The model room temperature ionic liquid, 1,3-dimethylimidazolium chloride, has been studied by neutron diffraction for the first time. The diffraction data are used to derive a structural model of this liquid using Empirical Potential Structure Refinement. The model obtained indicates that significant charge ordering is present in the liquid salt and that the local order in this liquid closely resembles that found in the solid state. As in the crystal structure, hydrogen-bonding interactions between the ring hydrogens and the chloride dominate the structure. The model is compared with the data reported previously for both simple alkyl substituted imidazolium halides and binary mixtures with AlCl3.

The structure of liquid 1, 3-dimethylimidazolium hexafluorophosphate is described in detail and compared with the structure of 1, 3-dimethylimidazolium chloride. In each case, the data were obtained from neutron diffraction experiments and analysed using an empirical potential structure refinement process. Overall, the structures are similar; however, significant differences arise from the variation in anion size.

The structures of three long alkyl chain imidazolium chloride salts have been determined by X-ray diffraction; one of the salts was isolated as two polymorphs. These compounds consist of regions containing interdigitated linear alkyl chains alternating with regions containing imidazolium headgroups, chloride ions, and solvent. The arrangement of the cation headgroup and the alkyl chain is strongly dependent on the thermal history of the salt and whether the C(2) position on the imidazolium ring is methylated or protiated. Structures are found where the headgroups within the same crystal are in different environments leading to a double bilayer structure which collapses on heating.

Alkyl-3-methylimidazolium tetrachloropalladate(II) salts ([C n -mim] 2 [PdCl 4 ], n~10, 12, 14, 16, 18) containing a single, linear alkyl-chain substituent on the cation have been synthesised and their behaviour characterised by differential scanning calorimetry, polarising optical microscopy and small-angle X-ray scattering. The salts display thermotropic polymorphism, exhibiting both crystal±crystal transitions and, for n~14±18, the formation of a thermotropic smectic liquid crystalline phase.

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