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  • Title: The richest collection of tautomeric polymorphs: the case of 2-thiobarbituric acid.
    Author: Chierotti MR, Ferrero L, Garino N, Gobetto R, Pellegrino L, Braga D, Grepioni F, Maini L.
    Journal: Chemistry; 2010 Apr 12; 16(14):4347-58. PubMed ID: 20183832.
    Abstract:
    Five new polymorphs and one hydrated form of 2-thiobarbituric acid have been isolated and characterised by solid-state methods. In both the crystalline form II and in the hydrate form, the 2-thiobarbituric molecules are present in the enol form, whereas only the keto isomer is present in crystalline forms I (reported in 1967 by Calas and Martinex), III, V and VI. In form IV, on the other hand, a 50:50 ordered mixture of enol/keto molecules is present. All new forms have been characterised by single-crystal X-ray diffraction, 1D and 2D ((1)H, (13)C, and (15)N) solid-state NMR spectroscopy, Raman spectroscopy and X-ray powder diffraction at variable temperature. It has been possible to induce keto-enol conversion between the forms by mechanical methods. The role of hydrogen-bond interactions in determining the relative stability of the polymorphs and as a driving force in the conversions has been ascertained. To the best of the authors' knowledge, the 2-thiobarbituric family of crystal forms represents the richest collection of examples of tautomeric polymorphism so far reported in the literature.
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