Abstract
The solid-state polymorphism of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], has been investigated via low-temperature and high-pressure crystallisation experiments. The samples have been characterised by single-crystal X-ray diffraction, optical microscopy and Raman spectroscopy. The solid-state phase behaviour of the compound is confirmed and clarified with respect to previous phase diagrams. The structures of the previously reported γ-form, which essentially exhibits a G′T cation conformation, as well as those of the elusive β- and α-forms, are reported. Crystals of the β-phase are twinned and the structure is heavily disordered; the cation conformation in this form is predominantly TT, though significant contributions from other less frequently encountered conformers are also observed at low temperature and high pressure. The cation conformation in the α-form is GT; the presence of the G′T conformer at 193 K in this phase can be eliminated on cooling to 100 K. Whilst X-ray structural data are overall in good agreement with previous interpretations based on Raman and NMR studies, they also reveal a more subtle interplay of intermolecular interactions, which give rise to a wider range of conformers than previously considered.
Highlights
Due to their unique physicochemical properties, roomtemperature ionic liquids (ILs) have attracted considerable interest in several branches of chemistry and chemical engineering
A low melting point, large liquid range and high thermal stability are some of the properties that make them attractive green solvents for synthetic and catalytic processes.[1,2,3]
ILs have found application as crystallisation media. Their nonvolatility makes them poor solvents for crystallisation by evaporation, they can be used in solvothermal, thermal shi, co-solvent, slow diffusion and electrocrystallisation of a variety of chemicals, including pharmaceuticals and biomolecules.[4,5,6]
Summary
Due to their unique physicochemical properties, roomtemperature ionic liquids (ILs) have attracted considerable interest in several branches of chemistry and chemical engineering. A low melting point, large liquid range and high thermal stability are some of the properties that make them attractive green solvents for synthetic and catalytic processes.[1,2,3] More recently, ILs have found application as crystallisation media. Their nonvolatility makes them poor solvents for crystallisation by evaporation, they can be used in solvothermal, thermal shi , co-solvent, slow diffusion and electrocrystallisation of a variety of chemicals, including pharmaceuticals and biomolecules.[4,5,6].
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