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The Richest Collection of Tautomeric Polymorphs: The Case of 2‐Thiobarbituric Acid
Author(s) -
Chierotti Michele R.,
Ferrero Luca,
Garino Nadia,
Gobetto Roberto,
Pellegrino Luca,
Braga Dario,
Grepioni Fabrizia,
Maini Lucia
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200902485
Subject(s) - tautomer , chemistry , enol , thiobarbituric acid , molecule , polymorphism (computer science) , hydrate , crystallography , hydrogen bond , raman spectroscopy , powder diffraction , diffraction , keto–enol tautomerism , stereochemistry , organic chemistry , catalysis , biochemistry , physics , antioxidant , genotype , optics , gene , lipid peroxidation
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.