Counterintuitive\nIsomerization of TFSI<sup>–</sup> and TFSI<sup>–</sup>–Cation Correlated Isomerization:\nInsights into the Low Melting Points of TFSI<sup>–</sup>‑Based\nIonic Liquids

Ionic liquids (ILs), particularly bis(trifluoromethane)sulfonamide\n(TFSI<sup>–</sup>)-based ILs, have attracted substantial attention\nin electrochemical energy storage, ionic gating for superconductivity,\nand iontronic sensing. However, underestimating TFSI<sup>–</sup> isomerization and overlooking TFSI<sup>–</sup>–cation\ncorrelation make the origin of their most characteristic property,\nlow melting points (<i>T</i><sub>m</sub>), ambiguous. Traditional\nstatic electronic structure calculations assume that <i>C</i><sub>2</sub>-symmetric <i>trans</i> enantiomers of TFSI<sup>–</sup> easily isomerize into <i>cis</i> enantiomers\nthrough four symmetrically equivalent pathways over a barrier of 7.1\nkJ mol<sup>–1</sup>. Herein, ab initio molecular dynamics (AIMD)\nsimulations combined with metadynamics reveal that the unusual oscillation\nof the central nitrogen atom promotes TFSI<sup>–</sup> to undergo\ncomplex isomerization. Specifically, asymmetric <i>trans</i>-to-<i>cis</i> diastereomers of TFSI<sup>–</sup> experience restricted interconversion (20–52 kJ mol<sup>–1</sup>) through four distinct asymmetric pathways. The adaptive oscillation\nand hybridization of chiral nitrogen boost n<sub>N</sub> →\nσ*<sub>S–C</sub> negative hyperconjugation for stabilizing\nconformational structures and enlarging energy barriers. The orientational\ndistortion of oxygen atoms’ lone pairs enhances conjugation\nbut breaks the <i>C</i><sub>2</sub>-symmetry. The coexistence\nof both helicity and chiral nitrogen breaks the enantiomeric relationship.\nFurthermore, Raman characterization and AIMD simulations confirm the\npositive correlation between the relative stability of <i>cis</i>-TFSI<sup>–</sup> and its countercation’s polarity.\nTFSI<sup>–</sup> and countercations make a mutual conformational\nselection instead of free isomerization. Surprisingly, <i>T</i><sub>m</sub> increases with the cation-dependent conformational rigidity\nof TFSI<sup>–</sup>, offering new fundamental insights into\nthe low <i>T</i><sub>m</sub> of ILs. This descriptor encoding\nthe dependence of thermal property on cation–anion correlated\nisomerization provides material design guidelines and property prediction\ncapability.