simple physical model for the simultaneous rationalisation of melting points and heat capacities of ionic liquids

The anal. of potential energy surfaces of ion pairs within the framework of an anharmonic oscillator model allows rationalization and prediction of m.ps. (Tmp) and heat capacities (Cp) of ionic liqs. (ILs) comprising di- and trialkylimidazolium or tetraalkylphosphonium cations and weakly coordinating BF4, PF6, or Tf2N 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 m.ps. of the ILs, comparable with differences in Tmp exptl. detd. for the same crystal. The theor. evaluation of IL heat capacities addnl. requires a consideration of conformational behavior of the corresponding cations. A similar conformational compn. of 1-butyl-3-methylimidazolium hexafluorophosphate and tetrafluoroborate at ambient temp. is demonstrated by the combined DFT-vibrational spectroscopy approach. A rough proportionality of Cp to 1/Tmp of ionic liqs. is suggested, provided that the conformational compn. of the ILs does not change on crystal-to-liq. transition.