Macholl Sven, Lentz Dieter, Börner Frank, Buntkowsky Gerd
Institut für Chemie und Biochemie, Freie Universität, Berlin Fabeckstrasse 34-36/Takustrasse 3, 14195 Berlin, Germany.
Chemistry. 2007;13(21):6139-49. doi: 10.1002/chem.200601843.
The molecular configuration and crystal structure of solid polycrystalline N,N''-diacetylbiuret (DAB), a potential nitrogen-rich fertilizer, have been analyzed by a combination of solid- and liquid-state NMR spectroscopy, X-ray diffraction, and DFT calculations. Initially a pure NMR study ("NMR crystallography") was performed as available single crystals of DAB were not suitable for X-ray diffraction. Solid-state 13C NMR spectra revealed the unexpected existence of two polymorphic modifications (alpha- and beta-DAB) obtained from different chemical procedures. Several NMR techniques were applied for a thorough characterization of the molecular system, revealing chemical shift anisotropy (CSA) tensors of selected nuclei in the solid state, chemical shifts in the liquid state, and molecular dynamics in the solid state. Dynamic NMR spectroscopy of DAB in solution revealed exchange between two different configurations, which raised the question, is there a correlation between the two different configurations found in solution and the two polymorphic modifications found in the solid state? By using this knowledge, a new crystallization protocol was devised which led to the growth of single crystals suitable for X-ray diffraction. The X-ray data showed that the same symmetric configuration is present in both polymorphic modifications, but the packing patterns in the crystals are different. In both cases hydrogen bonds lead to the formation of planes of DAB molecules. Additional symmetry elements, a two-fold screw in the case of alpha-DAB and a c-glide plane in the case of beta-DAB, lead to a more symmetric (alpha-DAB) or asymmetric (beta-DAB) intermolecular hydrogen-bonding pattern for each molecule.
通过固态和液态核磁共振光谱、X射线衍射以及密度泛函理论计算相结合的方法，对潜在的富氮肥料固态多晶型N,N''-二乙酰基缩二脲（DAB）的分子构型和晶体结构进行了分析。由于现有的DAB单晶不适用于X射线衍射，最初进行了一项纯粹的核磁共振研究（“核磁共振晶体学”）。固态13C核磁共振光谱揭示了通过不同化学程序获得的两种多晶型变体（α-DAB和β-DAB）的意外存在。应用了几种核磁共振技术对分子体系进行全面表征，揭示了固态中选定原子核的化学位移各向异性（CSA）张量、液态中的化学位移以及固态中的分子动力学。DAB在溶液中的动态核磁共振光谱揭示了两种不同构型之间的交换，这就引出了一个问题，即在溶液中发现的两种不同构型与在固态中发现的两种多晶型变体之间是否存在关联？利用这一知识，设计了一种新的结晶方案，从而生长出适用于X射线衍射的单晶。X射线数据表明，两种多晶型变体中都存在相同的对称构型，但晶体中的堆积模式不同。在这两种情况下，氢键导致形成DAB分子平面。额外的对称元素，α-DAB中的二重螺旋和β-DAB中的c滑移面，导致每个分子形成更对称（α-DAB）或不对称（β-DAB）的分子间氢键模式。
