Dipartimento di Chimica I.F.M. Università di Torino, V. Giuria 7, 10125 Torino, Italy.
Chemistry. 2010 Apr 12;16(14):4347-58. doi: 10.1002/chem.200902485.
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.
五种新的 2-硫代巴比妥酸多晶型物和一水合物已通过固态方法分离和表征。在晶型 II 和水合物形式中，2-硫代巴比妥酸分子以烯醇形式存在，而在晶型 I（1967 年由 Calas 和 Martinex 报道）、III、V 和 VI 中仅存在酮异构体。另一方面，在晶型 IV 中，存在 50:50 有序的烯醇/酮分子混合物。所有新形式均通过单晶 X 射线衍射、1D 和 2D（（1）H、（13）C 和（15）N）固态 NMR 光谱、拉曼光谱和变温 X 射线粉末衍射进行了表征。通过机械方法可以诱导形式之间的酮-烯醇互变异构。已经确定氢键相互作用在确定多晶型物的相对稳定性以及在转化中的驱动力方面的作用。据作者所知，2-硫代巴比妥酸晶型家族代表了迄今为止文献中报道的互变异构多晶型物最丰富的例子。
