The TACO Puzzle: A Phase-Transition Mystery Revisited

The organic salt (5-methyl-1-thia-5-azacyclo-octane-1-oxide) perchlorate (TACO) is known to undergo a single-crystal-to-single-crystal phase transition in the 276-298 K T range without a change in the external shape of the sample. Despite extensive computational and experimental investigations, no safe conclusions about the transition mechanism could be drawn till now. The two packing patterns are very similar, and symmetry is conserved, apart from an interchange of cell axes from P21/c (α-TACO, low-T) to P21/a (β-TACO, high-T). Yet, the phase transition implies a significant conformational rearrangement, coupled with ∼180°-wide rotations, of 1/2 of the cations, in conjunction with reorientation of the anions. Here, we analyze the crystal packing of the two phases in terms of pairwise molecule-molecule interaction energies, as derived from the PIXEL approach. Rigid-body molecular reorientations are simulated by solid-state Monte Carlo calculations, while the likelihood of conformational rearrangements is estimated through gas-phase density functional theory M06/6-311G(p,d) simulations. We demonstrate that rotational motion of the cations is not hampered by substantial energetic barriers, while the ring flip can be described as a two-step process with a main kinetic barrier of ∼45 kJ·mol-1, which might explain the metastable behavior of the β phase at low T. A possible mechanism of the phase transition is proposed, accounting for the present computational evidences in the context of the former experimental findings.