Self-aggregation and microhydration mechanisms of monoethanolamine: Far-infrared identification of large-amplitude hydrogen bond libration.

This article focuses on the self-aggregation and microhydration mechanisms of monoethanolamine (MEA) investigated through low-temperature far-infrared cluster spectroscopy in neon "quantum" matrices at 4 K, supported by high-level quantum chemical calculations. The study confirms that MEA homodimers adopt a compact "head-to-head" cyclic structure stabilized by two intermolecular OH⋯N hydrogen bonds formed at the expense of intramolecular OH⋯N bonds. For the first time, distinct far-infrared transitions linked to large-amplitude intramolecular OH torsional motion and intermolecular H2O librational motion in the MEA monohydrate are assigned, revealing a metastable conformation where water acts as an OH⋯O hydrogen bond donor to MEA's hydroxy group rather than the amino group, thereby strengthening the intramolecular OH⋯N bond via cooperativity. This microhydration behavior contrasts with previous theoretical predictions favoring more compact cyclic structures with water donating to the amino group, suggesting kinetic trapping in the cryogenic neon environment. The findings have implications for understanding hydrogen bond rearrangements in biomolecular recognition and encourage further vibrational spectroscopic studies under jet-cooled conditions to clarify MEA monohydrate conformations.