Experimental and Theoretical Studies on the Fragmentation of Gas

Article pubs.acs.org/JPCA

Experimental and Theoretical Studies on the Fragmentation of GasPhase Uranyl−, Neptunyl−, and Plutonyl−Diglycolamide Complexes Yu Gong,† Han-Shi Hu,‡ Linfeng Rao,† Jun Li,*,‡,§ and John K. Gibson*,† †

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Chemistry & Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China § William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡

S Supporting Information *

ABSTRACT: Fragmentation of actinyl(VI) complexes UVIO2(L)22+, NpVIO2(L)22+, and PuVIO2(L)22+ (L = tetramethyl-3-oxa-glutaramide, TMOGA) produced by electrospray ionization was examined in the gas phase by collision induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Cleavage of the C−Oether bond was observed for all three complexes, with dominant products being UVIO2(L)(L-86)+ with charge reduction, and Np VI O 2 (L)(L-101) 2+ and PuVIO2(L)(L-101)2+ with charge conservation. The neptunyl and plutonyl complexes also exhibited substantial L+ loss to give pentavalent complexes NpVO2(L)+ and PuVO2(L)+, whereas the uranyl complex did not, consistent with the comparative An 5forbital energies and the AnVIO22+/AnVO2+ (An = U, Np, Pu) reduction potentials. CID of NpVO2(L)2+ and PuVO2(L)2+ was dominated by neutral ligand loss to form NpVO2(L)+ and PuVO2(L)+, which hydrated by addition of residual water in the ion trap; UVO2(L)2+ was not observed. Theoretical calculations of the structures and bonding of the AnVIO2(L)22+ complexes using density functional theory reveal that the metal centers are coordinated by six oxygen atoms from two TMOGA ligands.

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INTRODUCTION Diglycolamides are considered promising extractants for actinide partitioning from nuclear waste. Compared with most other extractants, diglycolamides are easier to synthesize and are completely incinerable. A number of studies have been carried out on the structures, complexation, and extraction properties of diglycolamides in condensed phase.1 Similar to other extractants used for radioactive waste partitioning and transmutation,2 diglycolamides are not stable toward radiation. Although radiolytic decomposition may not significantly reduce their extraction efficiencies and the radiolysis products are relatively easily removed,3,4 it is nonetheless important to understand decomposition mechanisms of diglycolamides. The radiolytic stabilities of actinide partitioning extractants such as TODGA (N,N,N′,N′-tetraoctyl diglycolamide) and TEHDGA (N,N,N′,N′-tetra-2-ethylhexyl diglycolamide) have been studied in organic diluents.5−9 Fragments produced via C−N, C−O, C−H, and C−C bond cleavage in solution were identified for TODGA;7,8 TEHDGA was shown to be more stable toward radiolysis.9 It has been demonstrated that studies of the chemistry of gas phase ions can provide insights into condensed phase reaction mechanisms, and intermediates under solvent free conditions.10,11 Using collision induced dissociation (CID), detailed © 2013 American Chemical Society

information on the fragmentation of gas phase metal ion complexes can be obtained.12,13 CID of protonated TMOGA (TMOGA = tetramethyl-3-oxa-glutaramide, also referred to as TMDGA, N,N,N′,N′-tetramethyl diglycolamide; structure shown in Figure 1) and TODGA cations demonstrated relevance to condensed phase radiolysis.8 However, CID of diglycolamide actinide (An) complexes, where it is possible to

Figure 1. Structure of the TMOGA ligand, C8H16O3N2; red = O; blue = N; gray = C; shaded gray = H. Received: August 1, 2013 Revised: September 8, 2013 Published: September 9, 2013 10544

dx.doi.org/10.1021/jp4076977 | J. Phys. Chem. A 2013, 117, 10544−10550

The Journal of Physical Chemistry A

Article

Figure 2. Structure of the AnO2(L)22+ complexes from two perspectives. The side-view perspective at the right shows that the equatorial TMOGA ligands are distorted from planarity. The indicated U−O, Np−O, and Pu−O distances are in Å.

estimated to be on the order of 10−6 Torr. The helium buffer gas pressure in the trap is constant at ∼10−4 Torr.

probe the influence of the 5f metal centers on fragmentation channels, has not been reported. Diglycolamide ligands form stable complexes with lanthanide and actinide metals, some structures of which have been determined.14−18 The recent observation of Th(L)34+ (L = TMOGA) confirms that effective coordination can stabilize a highly charged metal ion against Coulomb explosion and hydrolysis in the gas phase.19 We here present a comparative study of the CID behaviors of UVIO2(L)22+, NpVIO2(L)22+, and PuVIO2(L)22+ in the gas phase using experimental and computational methods. These complexes were chosen as model systems because TMOGA is the simplest homologue of the family of diglycolamides, and the solid complex has been structurally characterized for uranium.15 CID of NpVO2(L)2+ and PuVO2(L)2+ was also performed for comparison with results for the corresponding dications; the crystal structure of the NpVO2(L)2+ complex has been reported.14

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COMPUTATIONAL DETAILS The theoretical calculations of the AnO2(L)22+ (An = U, Np, Pu) complexes, all its decomposed products, and various ligands were carried out using spin-unrestricted Kohn−Sham density functional theory (DFT).22,23 The local density approach (LDA)24,25 and generalized gradient approach with PBE exchange-correlation functional26 were used as implemented in Amsterdam Density Functional program (ADF 2010.01).27−29 The scalar relativistic (SR) and spin−orbit (SO) effects were taken into account using zero-order-regular approximation (ZORA).30 The frozen core approximation was applied to the [1s2−5d10] cores of U, Np, and Pu, and [1s2] cores of C, N, and O, with the rest of the electrons explicitly treated variationally. The uncontracted Slater basis sets with triple-ζ plus two polarization functions (TZ2P) were used for the valence electrons.31 All the geometries were fully optimized with molecular symmetries that all have real vibrational frequencies. The vibrational frequencies were computed analytically, and zero-point energy (ZPE) corrections were included in the calculations of relative energies. The Mulliken population analysis32 and Nalewajski−Mrozek bond orders33 were also performed to understand the An−L bonding. The Weinhold’s natural bond orbitals (NBO) and natural localized MOs (NLMO)34 were calculated based on PBE functional with the 6-31G* basis sets for H, C, N, and O,35,36 and scalar relativistic SDD pseudopotential and basis set with 30 valence electrons for U, Np, and Pu, respectively.37 These calculations were accomplished with NBO 3.1 and Gaussian03 programs.38,39

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EXPERIMENTAL SECTION The experiments were performed using an Agilent 6340 quadrupole ion trap mass spectrometer (QIT/MS) with the electrospray ionization (ESI) source located inside a radiological containment glovebox.20 The AnO2(L)22+ cations were produced by ESI of methanol solutions of the corresponding AnO2(ClO4)2 and TMOGA mixtures [AnO2(ClO4)2:TMOGA = 1:4, 200 μM AnO2(ClO4)2]. The 200 μM actinyl solutions were prepared from UO 2 (ClO 4 ) 2 , NpO 2 (ClO 4 ) 2 , and PuO2(ClO4)2 aqueous stock solutions with concentrations of approximately 20 mM such that the diluted solutions contained ∼1% water. The actinide isotopes employed were U-238, Np237, and Pu-242, which undergo alpha-decay with half-lives of 4.5 × 109 y, 2.1 × 106 y, and 3.8 × 105 y, respectively. The MSn capabilities of the QIT/MS allow isolation of ions with a particular mass-to-charge ratio, m/z, followed by either CID, in which ions are excited and undergo energetic collisions with helium, or by insertion of an ion−molecule reaction time without applying ion excitation. In high resolution mode, the instrument has a detection range of m/z 20−2200 with a mass width (fwhm) of m/z ≈ 0.3. Mass spectra were recorded in the positive ion accumulation and detection mode. The intensity distribution of ions in the mass spectra was highly dependent on instrumental parameters, particularly the RF voltage applied to the ion trap; the detailed parameters are provided in Supporting Information. The high-purity nitrogen gas for nebulization and drying in the ion transfer capillary was the boil-off from a liquid nitrogen Dewar. As has been discussed elsewhere,21 the background water pressure in the ion trap is

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RESULTS The first series of experiments was carried out under conditions optimized for trapping the dipositive complexes, AnVIO2(L)22+ (An = U, Np, Pu). ESI mass spectra of AnVIO2(ClO4)2 and TMOGA mixtures (1:4) in methanol revealed the presence of AnVIO2(L)22+ with relatively high intensities. Other peaks apparent in the ESI mass spectra at lower m/z were assigned as the impurities Na(L)+ and Ca(L)22+, and fragments of TMOGA, as were also observed in previous experiments.19 Neither AnVIO2(L)2+ nor AnVIO2(L)32+ were apparent; only dipositive complexes with two ligands to stabilize the +2 charge during ESI were evident. It was previously shown that three TMOGA ligands are necessary to stabilize Th4+ from solution to gas by ESI.19 10545



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Table 1. Bond Distances, Nalewajski−Mrozek Bond Orders, and Mulliken Charges of the UO2(L)22+, NpO2(L)22+, and PuO2(L)22+ Complexes Calculated with PBE ZORA/TZ2P distance (Å)a An−Oe L = TMOGA UO2(L)22+ NpO2(L)22+ PuO2(L)22+ a

2.75 2.72 2.71

bond order

An−Oc 2.45 2.44 2.44

An−Oe 0.28 0.29 0.12

Mulliken charge An−Oc 0.59 0.61 0.34

q(An)

q(Oe)

q(Oc)

2.14 2.06 2.10

−0.52 −0.64 −0.63 −0.63

−0.60 −0.71 −0.71 −0.71

Oe and Oc denote Oether and Ocarbonyl, respectively.

Structures and Bonding Analyses of the AnVIO2(L)22+ Complexes. The theoretically optimized structures of the AnVIO2(L)22+ complexes are shown in Figure 2, along with the An−O distances; the structures of the three complexes are similar. As is apparent in Figure 2, the two TMOGA ligands do not lie in the actinyl equatorial planes. The An−Ocarbonyl distances are only ∼0.03 Å longer than the An−O distances estimated from the single bond radius (∼2.34 Å),40 whereas the An−Oether distances are ∼0.3 Å longer. The calculated An− Ocarbonyl and An−Oether distances decrease from uranium to plutonium, as expected based on the decrease in atomic radii across the series. The Np−Oether bond distance is somewhat larger than expected in comparison with U−Oether and Pu− Oether distances. In contrast to the calculated gas-phase structure of the dipositive complex, in crystalline UVIO2(L)2(ClO4)2 the TMOGA ligands lie in the equatorial plane.15 The difference between the gas phase and solid state structures is likely due to crystal packing effects, which can overwhelm lower energy interactions that lead to distortions, which appear only in free gas phase complexes. The bond distances in the molecular compound, U−Oyl = 1.75 Å, U−Ocarbonyl = 2.42 Å, and U− Oether = 2.61 Å, are similar to the values calculated for the gasphase dipositive complex. To understand the structure and bonding of these complexes, we have performed bonding analysis to understand the fragmentation and energetics. Table 1 lists the bond distances, Nalewajski−Mrozek bond orders, and Mulliken charges of the AnO2(L)22+ complexes calculated with PBE ZORA/TZ2P. The results of the natural bond orbital (NBO) analysis of the three AnO2(L)22+ complexes are given in Tables S1−S3, Supporting Information. The calculated NLMO results show that TMOGA ligand has p-type and spx-type lone pairs on the carbonyl and ether oxygen atoms, as shown in our previous work.19 Upon coordination to AnO22+ actinyls, the An−Ocarbonyl and An−Oether interactions are mainly ionic, with weak covalent interactions as well. The NLMOs listed in Tables S1−S3, Supporting Information, show that there are triple An− O bonds in the axial bonding of actinyl, while in the equatorial plane the covalent interactions of An−Ocarbonyl are rather weak, and even weaker for the An−Oether interaction. CID of AnVIO2(L)22+ and AnVO2(L)2+ Complexes. To probe the fragmentation of the AnVIO2(L)22+ complexes, they were isolated and subjected to CID under comparable conditions; representative CID mass spectra are shown in Figure 3. The peak assignments are in accord with isotopic abundances of the 13C isotopomers. Peaks at higher m/z than AnVIO2(L)22+ correspond to monopositive cations resulting from charge reduction while peaks at lower m/z are due to dipositive actinyl complexes that have lost a portion of a TMOGA ligand, as well as to monopositive ions from L+ that have dissociated from a complex during CID. For UVIO2(L)22+, the dominant CID product is UVIO2(L)(L-86)+, with lesser

Figure 3. CID mass spectra of AnVIO2(L)22+. The asterisk denotes AnVO2(L)(H2O)+. The m/z value for L+ is 188.

yields of UVIO2(L)(OH)+ and the dipositive complexes UVIO 2(L)(L-73)2+, U VIO 2(L)(L-87)2+, and UVIO 2(L)(L101)2+. The fragmented ligands L − x (x = 73, 86, 87, 101) are depicted in Figure S1, Supporting Information. Positive ion fragments arising from L+ were negligible from the uranyl complex, consistent with the minuscule yield of UVO2L+. The expected fragment corresponding to the loss of (86)+ was below the low mass cutoff of the spectrometer. 10546



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The CID spectra of NpVIO2(L)22+ and PuVIO2(L)22+ are similar to one another but quite different from that of UVIO2(L)22+ (Figure 3). NpVO2(L)+ and PuVO2(L)+ were the major singly charged products, with minor yields of their hydrates; NpVIO2(L)(L-86)+ and PuVIO2(L)(L-86)+ were also minor products. In contrast to the uranyl complex, the dominant dipositive products were NpVIO2(L)(L-101)2+ and PuVIO2(L)(L-101)2+. Peaks at m/z 116 and 144 are assigned as fragments of L; HL+ was also apparent at m/z 189. Product yields and computed energies of the experimentally observed CID reactions are given in Table S4, Supporting Information. By modifying the instrumental parameters, monocationic species dominated the ESI mass spectra, while dications were essentially absent. The mass spectra from ESI of AnVIO2(ClO4)2 and TMOGA mixtures (1:4) in methanol reveals the presence of predominantly AnVO2(L)+ with lesser yields of AnVO2(L)2+ and AnVIO2(L)(ClO4)+, for An = Np and Pu. CID of NpVO2(L)2+ and PuVO2(L)2+ results primarily in neutral TMOGA loss to give NpVO2(L)+ and PuVO2(L)+, which subsequently hydrate due to the presence of water in the ion trap (Figure 4). The m/z 528 and 534 peaks observed upon

UVIO2(L)(CH3O)+, UVIO2(L)(OH)+, and UVIO2(L)(ClO4)+ were apparent.

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DISCUSSION CID of AnVIO2(L)22+ Complexes. The CID results for VI U O2(L)22+, NpVIO2(L)22+, and PuVIO2(L)22+ reveal two primary competitive fragmentation channels: L+ loss and C− Oether bond cleavage. Loss of L+ results in the appearance of AnVO2(L)+ (reaction 1), while AnVIO2(L)(L-101)2+ and AnVIO2(L)(L-86)+ arise from charge-conserving and chargereduction C−Oether bond cleavage processes, respectively (reactions 2a and 2b). The observed C−Oether bond cleavages can be compared with results from solution studies. Analysis of the radiolytic degradation products of TODGA revealed the existence of three fragmentation channels: C−N, C−COcarbonyl, and C−Oether bond cleavages.7 The C−N bond cleavage channel became important in the presence of nitric acid, while the C−COcarbonyl bond cleavage products were formed less efficiently. C−Oether bond cleavage was considered the major radiolysis reaction in the absence of nitric acid, an interpretation supported by electron paramagnetic resonance spectroscopic studies on TODGA and TMOGA.8 The observation of C−Oether bond cleavage instead of other fragmentation pathways for AnVIO2(L)22+ in the gas phase is consistent with the results from solution studies, reflecting that the intrinsic chemistry of diglycolamide ligands rather than the mode by which energy is deposited governs the experimentally observed fragmentation chemistry. The similarities between CID and radiolytic degradation of protonated diglycolamide cations have been discussed elsewhere.8 An VIO2 (L)2 2 + → An VO2 (L)+ + L+

(1)

An VIO2 (L)2 2 + → An VIO2 (L)(L − 86)+ + (86)+

(2a)

An VIO2 (L)2 2 + → An VIO2 (L)(L − 101)2 + + (101)

(2b)

Since the TMOGA ligands are coordinated to different metals in the AnVIO2(L)22+ complexes, it is possible to evaluate the influence of metal centers on fragmentation, which effects have not been addressed by solution studies. As shown in Figure 3, CID of NpVIO2(L)22+ and PuVIO2(L)22+ gave similar products, while U VI O 2 (L) 2 2+ behaved differently. For NpVIO2(L)22+ and PuVIO2(L)22+, L+ loss is much more favorable than is the formation of NpVIO2(L)(L-86)+ and PuVIO2(L)(L-86)+. In contrast, for UVIO2(L)22+ the major product is UVIO2(L)(L-86)+, with UVO2(L)+ as a very minor product. Concurrently, more HL+ and fragments arising from L+ were observed with the neptunyl and plutonyl complexes in the CID spectra because free L+ can either abstract an H-atom from background water in the ion trap or undergo secondary fragmentations. The observed differences among the AnVIO2(L)22+ complexes can be rationalized by the lowered energies of the 5f orbitals from U to Pu. From our DFT calculations on AnVIO2(L)22+, the energies of the lowest unoccupied 5f-based orbitals decrease as −9.34, −10.08, and −10.68 eV (An = U, Np, Pu), which makes the reduction of Np VI O 2 (L) 2 2+ and Pu VI O 2 (L) 2 2+ easier than that of UVIO2(L)22+. Indeed, the observed AnVIO22+/AnVO2+ reduction potential of uranyl (0.09 V) is much lower than those for neptunyl (1.16 V) and plutonyl (0.94 V),41 reduction from the VI to V oxidation states (reaction 1) should be much more favorable for NpVIO2(L)22+ and PuVIO2(L)22+ than for UVIO2(L)22+. Differences in CID patterns observed for other

Figure 4. CID mass spectra of NpVO2(L)2+ and PuVO2(L)2+. The abnormal asterisked peaks are not assigned.

CID of NpVO2(L)2+ and PuVO2(L)2+, identified by asterisks in Figure 4, are about twice as broad as other peaks and cannot be isolated despite their substantial intensities. Both peaks also appeared when NpVO2(L)+ and PuVO2(L)+ were isolated in the parent mass spectra, without CID. The origins of these abnormal features in the mass spectra are unknown. ESI of UVIO2(ClO4)2 and TMOGA did not produce detectable UVO2(L)2+, but monopositive cations such as UVO2(L)+, 10547



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freedom, which dissipate the hydration energy and enable collisional cooling prior to dissociation.21,47 Since both NpVO2(L)2+ and PuVO2(L)2+ are monopositive cations, the interactions between the actinide center and ligand are expected to be weaker than those in AnVIO2(L)22+, rendering An-L bond cleavage more facile. The ligands in AnVO2(L)2+ are evidently too weakly bound for in situ dissociation with retention of a fragment ligand, such that only ligand loss is observed. In the case of uranium, UVO2(L)+ was the only binary complex observed under conditions favoring monopositive cations; UVO2(L)2+ was not evident in the spectra. All other observed monopositive uranium species, such as UVIO2(L)(CH3O)+, UVIO2(L)(OH)+, and UVIO2(L)(ClO4)+, contain a bonding ligand or anion such that the highly stable U(VI) oxidation state is retained. Residual O2 in the ion trap resulted in the UVIO2(L)(O2)+ complex, which is presumed to be a superoxo complex based on a similar interpretation for the addition of O2 to other U(V) complexes.44,48

ligated uranyl, neptunyl, and plutonyl dications have similarly been attributed to the disparate VI/V reduction potentials.42,43 Another major difference between CID of UVIO2(L)22+ and NpVIO2(L)22+/PuVIO2(L)22+ is the preference for the formation of AnVIO2(L)(L-86)+ (reaction 2a) for the uranium complex, while AnVIO2(L)(L-101)2+ (reaction 2b) is the dominant dicationic fragment for the neptunium and plutonium complexes. Both of these products result from C−Oether bond cleavage of TMOGA in the AnVIO2(L)22+ complex, but hydrogen transfer occurs during the loss of m/z 101. AnVIO2(L)(L-86)+ can be considered as a complex formed by AnVIO22+ and (L-86)−, where (L-86)− represents a deprotonated alcohol. Fragment L-101 most likely corresponds to neutral N,N-dimethyl acetamide (Figure S1, Supporting Information). The differences revealed here indicate that fragmentation of TMOGA can induce charge reduction of the ligated complexes, with the processes highly dependent on the nature of metal center: reaction 1 is favored for neptunyl and plutonyl, whereas reaction 2a is favored for uranyl. In addition to the differences noted above, CID of UVIO2(L)22+ resulted in the formation of distinctive products (Figure 3) not seen for NpVIO2(L)22+ and PuVIO2(L)22+. The hydrolysis product, UVIO2(L)(OH)+ (reaction 3), was significant, whereas the hydrate, UVO2(L)(H2O)+, was not observed. Uranyl is known to form the hydroxide and retain the VI oxidation state much more readily than do neptunyl and plutonyl.43,44 Also, UVIO22+ was stabilized by additional ligands, as is evident from peaks due to UVIO2(L)(L-73)2+ and UVIO2(L)(L-87)2+ in addition to UVIO2(L)(L-101)2+ (Figure 3). The L-73 and L-87 fragment ligands likely result from C− COcarbonyl and C−Oether bond cleavage, respectively, and correspond to neutral ligands with two carbonyl groups (Figure S1, Supporting Information). The appearance of UVIO2(L)(L73)2+ and UVIO2(L)(L-87)2+ is in accord with more facile stabilization of UVIO22+ versus NpVIO22+ and PuVIO22+, in concurrence with previous results for the transfer of hexavalent actinyls from solution to gas phase.43,45,46 The formation of UVIO2(L)(L-73)2+ by C−COcarbonyl bond cleavage is in accord with this as a minor fragmentation channel for diglycolamide ligands upon radiolysis in solution.7 U VIO2 (L)2 2 + + H 2O → U VIO2 (L)(OH)+ + HL+

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An VO2 (L)2+ → An VO2 (L)+ + L

(4)

CONCLUSIONS Gas-phase dipositive U VI O 2 (L) 2 2+ , Np VI O 2 (L) 2 2+ , and PuVIO2(L)22+ complexes were produced by ESI; their CID fragmentation revealed similarities to radiolytic degradation of diglycolamides in solution. The results furthermore revealed influences of the metal centers on the fragmentation of TMOGA. Upon CID, Np VI O 2 (L) 2 2+ and Pu VI O 2 (L) 2 2+ primarily exhibited L + loss to give Np V O 2 (L) + and PuVO2(L)+, whereas UVIO2(L)22+ preferentially underwent C−Oether bond cleavage to form UVIO2(L)(L-86)+. These disparate results are consistent with the comparative AnVIO22+/ AnVO2+ reduction potentials, which indicate that U(VI) is substantially more resistant to reduction than are Np(VI) and Pu(VI). Cleavage of the C−Oether bond is also significant for NpVIO2(L)22+ and PuVIO2(L)22+ but the predominant pathway is to produce the charge conserving products NpVIO2(L)(L101)2+ and PuVIO2(L)(L-101)2+, rather than NpVIO2(L)(L86)+ and PuVIO2(L)(L-86)+. CID of UVIO2(L)22+ revealed that UVIO22+ can be stabilized by neutral L-87 and L-73 ligands, which are considered to result from C−Oether and C−COcarbonyl bond cleavage, respectively. Along with very minor UVO2(L)+, the hydrolysis product UVIO2(L)(OH)+ was observed, again revealing the propensity for oxidation of U(V) to U(VI). By modifying the experimental conditions, sufficient NpVO2(L)2+ and PuVO2(L)2+ were produced for isolation and CID; fragmentation was essentially exclusively by neutral L loss, presumably reflecting that the ligands are more weakly bound in the monopositive complexes. The structures of UVIO2(L)22+, NpVIO2(L)22+ and PuVIO2(L)22+ computed by DFT have the metal center coordinated by six oxygen atoms from two TMOGA ligands. An intriguing result is that the two TMOGA ligands do not lie in the equatorial planes of the actinyls but are instead rather distorted from planarity. The observation of C− Oether bond cleavage for AnVIO2(L)22+ in the gas phase is consistent with results for the degradation of TMOGA in solution.

(3)

The recently reported CID chemistry of Th(L)34+ revealed Th(L)2(L-86)3+ as the major product together with small amounts of Th(L)2(OH)3+ formed due to CID induced hydrolysis.19 This is quite similar to the products from CID of UVIO2(L)22+. For both complexes the L-86 ligand in the charge-reduced complex is presumed to be the alkoxide shown in Figure S1, Supporting Information, such that the initial oxidation state is retained. Such similarity between thorium and uranium is attributed to the highly stable Th(IV) and U(VI) oxidation states, which are more difficult to reduce than Np(VI) and Pu(VI) and are retained by formation of alkoxides upon charge reduction. CID of AnVO2(L)2+ Complexes. For comparison with the fragmentation of the dipositive AnVIO2(L)22+ complexes, NpVO2(L)2+ and PuVO2(L)2+ were subject to CID. Neutral ligand loss (reaction 4) was observed for both cations, as shown in Figure 4. Subsequent reactions with water in the ion trap resulted in the appearance of the m/z 475 and 480 peaks due to NpVO2(L)(H2O)+ and PuVO2(L)(H2O)+. It has been demonstrated that coordinatively unsaturated complexes that retain ligands can efficiently hydrate due to the vibrational degrees of

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ASSOCIATED CONTENT

S Supporting Information *

Complete author list for ref 39, detailed instrumental parameters, calculated natural localized molecular orbitals of 10548



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AnO2(L)22+ complexes, computed fragmentation energies, and proposed residual ligands from CID. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(J.K.G.) E-mail: [email protected]. *(J.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Element Chemistry, at LBNL under Contract No. DE-AC02-05CH11231 [to Y.G., L.R., and J.K.G.]. The theoretical work by H.S.H and J.L. was supported by NSFC (20933003 and 91026003) of China. The calculations were done using Tsinghua National Laboratory for Information Science and Technology and using the Molecular Science Computing capability at the EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at the PNNL, a multiprogram national laboratory operated for the Department of Energy by Battelle. We are grateful to Dr. Guoxin Tian for advice and assistance.

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Experimental and Theoretical Studies on the Fragmentation of Gas

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