Solid-State Analysis of Low-Melting 1,3-Dialkylimidazolium Hexafluorophosphate Salts (Ionic Liquids) by Combined X-ray Crystallographic and Computational Analyses

The interactions of ions in the solid state for a series of representative 1,3-dialkylimidazolium hexafluorophosphate
salts (either ionic liquids or closely related) have been examined by crystallographic analysis, combined with the theoretical estimation
of crystal-packing densities and lattice-interaction energies. Efficient close-packing of the ions in the crystalline states is observed,
but there was no compelling evidence for specific directional hydrogen-bonding to the hexafluorophosphate anions or the formation
of interstitial voids. The close-packing efficiency is supported by the theoretical calculation of ion volumes, crystal lattice energies,
and packing densities, which correlated well with experimental data. The crystal density of the salts can be predicted accurately
from the summation of free ion volumes and lattice energies calculated. Of even more importance for future work, on these and
related salts, the solid-state density of 1,3-dialkylimidazolium hexafluorophosphate salts can be predicted with reasonable accuracy
purely on the basis of on ab initio free ion volumes, and this allows prediction of lattice energies without necessarily requiring the
crystal structures.