Probing Conformational Evolution and Associated Dynamics of Mg(N(SO2CF3)2)2•Dimethoxyethane Adduct Using Solid-State 19F and 1H NMR

Bis(trifluoromethanesulfonimide) or TFSI is widely used as a counter anion in electrolyte design due to its structural flexibility and chemical stability. We studied the conformational variations and associated dynamics of TFSI in adduct of Mg(TFSI)(2) with dimethoxyethane (DME), a solvate crystalline material using solid-state H-1 and F-19 NMR. TFSI molecular motion in this solvate structure falls within the timescale of the F-19 NMR experiment, yielding spectroscopic signatures for unique TFSI conformers under the coordination environment of Mg2+ cation. Within the temperature range of -5 to 82 degrees C, we observe nine distinct TFSI sites in both crystalline and disordered regions using F-19 NMR, reflecting complexity of structural and dynamics of TFS1 anions within solvate structure. The four distinguishable sites in the disordered region for the two CF3 groups of the same TFSI molecule are identified using chemical shift analysis. The exchange rate constants from site to site are calculated through variable temperature F-19 NMR and twodimensional (2D) exchange spectroscopy (EXSY) experiments, along with respective activation enthalpies using Eyring's formulation. The flip rate of CF3 around the S-C bond is estimated as similar to 15 s(-1) at 8 degrees C with Delta H-not equal similar to 22 kJ/mol, but the rotation of the entire TFSI is 4.8 s(-1) at 8 degrees C with a significantly greater Delta H-not equal = 98 +/- 10 kJ/mol. Furthermore, the slow conversion of trans to cis conformers at a lower temperature (T <= 1 degrees C) in the crystalline region is monitored, with a conversion rate of similar to 2 x 10(-5) s(-1) at -5 degrees C. Density functional theory (DFT)-based calculations were performed to support further the assignment of experimental chemical shifts, and the activation energy E-a = 21.1 kJ/mol obtained for the cis to trans transition is consistent with experimental values. The combined set of F-19 and H-1 under both one-dimensional (1D) and 2D NMR methods demonstrated here can be further used for examining electrode-electrolyte interfaces to probe the motions of various constituents that can enable detailed studies of interfacial processes and dynamics. Ultimately, such studies will aid in the design and discovery of interfacial constructs in which directed defect chemistry, chemical moiety distribution, and nanostructure are employed to drive efficient charge transport.