title: The richest collection of tautomeric polymorphs: the case of 2-thiobarbituric acid.
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.
authors.affiliation: Dipartimento di Chimica I.F.M. Università di Torino, V. Giuria 7, 10125 Torino, Italy. michele.chierotti@unito.it
authors.forename: Michele R
authors.initials: MR
authors.lastname: Chierotti
authors.forename: Luca
authors.initials: L
authors.lastname: Ferrero
authors.forename: Nadia
authors.initials: N
authors.lastname: Garino
authors.forename: Roberto
authors.initials: R
authors.lastname: Gobetto
authors.forename: Luca
authors.initials: L
authors.lastname: Pellegrino
authors.forename: Dario
authors.initials: D
authors.lastname: Braga
authors.forename: Fabrizia
authors.initials: F
authors.lastname: Grepioni
authors.forename: Lucia
authors.initials: L
authors.lastname: Maini