A vibrational spectroscopy study of non-stoichiometric protic ionic liquids based on DBN and DBU superbases

The liquid structure of protic ionic liquids (PILs) composed of the superbases 1,5-diazabicyclo(4.3.0)non-5-ene (DBN) and 1,8-diazabicycloundec-7-ene (DBU) combined with carboxylic acids (acetic acid (HAc) and octanoic acid (HOct)) has been investigated by vibrational spectroscopy (infrared (IR) and Raman). The spectroscopic studies have been assisted by quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations of clusters of ions and molecules. To cover a broad range of base : acid ratios, IR and Raman spectra of different mixtures have been investigated: 10 : 1, 4 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 4, and 1 : 10. Characteristic bands in the spectra indicate the protonation of the base with increasing amounts of the acid, however, a small percent of neutral base is still present in the 1 : 1 composition that corresponds to the PIL. The Raman spectra show that the acid-base equilibrium shifts, allowing for more proton transfer, as the PIL is cooled. The Raman spectra of mixtures containing excess of acid reveal that the ion pairs and extended structures like [DBNH]⁺⋯[Ac]^-⋯[HAc] disrupt the acid liquid structure and decrease the population of cyclic acid dimers. The complex band shape characteristic of strong hydrogen bonds is seen in the high-frequency range of the IR spectra related to hydrogen stretching modes, which span the wide 1800-3600 cm^-1 range and depend heavily on the mixture's composition. Owing to the inherent anharmonic character of the vibrations in an AIMD simulation, the interpretation of the high-frequency range was aided by simulating clusters of ions and molecules of DBN : HAc, 4 : 4, 4 : 8, 2 : 8, and 1 : 10. In line with the experimental IR spectra, the AIMD simulations reproduce the decrease in intensity in the 3000-3500 cm^-1 range and the increase in intensity in the 1800-3000 cm^-1 range as the composition changes from 1 : 10 to 1 : 1. The AIMD simulations indicate that more mobile protons are responsible for this increased intensity in the lower-frequency side of the band. Direct inspection of N-H and O-H distances along the AIMD simulations reveals proton exchange between HAc and DBN in the 4 : 4 cluster, but prevalence of proton exchange between acetate and acetic acid in the other clusters with excess of acid.