Revealing the Microhydration Mechanisms of α-Hydroxy Carboxylic Acids: Identification of Large-Amplitude Torsional and Librational Motion

The molecular recognition mechanisms associated with the self-aggregation and microhydration processes of two different bifunctional α-hydroxy carboxylic acids, prototypical glycolic acid [CH2OHCOOH] and the doubly methylated variant 2-hydroxyiso-butyric acid [(CH3)2C(OH)COOH], have been investigated by low-temperature mid- and far-IR cluster spectroscopy of doped low polarizability neon matrices at 4 K complemented by systematic theoretical conformational sampling refined with high-level electronic structure theory at the DLPNO-CCSD(T)/aug-cc-pV5Z level. Several mid-IR perturbed vibrational fundamentals together with specific far-IR vibrational fundamentals associated with large-amplitude, anharmonic hindered intramolecular torsional and intermolecular librational (hindered overall rotational) motion of the α-hydroxy carboxylic acid monohydrates are unambiguously assigned based on a combination of mixing ratio dependencies, thermal annealing and the use of isotopically enriched H218O and D2O samples. These direct experimental spectroscopic probes of the formed intermolecular hydrogen bond motifs validate high-level quantum chemical predictions that the global intermolecular potential energy minima configurations involve six-membered cyclic cooperatively hydrogen-bonded C=O· · · H-Owater · · · H-Oacid "insertion" sequences in the monohydrate species of both α-hydroxy carboxylic acid analogues.