Thermodynamic and structural relationships between the two polymorphs of 1,3-dimethylurea.

IF 1.9 3区 化学 Acta Crystallographica Section B-structural Science Pub Date : 2013-02-01 Epub Date: 2012-12-20 DOI:10.1107/S0108768112049324
Christian Näther, Cindy Döring, Inke Jess, Peter G Jones, Christina Taouss
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引用次数: 15

Abstract

The title compound exists as polymorph (I), Fdd2 with Z = 8 [Pérez-Folch et al. (1997). J. Chem. Cryst. 27, 367-369; Marsh (2004). Acta Cryst. B60, 252-253], and as polymorph (II), P2(1)2(1)2 with Z = 2 [Martins et al. (2009). J. Phys. Chem. A, 113, 5998-6003]. We have redetermined both structures at somewhat lower temperatures [(I) at 180 K rather than room temperature; (II) at 100 K rather than 150 K]. For polymorph (I) the space group Fdd2 is confirmed rather than the original choice of Cc. The molecular structures of both polymorphs are essentially identical, with exact crystallographic twofold symmetry, approximate C(2v) symmetry, and a trans orientation of the H-N-C=O moiety. In both polymorphs the molecules associate into chains of rings with graph set C(4)[R(2)(1)(6)] via bifurcated hydrogen-bond systems C(N-H)(2)···O=C. In the polar structure (I) the chains are necessarily all parallel, whereas in (II) equal numbers of parallel and antiparallel chains are present. Further physical investigations [differential scanning calorimetry (DSC), powder investigations, solvent-induced phase conversions] were undertaken: these showed: (i) that the commercially available compound consists predominantly of polymorph (II), which on heating transforms into polymorph (I) by an endothermic reaction, so that both polymorphs are related by enantiotropism; (ii) that polymorph (I) represents the more stable modification at room temperature, where polymorph (II) is metastable, with the thermodynamic transition temperature lying somewhere between 253 K and room temperature. An apparent third polymorph, consisting of fibrous needles, was shown by powder diffraction to consist of a mixture of polymorphs (I) and (II).

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1,3-二甲基脲两种多晶型之间的热力学和结构关系。
标题化合物以多晶体形式存在(I), Fdd2, Z = 8 [p rez- folch et al.(1997)]。j .化学。中国医学杂志27,367-369;沼泽(2004)。Acta结晶。B60, 252-253],以及作为polymorph (II), P2(1)2(1)2 with Z = 2 [Martins et al.(2009)]。期刊。化学。[j].中国科学:自然科学版,2016,33(2):1 -6。我们在稍微低一些的温度下重新确定了这两种结构[(1)在180 K而不是室温;(II)在100k而不是150k时]。对于多晶型(I),确定了空间群Fdd2而不是原来选择的Cc,两种多晶型的分子结构基本相同,具有精确的晶体双重对称,近似的C(2v)对称,以及H-N-C=O部分的反取向。在这两种多晶型中,分子通过分岔氢键体系C(N-H)(2)··O=C结合成图集C(4)[R(2)(1)(6)]的环状链。在极性结构(I)中,链必然都是平行的,而在(II)中,平行链和反平行链的数量相等。进一步的物理研究[差示扫描量热法(DSC),粉末研究,溶剂诱导相变]进行了:这些表明:(1)市售化合物主要由多晶型(II)组成,加热后通过吸热反应转变为多晶型(i),因此两种多晶型都与对映性有关;(ii)在室温下,晶型(I)代表更稳定的改性,而晶型(ii)是亚稳态的,其热力学转变温度介于253 K和室温之间。粉末衍射表明,第三种多晶是由纤维针状物组成的,由多晶(I)和(II)的混合物组成。
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来源期刊
自引率
5.30%
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0
审稿时长
6-12 weeks
期刊介绍: Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials publishes scientific articles related to the structural science of compounds and materials in the widest sense. Knowledge of the arrangements of atoms, including their temporal variations and dependencies on temperature and pressure, is often the key to understanding physical and chemical phenomena and is crucial for the design of new materials and supramolecular devices. Acta Crystallographica B is the forum for the publication of such contributions. Scientific developments based on experimental studies as well as those based on theoretical approaches, including crystal-structure prediction, structure-property relations and the use of databases of crystal structures, are published.
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