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

Abstract

We have performed single crystal X-ray diffraction analyses of two 1-alkyl-3-methylimidazolium hexafluorophosphate ([C n mim]PF 6 ) salts, [C 1 mim]PF 6, and [C 3 mim]PF 6 . 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 [C 1 mim]PF 6 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 [C 3 mim]PF 6 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 [C 2 mim]PF 6 and [C 4 mim]PF 6 . When taken together, all crystallographic data for the [C n mim]PF 6 series ( n = 1–4) demonstrate the influence of the alkyl chain length dependence on structural packing in these salts. The free volume fractions for these structures are calculated and compared to the lattice energy simply based on formula unit volume. It is shown that the difference in the alkyl chain length of the imidazolium cations does not monotonically change with the lattice energy of the crystal structure, and the latter also depends on the packing efficiency of the constituent ions in the structure.