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Increased Stability of Nitroxide Radicals in Ionic Liquids: More than a Viscosity Effect

Abstract

Radical stability has been subject to continuous research due to its importance in polymerization as well as in all-organic batteries. Recently, the SOMO-HOMO conversion was identified as the main factor in controlling the stability of distonic radicals, for which the negative charge resides on the same molecule. Based on this finding, the idea of ionic liquids stabilizing radicals was hypothesized in this study. A series of ionic liquids were tested in EPR measurements of the 1-Hydroxy-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole-3-carboxylic acid radical. Unusually high rotational diffusion constants (τR), 4 times larger compared to conventional media such as DCM, were recorded at room temperature. This finding could only be explained by a strong interaction existing between the radical and ionic liquid ions, which was confirmed with quantum chemical calculations, with interaction energies falling between -17.1 kJ mol-1 for tetramethylphosphonium tetrafluorob-orate and -85.6 kJ mol-1 for 1,3-dimethylimidazolium triflate. Elevated temperature measurements performed at 80 °C reduced the viscosity of the ionic liquids to that of DCM, while the τR values remained relatively high, thus further confirm-ing that the rotational hindrance occurred due to radical-ionic liquid interactions. The calculated interaction energies between the radical and ionic liquids ions were also found to correlate well with experimental rotational diffusion constants, thus offering us a valuable tool in tailoring ionic liquids for enhanced stability of nitroxide radicals. The findings of this study showcase the ability of ionic liquids to reduce reactivity of nitroxides without the need for any chemical modification of the radical.

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Publication details

The article was received on 31 Jul 2018, accepted on 10 Oct 2018 and first published on 10 Oct 2018


Article type: Paper
DOI: 10.1039/C8CP04854A
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Increased Stability of Nitroxide Radicals in Ionic Liquids: More than a Viscosity Effect

    L. Wylie, Z. L. Seeger, A. N. Hancock and E. I. Pas, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP04854A

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