Infrared Spectroscopy of Li plus Solvation in EmimBF4 and in Propylene Carbonate: Ab Initio Molecular Dynamics and Experiment

Infrared (IR) spectra of solutions of the lithium salt LiBF4 in t h e ionic liquid 1-ethyl-3-methylimidazol i u m tetrafluoroborate (EmimBF4) and in t h e organic sol v e n t propylene carbonate (PC) are studied via infrared spectroscopy and ab initio molecula r 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 t h e development of a new peak at 980 cm-1. Computational analysis indicates that hindered translational motions of Li+ in its solvation cage are ma i n l y responsible for the former, w h i l e 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 molecula r 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.

Automated summary

The infrared spectra of solutions containing LiBF4 in an ionic liquid and an organic solvent are studied. The presence of LiBF4 leads to changes in the spectra, including the appearance of new bands and broadening of the IR structure. Computational analysis reveals that the motion of Li+ ions and structural changes in the solution are responsible for these spectral changes. Additionally, the molecular motions in the lower frequency regions are correlated with the spectral changes.