Structural aspects of the β-polymorph of (E)-4-formylcinnamic acid: Structure determination directly from powder diffraction data and elucidation of structural disorder from solid-state NMR

Among the derivatives of (E)-cinnamic acid for which the solid-state photochemical properties have been studied, (E)4-formylcinnamic acid (1) has already received much attention. Given the inability to prepare single crystals of the β-polymorph of 1 that are of suitable size and quality for structural characterization by single-crystal X-ray diffraction, the structure of this material was determined directly from powder X-ray-diffraction data by means of the genetic-algorithm technique for structure solution, followed by Rietveld refinement. High-resolution solid-state ¹³C-NMR was also applied to elucidate details of structural disorder concerning the orientation of the formyl group, and provided independent support for the disorder model established form the Rietveld refinement. The reported structure establishes that the β-phase of 1 is not structurally anomalous among photoreactive (E)-cinnamic acid crystals, and finally resolves a long-standing controversy concerning the structural properties of this material.