Molecular Dynamics in Tetrafluoridoborate Salts of Divalent Metals Studied by Nuclear Magnetic Resonance Spectroscopy.

Fluoridoborate (BF₄ ⁻) salts are linchpins across synthesis and catalysis, key constituents of ionic liquids, and promising electrolytes and additives in next-generation multivalent rechargeable batteries. Despite this reach, quantitative links between crystal chemistry and anion motion remain scarce. Here, we present a molecular dynamics study of four such salts, namely M(BF₄)₂ (M = Ca²⁺, Sr²⁺, Ba²⁺, Cd²⁺), by means of nuclear magnetic resonance spectroscopy (NMR). We measured the temperature dependence of static ¹⁹F NMR spectra and spin-lattice relaxation and determined the values of activation energies for the thermally-activated reorientations for the BF₄ ⁻ tetrahedra. We draw a comparison with systems with similar geometries, such as the well-studied complex metal borohydrides.