Revisited vibrational assignments of imidazolium-based ionic liquids

Imidazolium-based ionic liquids (ILs) involving anions of variable coordinating strength have been investigated using infrared (IR) and Raman spectroscopies, density functional theory (DFT) calculations and selective deuteration of the imidazolium CH groups. Particular emphasis has been placed on the vibrational assignments of the anion and cation internal vibrations, a prerequisite before any interpretation of spectral changes due to ion-ion interactions in these unconventional liquids. The vibrations of highly symmetric and weakly coordinating anions, such as PF(6)(-), have unperturbed wavenumbers, but unexpected IR or Raman activity for some modes, showing that the anion is subjected to an anisotropic electric field. The stretching as well as the in-plane and out-of-plane bending modes of the imidazolium CH groups are anharmonic. They give broad bands that reflect a large distribution of interactions with the surrounding anions. All the bending modes are mixed with ring vibrations and the stretching modes are complicated by Fermi resonance interactions with overtones and combination of in-plane ring modes. However, the stretching vibration of the quasi-diatomic C((2))-D bond appears to be a good spectroscopic probe of the increasing cation-anion interactions when the coordinating strength of the anion increases. The broad absorption observed in the far IR with weakly coordinating anions remains practically unchanged when the acidic C((2))-H imidazolium bond is methylated and even when the imidazolium cation is substituted by tetra-alkyl ammonium or pyrrolidinium cations. It is concluded that this absorption is a general feature of any IL, coming from the relative translational and librational motions of the ions without needing to invoke C((2))-H anion hydrogen bonds