The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray diffraction analysis and in solution by (1)H NMR spectroscopy, conductivity, and microcalorimetry. In the solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type {[(DAI)(3)(X)](2+)[(DAI)(X)(3)](2-)}(n) (where DAI is the dialkylimidazolium cation and X is the anion) formed through hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.

The crystal structure of 1-nbutyl-3-methylimidazolium tetraphenylborate molten salt (1) shows C-H ± p interactions between the hydrogens of the imidazolium cation and the phenyl rings of the tetraphenylborate anion. The imidazolium ring is surrounded by three tetraphenylborate anions that are connected with the same cation by C-H ± p (phenyl rings) interactions. The nearest inter-ion interaction is found between the N-CH-N proton of the cation and the B-phenyl centroid (2.349 ) with a nearly T-shaped geometry. The inter-ionic solution structure of 1 has been investigated by the detection of inter-ionic contacts in 1 H NOESY NMR spectra between the protons of the cation and the anion. The 1 H-NMR spectra of molten salt 1 is almost inde-pendent of its concentration in [D 6 ]DMSO solution, the imidazolium proton chemical shifts are in the expected region and there are no observable NOE effects between the protons of the cation with those of the anion, indicating that 1 behaves in [D 6 ]DMSO as a solvent-separated ion pair. In CDCl 3 the 1 H-NMR spectra of 1 are concentration dependent and all the imidazolium protons are shielded as compared with those observed in [D 6 ]DMSO. Moreover, the 1 H NOESY NMR spectra show all the peaks affected by the interaction between the protons of the imidazolium cation and those of the anion, indicating that in CDCl 3 1 possesses a contact ion pair structure. The NCHN proton of the cation exhibits the greatest shielding (up to À 4.5 ppm), an indication of the existence of C-H ± p interactions, even in solution. The calculated distance of this proton to the phenyl centroid is 2.3 for a C-H ± p angle of 1808. The apparent volumes for the cation and anion, calculated from the measured 13 C-NMR relaxation times, increase from 38 and 140 3 in [D 6 ]DMSO to 360 and 600 3 in CDCl 3 , respectively; this indicates the formation of floating aggregates of the type (1) n in CDCl 3 via weak hydrogen bonds, with increasing concentration.

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