The cation structural changes of two 1-butyl-3-methylimidazolium halides, [BMIM]Cl and [BMIM]Br, are investigated by in situ Raman spectroscopy, infrared spectroscopy, and synchrotron X-ray diffraction as a function of hydrostatic pressure. The [BMIM]⁺ cations of [BMIM]Cl take the TT conformation under ambient conditions, undergo significant changes at 3.4 GPa, which is related to the potential formations of some new cation conformations, and take another conformation from 6.8 to 35.6 GPa. Correspondingly, crystalline [BMIM]Cl has a structure transition at 3.4 GPa, originating directly from the cation conformational changes. The [BMIM]⁺ cations totally transform into the GT conformer, if the diamond anvil cell-loaded sample is treated at 423 K for 12 h in a furnace before being cooled down to room temperature. It is the first time to observe the complete conformational transition from one conformer (TT) to another (GT) for the [BMIM]⁺ salts in experiments. The cations preserve the GT conformation as pressure reaches 44.6 GPa for heat-treated [BMIM]Cl. As a comparison, without and with the heat treatment, the [BMIM]⁺ cations of [BMIM]Br keep the GT conformation at high pressures up to 29.6 GPa, as it is a relatively stable state. Moreover, a notable photoluminescence emerges at high pressures over 20.0 GPa and is maintained in the decompression process, as a result of the pressure-induced polymerization of a small portion of the [BMIM]⁺ cations.

中文翻译：

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高压下1-丁基-3-甲基咪唑鎓卤化物的阳离子构象变化

通过原位拉曼光谱，红外光谱和同步加速器X射线衍射研究了两种1-丁基-3-甲基咪唑鎓卤化物[BMIM] Cl和[BMIM] Br的阳离子结构变化与静水压力的关系。[BMIM] Cl的[BMIM] ⁺阳离子在环境条件下呈TT构象，在3.4 GPa时发生显着变化，这与某些新阳离子构象的潜在形成有关，并在6.8至35.6 GPa处呈另一构象。相应地，结晶[BMIM] Cl在3.4 GPa处具有结构转变，直接源自阳离子构象变化。[BMIM] ⁺阳离子完全转化为GT如果将装有钻石砧细胞的样品在炉中以423 K的温度处理12小时，然后冷却至室温。在实验中，这是第一次观察到[BMIM] ⁺盐从一个构象体（TT）到另一个构象体（GT）的完全构象转变。对于经热处理的[BMIM] Cl，当压力达到44.6 GPa时，阳离子会保留GT构象。作为比较，在不进行热处理的情况下，[BMIM] Br的[BMIM] ⁺阳离子保持GT在高达29.6 GPa的高压下，它是一个相对稳定的状态。此外，由于压力诱导的[BMIM] ⁺阳离子的少量聚合，在超过20.0 GPa的高压下会出现显着的光致发光，并在减压过程中得以维持。