Unanticipated guest motion during a phase transition in a ferroelastic inclusion compound.

Urea inclusion compounds (UICs) have been used as tools to understand ferroelastic domain switching and molecular recognition during crystal growth. Although the vast majority of UICs contain helical arrangements of host H-bonds, those containing guests with the formula X(CH2)6Y (X, Y = Br, Cl, CN, NC) adopt an alternative P21/n packing mode in which the host molecules exist as stacked loops of urea hexamers. Such structures may be further separated into two classes, ones distorted away from hexagonal symmetry along [100] (Br(CH2)6Br, Br(CH2)6Cl, and Cl(CH2)6Cl) and those distorted along [001] (e.g. NC(CH2)6CN). In each of these systems, guests exist as equilibrium mixtures of gauche conformers whose populations control the direction and magnitude of the observed distortion. Such UICs are potentially ferroelastic, but the n-glide requires that domains are not related by a simple rotation−translation mechanism as in the helical systems. Ferroelastic (degenerate) domain reorientation would necessitate a lar...