Infrared Spectroscopy of Li⁺ Solvation in EmimBF₄ and in Propylene Carbonate: Ab Initio Molecular Dynamics and...

Infrared (IR) spectra of solutions of the lithium salt LiBF₄ in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF₄) and in the organic solvent propylene carbonate (PC) are studied via infrared spectroscopy and ab initio molecular dynamics (AIMD) simulations. The measurements show that the major effects of LiBF₄ in both solutions, compared to their neat counterparts, are the appearance of a new broad band in the 300-450 cm^-1 frequency region and a broadening of the IR structure in the 900-1200 cm^-1 region with the development of a new peak at 980 cm^-1. Computational analysis indicates that hindered translational motions of Li⁺ in its solvation cage are mainly responsible for the former, while the latter is due to Li⁺-induced structural changes and accompanying vibrational frequency shifts of constituent ions and molecules of the solutions. In addition, molecular motions in these and lower-frequency regions are generally correlated, disclosing the collective nature of the vibrational dynamics, which involve multiple ions/molecules. Herein, a detailed analysis of these features via AIMD simulations of the spectrum and its components arising from auto- and cross-correlations of motions of constituent molecular species, combined with generalized normal modes of the solutions and normal modes of small Li⁺-containing clusters, is presented. Other minor spectral changes caused by the lithium salt as well as the interaction-induced effect on IR spectra are also discussed.