The mark of hydrogen-bond interactions in liquid methanol dynamics

Methanol is a hydrogen-bonded liquid of enormous importance in pure and applied physics and chemistry, and is the object of innumerable studies. Yet, for a long time the fundamental aspects of its molecular dynamics have been known only to a very poor extent. However, our detailed analysis of molecular dynamics simulation results for the system at two different temperatures (300 K and 200 K), has shown that methanol exhibits a very rich dynamical behavior, which classifies it as a fluid partly similar to the most important hydrogen-bonded liquid: the water. Besides the normal viscoelastic behaviour typical of a large variety of liquids, three more excitations were detected in the molecular centre-of-mass dynamic structure factor. Following the explanation proposed in the pioneering works about liquid water, the low-frequency one has been interpreted as the manifestation of a coupling between longitudinal and transverse dynamics. This fact seems to be a quite common feature of molecular liquids and, as recently shown, also of liquid metals. The other two excitations appear to be of a completely different nature: they have an optic-like character and are almost non dispersive. The analysis of simulations carried out by "switching off" the hydrogen-bond interactions allowed us to unambiguously determine the nature of such excitations and to deepen the role of hydrogen bond in liquid dynamics.