Structure and dynamics elucidation of ionic liquids using multidimensional Laplace NMR

Abstract

aWe demonstrate the ability of multidimensional Laplace NMR (LNMR), comprising relaxation and diffusion experiments, to reveal essential information about microscopic phase structures and dynamics of ionic liquids that is not observable using conventional NMR spectroscopy or other techniques.Ionic liquids (ILs) are salts that consist of ions and have, by definition, melting points below 100 1C. They have unique physical and chemical properties such as high ionic conductivity, negligible vapour pressure, nonflammability, broad liquid phase temperature ranges, and high thermal stability, which all make ILs attractive in many scientific and technological applications. The applications include organic synthesis and catalysis, gas separation, extraction of metals, lubrication, electrochemistry, crystallization media for pharmaceutically active compounds and functional materials, etc. 1 NMR relaxation and diffusion experiments provide versatile information about the dynamics and structure of substances such as proteins, polymers, liquid crystals and porous media.2 They may also improve chemical resolution by distinguishing different components in complex systems without spectral resolution. 3 The relaxation and diffusion data consist of exponentially decaying components and the processing requires a Laplace inversion in order to determine diffusion coefficient and relaxation time distributions. 2 Therefore, these methods are referred to as Laplace NMR (LNMR). Like in traditional NMR spectroscopy, a multidimensional approach significantly enhances the resolution and information content of LNMR. 4 The approach makes it possible to correlate diffusion coefficients and relaxation times, and enables the investigation of chemical exchange even in the case when the exchanging sites are not resolved in the spectrum. The method requires a reliable and robust multidimensional Laplace inversion algorithm for extracting the diffusion coefficient and relaxation time distributions from the experimental data. 5NMR relaxation and diffusion experiments have been widely used to investigate physicochemical properties, hydrogen bonding, aggregation, solvation dynamics and atomic level interactions of ILs.6 However, to the best of our knowledge, the ability of Laplace inversion algorithms to provide distributions of relaxation times and diffusion coefficients has not yet exploited in studies of ILs. Furthermore, the potential of multidimensional LNMR to provide unique information about correlations and exchange is yet unexplored in the IL context. In this work, we demonstrate that a combination of several one-dimensional (1D) and two-dimensional (2D) LNMR experiments can provide important microscopic information about the phase structures of ILs, which is not possible using conventional NMR spectroscopy or other methods. We concentrate on the investigation of a halogen-free orthoborate based ionic liquid (hf-BIL).7 The affinity to absorb water, high polarity and toxicity make halogen containing ILs undesirable in many applications, and h...

Mechanisms and Beyond: Elucidation of Fluxional Dynamics by Exchange NMR Spectroscopy

“…Low-temperature 1 H-1 HE XSY studies and crossover experiments revealed that the observed equilibration was due to ion-pair rearrangement and ligand flip, with as ignificantly lower barrierf or the ligand flip process. Te lkki and co-workers [131] demonstrated the utility of Laplace NMR spectroscopy [132] for studying chemical exchange. Laplace-transformation LNMR gives information on relaxation T 2 :i onic liquid 81 hast wo different phases,o ne slower than the other.A2D T 2 -T 2 domain experiment showed exchange of ions between the phases, with an "ion lifetime" of 0.92 sf or the fast phase and 55 sf or the slow phase.…”

Diffusion of Ions in Phosphonium Orthoborate Ionic Liquids Studied by 1H and 11B Pulsed Field Gradient NMR

“…The usefulness of NMR methods to study ion diffusion in ILs has been validated in a number of previously reported works (Annat et al, 2007;Burrell et al, 2010;Frise et al, 2011;Hayamizu et al, 2011;Filippov et al, 2013Filippov et al, , 2014Filippov et al, , 2015Filippov et al, , 2016Taher et al, 2014;Javed et al, 2017;. The diffusion properties of one tetraalkylphosphonium orthoborate-based IL that contains bis(mandelato)borate anions, [P 6,6,6,14 ][BMB], has been the most thoroughly investigated (Filippov et al, 2013;Javed et al, 2017).…”

Hyperpolarized Laplace NMR

“…(b) T 2 – T 2 exchange map of water in a porous sandstone sample . (c) D – T 2 correlation and (d) T 2 – T 2 exchange maps of a halogen‐free orthoborate‐based ionic liquid . Panel (a) reprinted with permission from J. Cox, P. J. McDonald, B.…”

“…Figure c shows a D – T 2 correlation map of a halogen‐free orthoborate‐based ionic liquid . The map reveals two coexisting dynamic “phases” with significantly different diffusion coefficients.…”