Applying Molecular Dynamics for the Search of New Frank-Kasper Phases of 4-aminopyridinium Chloride.

In a previous crystallographic study on 4-aminopyridinium chloride, Frank-Kasper (FK) phases were serendipitously produced for the first time in a small organic molecule, showing that a simple organic salt can crystallise as hydrate phases of extraordinary complexity. These results raised questions on the mechanisms of formation of such phases, for instance, whether they arise from a specific pre-organization in the liquid state, probably following a non-classical nucleation path and, most importantly, opened discussions on whether this family of structures can be extended. Here we report the results from a classical molecular dynamics investigation aimed to give more insights on this matter. In particular, we modelled the thermal behaviour of an arbitrary FK phase of 4-aminopyridinium chloride prior the melting and after cooling the molten phase, exploring the mechanisms of dehydration of these phases and nucleation from the melt. The results, also confirmed by experiments, suggest that the dehydration of these phases can occur without dramatic changes in the crystal packing. This is also confirmed by simulating a stable anhydrous FK phase of 4-aminopyridinium chloride. Most importantly, molecular dynamics simulated by cooling the melt of this anhydrous phase, showed the formation of mainly two types of aggregates identical to those observed in the experimental crystal structures. These results confirm that other unknown FK structures might be obtained for our system, either from the melt or by dehydration of the known phases. Moreover, they provide further evidence that their crystallisation follows a non-classical path with a probable formation of two types of highly symmetric clusters that assemble into the solid-state only when the relative ratio of these reaches a threshold.