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

Journal Article · · Journal of Physical Chemistry. C
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  1. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States). Joint Center for Energy Storage Research (JCESR)
  2. Argonne National Laboratory (ANL), Lemont, Illinois 60439, United States; The Joint Center for Energy Storage Research (JCESR), Lemont, Illinois 60439, United States
  3. Argonne National Laboratory (ANL), Argonne, IL (United States). Joint Center for Energy Storage Research (JCESR)
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Bis(trifluoromethanesulfonimide) or TFSI is widely used as a counter anion in electrolyte design due to its structural flexibility and chemical stability. Here 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 1H and 19F NMR. TFSI molecular motion in this solvate structure falls within the timescale of the 19F NMR experiment, yielding spectroscopic signatures for unique TFSI conformers under the coordination environment of Mg2+ cation. Within the temperature range of -5 to 82 °C, we observe nine distinct TFSI sites in both crystalline and disordered regions using 19F 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 19F NMR and two-dimensional (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 ~ 15 s-1 at 8 °C with ΔH≠ ~ 22 kJ/mol, but the rotation of the entire TFSI is 4.8 s-1 at 8 °C with a significantly greater ΔH≠ = 98 ± 10 kJ/mol. Furthermore, the slow conversion of trans to cis conformers at a lower temperature (T ≤ 1 °C) in the crystalline region is monitored, with a conversion rate of ~ 2 x 10-5 s-1 at -5 °C. Density functional theory (DFT)-based calculations were performed to support further the assignment of experimental chemical shifts, and the activation energy Ea = 21.1 kJ/mol obtained for the cis to trans transition is consistent with experimental values. The combined set of 19F and 1H 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.
Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); USDOE Office of Science (SC), Biological and Environmental Research (BER)
Grant/Contract Number:
AC02-06CH11357; AC06-76RL01830
OSTI ID:
2205167
Journal Information:
Journal of Physical Chemistry. C, Journal Name: Journal of Physical Chemistry. C Journal Issue: 9 Vol. 124; ISSN 1932-7447
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English

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