Gelbrich Thomas, Braun Doris E, Oberparleiter Stefan, Schottenberger Herwig, Griesser Ulrich J
Institute of Pharmacy, University of Innsbruck, Innrain 52c, 6020 Innsbruck, Austria.
Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, Innrain 80, 6020 Innsbruck, Austria.
Crystals (Basel). 2016 Apr 22;6(4):47. doi: 10.3390/cryst6040047.
The crystal structure of the methanol hemisolvate of 5,5-dibromobarbituric acid () displays an H-bonded layer structure which is based on N-H⋯O=C, N-H⋯O(MeOH) and (MeOH)O-H⋯O interactions. The barbiturate molecules form an H-bonded substructure which has the topology. 5,5'-Methanediylbis(5-bromobarbituric acid) , obtained from a solution of 5,5-dibromobarbituric acid in nitromethane, displays a N-H⋯O=C bonded framework of the type. The conformation of the pyridmidine ring and the lengths of the ring substituent bonds C5-X and C5-X' in crystal forms of 5,5-dibromobarbituric acid and three closely related analogues (X = X' = Br, Cl, F, Me) have been investigated. In each case, a conformation close to a C5-endo envelope is correlated with a significant lengthening of the axial C5-X' in comparison to the equatorial C5-X bond. Isolated molecule geometry optimizations at different levels of theory confirm that the C5-endo envelope is the global conformational energy minimum of 5,5-dihalogenbarbituric acids. The relative lengthening of the axial bond is therefore interpreted as an inherent feature of the preferred envelope conformation of the pyrimidine ring, which minimizes repulsive interactions between the axial substituent and pyrimidine ring atoms.
5,5-二溴巴比妥酸甲醇半溶剂化物的晶体结构显示出一种基于N-H⋯O=C、N-H⋯O(甲醇)和(甲醇)O-H⋯O相互作用的氢键层状结构。巴比妥酸盐分子形成了具有 拓扑结构的氢键子结构。由5,5-二溴巴比妥酸在硝基甲烷中的溶液制得的5,5'-亚甲基双(5-溴巴比妥酸)显示出 型的N-H⋯O=C键合骨架。对5,5-二溴巴比妥酸及其三个密切相关类似物(X = X' = Br、Cl、F、Me)晶体形式中嘧啶环的构象以及环取代基键C5-X和C5-X'的长度进行了研究。在每种情况下，与赤道C5-X键相比，接近C5-内型信封式构象与轴向C5-X'的显著延长相关。在不同理论水平下进行的孤立分子几何结构优化证实，C5-内型信封式构象是5,5-二卤代巴比妥酸的全局构象能量最小值。因此，轴向键的相对延长被解释为嘧啶环优选信封式构象的固有特征，它使轴向取代基与嘧啶环原子之间的排斥相互作用最小化。
