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 (1H, 13C, and 15N) 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.

The Richest Collection of Tautomeric Polymorphs: The Case of 2-Thiobarbituric Acid

CHIEROTTI, Michele Remo;GOBETTO, Roberto;
2010-01-01

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 (1H, 13C, and 15N) 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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http://onlinelibrary.wiley.com/doi/10.1002/chem.200902485/abstract;jsessionid=16EAE9DEC608F4A3BAC44710F4EC33D2.d02t02
hydrogen bonds; polymorphism; solid-state NMR spectroscopy; solid-state structures; tautomerism; thiobarbituric acid
Michele R. Chierotti; Luca Ferrero; Nadia Garino; Roberto Gobetto; Luca Pellegrino; Dario Braga; Fabrizia Grepioni; Lucia Maini
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/76538
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