The crystal structure of the high-temperature phase (HT-form or form III) of barbituric acid has been solved from X-ray powder diffraction data. It was found to be monoclinic, space group C2/c, a = 8.5302(3) Å, b = 6.8167(3) Å, c = 9.3304(4) Å, β = 89.865(2)°, volume = 542.54(4) Å3 at T = 236(1) °C. The crystal structure is closely related to the also monoclinic structure of form II. A combined Rietveld refinement of X-ray and neutron powder diffraction data revealed fine details of the disorder aspects of the structure. 1H variable temperature and 13C MAS solid-state NMR confirmed those and proved the dynamic nature of the rotational disorder of the molecules. Energy optimizations of ordered subgroup structures with dispersion-corrected DFT gave insights into the underlying reason for the disorder. The reorientation of the unique 2-fold symmetry axis at the II → HT phase transition derives from both the anisotropy of the hydrogen bonds and the resulting anisotropy of the thermal rotational disorder. The unusual thermal expansion behavior of form II also finds its explanation in the incipient disorder.
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