern is remarkably systematic and allows to extract physically plausible, effective normal mode coupling constants which are relevant for the initial energy flow out of the excited OH oscillator. A remaining ambiguity in the coupling pattern for the weakest transition invites detailed anharmonic quantum dynamics studies, but it still allows for robust deperturbed positions of the uncoupled oscillators for 8 amine monohydrates, which are valuable as experimental benchmarks for databases and for the training phase of theory blind challenges on microhydration. The more isolated hydrogen-bonded OH stretching vibration of a second water molecule is also assigned to widen the scope of a future theory challenge addressing the wavenumber of hydrogen-bonded OH groups. Such blind challenges thus remain accessible not only to fully anharmonic, but also to scaled harmonic and machine learning approaches which may try to average over the anharmonic details.