Frustration of H-Bonding and Frustrated Packings in a Hexamorphic Crystal System with Reversible Crystal–Crystal Transitions
Emmanuele Parisi, Emanuela Santagata, Elena Simone, Fabio Borbone, Roberto Centore
Abstract
High Resolution Image Download MS PowerPoint Slide The study of transitions between polymorphic phases is a less investigated chapter of the widely studied book of polymorphism. In this paper, we discuss the phase behavior of a new compound that has been rationally designed to show frustration of H-bonds for the strong amide N–H donor, which cannot be involved in H-bonding nor in van der Waals interactions. The compound ( ImB ) is a showcase of almost all possible cases of transitions between polymorphs [monotropic/enantiotropic, fast/slow, diffusive/displacive, and single-crystal-to-single-crystal (SCSC)] and of relation between polymorphs with different Z ′. Six crystal phases (I, II, III, IV, V, and VI) were identified for it with five crystal–crystal transitions. Two transitions are reversible/SCSC/fast. Of the three monotropic transitions, all non-SCSC, one is slow, and the others are fast. Of the two enantiotropic SCSC transitions, one does not exhibit undercooling, while the other shows strong undercooling. Phase III, with Z ′ = 6, is stable at room temperature between phase II ( Z ′ = 1), stable at high temperature, and phase IV ( Z ′ = 2), stable at low temperature. All six polymorphs are based on the same O–H···O═C H-bonding synthon, which produces infinite chains in five polymorphs and ring tetramers in one. The sequence of reversible SCSC transitions IV ⇆ III ⇆ II involves a remarkable ping pong of the symmetry rules by which H-bonded chains are built. Based on all of this, a possible roadmap for prediction of SCSC transitions in crystals is shortly outlined.