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.