The effect of hydrogen bonding on torsional dynamics: a combined far-infrared jet and matrix isolation study of methanol dimer.

The effect of strong intermolecular hydrogen bonding on torsional degrees of freedom is investigated by far-infrared absorption spectroscopy for different methanol dimer isotopologues isolated in supersonic jet expansions or embedded in inert neon matrices at low temperatures. For the vacuum-isolated and Ne-embedded methanol dimer, the hydrogen bond OH librational mode of the donor subunit is finally observed at ~560 cm(-1), blue-shifted by more than 300 cm(-1) relative to the OH torsional fundamental of the free methanol monomer. The OH torsional mode of the acceptor embedded in neon is observed at ~286 cm(-1). The experimental findings are held against harmonic predictions from local coupled-cluster methods with single and double excitations and a perturbative treatment of triple excitations [LCCSD(T)] and anharmonic. VPT2 corrections at canonical MP2 and density functional theory (DFT) levels in order to quantify the contribution of vibrational anharmonicity for this important class of intermolecular hydrogen bond vibrational motion.