Correlating Intermolecular Cross-Relaxation Rates with Distances and Coordination Numbers in Ionic Liquids

Abstract

The HOESY NMR experiment is commonly used to probe ion associations in ionic liquids and their mixtures. The parameter measured in this experiment is the heteronuclear cross-relaxation rate σ, which has dimensions of s–1. For intramolecular NOEs this scales as r –6 where r is the internuclear distance, but in the intermolecular case (as typically probed in studies of ionic liquids), theory predicts a more complex behavior including a distance dependence that is affected by the relative frequencies of the nuclei involved. Specifically, for nuclei with similar resonance frequencies such as 1H and 19F, it has been predicted that intermolecular NOEs will be sensitive to longer range distances than for nuclei with very different frequencies such as 1H and 7Li. In this contribution, we test this theory using a combination of quantitative HOESY analysis and molecular dynamics simulations carried out on two different ionic liquid electrolyte systems. In agreement with theoretical predictions, we find excellent correlations between the experimentally measured 1H–7Li NOEs and carbon–lithium distances below 4 Å, while longer distances (>6 Å) must be considered in order to obtain good correlations between 1H–19F NOEs and carbon–fluorine coordination numbers. This demonstrates the utility of HOESY NMR in understanding structure and interactions in ionic liquids while also illustrating that care must be taken in interpreting the measured cross-relaxation rates.

Investigating Intermolecular Interactions in a DME-Based Hybrid Ionic Liquid Electrolyte by HOESY NMR

“…Gabl et al (2013) proposed that intermolecular NOEs measured between nuclear spins with similar frequencies, such as 1 H and 19 F, are sensitive to longer distances than nuclear spins whose frequencies are far apart. Our very recent work corroborates this prediction (Martin et al, 2018a), demonstrating the validity of interpreting intermolecular NOEs in terms of distances provided that such effects are taken into consideration.…”

“…Cross-relaxation rates for 7 Li- 1 H and 19 F- 1 H for each sample were measured at 20°C using a 1 H-detected HOESY pulse sequence, following an identical procedure to that previously reported (Martin et al, 2018a) and with the same NMR spectrometer and probe used for the longitudinal relaxation measurements. Mixing times for the HOESY build-up curves were varied over 22 values ranging from 3.7 ms to 4.5 s and 440 scans were recorded for each mixing time.…”

Enhanced Ion Transport in an Ether Aided Super Concentrated Ionic Liquid Electrolyte for Long-Life Practical Lithium Metal Battery Applications

“…Results for the neat ionic liquid C3mpyrFSI were taken from our previous work in which the computational details are reported. 26 The 80IL20DME system consisted of 108 C3mpyrFSI ion pairs (IPs), 86 LiFSI ion pairs and 43 DME molecules. All of these molecules are packed in a cubic simulation box randomly using Packmol code.…”

Cation‐Structure Effects on Zinc Electrodeposition and Crystallographic Orientation in Ionic Liquids

“…[36,37] Of course, the modification of side chain of cations can also increase the efficiency of electrochemical reactions not only through the tuned physicochemical properties such as the reduction of viscosity but also through the active involvement of the functional groups. [38][39][40][41] For instance, in 1-((2-methoxyethoxy)methyl)-1-methylpiperidinium bis(trifluoromethanesulfonyl)amide (Pip 1,1O2O1 -TFSA), where two ether oxygen is introduced, the average solvation number of TFSA anions to Li + is reduced compared to that in 1-hexyl-1methlypiperidinium bis(trifluoromethanesulfonyl)amide (Pip 1,6 -TFSA) with only alkyl group of same chain length, leading to high electrode performance as well as rapid charge/discharge properties due to the improved Li + diffusion in the electrolyte. [42][43][44] The number and position of oxygen atoms in functional groups for pyrrolidinium-based ionic liquid have been investigated systematically using the combination of spectroscopic analyses and molecular dynamics (MD) calculations.…”