Article pubs.acs.org/JPCB
Cite This: J. Phys. Chem. B 2018, 122, 1992−2000
Evidence for Indirect Action of Ionizing Radiation in 18-Crown‑6 Complexes with Halogenous Salts of Strontium: Simulation of Radiation-Induced Transformations in Ionic Liquid/Crown Ether Compositions Olga A. Zakurdaeva,† Sergey V. Nesterov,*,†,‡ Natalya A. Sokolova,§ Pavel V. Dorovatovskii,∥ Yan V. Zubavichus,∥ Victor N. Khrustalev,⊥ Andrey F. Asachenko,⊥,# Gleb A. Chesnokov,‡,# Mikhail S. Nechaev,‡,# and Vladimir I. Feldman‡,† †
Enikolopov Institute of Synthetic Polymer Materials of RAS, ul. Profsoyuznaya, 70, Moscow 117393, Russia Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia § Zelinsky Institute of Organic Chemistry of RAS, Leninsky Prospect, 47, Moscow 119991, Russia ∥ NRC Kurchatov Institute, Acad. Kurchatov Sq., 1, Moscow 123182, Russia ⊥ Peoples’ Friendship University of Russia (RUDN University), ul. Miklukho-Maklay, 6, Moscow 117198, Russia # Topchiev Institute of Petrochemical Synthesis of RAS, Leninsky Prospect, 29, Moscow 119991, Russia ‡
S Supporting Information *
ABSTRACT: Ionic liquid/crown ether compositions are an attractive alternative to traditional extractants in the processes for spent nuclear fuel and liquid radioactive wastes reprocessing. These compositions are exposed to ionizing radiation, and their radiation stability, especially in the presence of metal salts, is a crucial issue. In the present study, the macrocyclic 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes simulating the components of metal loaded ionic liquid/crown ether extractants were synthesized and their structures were characterized by FTIR spectroscopy and single-crystal X-ray diffraction analysis. Inclusion of Sr2+ cation into the 18C6 cavity resulted in more symmetric D3d conformations of the macrocycle. The structural transformations of the crown ether were accompanied by an elongation of polyether CO bonds that could increase the possibility of radiolytic cleavage of the macrocycle. However, EPR study of the synthesized compounds subjected to X-ray irradiation revealed predominant formation of macrocyclic −CH2−Ċ H−O− radicals. This result demonstrated an evidence for indirect action of ionizing radiation on individual components of the complexes and was reasonably described by a positive “hole” transfer from primary macrocyclic radical cation to fluorous anion at the primary stages of radiolysis and a subsequent interaction of fluorine atom with 18C6 macrocycle in secondary radical reactions. The observed effects may be partially responsible for enhanced sensitivity of the ionic liquid/crown ether extractants to ionizing radiation due to chemical blocking of the crown ether with radiolytic HF, radiation-chemical degradation of the 18C6, and precipitation of a low-soluble SrF2.
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INTRODUCTION Requirements for environmental protection demand a design of new advanced extractants for selective removal of radionuclides from spent nuclear fuel and radioactive wastes.1 Since the pioneer study of Dai and coauthors,2 the room temperature ionic liquids (RTIL) were considered as an alternative to the traditional solvents, such as branched alkanes, aliphatic alcohols, aromatics, etc., in the compositions of extractants based on crown ethers (CE), which are particularly attractive for the removal of radioactive 90Sr. In this situation, the RTIL/CE solutions would expose to ionizing irradiation, and radiation stability of the whole system is one of the key issues for its practical applicability. Although the radiation stability of neat RTIL at least is acceptable,3,4 the multicomponent RTIL/CE compositions have demonstrated a sharp decrease in cation binding ability due to radiation destruction under irradiation.5−7 Up to now, several © 2018 American Chemical Society
concepts were developed to explain a sensitivity of the RTIL/CE extractants to ionizing radiation. One of them relies on chemical “blocking” the macrocycle with acidic stable radiolytic products generated from ionic liquid.7 Actually, the reaction of acidic halogenous species with polyether ring produces a protonated form of the CE, and this effect may be responsible for decrease in the efficiency of strontium removal after the irradiation of RTIL/CE solutions. However, the accumulation of acids has been shown to be insignificant, for example, 0.7% for [C4mim][PF6] irradiated to 500 kGy,4 while an enhanced degradation of the functional characteristics in this dose range may be appreciable. In particular, the extraction ability of the system irradiated to Received: November 22, 2017 Revised: January 10, 2018 Published: January 12, 2018 1992
DOI: 10.1021/acs.jpcb.7b11498 J. Phys. Chem. B 2018, 122, 1992−2000
Article
The Journal of Physical Chemistry B
radiation destruction and a measurable contribution of the anions to the total mechanism of radiolysis. These processes can play an important role in radiolysis of macrocyclic extractants of more complicated chemical structure, such as DCH18C6 in “free” and complexed states. Here, we report the data on synthesis and structure of the macrocyclic complexes 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2, which are suitable to mimic the radiolysis of RTIL/CE extractants loaded with metal salts and discuss the mechanism of formation and transformations of radical species generated in these complexes under irradiation.
this dose has decreased by a factor of 3 as measured in terms of strontium distribution coefficient.7 On the other hand, it seems reasonable to expect masking the negative effect from the protonation of the CE with acidic products, when subsequent extraction stages with concentrated 3 M HNO3 occur,6 but this is not the case. A high radiation destruction of the crown ethers in γ-irradiated RTIL/CE solutions has been alternatively ascribed to covalent addition of radical species produced from anions of RTIL to a macrocycle.5 Despite the possibility of a contribution from these reactions into the secondary processes of radiolysis, one can expect more efficient interaction of these radical intermediates with RTIL cations. In view of the above-mentioned issues, it appears that some additional radiation-chemical channels may be involved in mechanism of radiolysis and degradation of RTIL/CE systems. For example, Shkrob and coauthors8 have suggested that undamaged CE reacted with acidic stable products of solvent radiolysis to produce protonated macrocyclic complexes, which are very efficient scavengers for the secondary electrons formed from the macrocycle and the RTIL. The subsequent neutralization of protonated complexes with these electrons generates ”additional” H atoms, which are able to react with CE with accumulation of macrocyclic −CH2−Ċ H−O− radicals. It enhances the total radiation destruction of the system. Another important aspect of radiation-chemical tests of the RTIL/CE extractants relates to the actual composition of the system, which would be exposed to ionizing radiation. The crown ether molecules are partly bound in macrocyclic complexes with metal cations under agitation of the organic RTIL/CE phase with the aqueous metal-loaded phase. The composition of the macrocyclic complex has been demonstrated to vary strongly depending on hydrophobicity of RTIL cations.9,10 In particular, an anionic component of the complex may originate from either metal salt dissolved in aqueous phase or RTIL itself. The latter case occurs, for example, upon metal cation extraction with RTIL through a cation exchange mechanism. As a result, the organic phase is loaded with macrocyclic complex having composition of CE·Sr2+(XFn−)2, where in XFn− is anion of RTIL, such as BF4−, PF6−, NTf2−, and so forth. As a rule, the complexation with metal cation during extraction process is ignored in radiation-chemical tests currently running, although the contribution of complexed macrocycle to extractant radiolysis may be measurable and understanding the structure of macrocyclic complex is of crucial importance to correct description of the radiolytic degradation mechanism. Nevertheless, to the best of our knowledge the experimental data on cation coordination and conformation of the macrocycle in the strontium complexes comprising RTIL anions is still lacking. From this point of view, the complexes of 18-crown-6 (18C6) with Sr(BF4)2 and Sr(PF6)2 salts are of special interest. Although 18C6 and RTIL based on tetrafluoroborate and hexafluorophosphate anions are not applicable for the strontium extraction due to their solubility in water, 18C6·Sr(XFn)2 complexes can be used as model compounds to demonstrate the energy transfer effects in metal loaded RTIL/CE extractants. They are characterized by a marked difference in ionization potentials of the organic ligand and anion XFn− and a high affinity of the anion to secondary electrons generated under irradiation.11,12 Therefore, one can expect a positive hole transfer from ionized macrocycle to anion and a dissociative electron capture at the primary stages of radiolysis. Further reactions of the radicals produced from the anion may initiate additional channels of CE
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EXPERIMENTAL SECTION All reagents were purchased from Sigma-Aldrich. SrCO3, HBF4, HPF6, methanol, diethyl ether, and acetonitrile were of analytical grade and used as received. Crown ether 18-crown-6 (18C6) was preliminary purified by complexing with acetonitrile.13 Synthesis. 18C6·Sr(BF4)2. To a suspension of 0.465 g (3.15 mmol) of SrCO3 in 30 mL of deionized H2O 0.79 mL (1.098 g, 6.0 mmol) of 48% aq. HBF4 was added dropwise. The reaction mixture was stirred at room temperature for 10 min (until acid neutralization) and filtered. The resulting solution was evaporated in vacuum; the obtained solid was dissolved in 30 mL of methanol, followed by addition of 0.872 g (3.3 mmol) of 18-crown-6. The resulting solution was stirred for 10 min, concentrated to ∼5 mL, filtered, and washed with 3 × 5 mL of diethyl ether. Obtained white solid was recrystallized from minimal amount of warm methanol to give 1.452 g (92%) of colorless crystals. Suitable for X-ray diffraction crystals were grown from a saturated solution of title compound in MeOH by slow diffusion of diethyl ether. 1 H NMR (600 MHz, methanol-d4) δ 3.88 (s, 1H). 13C{1H} NMR (151 MHz, methanol-d4) δ 70.92. 19F NMR (376 MHz, methanol-d4) δ −77.04 to −77.20 (m). 18C6·Sr(PF6)2. A suspension of 0.465 g (3.15 mmol) of SrCO3 and 0.872 g (3.3 mmol) of 18-crown-6 in 30 mL of MeOH was placed into a polypropylene beaker. To the reaction mixture 0.82 mL (1.347 g, 6 mmol) of 65% aq. HPF6 was added dropwise. The obtained solution was stirred for 10 min. After filtering the reaction mixture through a plug of Celite, 60 mL of diethyl ether was added slowly. The precipitated solid was filtered off and washed with additional portions of diethyl ether (3 × 5 mL). To purify the product, the obtained white solid was recrystallized from MeOH by diffusion of diethyl ether to give 1.481 g (77%) of colorless crystals. Suitable for X-ray diffraction crystals were grown from a saturated solution of title compound in acetonitrile by slow diffusion of diethyl ether. 1 H NMR (600 MHz, methanol-d4) δ 3.84 (s, 1H). 13C{1H} NMR (151 MHz, methanol-d4) δ 71.04. 19F NMR (376 MHz, methanol-d4) δ 1.94 (d, J = 707.2 Hz). 31P{1H} NMR (162 MHz, methanol-d4) δ −144.59 (hept, J = 707.7 Hz). Characterization. The synthesized complexes were characterized by DSC/TGA, NMR and FTIR-spectroscopy. Netzsch STA 443 F3 device was used for DSC/TGA analysis of the samples at scan rate of 5 K/min under dry argon flow. Weight of the complexes placed in alumina crucibles was ∼7 mg. NMR spectra were measured on Bruker “Avance 600” (600 MHz 1H, 151 MHz 13C) and Bruker “Avance 400” (376 MHz 19F, 162 MHz 31P). The 1H chemical shifts were referenced to the residual undeuterated solvent peak, 13C chemical shifts were referenced to the solvent peak, 19F NMR chemical shifts were referenced to external standard CF3CO2H (0.0 ppm), and 31P NMR chemical shifts were referenced to external standard 85% 1993
DOI: 10.1021/acs.jpcb.7b11498 J. Phys. Chem. B 2018, 122, 1992−2000
Article
The Journal of Physical Chemistry B
concentrations of paramagnetic species were calculated by double integrating the EPR spectra and using EPR signals from Mn2+ as a standard. Relative error of determination of the concentration of radicals was within 10%.
H3PO4 (0.0 ppm). FTIR spectra were measured using Nicolet iS50FT-IR spectrometer in the range 4000−400 cm−1 with a resolution of 2 cm−1. X-ray Diffraction Analysis. X-ray diffraction data were collected on the “Belok” beamline (λ = 0.96990 Å) of the National Research Center “Kurchatov Institute” (Moscow, Russian Federation) using a Rayonix SX165 CCD detector. A total of 360 images for each complex were collected using an oscillation range of 1.0° and φ scan mode, and corrected for absorption using the Scala program.14 The data were indexed, integrated and scaled using the utility iMOSFLM in CCP4 program.15 The structures were determined by direct methods and refined by full-matrix least-squares technique on F2 with anisotropic displacement parameters for non-hydrogen atoms. The hydrogen atoms were placed in calculated positions and refined within riding model with fixed isotropic displacement parameters [Uiso(H) = 1.2Ueq(C)]. All calculations were carried out using the SHELXTL program.16 Crystallographic data for 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 have been deposited with the Cambridge Crystallographic Data Center, CCDC 1568990 and CCDC 1568991, respectively. Radiolysis Study. The macrocyclic complexes were placed in EPR ampules manufactured from the special SK-4B glass (Russia), which gives no measurable EPR signal under irradiation. The samples were evacuated to a residual pressure of ∼0.13 Pa and sealed. X-ray irradiation of the samples was carried out at 77 K using a 5-BKhV-6 tube with a tungsten anode. The absorbed doses were calculated using the data of ferrous sulfate dosimetry (fsd) and taking into account conversion factors (K), Dsampl =Dfsd·K, where K = (μen/ρ)sampl/(μen/ρ)fsd; (μen/ρ)sampl and (μen/ρ)fsd are the mass energy-absorption coefficients for the irradiated substance and the ferrous sulfate solution, respectively. The μen/ρ values for elements and ferrous sulfate solution were taken from the NIST database.17 The conversion factors K of 0.557, 10.33, and 9.41 were calculated for 18C6 and its complexes with Sr(BF4)2 and Sr(PF6)2, respectively. EPR spectra of the radical species were measured using an EPR spectrometer with a 100 kHz high-frequency modulation (SPIN, Russia) at 77 K. Thermal annealing of the irradiated samples was carried out in thermocouple controlled oven. The g-values were determined from a comparison with DPPH and Mn2+ standards. Relative
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RESULTS AND DISCUSSION FTIR Analysis and Single Crystal Structure. FTIR spectra of 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes in the range of 1400−500 cm−1 (Figure 1 and Supporting Information, Table S1) demonstrate definite distinctions from that of “free” 18C6, which was identical to spectra described previously.18−20 As follows from the published data, the most intensive absorption bands at 1129 and 1105 cm−1 in the spectrum of uncomplexed 18C6 correspond to νas(COC) vibrational mode being sensitive to complexing with a metal cation. Indeed, these vibrational frequencies were measurably shifted to low-frequency region in the FTIR spectra of the complexes synthesized in present study, and intensive maximums revealed at ∼1094 and 1104 cm−1 for 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2, respectively. This shift is caused by an elongation of CO bonds of the macrocycle upon complexing with Sr2+ cations. In the case of 18C6·Sr(BF4)2, the region of 1150−1090 cm−1 is rather complicated for more detailed interpretation because the νas(COC) bands from the macrocycle are overlapped on the bands from asymmetric stretching mode of BF4− (ν3). The range of 1000−800 cm−1 in the FTIR spectra of these macrocyclic complexes is more informative due to its conformational sensitivities. The rocking vibrations of the methylene groups in 18C6 macrocycle has been demonstrated to depend on a conformation of −OCH2CH2O− moiety,18,19,21 which could be characterized with the values of torsion angles in this structural unit. In particular, an inclusion of potassium cation into 18C6 cavity is well-known to result in T−G−T-conformation of −OCH2CH2O− units (D3d symmetry) that accompanies with a decrease in the number of the respective absorption bands in FTIR spectrum.18 The same trend is clearly visible in the spectra of the 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes. Instead of a sharp doublet at 991 and 944 cm−1 revealed in free 18C6 (Ci symmetry of the macrocycle), the single lines were observed at 972 and 973 cm−1 in 18C6·Sr(BF4)2 and 18C6· Sr(PF6)2 complexes, respectively. This spectroscopic feature
Figure 1. FTIR spectra of 18C6 (a, black solid line), 18C6·Sr(BF4)2 (b, gray solid line) and 18C6·Sr(PF6)2 (c, black dash line) in mineral oil. 1994
DOI: 10.1021/acs.jpcb.7b11498 J. Phys. Chem. B 2018, 122, 1992−2000
Article
The Journal of Physical Chemistry B
(the Sr−F distances of 2.560(3), 2.632(3) and 2.583(3), 2.637(3) Å, respectively, for the molecules A and B) are arranged above and under the crown ring closing the ten-coordinated environment of the strontium cation. The chelate SrFBF four-membered rings of the two BF4− anions are almost planar (rms deviations are 0.046, 0.032 and 0.078, 0.015 Å, respectively, for the molecules A and B) and practically perpendicular to each other (the dihedral angles are 88.4(2) and 80.9(2)°, respectively, for the molecules A and B). Thus, complex 18C6· Sr(BF4)2 can be described as a peg top with the 18-crown-6 ether in the equatorial plane and the two BF4− anions in its vertexes. In contrast to 18C6·Sr(BF4)2, 18C6·Sr(PF6)2 complex crystallizes in monoclinic P21/n space group, with the two crystallographically independent molecules C and D (Figure 3 and Supporting Information, Tables S2, S8−S12). The molecules C and D are distinguished by a coordination type of the two PF6− anions to the strontium cation. So, in the molecule C, one PF6− anion is k3-coordinated and the other PF6− anion is k2-coordinated, whereas the molecule D has two k2-coordinated PF6− anions. As in 18C6·Sr(BF4)2 complex, the strontium cation in 18C6·Sr(PF6)2 is disposed within the 18-crown-6 polyether ring and coordinated to all six oxygen atoms. Interestingly, despite the more symmetrical conformation of the crown macrocycle in 18C6·Sr(PF6)2 complex, the Sr−O distances (2.702(2)−2.765(2) and 2.686(2)−2.761(2) Å, respectively, for the molecules C and D) are very close to those observed in complex 18C6·Sr(BF4)2. However, the SrF distances for the k2-coordinated PF6− anions (2.642(3), 2.668(2) and 2.600(2), 2.658(2) Å, respectively, for the molecules C and D) are somewhat longer than those found for the k2-coordinated BF4− anions in 18C6·Sr(BF4)2. Expectedly, the SrF distances for the k3-coordinated PF6− anion (2.707(2)−2.820(3) Å) are significantly longer than those for the k2-coordinated PF6− anions. The characteristic feature of the 18C6·Sr(PF6)2 complex is a “butterfly” conformation of the chelate SrF PF four-membered rings for the k2-coordinated PF6− anions (the fold angles on the F···F line are 23.21(10) and 10.21(15)°, respectively, for the molecules C and D). Furthermore, in contrast to complex 18C6·Sr(BF4)2, the chelate SrFPF four-membered rings of the two PF6− anions in the molecule D are in the eclipsed configuration. Thus, complex 18C6·Sr(PF6)2 can be also described as a peg top with the 18-crown-6 ether in the equatorial plane but with the different coordination types of the apical PF6− anions. Besides the peculiarities in the anion coordination, the conformations of polyether rings in 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes demonstrate key differences in comparison with free 18C6 due to the inclusion of Sr2+ cation into the macrocycle cavity. The CO distances in these complexes are elongated and the CC distances are slightly shortened (see Supporting Information, Table S13), and the values of all OCCO and COCC torsion angles in both complexes correspond to gauche (G) and trans (T) configuration of −OCH2CH2O− units, respectively. It is worth nothing that scattering in the values of torsion angles in 18C6·Sr(BF4)2 complex is higher. This agrees with our results of FTIR analysis revealed the lower symmetry of macrocycle in 18C6·Sr(BF4)2. Nevertheless, both complexes demonstrate D3d symmetry of 18C6 macrocycle with rather uniform conformational composition of polyether ring including six T−G−T moieties. The analysis given above allowed us to conclude the following. The complexation of 18C6 with the strontium salts
implies the formation of the more regular macrocyclic structure in these compounds, which is close to D3d, as compared to free 18C6. At the same time, the singlet line at 972 cm−1 in the FTIR spectrum of 18C6·Sr(BF4)2 is markedly broadened, and this finding testifies on lower symmetry of macrocycle in this complex relative to 18C6·Sr(PF6)2. It may be consequently concluded that the conformation of 18C6·Sr(BF4)2 complex falls in between disordered Ci structure of free 18C6 and most symmetrical D3d conformation of 18C6·Sr(PF6)2 complex. These conclusions are in a good agreement with the results of single-crystal X-ray diffraction study of 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes (Figures 2 and 3).
Figure 2. Molecular structure of complex 18C6·Sr(BF4)2. The two crystallographically independent molecules A and B are presented.
Figure 3. Molecular structure of complex 18C6·Sr(PF6)2. The two crystallographically independent molecules C and D are presented.
The 18C6·Sr(BF4)2 complex was found to crystallize in triclinic P1̅ space group, with the two crystallographically independent molecules A and B having similar geometries (Figure 2). Crystallographic data and the full geometrical parameters of this compound are available as Supporting Information (Tables S2−S7). The X-ray structural analysis reveals the strontium cation within the 18-crown-6 ether ring coordinated to all six oxygen atoms. Remarkably, the position of the strontium atom almost exactly corresponds to the center of mass of the six oxygen atoms, with the SrO distances of 2.677(4)−2.752(4) and 2.698(4)−2.731(4) Å, respectively, for the molecules A and B. The two k2-BF4− anions 1995
DOI: 10.1021/acs.jpcb.7b11498 J. Phys. Chem. B 2018, 122, 1992−2000
Article
The Journal of Physical Chemistry B comprising BF4− and PF6− anions results in an elongation of the C−O bonds in the macrocycle. This bond elongation correlates with the shifts of 10−20 cm−1 to lower vibrational frequencies in νas(COC) absorption bands in FTIR spectra of these complexes in comparison with the spectrum of 18C6. The shifts of the νas(COC) absorption bands also depend on SrO distances in the complexes. Indeed, the average Sr−O distance in 18C6·Sr(BF4)2 is less than this distance in 18C6· Sr(PF6)2 (Supporting Information, Table S13). It suggests that the polyether oxygen atoms interact with strontium cation more strongly in tetrafluoroborate complex. In accord with this finding, the shift of νas(COC) vibration band in FTIR spectrum of 18C6·Sr(BF4)2 (νmax ≈ 1094 cm−1) is more pronounced in comparison with that of 18C6·Sr(PF6)2 (νmax = 1104 cm−1). It is an important point from radiation-chemical point of view, because elongation of CO bond length leads to its weakening. Mechanism of Radiolysis. From general consideration of the chemical structure of macrocyclic 18C6·Sr(XFn)2 complexes under study, one can expect that two components, namely anionic (XFn−) and macrocyclic (18C6), should be sensitive to ionizing radiation. It is worth noting that the primary absorption of X-rays in the studied energy range is rather selective due to very strong dependence of mass absorption coefficients on atomic number of the constituent chemical elements, so it is essentially dominated by the heavy Sr ions.17 Meanwhile, the observed chemical events are mostly induced by secondary electrons (about 103 electrons per one primary photon), which cause a nonselective ionization of macrocyclic molecules and anions, so the direct ionization effect is roughly proportional to the electron fraction of the components. The close values of electron fractions of the macrocycle and anions (BF4− and PF6−) (Table 1) imply a comparable effect of direct action of radiation
This process will induce an additional generation of reactive atomic fluorine via subsequent reaction 3. An additional channel of the anion radiolysis is initiated by low-energy secondary electrons. The BF4− and PF6− anions have been reported to capture efficiently such electrons through a dissociative mechanism leading to the formation of fluoride anions and XFn−1•− radical anions11,12 XFn− + e− → XFn − 1•− + F−
F• and XFn−1•− species generated in reactions 3 and 5 respectively, have been identified in irradiated crystalline tetrafluoroborate and hexafluorophosphate salts.28−33 These paramagnetic products are characterized by extended anisotropic spectra and specific high values of the hyperfine splitting constants on X atoms in XFn−1•− (see Table 2). Table 2. EPR Spectroscopic Parameters of the Major Paramagnetic Species Resulted from the XFn− Anions
18C6
IPanion, eV23,24
IP18C6, eV26,27
18C6·Sr(BF4)2 18C6·Sr(PF6)2
0.311 0.431
0.545 0.450
7.3 8.0
9.7
(3)
The fluorine atoms are very reactive, and their role in subsequent radiation-chemical transformations of the macrocyclic complexes will be discussed later. The radiolysis mechanism of the 18C6·Sr(XFn)2 complexes is further complicated due to marked difference in ionization potentials (IP) of the anions and organic ligand (Table 1). First, it implies positive charge or “hole” migration from the macrocyclic radical cation to the anion at the early stage of radiolysis as expressed by reaction 4 18C6+• + XFn− → 18C6 + XFn•
g-factor
references
g = 2.0016 ± 0.0006 gxx = 2.151, gyy = 2.359, gzz = 1.971
28 29
g = 2.0021
32
g = 2.0014
33
Taking into account reactions 1 and 3−5, the radiolysis of 18C6·Sr(XFn)2 complexes should be a rather complicated process with a significant contribution of the intermediates resulted from BF4− and PF6− anions to the total yield of the stabilized radical species. Similar situation has been found previously for the radiolysis of dicyclohexano-18-crown-6 (DCH18C6) complexes with alkaline earth metal nitrates.34 Despite the significant electron fractions of macrocycle in those complexes (e.g., εDCH18C6 = 0.67 in DCH18C6·Sr(NO3)2 complex), NO32− dianions were revealed to be predominant radical species stabilized under irradiation in the dose range below 40 kGy. The observed effect has been attributed to the absorbed energy transfer from the macrocycle to nitrate-anions at the early stages of radiolysis. Nevertheless, in contrast to the radiolysis of the “nitrate” complexes the experimental EPR spectra of 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 irradiated at 77 K demonstrate almost identical isotropic “triplet” patterns with observed splitting of 2.0 mT, relative line intensities being close to 1:2:1 and g-factors of ∼2.0035 (Figures 4a and 5a). The spectroscopic features of the measured triplet signals allowed us to conclude that they belong solely to C-centered radicals. Moreover, the experimental EPR spectra do not reveal the signals from the products of the anion radiolysis (such as XFn‑1•‑ radical anions or fluorine atoms). In fact, the free fluorine atoms could be hardly detected by EPR in macroscopically disordered media, such as the polycrystalline complexes, due to very large anisotropy of both g and a values (see Table 2). However, most likely they disappear in the secondary reactions with the 18C6 macrocycle occurring even at low (77 K) temperature. In line with this finding, the paramagnetic intermediates of BF4− and PF6− radiolytic destruction have not been detected by EPR under the low-temperature radiolysis of neat RTILs.23 The assignment of the experimental EPR spectra to C-centered radicals was further supported by postirradiation thermal
The “hole” products of general formula XFn•, such as BF4• and PF6•, generated from the anions are known to be thermodynamically unstable, so they should undergo a fast dissociation (Ediss. = 0.02 eV) resulting in the formation of stable Lewis superacids and atomic fluorine12,22−25 XFn• → XFn − 1 + F•
splitting constants, mT isotropic, 177.1 ± 10.0 anisotropic, azz = 102.1 mT, axx = 4.4 mT, ayy = 12.3 mT a(19F) = 17.8 mT; a(11B) = 15.3 mT a(19F) = 19.8 mT; a(31P) = 135.4 mT
PF5•−
electron fraction (ε) anion XFn−
species atomic F
BF3•−
Table 1. Electron Fractions of Anions and Macrocycle in the Macrocyclic Complexes complex
(5)
(4) 1996
DOI: 10.1021/acs.jpcb.7b11498 J. Phys. Chem. B 2018, 122, 1992−2000
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The Journal of Physical Chemistry B
in the EPR spectra shown in Figures 4b,c and 5b,c. The difference spectrum (b−c) demonstrates a triplet pattern with an observed splitting of 1.9−2.0 mT (Figures 4d and 5d). As it has been demonstrated in our previous study,36 it belongs to a macrocyclic −CH2−Ċ H−O− radical with the following constants of hyperfine splitting: a(Hα) = 1.6 mT, a(Hβ1) = 2.2 mT, and a(Hβ2) = 0.4 mT. Apparently, these intermediates result from deprotonation of the primary macrocyclic radical cations in the reaction 6 and are generated in the secondary reactions 7−9 of 18C6 with H and F atoms. Accordingly, the observed transformations in the experimental spectra upon thermal annealing the samples at room temperature (Figures 4b,c and 5b,c) are reasonably described by decay of −CH2−Ċ H−O− species in reaction 10.
Figure 4. EPR spectra of 18C6·Sr(BF4)2 complex irradiated at 77 K and subjected to thermal annealing at 298 K: (a) initial EPR spectrum, (b) after 7 min of thermal annealing, (c) after 13 min of thermal annealing, (d) difference spectrum (b−c), (e) after 140 min of thermal annealing. Tmeasur. = 77 K. Absorbed dose was 45 kGy.
̇ −O−]* → [−O−CH 2−CH ̇ 2 + OCH−CH 2−O−]→ [−CH 2−CH ̇ −C(H)O →−O−CH 2−CH3 + −O−CH
(6a)
(18C6)H+ + e− → 18C6 + H
(7)
̇ −O−+H 2 H + 18C6 → −CH 2−CH
(8)
̇ −O−+HF F + 18C6 → −CH 2−CH
(9)
̇ −O− → Products 2−CH 2 − CH
(10)
In reactions 6 and 6a, 18C6 and [−CH2−Ċ H−O−]* present the primary radical cation and the macrocyclic radical in excited state, respectively. The triplet pattern of the difference spectrum, as depicted in Figures 4d and 5d, remained unchanged throughout thermal annealing the irradiated complexes until complete disappearance of the macrocyclic radicals. This is a key point, because it provides a convincing demonstration that the decay of macrocyclic radicals in reaction 10 does not lead to generation of new radical species. Ultimately, the EPR spectra of both irradiated complexes after 140 min of thermal annealing at 298 K look like a doublet with an observed splitting of 1.7−1.8 mT, g-factor of 2.0045 and relative line intensities ∼1:1 (Figures 4e and 5e). Further annealing the samples at this temperature did not result in any transformation of the doublet. This signal belongs to acyclic −Ċ H−C(H)O radicals formed in reaction 6a at direct action of ionizing radiation on macrocycle.36 Thus, thermal treatment of the irradiated samples at 298 K induced the disappearance of macrocyclic radicals, while the decay of −Ċ H−C(H)O radicals at this temperature was negligible. Accordingly, one can conclude that two principal radical species, macrocyclic −CH2−Ċ H−O− and acyclic −Ċ H−C(H)O radicals, were stabilized in irradiated 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes. The same products were also detected in “free” 18C6, which was irradiated at 77 K and then subjected to thermal annealing at 273 K (Figure 6). However, a marked difference between the EPR patterns of 18C6 and its complexes with the strontium salts clearly implies different proportions of macrocyclic and acyclic radical species. It is worth noting that stability of the acyclic −Ċ H−C(H)O radicals at the temperature of thermal annealing makes it possible to estimate their contributions to the total composition of the radicals measured immediately after irradiation at 77 K. +•
Figure 5. EPR spectra of 18C6·Sr(PF6)2 complex irradiated at 77 K and subjected to thermal annealing at 298 K: (a) initial EPR spectrum, (b) after 3 min of thermal annealing, (c) after 21 min of thermal annealing, (d) difference spectrum (b−c), (e) after 140 min of thermal annealing. Tmeasur. = 77 K. Absorbed dose was 41 kGy.
annealing of the irradiated samples. Recently, we have found that in the case of radiolysis of macrocyclic complexes of DCH18C6 with alkaline earth metal chlorides the thermal reactivity of different trapped radicals varies significantly.35 The analysis of the EPR spectra of the samples subjected to a postirradiation thermal treatment allowed us to determine a temperature range in which a “selective” decay of individual radicals occurs. In this way, it was possible to separate the EPR signals resulting from the corresponding species and to determine their structures.35 In this work, similar procedure was applied to 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes. Our preliminary study of their thermal stability by DSC/TGA methods revealed that the onset temperatures of the decomposition for these compounds were above ∼580 K (Supporting Information, Figure S1 and S2). This high thermal stability provides an opportunity to initiate a “selective” thermal annealing the radicals stabilized in these complexes. Indeed, increasing the temperature from 77 to 298 K resulted in gradual irreversible transformations 1997
DOI: 10.1021/acs.jpcb.7b11498 J. Phys. Chem. B 2018, 122, 1992−2000
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complexes was negligible after annealing time exceeding 50 min. This stage of thermal annealing gave flat regions on the kinetic curves 1 and 2 (Figure 7), which corresponded to the fractions of acyclic radicals stabilized in 18C6·Sr(BF4)2 and 18C6· Sr(PF6)2. Similarly, a bold arrow in Figure 8 shows the point of complete disappearance of the macrocyclic radicals on the decay curve and the fraction of −Ċ H−C(H)O radicals in irradiated “free” 18C6. The proportions of acyclic species of approximately 9% and 15% for 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2, respectively, are obviously lower than that in 18C6 (>35%, see Figure 8). It should be emphasized that the −Ċ H−C(H)O radicals were produced from primary species in reaction 6a under direct action of ionizing radiation. Thus, the macrocycle cleavage occurring in these complexes at the early stages gives a measurably smaller contribution to the total accumulation of radical intermediates in comparison with radiolysis of free 18C6, despite a measurable CO bond elongation in the complexes as follows from the data of FTIR spectroscopy and singlecrystal X-ray diffraction study discussed above. It is reasonable to suggest that the indirect action of ionizing radiation plays an essential role in the radiolysis of these complexes, and this process comprises in particular the positive “hole” transfer from primary macrocyclic radical cation to anion of the complex (reaction 4) and the interaction of the radiolytic fluorine atoms resulted from a dissociation of XFn• (reaction 3) with 18C6 macrocycle (reaction 9). Reaction 4 decreases the yield of the acyclic −Ċ H−C(H)O radicals in the complexes, while the products generated from anions initiate the formation of an additional amount of the macrocyclic −CH2−Ċ H−O− radicals. In general, the radiation-chemical transformations occurring in 18C6·Sr(XFn)2 complexes can be illustrated by a simplified Scheme 1 given below. Another important conclusion follows from a comparison of the kinetics of radical accumulation in the complexes exposed to ionizing radiation at 77 K (Figure 9). The concentrations of
Figure 6. EPR spectra of “free” 18C6 irradiated at 77 K: (a) initial EPR spectrum, (b) after 1 min of thermal annealing at 273 K. Tmeasur. = 77 K. Absorbed dose was 40 kGy.
Figure 7. Kinetics of the radical decay in 18C6·Sr(BF4)2 (1) and 18C6·Sr(PF6)2 (2) X-ray irradiated at 77 K and subjected to thermal annealing at 298 K.
Figure 9. Accumulation of the paramagnetic species in 18C6·Sr(BF4)2 (solid line and “●” symbols) and 18C6·Sr(PF6)2 (dash line and “□” symbols) complexes under X-ray irradiation at 77 K.
intermediates shown in the figure were normalized using Mn2+ standard that makes it possible to compare the total radiationchemical yields of radicals (GΣ). The close values of the slopes for these accumulation curves point to approximately equal GΣ in these complexes. Nevertheless, the 18C6·Sr(PF6)2 complex demonstrated lower stability to radiolytic cleavage of the macrocycle, as compared to 18C6·Sr(BF4)2, in view of larger fraction of the acyclic
Figure 8. Kinetics of the radical decay in irradiated 18C6 under thermal annealing at 273 K.
From quantitative point of view, it is illustrated by kinetic curves of the radical decay as given in Figures 7 and 8. An analysis of experimental EPR spectra revealed that the concentration of macrocyclic −CH2−Ċ H−O− radicals in irradiated 1998
DOI: 10.1021/acs.jpcb.7b11498 J. Phys. Chem. B 2018, 122, 1992−2000
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should bear in mind the difference between the IP values of macrocycle and anions of RTIL (BF4−, PF6−, NTf2−, and so forth). From the one hand, the positive “hole” transfer from the crown ether to the anions protects the macrocycle from radiation destruction. However, this protective effect will be compensated by a degradation of CE and RTIL/CE extractants as a whole, which is induced by highly reactive intermediates of anion radiolysis and other factors. In particular, reactions 5 and 9 are of special importance for the radiation resistance of these systems. The dissociative capture of secondary electrons in reaction 5 under irradiation of the real liquid-phase extractants would induce an accumulation of fluoride anions in the irradiated systems. Assuming that the strontium concentration in radioactive wastes under reprocessing could reach up to 0.2−0.5 g/L,37,38 one may conclude that the “excessive” radiolytic F− anions can react with Sr2+ cations with a precipitation of low-soluble SrF2 (solubility in water of 0.117 g/L39) as a solid phase. To the best of our knowledge, the contribution of this reaction to the radiolytic degradation of strontium loaded extractants based of “halogenated” RTIL was not considered previously. Additional channels of the CE degradation are initiated by the fluorine atom formed in reaction 9. They include chemical “blocking” and radiation-chemical transformations of the macrocycle. The former term is related to the generation of stable HF being able to protonate CE and to prevent its complexing with metal cations. This process is reversible and can be overcome by an alkaline treatment of the irradiated extractant. The radiationchemical degradation is irreversible process and results in accumulation of macrocyclic stable products of radiolysis, and their removal from the irradiated extractants is individual separation task. An estimation of the quantitative contributions from each of the degradation channels to the radiation destruction of the RTIL/CE systems is a key issue for the modern radiation chemistry of these extractants.
radicals stabilized in this compound (Figure 7). This result is rather unexpected taking into account a similarity in the channels of radiation-chemical transformations, as depicted in Scheme 1, and the values of electron fractions (ε) of macrocycle Scheme 1. Radiation-Chemical Transformations in Macrocyclic 18C6·Sr(XFn)2 Complexes
in 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes (Table 1). Actually, 18C6·Sr(BF4)2 should be more sensitive to radiolytic cleavage of the macrocycle due to the higher value of ε18C6. The experimental data, however, demonstrated an opposite trend. Generally speaking, some examples of enhanced radiationinduced degradation of organic molecules in the presence of hexafluorophosphate salts are known. For instance, it has been previously reported that PF6− anions promoted the radiolysis of propylene carbonate (PC).11 The effect has been explained by efficient dissociative scavenging of the solvated electron by PF6− (as depicted by reaction 5) and an electron transfer from PF5•− to PC molecule followed by its degradation. However, the probability of the electron transfer from anion to a neutral 18C6 molecule is low, so electron transfer is hardly responsible for the lower radiation stability of 18C6·Sr(PF6)2 complex to macrocycle cleavage. It seems reasonable to suggest that a conformational factor may contribute to destruction of hexafluorophosphate complex. Indeed, FTIR spectroscopy and X-ray analysis reveal a higher symmetry of the macrocycle and the largest elongation of the polyether C−O bonds in the 18C6· Sr(PF6)2 (Figures 1−3 and Supporting Information, Tables S1 and S13). Notwithstanding the correlation between this finding for the ground-state molecular structure and the radiationinduced transformations occurring through intermediate formation of radical species is not straightforward, one may speculate that the peculiarities in the molecular distortions may be conserved in radicals produced from specific molecular conformations in rigid systems.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b11498. Characteristic vibration frequencies in FTIR spectra (Table S1), single crystal structures (Tables S2−S13), and TGA/DSC analysis (Figure S1, S2) of 18C6·Sr(BF4)2 and 18C6·Sr(PF6)2 complexes (PDF) 18C6·Sr(BF4)2 (CIF) 18C6·Sr(PF6)2 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +7 495 332 58 36. ORCID
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Sergey V. Nesterov: 0000-0001-5840-0670 Victor N. Khrustalev: 0000-0001-8806-2975 Mikhail S. Nechaev: 0000-0001-8941-4856 Vladimir I. Feldman: 0000-0002-5407-8685
CONCLUSIONS As discussed above, the extractants based of RTIL/CE solutions have demonstrated enhanced radiation sensitivity in a number of cases.5−7 It is one of the serious drawbacks for a practical applicability of these compositions in nuclear reprocessing. In our opinion, the relatively high degradation of the functional characteristics of RTIL/CE under their irradiation may be partially explained by the effects of indirect action of ionizing radiation on individual components of the system. First, one
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by Program No. 1.8II of the Division of Chemistry of Russian Academy of Sciences and by 1999
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