Structural and Conformational Analysis of 1-Butyl-3-methylimidazolium Nitrate,Journal of Chemical Crystallography

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Structural and Conformational Analysis of 1-Butyl-3-methylimidazolium Nitrate
Journal of Chemical Crystallography ( IF 0.4 ) Pub Date : 2021-08-16 , DOI: 10.1007/s10870-021-00899-w
Dean H. Johnston ₁ , Mallory Gasbarre ₁ , Carolanne E. Norris ₁

Affiliation

The crystal structures of two polymorphic forms of 1-butyl-3-methylimidazolium nitrate are reported. The form observed at 100 K and 200 K (Ia and Ib) crystallizes in the \(P{\bar{1}}\) space group and contains two independent imidazolium cations and nitrate anions. At 100 K the butyl chain of one cation adopts a TT (trans–trans) conformation and the other cation adopts a G′G′ (gauche–gauche) conformation. At 200 K, the G′G′ chain is disordered with about twelve percent of the GT conformation present. A different polymorph (Form II, also crystallizing in the \(P{\bar{1}}\) space group with two independent ion pairs) is present at 273 K displaying significant disorder in the butyl chains with a mixture of G′T, G′G′, and GT conformers. Raman spectra were collected on samples between 100 and 350 K and show changes in band frequencies and intensities consistent with conversion between different butyl chain conformations. Hydrogen bond interactions are present between cation C–H’s and oxygen atoms of the nitrate ions, with significant lengthening observed for three of the six close contacts (and formation of one new contact) upon conversion to the higher-temperature form. The structural details revealed in this study shed light on the intermolecular forces and the conformational changes that accompany phase changes in 1-butyl-3-methylimidazolium nitrate.

Graphic Abstract

X-ray diffraction and Raman spectroscopy of the ionic liquid 1-butyl- 3-methylimidazolium nitrate at temperatures from 100 to 300 K show evidence of two different polymorphs and significant temperature-dependent conformational changes.

中文翻译：

1-丁基-3-甲基咪唑硝酸盐的结构和构象分析

报道了硝酸 1-丁基-3-甲基咪唑鎓的两种多晶型物的晶体结构。在 100 K 和 200 K（Ia和Ib）下观察到的形式在\(P{\bar{1}}\)空间群中结晶，并包含两个独立的咪唑鎓阳离子和硝酸根阴离子。在 100 K 时，一个阳离子的丁基链采用 TT（反式-反式）构象，另一个阳离子采用 G'G'（gauche - gauche）构象。在 200 K 时，G'G' 链无序，大约 12% 的 GT 构象存在。不同的多晶型物（形式II，也在\(P{\bar{1}}\)具有两个独立离子对的空间群）存在于 273 K 处，在丁基链中显示出显着的无序，其中包含 G'T、G'G' 和 GT 构象异构体的混合物。拉曼光谱是在 100 到 350 K 之间的样品上收集的，并且显示了与不同丁基链构象之间的转换一致的带频率和强度的变化。阳离子 C-H 和硝酸根离子的氧原子之间存在氢键相互作用，在转化为高温形式后，六个紧密接触中的三个观察到显着延长（并形成一个新接触）。本研究揭示的结构细节揭示了伴随硝酸 1-丁基-3-甲基咪唑鎓相变的分子间作用力和构象变化。