Understanding the\nMid-Infrared Spectra of Protic Ionic\nLiquids by Density Functional Theory

Protic ionic liquids (PILs) are promising candidates\nas electrolytes\nfor proton exchange polymer membrane fuel cells. In order to optimize\ntheir properties, a detailed understanding of the molecular interactions\nwithin the bulk and at the electrode–electrolyte interface\nis needed, which can be obtained by infrared spectra. A prerequisite\nfor extracting information on the molecular structure and inter- or\nintramolecular interactions from an experimental spectrum is a reasonable\ninterpretation of the observed spectral features. Here, we employed\ndensity functional theory to understand the vibration modes of PILs\ncomposed of ammonium cations and different counteranions. Different\nfrom the previous calculation methods performed on small cluster model\nsystems consisting of isolated species, a periodically repeated system\nof four ion pairs was used in order to approximate the bulk liquid\nenvironment. The computed frequencies and IR intensity match well\nwith the corresponding experimental spectra, allowing for its proper\ninterpretation, especially the characteristic features of the interionic\ninteraction. The presented approach enables accurate computation of\na variety of ionic liquid systems in a highly efficient way.