Highly localized water librational transitions as sensitive far-infrared spectroscopic observables for hydrogen-bonded ether and amine monohydrates

A combined mid-IR and Raman jet investigation of strongly hydrogen-bonded monohydrates of tertiary amines [Lwin et al., Phys. Chem. Chem. Phys. 27, 5808-5820 (2025)] recently reported strong anharmonic vibrational resonances. These resonances involve the spectrally redshifted and strongly IR-active hydrogen-bonded O-H stretching fundamental (OHb) of H2O and several overtone and combination states involving quanta of both intramolecular and intermolecular modes, blurring the suitability of the conventional OHb fundamentals as reliable empirical indicators of the intermolecular hydrogen bond strengths. It is shown that these universal anharmonic vibrational resonances are also observed for hydrogen-bonded monohydrates of both primary and secondary amines. The present work demonstrates that isolated vibrational transitions associated with large-amplitude intermolecular out-of-plane H2O librational motion prove to be alternative robust resonance-free far-IR spectroscopic observables that correlate strongly with intermolecular hydrogen bond strength across an extended series of ether and amine monohydrates. The observed H2O librational spectral signatures are compared with a comprehensive local energy decomposition analysis of the total interaction energies predicted at the DLPNO-CCSD(T)/aug-cc-pV5Z level. The combined results show that the librational transition energies are strongly correlated with the total non-dispersive energy contributions to the directional local hydrogen bond interactions across both classes of monohydrates and to a lesser extent correlated with the additional non-directional, long-range London dispersion forces introduced by bulky alkyl substituents.