A molecular dynamics view of some kinetic and structural aspects of melting in the acetic acid crystal

Abstract

Melting of the acetic acid crystal was simulated by NPT molecular dynamics calculations, starting from an ordered crystal box and increasing the simulation temperature until the transition to the melt was induced. The potential energy was calculated using the OPLS all atom force field. The time evolution of some key geometrical quantities indicates that premelting events start with single-molecule rotational flips and hydrogen-bond breaking, followed by cross-linking and formation of cyclic dimers. Melting follows as density further decreases and translational and rotational diffusion sets in. Thus, localized breaking of stronger intermolecular bonds precedes the overcoming of weaker (dispersive) but highly cooperative intermolecular bonds. The melting of a crystal with 2.5% vacancies follows the same path, only at a lower temperature, which happens to coincide with the actual melting temperature of the acetic acid crystal. Surface melting is not an indispensable mechanism, and a small defect concentration may lead to a state intermediate between crystal and melt with partial translational structuring and little or no ordered hydrogen-bonding patterns. As the intermolecular barrier to methyl rotation is negligible, at ordinary temperatures the C–H…O “hydrogen bond” turns in fact into a just scarcely effective oxygen–methyl attractive bias.