Molecular Dynamics in Ionic Liquid/Radical Systems

Abstract

Molecular dynamics of the ionic liquid 1-ethyl-3-methylimidazolium bis­(trifluoromethyl sulfonyl)­imide (Emim-Tf2N) with either of the four organic stable radicals, TEMPO, 4-benzoyloxy-TEMPO, BDPA, and DPPH, is studied by using Nuclear Magnetic Resonance (NMR) and Dynamic Nuclear Polarization (DNP). In complex fluids at ambient temperature, NMR signal enhancement by DNP is frequently obtained by a combination of several mechanisms, where the Overhauser effect and solid effect are the most common. Understanding the interactions of free radicals with ionic liquid molecules is of particular significance due to their complex dynamics in these systems, influencing the properties of the ion-radical interaction. A combined analysis of EPR, DNP, and NMR relaxation dispersion is carried out for cations and anions containing, respectively, the NMR active nuclei 1H or 19F. Depending on the size and the chemical properties of the radical, different interaction processes are distinguished, namely, the Overhauser effect and solid effect, driven by dominating dipolar or scalar interactions. The resulting NMR relaxation dispersion is decomposed into rotational and translational contributions, allowing the identification of the corresponding correlation times of motion and interactions. The influence of electron relaxation time and electron–nuclear spin hyperfine coupling is discussed.

Solid-like Dynamic Nuclear Polarization Observed in the Fluid Phase of Lipid Bilayers at 9.4 T

“…A similar trend has been observed at X-band in ionic liquids using nitroxide and BDPA radicals as polarizing agents. 32 Surprisingly, we did not observe any Overhauser DNP enhancement in the BDPA/DMPC samples under our experimental conditions, differently from observations of Overhauser-like positive DNP enhancements observed at lower temperatures in solids. 21−26 Indeed, the negative dipolar-coupling induced proton Overhauser DNP of BDPA in liquid solutions is much less efficient at high magnetic fields compared to nitroxide radicals.…”

“…This will on the one hand significantly reduce the possibility of efficiently saturating allowed and forbidden EPR transitions and on the other hand lead to much less defined field profiles for the SE. A similar trend has been observed at X-band in ionic liquids using nitroxide and BDPA radicals as polarizing agents …”

“…This is only possible, if the anisotropic part of the hyperfine interaction is not averaged out, as usual is the case in liquid samples by fast rotational and translational motion of the radical. It has been shown at X-band frequencies that for viscous samples a transition from Overhauser effect to SE DNP can be observed if the rotational correlation time of the radical–target complex becomes slow enough. − The lateral diffusion of lipid molecules and the lipid reorientation in the bilayers are indeed slow enough even in the fluid phase of DMPC at our experimental conditions. A second important requirement to observe efficient SE DNP is a high enough microwave B 1 field strength.…”

Stable organic radicals and their untapped potential in ionic liquids

“…[43] Recent theoretical studies have highlighted that the propensity of the aminoxy anion to undergo side reactions can be decreased through replacement of the aqueous media with an appropriate IL. [44][45][46][47][48] Tailoring the IL, rather than substitution of the nitroxide, can decrease reactivity by increasing diffusion rates, [44] increasing the redox potential of the nitroxide, 4.7-5.5 eV (cf. 2.2 eV in aqueous media), [45] and improving reversibility in flow systems.…”

Three mechanisms of room temperature dynamic nuclear polarization occur simultaneously in an ionic liquid

“…2 is complex and cannot be described using only the EPR lineshape, which in Fig. 2 corresponds to the 19 F OE contribution. The three DNP mechanisms, namely OE, SE and CE/TM, are described as follows: 6 where g(o MW ) is the EPR lineshape, o MW and o n are microwave frequency and Larmor nuclear frequency, respectively.…”