Comprehensive Conformational and Rotational Analyses of the Butyl Group in Cyclic Cations: DFT Calculations for Imidazolium, Pyridinium, Pyrrolidinium, and Piperidinium

Abstract

The flexibility and conformational variety of the butyl group in cations of ionic liquids (ILs) play an important role in dictating the macroscopic and microscopic properties of ILs. Here we calculate potential energy surfaces for the dihedral angles of the butyl group in four different types of cyclic cations, imidazolium, pyridinium, pyrrolidinium, and piperidinium, using the density functional theory method. The calculation results highlight the role of the butyl group in these cations by comparison of five-membered and six-membered rings, and of aromatic and alicyclic rings, in terms of stable conformations and rotational barriers. A striking result is that the butyl group rotation in pyrrolidinium induces pseudorotation of the ring whereas such a phenomenon does not occur in piperidinium. This difference is thought to be because of the relationship in rotational activation energy between the butyl group (10-40 kJ mol-1) and the ring (<6 kJ mol-1 for pyrrolidinium and 40-50 kJ mol-1 for piperidinium). The calculated stable conformers are compared with the ones observed in crystals recorded in the Cambridge Structural Database. Although conformers with lower calculated energy generally have higher chances to be experimentally observed, roughly independent of the cation structure, some calculated conformers deviate from this trend and show very low population. It is found that not only low energy but also high rotational activation energy (i.e., long lifetime) is required to observe conformers in crystalline states. In the last part of this article, to identify conformers in real systems, the applicability of the calculated Raman bands of cations with different butyl group conformations is discussed.