Determination of Missing Crystal Structures in the 1-Alkyl-3-methylimidazolium Hexafluorophosphate Series: Implications on Structure–Property Relationships

We have performed single crystal X-ray diffraction analyses of two 1-alkyl-3-methylimidazolium hexafluorophosphate ([Cnmim]PF6) salts, [C1mim]PF6, and [C3mim]PF6. The thermodynamically metastable crystalline phase (β phase) of the former was obtained by crystallization from the melt in a capillary for the crystallographic analysis. Both the cation and the anion in the β phase of [C1mim]PF6 show higher point group symmetry than those in the stable α phase. This result is shown to be consistent with the fact that the melting point of the β polymorph (314 K) is significantly lower than that of the α polymorph (364 K). The crystal structure of [C3mim]PF6 seems to be typical of that characteristic of the crystals of imidazolium-based room temperature ionic liquids. The cation conformation is the one in between those characteristic of the crystal structures of [C2mim]PF6 and [C4mim]PF6. When taken together, all crystallographic data for the [Cnmim]PF6 series (n = 1–4) demonstrate the influence of the alkyl chai...