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Incommensurately modulated structure of the halogen bonded cocrystal of urotropine and octafluoro-1, 4-diiodobutane

Aperiodic crystals are crystals which do not possess 3D translation symmetry. An incommensurately modulated structure can be considered a perturbed periodic crystal. Urotropine (hexamethylenetetramine) is a quasi-spherical molecule that is known for forming co-crystals of modulated superstructures with long linear coformers such as aliphatic diacids, of wich cocrystal with azelaic acid was found to have an invommensurately modulated superstructure. Here we present a halogen bonded cocrystal of urotropine and a linear halogen bond donor, octafluoro-1, 4-diiodobutane (ofib). The cocrystal was prepared by means of LAG (liquid-assisted grinding) and from solution. X-ray diffraction at room temperature has shown that the structure of the cocrystal is incommensurately modulated with a monoclinic sublattice (a = 5.976 Å, b = 24.82 Å, c = 6.015 Å, β = 105.82°, Z = 2) and the modulation vector q(298 K) = (0.0808, 0, 0.2063). Repeated measurement at 150K has shown only minor changes in the sublattice parameters (a = 5.911 Å, b = 24.21 Å, c = 5.947 Å, β = 103.94°, Z = 2) and a considerable change in the modulation vector q(150 K) = (0.0824, 0, 0.1907). Interestingly, it was possible to solve and refine both the room temperature and low temperature structures as periodic approximates – the room temperature one can be modeled as a centrosymmetric monoclinic structure with Z = 20, and the low temperature as an acentric monoclinic one with Z = 22 (Figure 1). Subsequent measurements oft he unit cell parameters and the modulation vector have shown that the change occurs relatively abruptly between 0 °C and the room temperature. There was, however, no corresponding thermal event discernable in the DSC curve bellow the room temperature, but a reversible phase transition at 35-40 °C has been detected, which has prompted us to remeasure the crystal at 325 K. The crystal structure of the high temperature phase was found to be a non-modulated orthorhombic structure with severe disorder in difluoromethylene groups of the ofib molecules.

cocrystals ; incommensurate crystal ; octafluoro-1, 4-diiodobutane

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