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A variable-temperature X-ray diffraction and theoretical study of conformational polymorphism in a complex organic molecule (DTC)

机译：复杂有机分子（DTC）中的变温X射线衍射和构象多态性的理论研究

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摘要

Two conformational crystal polymorphs of 3-diethylamino-4-(4-methoxyphenyl)-1,1-dioxo-4 H -1λ ~(6) ,2-thiazete-4-carbonitrile (DTC) have been analyzed in the 100 K-room temperature range by single crystal X-ray diffraction and high quality DFT calculations. DTC has strongly polar as well as aliphatic substituents but no hydrogen bonding groups, and thus qualifies as a test molecule for the relative importance of electrostatic vs. dispersion–repulsion terms. The two polymorphs have the same P 2 _(1) / n space group and differ by a flipping of the –OCH _(3) group, the two conformations being almost equi-energetic and separated by a low barrier. The system is monotropic in the observed temperature range with nearly identical thermal expansion coefficients and energy–temperature slopes, one phase consistently predicted to be more stable in agreement with the relative ease of appearance. Energy decompositions show that the electrostatic term is dominant and stabilizes with decreasing temperature. Dispersion and repulsion show the expected behavior, the former becoming more stabilizing at lower temperature in contrast with increasing repulsion at higher density. Absolute values and trends are very similar in the two phases, explaining the small total energy difference. Geometrical analyses of intermolecular contacts using fingerprint plots, as well as the study of molecular dipole moments as a function of T in the framework of the Quantum Theory of Atoms in Molecules, reveal more details of phase stability.
机译：在100 K-中分析了3-二乙氨基-4-（4-甲氧基苯基）-1,1-二氧代-4 H-1λ〜（6），2-噻嗪-4-腈（DTC）的两个构象多晶型通过单晶X射线衍射和高质量DFT计算得出室温范围。 DTC具有强极性和脂肪族取代基，但没有氢键基团，因此可以作为静电对分散-排斥项相对重要性的测试分子。这两个多晶型物具有相同的P 2 _（1）/ n空间组，并且通过翻转–OCH _（3）组而有所不同，这两个构象几乎相等，并且被低势垒隔开。该系统在观察到的温度范围内是单向性的，具有几乎相同的热膨胀系数和能量-温度斜率，据一致预测，一个相与相对易出现性相比，将更稳定。能量分解表明，静电项占主导地位，并随着温度的降低而稳定。分散和排斥显示出预期的行为，与较高密度下增加的排斥相反，前者在较低温度下变得更加稳定。这两个阶段的绝对值和趋势非常相似，这说明总能量差异很小。使用指纹图对分子间接触进行几何分析，以及在分子的原子量子理论框架内研究分子偶极矩作为T的函数，揭示了相稳定性的更多细节。

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