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Correlating intermolecular cross-relaxation rates with distances and coordination numbers in ionic liquids

journal contribution
posted on 20.12.2018, 00:00 authored by Pierre Alexandre Martin, Fangfang ChenFangfang Chen, Maria ForsythMaria Forsyth, Michaël Deschamps, Luke O'DellLuke O'Dell
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.

History

Journal

Journal of physical chemistry letters

Volume

9

Issue

24

Pagination

7072 - 7078

Publisher

American Chemical Society

Location

Washington, D.C.

eISSN

1948-7185

Language

eng

Publication classification

C1 Refereed article in a scholarly journal

Copyright notice

2018, American Chemical Society