A.E.Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov str. 8, 420088 Kazan, Russia.
Phys Chem Chem Phys. 2010 Nov 7;12(41):13780-7. doi: 10.1039/c0cp00881h. Epub 2010 Sep 17.
The analysis of potential energy surfaces of ion pairs within the framework of an anharmonic oscillator model allows rationalization and prediction of melting points (T(mp)) and heat capacities (C(p)) of ionic liquids (ILs) comprising di- and trialkylimidazolium or tetraalkylphosphonium cations and weakly coordinating BF(4), PF(6), or Tf(2)N anions. Multiple short contacts between the counterions are demonstrated to be typical for the imidazolium based ILs. Differences in the types of contacts result in moderate changes of melting points of the ILs, comparable with differences in T(mp) experimentally determined for the same crystal. The theoretical evaluation of IL heat capacities additionally requires a consideration of conformational behaviour of the corresponding cations. A similar conformational composition of 1-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate at ambient temperature is demonstrated by the combined DFT-vibrational spectroscopy approach. A rough proportionality of C(p) to 1/T(mp) of ionic liquids is suggested, provided that the conformational composition of the ILs does not change on crystal-to-liquid transition.
在非谐振荡器模型的框架内分析离子对的势能面，可以合理推断和预测包含二烷基和三烷基取代的咪唑鎓或四烷基鏻阳离子以及弱配位 BF(4)、PF(6)或 Tf(2)N 阴离子的离子液体 (ILs) 的熔点 (T(mp)) 和热容 (C(p))。多个反离子之间的短接触被证明是基于咪唑鎓的 ILs 的典型特征。接触类型的差异导致 ILs 的熔点发生适度变化，与相同晶体中实验测定的 T(mp)差异相当。理论评估 IL 热容还需要考虑相应阳离子的构象行为。通过结合 DFT-振动光谱方法，证明了在环境温度下 1-丁基-3-甲基咪唑六氟磷酸盐和四氟硼酸盐具有相似的构象组成。建议离子液体的 C(p)与 1/T(mp)呈粗略的比例关系，前提是 ILs 的构象组成在晶体到液体的转变过程中不会发生变化。
