Molecular motions of acetonitrile molecules in the solvation shell of lithium ions

Lithium ion solutions in organic solvents have become ubiquitous because of their use in energy storage technologies. The widespread use of lithium salts has prompted a large scientific interest in elucidating the molecular mechanisms, giving rise to their macroscopic properties. Due to the complexity of these molecular systems, only few studies have been able to unravel the molecular motions and underlying mechanisms of the lithium ion (Li⁺) solvation shell. Lately, the atomistic motions of these systems have become somewhat available via experiments using ultrafast laser spectroscopies, such as two-dimensional infrared spectroscopy. However, the molecular mechanism behind the experimentally observed dynamics is still unknown. To close this knowledge gap, this work investigated solutions of a highly dissociated salt [LiTFSI: lithium bis(trifluoromethanesulfonyl)imide] and a highly associated salt (LiSCN: lithium thiocyanate) in acetonitrile (ACN) using both experimental and theoretical methods. Linear and non-linear infrared spectroscopies showed that Li⁺ is found as free ions and contact ion pairs in ACN/LiTFSI and ACN/LiSCN systems, respectively. In addition, it was also observed from the non-linear spectroscopy experiments that the dynamics of the ACN molecules in the Li⁺ first solvation shell has a characteristic time of ∼1.6 ps irrespective of the ionic speciation of the cation. A similar characteristic time was deducted from ab initio molecular dynamics simulations and density functional theory computations. Moreover, the theoretical calculations showed that molecular mechanism is directly related to fluctuations in the angle between Li⁺ and the coordinated ACN molecule (Li⁺⋯N≡C), while other structural changes such as the change in the distance between the cation and the solvent molecule (Li⁺⋯N) play a minor role. Overall, this work uncovers the time scale of the solvent motions in the Li⁺ solvation shell and the underlying molecular mechanisms via a combination of experimental and theoretical tools.