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

Abstract

Infrared (IR) spectra of solutions of the lithium salt LiBF ₄ in diglyme, CH ₃O(CH ₂CH ₂O) ₂CH ₃, are studied via IR spectroscopy and ab initio molecular dynamics (AIMD) simulations. Experiments show that the major effects of LiBF ₄, compared to neat diglyme, are the appearance of a new broad band in the 250–500 cm ^–1 frequency region and a broadening and intensity enhancement of the diglyme band in the 900–1150 cm ^–1 region accompanied by a red-shift. Computational analysis indicates that hindered translational motions of Li ⁺ in its solvation cage are mainly responsible for the new far-IR band, while the changes in the mid-IR are due to Li ⁺-coordination-dependent B–F stretching vibrations of BF ₄ ^– anions coupled with diglyme vibrations. Molecular motions in these and lower frequency regions are generally correlated, revealing 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, combined with analysis of the generalized normal modes of the solution components, is presented. Other minor spectral changes as well as diglyme conformational changes induced by the lithium salt are also discussed.