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

Melting of the acetic acid crystal was simulated by MPT molecular dynamics calculations, starting from an ordered crystal box and increasing the simul ation temperature until the transition to the melt was induced. The potenti al energy was calculated using the OPLS all atom force field. The time evol ution of some key geometrical quantities indicates that premelting events s tart with single-molecule rotational flips and hydrogen-bond breaking, foll owed by cross-linking and formation of cyclic dimers. Melting follows as de nsity further decreases and translational and rotational diffusion sets in. Thus, localized breaking of stronger intermolecular bonds precedes the ove rcoming of weaker (dispersive) but highly cooperative intermolecular bonds. The melting of a crystal with 2.5% vacancies follows the same path, only a t a lower temperature, which happens to coincide with the actual melting te mperature of the acetic acid crystal. Surface melting is not am indispensab le mechanism, and a small defect concentration may lead to a state intermed iate between crystal and melt with partial translational structuring and li ttle or no ordered hydrogen-bonding patterns. As the intermolecular barrier to methyl rotation is negligible, at ordinary temperatures the C-H...O "hy drogen bond" turns in fact into a just scarcely effective oxygen-methyl att ractive bias. (C) 1999 Elsevier Science B.V. All rights reserved.