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

Abstract

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.

Infrared Spectroscopy of Li+ Solvation in Diglyme: Ab Initio Molecular Dynamics and Experiment

“…Figure b shows that very similar spectral changes are obtained with another lithium salt, LiTfO (TfO – = CF 3 SO 3 – ). Although we focus exclusively on diglyme mixtures with LiBF 4 , the LiTfO example in Figure is used to highlight the fact that development of a broad band in the ∼250–500 cm –1 region is observed upon addition of various lithium salts to a wide range of different solvents, including organic solvents and ILs, though details of the band structure may differ (see Figure S1 in the Supporting Information). ,− ,− This feature arises primarily from hindered translational motions (“rattling”) of Li + ions in differing solvation environments. ,− ,− In the following section, we thus start with the analysis of Li + solvation, and how it is coordinated by ions and diglyme molecules.…”

“…Those with one or no Li + -coordinating O atoms are considered unbound diglyme (uG), as done in ref . BF 4 – , on the other hand, is defined as a coordinating anion if one or more of its F atoms coordinate to Li + , as in previous studies. , The three most populous Li + complexes are a Li + ion coordinated by one diglyme molecule and two BF 4 – anions (LiG 1 B 2 , 76%) and a Li + ion coordinated either by one diglyme and one BF 4 – anion (LiG 1 B 1 , 14%) or by two diglyme molecules (LiG 2 , 8%). This means that 90% of the Li + solvation cages consist of one diglyme molecule and one or two BF 4 – anions.…”

Spectroscopic and Chemical Properties of Ionic Liquids: Computational Study

“…Reprinted with permission from ref. [30] (Copyright 2022 American Chemical Society). ð Þa w ð Þ; eq 2; units: 10 3 cm À 1 ); (c) generalized normal modes for CÀ H stretching vibrations of the Emim + ring.…”

“…[28,29] The AIMD results for pure EmimBF 4 (1-ethyl-3-methylimidazolium tetrafluoroborate) and its mixture with LiBF 4 in the far-IR region are compared with experimental results in Figure 4. [30] The peaks labeled as sa# and sc# (e. g., sa2 and sc10) in the simulated spectra are related to vibrational modes a# (anion) and c# (cation), respectively (see below). Simulation and experiment show good overall agreement though the former results are generally red-shifted by ≲ 50 cm À 1 compared to the latter for…”

“…Mid-IR spectra of pure EmimBF 4 and the EmimBF 4 + LiBF 4 mixture in the CÀ H stretching region: (a) Experimental results; (b) AIMD results (n wð Þa w ð Þ; eq 2; units: 10 3 cm À 1 ); (c) generalized normal modes for CÀ H stretching vibrations of the Emim + ring. Reprinted with permission from ref [30]. (Copyright 2022 American Chemical Society).…”