Conformational Flexibility
and Cation–Anion Interactions in 1-Butyl-2,3-dimethylimidazolium
Salts
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Abstract
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 nm<sup>3</sup> agreed
favorably, thus facilitating reliable predictions of volume-derived
properties
