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 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.