Radiation Induced Redox Reactions and Fragmentation of Constituent Ions in Ionic Liquids. 2. Imidazolium Cations

Abstract

In part 1 of this study, radiolytic degradation of constituent anions in ionic liquids (ILs) was examined. The present study continues the themes addressed in part 1 and examines the radiation chemistry of 1,3-dialkyl substituted imidazolium cations, which currently comprise the most practically important and versatile class of ionic liquid cations. For comparison, we also examined 1,3-dimethoxy- and 2-methyl-substituted imidazolium and 1-butyl-4-methylpyridinium cations. In addition to identification of radicals using electron paramagnetic resonance spectroscopy (EPR) and selective deuterium substitution, we analyzed stable radiolytic products using (1)H and (13)C nuclear magnetic resonance (NMR) and tandem electrospray ionization mass spectrometry (ESMS). Our EPR studies reveal rich chemistry initiated through "ionization of the ions": oxidation and the formation of radical dications in the aliphatic arms of the parent cations (leading to deprotonation and the formation of alkyl radicals in these arms) and reduction of the parent cation, yielding 2-imidazolyl radicals. The subsequent reactions of these radicals depend on the nature of the IL. If the cation is 2-substituted, the resulting 2-imidazolyl radical is relatively stable. If there is no substitution at C(2), the radical then either is protonated or reacts with the parent cation forming a C(2)-C(2) σσ*-bound dimer radical cation. In addition to these reactions, when methoxy or C(α)-substituted alkyl groups occupy the N(1,3) positions, their elimination is observed. The elimination of methyl groups from N(1,3) was not observed. Product analyses of imidazolium liquids irradiated in the very-high-dose regime (6.7 MGy) reveal several detrimental processes, including volatilization, acidification, and oligomerization. The latter yields a polymer with m/z of 650 ± 300 whose radiolytic yield increases with dose (~0.23 monomer units per 100 eV for 1-methyl-3-butylimidazolium trifluorosulfonate). Gradual generation of this polymer accounts for the steady increase in the viscosity of the ILs upon irradiation. Previous studies at lower dose have missed this species due to its wide mass distribution (stretching out to m/z 1600) and broad NMR lines, which make it harder to detect at lower concentrations. Among other observed changes is the formation of water immiscible fractions in hydrophilic ILs and water miscible fractions in hydrophobic ILs. The latter is due to anion fragmentation. The import of these observations for use of ILs as extraction solvents in nuclear cycle separations is discussed.

Do TFSA Anions Slither? Pressure Exposes the Role of TFSA Conformational Exchange in Self-Diffusion

“…Complete details regarding the synthesis can be found in the Supporting Information of Shkrob, et al . 33 Their structures are shown in Figure 1. The syntheses of EMIM FSI and EMIM BF 4 are reported in the Supporting Information.…”

“…JLH originally prepared the selectively deuterated imidazolium TFSA ILs and BMIM TFSA for the work described in Ref. 33. SR prepared the EMIM FSA and EMIM BF 4 salts.…”

Photocatalytic Reverse Semi‐Combustion Driven by Ionic Liquids

“…The possible involvement of radical species during the photolysis of BMIm.CO 2 (Figure 1b)w as verified by using electron spin resonance( ESR), and the predicted ESR signal of the imidazolium radical cation [BMIm]C + [20] was observed at 3365 G after 15 min of irradiation at 254 nm (À196 8C). The simulated and detected ESR signals are in excellent agreement (Figures 1b and S4).…”

“…[15,16] Hence, ILs may constitute an attractive materialf or CO 2 photoreduction. [20] In addition, CO 2 radical anion ([CO 2 ]C À )c ould be pro-The simple photolysis of CO 2 in aqueous solutionst og enerate CO and/or hydrocarbons and derivatives in the presence of a catalysti sc onsidered to be ac leana nd efficient approachf or utilizing CO 2 as aC 1b uildingb lock. [18] The objective of this work is to develop IL photocatalysts that would promote the formation of activated-CO 2 intermediates, which then react with water photoproducts to produce CO akin to electrochemically inducedp athways.…”

Iron Fluoroanions and Their Clusters by Electrospray Ionization of a Fluorinating Ionic Liquid

“…Finding degradation products has been a common objective for ESI studies of ILs, for example measurement of hexafluorophosphate degradation products caused by electrolysis of water [50], or radiolysis products [51]. Normally, ESI-MS studies are conducted by diluting the ILs in polar or even nonpolar solvents [42,52].…”