Hydrogen-bonding and symmetry breaking in the protic ionic liquid 1-ethylimidazolium nitrate

The strong cation-anion interaction in the protic ionic liquid 1-ethylimidazolium nitrate, [C2im][NO3], is revealed in different regions of the infrared spectrum (IR). The IR band related to the anion asymmetric stretching mode νas(NO3) splits due to degeneracy lifting, the symmetry forbidden symmetric stretching mode νs(NO3) acquires significant IR activity, and the cation ν(NH) stretching mode produces a broad band characteristic of strong hydrogen bond (H-bond). At high temperatures (up to 120 °C), these spectral features remain in the IR spectrum, indicating the strong cation-anion interaction. The size of the νas(NO3) splitting, the relative strength of νs(NO3), and the intermolecular H-bond stretching mode ν(NH…O) seen in the far-IR range are all accounted for in a quantum chemistry calculation of an ionic pair. The complex spectral pattern in the high-frequency range of the IR spectrum, where NH and CH stretching modes occur in overlapping bands, can be explained using a simple model coupling the ν(N–H) and ν(NH…O) modes. The parameters of the model, including the dependence of the ν(NH) frequency on the N…O distance of the H-bond, are also derived from the ionic pair calculation. The IR spectrum thus gives experimental evidence that using an ionic pair to explore the H-bond influence on the electronic structure of [C2im][NO3] is reasonable.