Hydrogen-Bond Network Rearrangements Induced by Selective O–H Stretching Excitation in Neon-Embedded Monoethanolamine Monohydrate

The present work experimentally demonstrates that the selective excitation of the hydrogen-bonded O–H stretching fundamental (OHb) of the H2O subunit induces a concerted hydrogen-bond network rearrangement in the monoethanolamine (MEA) monohydrate embedded in neon matrices. The initially formed global potential energy minimum conformation of the MEA monohydrate adopts a cyclic cooperatively hydrogen-bonded “insertion” configuration, in which the H2O molecule simultaneously acts as a hydrogen-bond donor to the amino group and as an acceptor for the hydroxy group of MEA. The detection of this energetically favored “insertion″ conformation is based on unambiguous assignments of characteristic intramolecular vibrational transitions associated with the red-shifted O–H···O hydrogen-bonded O–H stretching motion (3507 cm–1) and the blue-shifted NH2 wagging motion (960 cm–1) of MEA together with the intermolecular large-amplitude in-plane librational (hindered overall c-type rotational) motion of H2O (452 cm–1). In addition, a strong anharmonic vibrational resonance involving the highly red-shifted O–H···N hydrogen-bonded OHb fundamental state (3262 cm–1), the HOH bending overtone state (3173 cm–1) and a ternary inter-intramolecular combination state (3348 cm–1) involving the intermolecular stretching level is rationalized by isotopic H/D and 16O/18O substitutions and is consistent with recently recognized anharmonic vibrational resonances in amine monohydrates. Upon selective infrared irradiation enabled by complementary optical filters, the excitation within this strongly resonance-coupled O–H stretching vibrational triad is shown to trigger a conformational transformation into a metastable open “addition″ configuration, which is subsequently trapped in the neon matrix cages consistent with previous findings [J. Chem. Phys., 2024, 161, 154301]. The experimental observations suggest that vibrational energy deposited in the intramolecular O–H stretching coordinate is redistributed (IVR) into one or more large-amplitude intermolecular vibrational coordinates, enabling barrier crossing on the underlying intermolecular potential energy surface prior to energy dissipation into the neon host.