辐照离子液体中自由基的EPR研究及其对离子液体萃取体系辐射稳定性的影响
EPR Study of Radicals in Irradiated Ionic Liquids and Implications for the Radiation Stability of Ionic Liquid-Based Extraction Systems
摘要：
用低温电子顺磁共振（EPR）研究了由铵、磷、吡咯烷和咪唑阳离子与双三氟甲磺酰胺、双氰胺和双硼酸根阴离子组成的室温离子液体（IL）的辐射和光化学。几类自由基已被确定和有关的主要放射性产生的电子和空穴的反应。在磷、铵和吡咯烷鎓阳离子的脂族链中观察到大量的末端和倒数第二个C-中心自由基，但咪唑鎓阳离子没有。这种模式可以通过捕获在阳离子（自由基二价阳离子）上的空穴的有效去质子化来解释，该空穴与快速电荷转移到附近的阴离子竞争。后者导致形成稳定的N-或O-中心的自由基。电子与质子杂质（对于非芳香族阳离子）反应产生H原子或芳香族部分（对于咪唑阳离子）。双（三氟甲磺酰基）酰胺阴离子的激发显示产生三氟甲基自由基;然而，在辐解中该自由基的产率较低（< 10%的烷基自由基产率）。就其辐射化学而言，纯离子液体似乎是有机液体和离子固体之间的中间体。添加10-40重量%的磷酸三烷基酯（核循环分离的常用萃取剂）对离子液体的碎裂具有相对较小的影响。磷酸盐的辐射脱烷基作用显著，但衍生自磷酸盐的烷基自由基片段的产率小于溶剂驱动的自由基片段产率的4%。我们讨论了这些结果的辐射稳定性的核循环提取系统的基础上IL稀释剂的影响。
英文摘要：
The radiation- and photo- chemistry of room temperature ionic liquids (ILs) composed of ammonium, phosphonium, pyrrolidinium, and imidazolium cations and bis(triflyl)amide, dicyanamide, and bis(oxalato)borate anions, have been studied using low-temperature Electron Paramagnetic Resonance (EPR). Several classes of radicals have been identified and related to reactions of the primary radiolytically generated electrons and holes. Large yields of terminal and penultimate C-centered radicals are observed in the aliphatic chains of the phosphonium, ammonium and pyrrolidinium cations, but not for imidazolium cation. This pattern can be accounted for by efficient deprotonation of a hole trapped on the cation (the radical dication) that competes with rapid charge transfer to a nearby anion. The latter leads to the formation of stable N- or O-centered radicals. The electrons either react with the protic impurity (for nonaromatic cations) yielding H atoms or the aromatic moiety (for imidazolium cations). Excitation of bis(triflyl)amide anion is shown to yield trifluoromethyl radical; the yield of this radical in radiolysis, though, is low (< 10% of the alkyl radical yield). In terms of their radiation chemistry, neat ILs appear to be intermediate between organic liquids and ionic solids. Addition of 10-40 wt% of trialkylphoshate (a common extraction agent for nuclear cycle separations) has relatively little effect on the fragmentation of the ILs. Radiation induced dealkylation of the phosphate is prominent, but the yield of the alkyl radical fragments derived from the phosphates is < 4% of the yield of the radical fragments drived from the solvent. We discuss the implication of these results for the radiation stability of nuclear cycle extraction systems based upon the IL diluents.
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