Structure of LiBF4 Solvate Complexes in Ethylene Carbonate, Based on High-Resolution NMR and Quantum-Chemical Data

Abstract

The ion solvation of LiBF₄ in ethylene carbonate is studied via high resolution NMR, conductometry, and quantum-chemical simulation. ⁷Li, ¹¹B, ¹⁹F, ¹³C, and ¹⁷O NMR spectra are acquired for LiBF₄ solutions in ethylene carbonate, and their conductivity is measured in the concentration range of 0.07–1.77 mol kg^–1 at 40°C. Molecular models of solvate complexes of a Li⁺BF₄⁻ ion pair containing n ethylene carbonate molecules are constructed. The calculated ¹¹B chemical shifts are virtually independent of n, which can provide a relationship between ¹¹B experimental shifts and degree of dissociation (α). The α value is estimated from a theoretical change in chemical shift of −0.414 ppm when a BF₄⁻ ion transitions from a free state to an associated one of the contact ion pair. The α values are in reasonable agreement with the degree of dissociation for LiBF₄ in propylene carbonate, found from the Walden product of the equivalent electrical conductivity of a solution by its viscosity.