Microscopic molecular organization and heterogeneities in ionic liquids (ILs) are of significant fundamental and applied interest. Although many theoretical studies have been dedicated to this topic, the development of experimental methods for studying such heterogeneities is still in demand. In this work we propose a new approach for the characterization of microscopic rigidity and heterogeneities in ILs using stochastic librations (small angle motions) of the nitroxide radicals as a probe. Stable nitroxides are dissolved in ILs, which are then shock-frozen and investigated using pulse Electron Paramagnetic Resonance (EPR) at variable temperatures. Stochastic molecular librations of nitroxides depend on local rigidity of a medium and manifest themselves in different electron dephasing times across the EPR spectrum. The use of advanced spiro-cyclohexane-substituted nitroxides allows implementation of this approach at temperatures up to ∼150 K and above. Remarkably, we have found that librational motions in ILs arise at much lower temperatures (∼75 K for [Bmim]BF₄, [Bmim]PF₆, [C₁₀mim]BF₄) compared to common organic solvents. This can be rationalized by smaller local rigidity of the medium in ILs, most likely due to localization of nitroxides in low-density heterogeneities formed by non-polar alkyl chains. The structure, size, solubility and other properties of nitroxides are adjustable for particular tasks, therefore the proposed approach can potentially be implemented to probe the rigidity and heterogeneities of any ILs, thus providing vital insights into their molecular-scale self-organization.

中文翻译：

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通过溶解氮氧化物的随机分子释放探测离子液体的微观刚度和异质性

离子液体（ILs）中的微观分子组织和异质性具有重要的基础和应用意义。尽管许多理论研究致力于该主题，但仍需要开发用于研究此类异质性的实验方法。在这项工作中，我们提出了一种新方法，该方法使用一氧化氮自由基的随机释放（小角度运动）作为探针来表征IL的微观刚度和异质性。将稳定的一氧化氮溶解在IL中，然后将其骤冷并在可变温度下使用脉冲电子顺磁共振（EPR）进行研究。一氧化氮的随机分子释放取决于介质的局部刚度，并在整个EPR谱图中以不同的电子移相时间表现出来。使用先进的螺-环己烷取代的氮氧化物可以在高达约150 K或更高的温度下实施该方法。值得注意的是，我们发现IL的自由运动是在较低的温度下发生的（[Bmim] BF约为75 K与普通的有机溶剂相比，图₄中的[Bmim] PF ₆，[C ₁₀ mim] BF ₄）为低。这可以通过IL中较小的介质局部刚度来合理化，这很可能是由于氮氧化物在由非极性烷基链形成的低密度异质性中的定位所致。氮氧化物的结构，大小，溶解度和其他属性可针对特定任务进行调整，因此，所提出的方法可以潜在地用于检测任何IL的刚性和异质性，从而提供对其分子尺度自组织的重要见解。