Radiation Induced Redox Reactions and Fragmentation of Constituent Ions in Ionic Liquids. 1. Anions

Abstract

Room temperature ionic liquids (IL) find increasing use for the replacement of organic solvents in practical applications, including their use in solar cells and electrolytes for metal deposition, and as extraction solvents for the reprocessing of spent nuclear fuel. The radiation stability of ILs is an important concern for some of these applications, as previous studies suggested extensive fragmentation of the constituent ions upon irradiation. In the present study, electron paramagnetic resonance (EPR) spectroscopy has been used to identify fragmentation pathways for constituent anions in ammonium, phosphonium, and imidazolium ILs. Many of these detrimental reactions are initiated by radiation-induced redox processes involving these anions. Scission of the oxidized anions is the main fragmentation pathway for the majority of the practically important anions; (internal) proton transfer involving the aliphatic arms of these anions is a competing reaction. For perfluorinated anions, fluoride loss following dissociative electron attachment to the anion can be even more prominent than this oxidative fragmentation. Bond scission in the anion was also observed for NO(3)(-) and B(CN)(4)(-) anions and indirectly implicated for BF(4)(-) and PF(6)(-) anions. Among small anions, CF(3)SO(3)(-) and N(CN)(2)(-) are the most stable. Among larger anions, the derivatives of benzoate and imide anions were found to be relatively stable. This stability is due to suppression of the oxidative fragmentation. For benzoates, this is a consequence of the extensive sharing of unpaired electron density by the π-system in the corresponding neutral radical; for the imides, this stability could be the consequence of N-N σ(2)σ(*1) bond formation involving the parent anion. While fragmentation does not occur for these "exceptional" anions, H atom addition and electron attachment are prominent. Among the typically used constituent anions, aliphatic carboxylates were found to be the least resistant to oxidative fragmentation, followed by (di)alkyl phosphates and alkanesulfonates. The discussion of the radiation stability of ILs is continued in the second part of this study, which examines the fate of organic cations in such liquids.

The low temperature radiolysis of cis-syn-cis- dicyclohexano-18-crown-6 complexes with alkaline earth metal nitrates: An evidence for energy transfer to the macrocyclic ligand

“…The parameters of the species responsible for these signals are given in Table 1. They should be attributed to the NO 3 2− radical dianions, in reasonable agreement with previously published data for irradiated AEM nitrates and other systems containing nitrateanions (Bannov and Miklin, 1997;Harrison and Ng, 1973;Shkrob et al, 2011;Žďénský and Šroubek, 1964). Thus, NO 3 2− appears to be the principal paramagnetic product in macrocyclic complexes irradiated to absorbed doses in the range of 10-40 kGy.…”

“…It is obvious that such a spectrum cannot belong to any radical resulting from the radiolysis of nitrate moiety. Indeed, it is well known that nitrate NO 3 and nitrosyldioxyl ONOO radicals (major hole products in irradiated nitrates (Bannov and Miklin, 1997;Nevostruev, 1997, 2003;Pak and Nevostruev, 2000;Shkrob et al, 2011)) are characterized by insignificant contribution of 14 N orbital to the localization of spin density and small nitrogen hyperfine coupling constants. The corresponding values of a ⊥ ¼ 0.36 mT, a || ¼0.41 mT, Fig.…”

The First Lithium-ion Battery with Ionic Liquid Electrolyte Demonstrated in Extreme Environment of Space

“…Shkrob et al have reported the radiation stability of ionic liquids with reference to their application to nuclear fuel processing. [41][42][43] Sulfonyl imide anions typified by bis(trifluoromethylsulfonyl)imide, whose structure contains SO 2 N É , can be easily oxidized through production of an imidyl radical (CF 3 SO 2 N•), which usually involves chain radiolysis of ionic liquids. 41,42 An irreversible radiolytic reaction of alkylated ammonium cations (and perhaps other aliphatic cations) can occur through elimination of their alkyl chain in a high-dose environment.…”

“…41 Even alkylated imidazolium cations, one of the typical aromatic cations comprising many ionic liquids, can decompose radiolytically via several degradation pathways. 43 It is important to understand ionic liquids' radiation stabilities in space environment because they are now investigated for aiming at the application of not only batteries but also other space technology, such as space propulsion, [44][45][46][47] space lubricants, [48][49][50] and artificial muscles. 51 We did not observe any apparent change in shape and volume for three cells.…”

Applications of EPR and ENDOR Spectrum Simulations in Radiation Research

“…of metal ions in unusual valence states [111] and from technological points of view [112], for instance in treatment of nuclear waste. EPR spectra of transition metal complexes have frequently been simulated exactly in recent treatments [43,[58][59][60][61][62][113][114][115][116][117].…”