Ab Initio Molecular Dynamics Simulation of Infrared Absorption Spectra of H3O+ and H5O2+ in Nonaqueous Solutions of Trifluoromethanesulfonic Acid Hydrates

Infrared absorption spectra of hydrated protons are very sensitive to their different structures that change with the number of water molecules as well as their hydrogen bonds with anions and/or solvent molecules. Hydronium ion (H3O+) in nonaqueous solutions of trifluoromethanesulfonic (triflic) acid monohydrate shows largely red-shifted and very broad OH stretching bands in their infrared absorption spectra. We found that four triflate anions and four hydronium ions form a hydrogen-bonded cluster in which sulfur and oxygen atoms of anions and cations have a cubic structure with red-shifted OH stretching bands; from its ab initio molecular dynamics simulation large widths of the OH stretching absorption bands were successfully reproduced. Based on the very similar infrared absorption bands of perfluorinated sulfonic acid ionomer membranes with low water content their acid sites are supposed to have very similar cluster configurations.