Conformational Flexibility and Cation–Anion Interactions in 1-Butyl-2,3-dimethylimidazolium Salts

The butyl group in 1-butyl-2,3-dimethylimidazolium (BMMI) salts, a common group of low-melting solids, was found to exhibit different conformations in the solid state. Crystal structures of pure BMMI azide, thiocyanate, propynoate, hexachlorocerate­(IV), chlorocyanocuprate­(I), nonachlorodititanate­(IV), and mixed azide/chloride and cyanide/chloride salts were determined by single crystal X-ray diffraction, and their butyl chain conformations were examined. The twist angle of the C­(α)–C­(β) bond out of the plane of the imidazole ring ranges from 57° to 90°, whereas the torsion angle along the C­(α)–C­(β) bond determines the overall conformation: 63° to 97° (gauche) and 170° to 179° (trans). The preferred conformations of the butyl group are trans–trans and gauche–trans, but trans–gauche and gauche–gauche were also observed. More than one conformer was present in disordered structures. Numerous polar hydrogen bonds between cations and anions were identified. Five structures exhibit stacking of the aromatic imidazole systems, indicated by parallel alignment of pairs of cations with short centroid–centroid distances due to π–π interactions, which is surprisingly frequent. Not only imidazole ring protons are involved in the formation of short CH···X hydrogen bonds, but also interactions between methylene and methyl groups of the alkyl chain and the anion are visible. Hirshfeld surface analysis revealed that nonpolar H···H contacts represent the majority of interactions. The volume-based lattice potential energy, enthalpy, entropy, and free energy were calculated by density functional theory. Calculated and experimental molecular volumes in the range from 0.27 to 0.70 nm³ agreed favorably, thus facilitating reliable predictions of volume-derived properties