Hydrogen-bond rearrangements in the self-association and microhydration of ethylene glycol: A combined infrared and theoretical conformational sampling investigation

The hydrogen-bond rearrangements involved in the self-association and microhydration of the simplest vicinal diol, ethylene glycol (EG), have been explored by low-temperature mid- and far-IR cluster spectroscopy in doped neon "quantum" matrices at 4 K complemented by high-level quantum chemical conformational sampling. In addition to the reproduction of previous mid-IR jet assignments of the highly concerted hydrogen-bonded O-H stretching transitions, new distinct far-IR observations have been unambiguously attributed to transitions associated with concerted and highly anharmonic large-amplitude hindered OH (OD) torsional motion of (EG)2 and (EG-d2)2, respectively. These observations confirm the formation of a highly S4 symmetric global intermolecular potential energy minimum in the cryogenic neon environment associated with a very compact intermolecular hydrogen-bonded cyclic structure. In this conformation of (EG)2, the two intramolecular hydrogen bonds are rearranged into four new identical strongly cooperative intermolecular hydrogen bonds upon complexation as previously observed in supersonic jets. By means of selective complexation between EG and isotopically enriched H218O and D2O samples, the IR-active intramolecular hydrogen-bonded O-H stretching transitions furthermore are assigned unambiguously for the EG monohydrate. These spectroscopic observations reveal a cyclic cooperatively hydrogen-bonded structure, where the monomeric intramolecular hydrogen bond of EG is disrupted upon microhydration. In this detected conformation of the EG monohydrate, one hydroxy group acts as an intermolecular hydrogen bond donor to the H2O subunit and the vicinal hydroxy group as an intermolecular hydrogen bond acceptor to the H2O subunit in the cryogenic neon environment. The experimental findings are supported by quantum chemical analysis of the conformational potential energy landscape at the CCSD(T)-F12/cc-pVQZ-F12 level.