The collective dynamics of a hydrogen-bonded liquid: methanol

Methanol is a hydrogen-bonded liquid of enormous importance in pure and applied physics and chemistry, and is the object of innumerable studies. Yet, the fundamental aspects of its molecular dynamics are still known only to a very poor extent. The study of the collective dynamics is hindered by the weakness of the acoustic excitations, which has led to the wrong conclusion that sound modes propagate only in a surprisingly narrow range of small wave vector values. Combined molecular dynamics simulations and neutron Brillouin scattering measurements reveal, however, quite a different situation. Methanol is shown, for the first time, to feature the normal viscoelastic behaviour typical of a large variety of liquids, including the arrest of acoustic propagation near the peak of the static structure factor. Besides this, however, two more excitations are detected in the molecular centre-of-mass dynamics structure factor, at frequencies, respectively, one lower and one higher than the acoustic frequency, both with negligible dispersion. The rich translational dynamics revealed by this study classifies methanol as a fluid partly similar to the most important hydrogen-bonded liquid: water.