Research Article

Received: 3 August 2011 Revised: 12 October 2011 Accepted: 18 October 2011 Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/jms.2011

Electron transfer dissociation of dipositiveuranyl and plutonyl coordination complexesDaniel Rios,a Philip X Rutkowski,a David K. Shuh,a Travis H. Bray,a

John K. Gibsona* and Michael J. Van Stipdonkb*

Reported here is a comparison of electron transfer dissociation (ETD) and collision-induced dissociation (CID) of solvent-coordinated dipositive uranyl and plutonyl ions generated by electrospray ionization. Fundamental differences between

the ETD and CID processes are apparent, as are differences between the intrinsic chemistries of uranyl and plutonyl. Reductionof both charge and oxidation state, which is inherent in ETD activation of [AnVIO2(CH3COCH3)4]

2+, [AnVIO2(CH3CN)4]2, [UVIO2

(CH3COCH3)5]2+ and [UVIO2(CH3CN)5]

2+ (An=U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V)complexes add O2, whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantlydoubly-charged products through loss of coordinating ligands. Singly-charged CID products of [UVIO2(CH3COCH3)4,5]

2+,[UVIO2(CH3CN)4,5]

2+ and [PuVIO2(CH3CN)4]2+ retain the hexavalent metal oxidation state with the addition of hydroxide or

acetone enolate anion ligands. However, CID of [PuVIO2(CH3COCH3)4]2+ generates monopositive plutonyl(V) complexes,

reflecting relatively more facile reduction of PuVI to PuV. Copyright © 2011 John Wiley & Sons, Ltd.

Supporting information may be found in the online version of this article.

Keywords: ETD; CID; Uranyl; Plutonyl; Actinides; Electrospray; Plutonium; Uranium

* Correspondence to: John K. Gibson, Chemical Sciences Division, Lawrence BerkeleyNational Laboratory, Berkeley, CA 94720 USA. E-mail: jkgibson@lbl.gov, mike.vanstipdonk@wichita.edu

a Chemical Sciences Division, The Glenn T. Seaborg Center, Lawrence BerkeleyNational Laboratory, Berkeley, CA 94720, USA

b Department of Chemistry, Wichita State University, Wichita, KS 67260-0051,USA

124

INTRODUCTION

The first uranyl ions transferred from solution to the gas phaseusing electrospray ionization (ESI) were monopositive, pentava-lent [UVO2]

+ species.[1] Since then, the experimental methodshave been refined to allow transfer of pentavalent and hexava-lent uranyl complex ions, monopositive and dipositive, fromsolution to the gas phase for subsequent study of fragmentationbehavior and intrinsic reactivity using tandem mass spectrometry(MS).[2] Of particular relevance to the present work, ESI of solutionsof [UVIO2]

2+ in water/acetone and water/acetonitrile have revealedthe affinity of gas-phase uranyl and plutonyl ions for more basicorganic ligands over H2O, and coordination numbers upto five, as in [AnVIO2(CH3COCH3)5]

2+ and [AnVIO2(CH3CN)5]2+

(An =U, Pu).2e, [3]

A central object of actinide chemistry is to identify variations inchemistry across the series, particularly for the early actinides,U, Np and Pu, which exhibit a variety of oxidation states under mod-erate conditions, from AnIII to AnVI.[4] Among the well-establishedvariations in the chemistry of the early actinides are the standardreduction potentials of the actinyl ions, AnVIO2

2+ ➔ AnVO2+: +0.17V

for UVIO22+; +1.24V for NpVIO2

2+; +1.02V for PuVIO22+; and +1.60V

for AmVIO22+.[5] One of the goals is to use the methods developed

during initial experiments with uranyl species to explore theintrinsic chemistry of transuranic elements in the gas phase.Several unusual oxidation states can appear in gas-phase specieswhich may not be known in condensed-phase chemistry. Estab-lishing clear links between oxidation/reduction chemistry inelementary gas-phase species and complex condensed-phasespecies can enable additional insights into the fundamentalnature of an element’s chemistry. In the present work, an aim isto establish such a connection for hexavalent and pentavalenturanyl and plutonyl species.

J. Mass. Spectrom. 2011, 1247–1254

The prevalent oxidation state of plutonium in aqueous solu-tions is PuIV, but hexavalent plutonyl, [PuVIO2]

2+, is stable andcan be a significant solution species.[6] Early plutonium ESI/MSwork focused on the speciation of PuIV in the gas phase.[7] Thefirst report appeared in 2003, where ESI and mass spectrometrywere used to elucidate the molecular structure of radiolyticproducts of irradiated TRPO-kerosene systems.7a Walther et al.subsequently employed ESI/MS to characterize tetravalent pluto-nium polymer growth.[8]

Transfer of solvent-coordinated PuVIO22+ ions to the gas phase by

ESI of plutonyl perchlorate in acetone/water and acetonitrile/watersolvent mixtures was recently reported.3b Ion complexes wereisolated in an ion trap for mass analysis and study of fragmentationbehavior using collision-induced dissociation (CID). Parallel studiesof uranyl species revealed inherent differences between thecoordination and redox chemistries of Pu and U in the gas phase.

CID of dipositive actinyl(VI) complexes proceeds by heating,without any inherent modification in charge or actinide oxidationstate, although both may change. The importance of differentfragmentation routes—simple ligand loss with retention ofcharge and hexavalent oxidation state, charge and oxidationstate reduction, or charge reduction with retention of the hexava-lent oxidation state as in hydroxides or acetone enolates—will

Copyright © 2011 John Wiley & Sons, Ltd.

7

D. Rios et al.

1248

vary with the actinide, and the nature and number of coordinat-ing basic ligand. For example, it might be anticipated that agreater number of basic ligands, as in pentacoordinate versustetracoordinate complexes, would tend to stabilize the higherhexavalent oxidation state. In electron transfer dissociation(ETD), fluoranthene anion (C16H10

� ) is introduced into an ion trapfor exothermic electron transfer to multiply-charged cations.While CID proceeds by relatively gradual ion heating on the time-scale of milliseconds, ETD proceeds by prompt deposition ofenergy into a multiply-charged cation complex. ETD activationinherently results in both charge and oxidation state reduction,from AnVIO2

2+ to AnVO2+ for actinyl complexes. For complexes such

as actinyls where charge and oxidation state retention is domi-nant in CID, different behavior will necessarily appear in ETD.To date, the great majority of ETD applications have aimed at

improving the characterization of gas-phase, multiply-charged(protonated) biomolecules[9], and metal complexes[10] whichcontain coordinating ligands of biological significance. An ETDversus CID study of protein ubiquitination revealed that thefragmentation efficiency is lower for ETD compared to CID, withthe conclusion that these fragmentation methods should beused as complementary to one another for obtaining informationabout ubiquitinated side chains.[11] Comparative fragmentationof doubly-charged actinyl complexes by ETD and CID hasnot been reported. In the present study, ETD and CID wereapplied to the gas-phase complexes, [UVIO2(CH3CN)4,5]

2+, [UVIO2

(CH3COCH3)4,5]2+, [PuVIO2(CH3COCH3)4]

2+ and [PuVIO2(CH3CN)4]2+.

Comparative dissociation channels reveal differences betweenETD and CID, as well as between uranyl and plutonyl.

Experimental section

The transuranium actinide ESI quadrupole ion trap MS (ESI-QIT/MS) at Lawrence Berkeley National Laboratory (LBNL) is describedin the Supporting Information.Plutonyl solutions in acetone/water and acetonitrile/water

were prepared at a concentration of 80 mM in an organic sol-vent/water ratio of 90/10 from a [242PuVIO2(ClO4)2] stock solutionin water; a 180mM solution in water/acetone was also preparedto establish that there were negligible concentration effects inthis range. Uranyl solutions of 180 mM concentration were pre-pared using aqueous [238UVIO2(ClO4)2] to give solutions withcompositions of 90% acetone/10% water and 90% acetonitrile/10% water. The alpha-emitting actinide radionuclides, Pu-242and U-238, were handled in a radiological glove box in the HeavyElement Research Laboratory at LBNL.The ESI/MS and CID experiments were performed with an

Agilent 6340 QIT/MS, with the ESI source housed within a radio-logical-containment glove box. In high-resolution mode, theinstrument has a detection range of m/z 50 – 2200 and a resolu-tion of m/z ~0.25. Mass spectra were recorded in the positive ionaccumulation and detection mode using the following instru-mental parameters: nebulizer gas pressure, 12 psi; capillaryvoltage and current, �4500 V, 1.221 nA; end plate voltage offsetand current, �500 V, 22.5 nA; dry gas flow rate, 5 l/min; dry gastemperature, 100 �C; capillary exit, 75 V; skimmer, 29.2 V; octopole1 and 2DC, 11.46 V and 7.40 V; octopole RF amplitude, 50.0 Vpp;lens 1 and 2, �2.3 V and �77.5 V; trap drive, 49.9. Minor changesto these parameters were made as necessary to maximize theintensity of specific precursor ions.Solutions were injected into the electrospray capillary via a

syringe pump at a rate of 60 mL min�1. Nitrogen gas was used

wileyonlinelibrary.com/journal/jms Copyright © 2011 Jo

for nebulization and drying in the ion transfer capillary. CIDexperiments were performed using the He trapping gas at apressure of ~10�4 Torr; tickle voltages ranged between 0.1 Vand 1.0 V applied for 40ms. The background water pressure inthe ion trap was ~10�6 Torr. ETD was performed using fluor-anthene anion, C16H10

� , as the transfer reagent, which was gatedfrom the incorporated negative chemical ionization source intothe ion-transfer optics. Typical ETD reaction times ranged from40ms to 500ms.

RESULTS AND DISCUSSION

The gas-phase behavior, primarily ion-molecule reactions andfragmentation induced by collisional activation of acetonitrile-and acetone-coordinated uranyl ions, has been the focus ofearlier studies.2e, [3] In general, multiple-stage CID experimentshave shown that the doubly-charged species tend to generateproduct ions in which H2O replaces acetonitrile or acetoneligands eliminated in fragmentation reactions; or undergo chargereduction reactions to furnish cations such as [UVIO2(OH)]

+ and[UVO2]

+ coordinated by acetonitrile, acetone and/or H2O. Incontrast to the dipositive complexes, elimination of all coordinat-ing neutral ligands to give bare [UVIO2(OH)]

+ and [UVO2]+ can be

achieved in CID of singly-charged complexes.As ETD is necessarily applied to multiply-charged cations, in

the present study, the fragmentation behavior of doubly-chargedcomplexes containing either the UVI or PuVI dioxo cations wasexamined. For multiply-protonated peptide ions, ETD presumablycauses formation of H radicals, which migrate and induce (disso-ciation) reactions, ultimately producing fragments that revealamino acid sequences. For metal-cation complexes, electron trans-fer reduces the metal ion oxidation state. The electron affinity offluoranthene (fln) is much below that of the dipositive actinyl com-plexes, i.e. EA[fln]<< EA[AnVIO2(ligand)n]

2+, such that the electrontransfer process deposits substantial energy in the charge-reducedcomplex; this excess energy can initiate dissociation reactions. AsEA[fln]=0.6 eV[12] and EA[UVIO2

2+] =14.6� 0.4 eV,[13] electron transferfrom fln� to [UVIO2]

2+ would deposit ~14eV in the resulting mono-positive uranyl cation. Because the EA of base-ligated complexes of[UVIO2]

2+ is lower than of the bare dipositive cation, the energy de-posited upon ETDwill be somewhat less than 14eV, but nonethelesssubstantial (i.e. >> 1eV).

Throughout the discussion of results, ‘aco’ represents acetone(CH3COCH3), ‘aco-H’ represents deprotonated acetone enolate(CH3CO=CH2

�), and ‘acn’ represents acetonitrile (CH3CN). ESIusing solutions of 180 or 80mM 238UVIO2

2+ or 242PuVIO22+ in 90/10

acetonitile/water or acetone/water generated complexes consist-ing of 238UVIO2

2+ or 242PuVIO22+ coordinated by acetone or acetoni-

trile with general formulae [AnVIO2(acn)4,5]2+ and AnVIO2(aco)4,5]

2+.The [AnVIO2(L)4]

2+ (An=U or Pu) and [UVIO2(L)5]2+ (L = acn or

aco) ions were apparent in parent ESI spectra in abundancesadequate for isolation and CID; abundances of the penta-coordi-nated plutonyl complexes were inadequate for ETD fragmenta-tion. The fragment ions generated by ETD and CID of eachprecursor ions included in this study are compiled in Tables 1and 2. ETD and CID spectra for [UVIO2(acn)4]

2+, [UVIO2(aco)4]2+,

[PuVIO2(acn)4]2+ and [PuVIO2(aco)4]

2+ are shown in Figs. 1–4, andETD and CID spectra for [UVIO2(acn)5]

2+ and [UVIO2(aco)5]2+ are

shown in Figs. S6 and S7 (Supporting Information). HydratedETD and CID products result from reactions with backgroundwater in the ion trap. CID of acetone complexes resulted in

hn Wiley & Sons, Ltd. J. Mass. Spectrom. 2011, 1247–1254

Table 1. Product ions generated by ETD and CID of [UVIO2(acn)4,5]2+ and [UVIO2(aco)4,5]

2+

Product ions

Precursor m/z ETD m/z CID

[UVIO2(acn)4]2+ 270 [UVO2]

+ 205.5 [UVIO2(acn)3(H2O)]2+

311 [UVO2(acn)]+ 214.5 [UVIO2(acn)3(H2O)2]

2+

329 [UVO2(acn)(H2O)]+ 287 [UVIO2(OH)]

+

347 [UVO2(acn)(H2O)2]+ 328 [UVIO2(OH)(acn)]

+

352 [UVO2(acn)2]+ 369 [UVIO2(OH)(acn)2]

+

370 [UVO2(acn)2(H2O)]+ 387 [UVIO2(OH)(acn)2(H2O)]

+

384 [UVIO2(acn)2(O2)]+

393 [UVO2(acn)3]+

[UVIO2(acn)5]2+ 311 [UVO2(acn)]

+ 217 [UVIO2(acn)4]2+

329 [UVO2(acn)(H2O)]+ 226 [UVIO2(acn)4(H2O)]

2+

347 [UVO2(acn)(H2O)2]+ 235 [UVIO2(acn)4(H2O)2]

2+

352 [UVO2(acn)2]+

370 [UVO2(acn)2(H2O)]+

384 [UVIO2(acn)2(O2)]+

393 [UVO2(acn)3]+

402 [UVIO2(acn)2(H2O)(O2)]+

[UVIO2(aco)4]2+ 270 [UVO2]

+ 231 [UVIO2(aco)3(H2O)]2+

328 [UVO2(aco)]+ 242 [UVIO2(mo)(aco)2]

2+

346 [UVO2(aco)(H2O)]+ 270 [UVO2]

+

364 [UVO2(aco)(H2O)2]+ 287 [UVIO2(OH)]

+

378 [UVIO2(aco)(H2O)(O2)]+ 327 [UVIO2(aco-H)]

+

386 [UVO2(aco)2]+ 345 [UVIO2(OH)(aco)]

+

404 [UVO2(aco)2(H2O)]+ 403 [UVIO2(OH)(aco)2]

+

418 [UVIO2(aco)2(O2)]+ 421 [UVIO2(OH)(aco)2(H2O)]

+

422 [UVO2(aco)2(H2O)2]+ 443 [UVIO2(aco-H)(aco)2]

+

436 [UVIO2(aco)2(H2O)(O2)]+

444 [UVO2(aco)3]+

[UVIO2(aco)5]2+ 386 [UVO2(aco)(H2O)]

+ 242 [UVIO2(mo)(aco)2]2+

404 [UVO2(aco)2(H2O)]+ 251 [UVIO2(aco)4]

2+

418 [UVIO2(aco)2(O2)]+ 260 [UVIO2(aco)4(H2O)]

2+

422 [UVO2(acn)2(H2O)2]+ 271 [UVIO2(mo)(aco)3]

2+

436 [UVIO2(aco)2(H2O)(O2)]+ 368 [UVO2(mo)]+

444 [UVO2(aco)3]+ 385 [UVIO2(aco-H)(aco)]

+

462 [UVO2(aco)3(H2O)]+ 403* [UVIO2(OH)(aco)2]

+

476 [UVIO2(aco)3(O2)]+ 421 [UVIO2(OH)(aco)2(H2O)]

+

484 [UVO2(mo)(aco)2]+ 443 [UVIO2(aco-H)(aco)2]

+

502 [UVO2(aco)4]+ 461* [UVIO2(OH)(aco)3]

+

501 [UVIO2(aco-H)(aco)3]+

Table 2. Product ions generated by ETD and CID of [PuVIO2(acn)4]2+ and [PuVIO2(aco)4]

2+

Product ions

Precursor m/z ETD m/z CID

[PuVIO2(acn)4]2+ 274 [PuVO2]

+ 207.5 [PuVIO2(acn)3(H2O)]2+

315 [PuVO2(acn)]+ 216.5 [PuVIO2(acn)3(H2O)2]

2+

333 [PuVO2(acn)(H2O)]+ 291 [PuVIO2(OH)]

+

351 [PuVO2(acn)(H2O)2]+ 332 [PuVIO2(OH)(acn)]

+

374 [PuVO2(acn)2(H2O)]+ 373 [PuVIO2(OH)(acn)2]

+

374 [PuVO2(acn)2(H2O)]+

[PuVIO2(aco)4]2+ 332 [PuVO2(aco)]

+ 233 [PuVIO2(aco)3(H2O)]2+

350 [PuVO2(aco)(H2O)]+ 244 [PuVIO2(mo)(aco)2]

2+

368 [PuVO2(aco)(H2O)2]+ 332 [PuVO2(aco)]

+

390 [PuVO2(aco)2]+ 390 [PuVO2(aco)2]

+

408 [UVO2(aco)2(H2O)]+ 408 [PuVO2(aco)2(H2O)]

+

ETD of Uranyl/Plutonyl Complexes

J. Mass. Spectrom. 2011, 1247–1254 Copyright © 2011 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jms

1249

[UVIO2(aco)4]2+

[UVIO2(aco)(H2O)(O2)]+

[UVO2(aco)2]+

[UVO2(aco)2(H2O)]+

[UVIO2(aco)2(O2)]+

[UVO2(aco)2(H2O)2]+

[UVIO2(aco)2(H2O)(O2)]+[UVO2(aco)3]+

x2(a)

D. Rios et al.

1250

products corresponding to water elimination; as discussedpreviously3b, we speculate that these products may comprise amesityl oxide ligand, CH3C(O)CH=C(CH3)2, which is a dehydratedacetone dimer and is denoted here as ‘mo’. The minor asteriskedpeaks in ETD and CID spectra were unassigned and are likely frag-ments from unknown ‘impurity’ constituents in the isolated ionpacket.

[UVO2]+

[UVIO2(aco)4]2+

[UVIO2(mo)(aco)2]2+

[UVIO2(aco)3(H2O)]2+

[UVIO2(OH)]+

[UVIO2(aco-H)]+

[UVIO2(OH)(aco)]+[UVO2]+

[UVIO2(OH)(aco)2]+

[UVIO2(OH)(aco)2(H2O)]+

[UVO2(aco)]+

[UVO2(aco)(H2O)]+

[UVO2(aco)(H2O)2]+

(b)

Figure 2. (a) ETD and (b) CID of [UO2(aco)4]2+.

ETD of acn- and aco-coordinated [UVIO2]2+ and [PuVIO2]

2+

A doubly-charged product ion with m/z corresponding to [UVIO2

(acn)4]2+ was observed in the ETD spectrum of [UVIO2(acn)5]

2+

(Fig. S6a). This dipositive fragment ion cannot result from ETDwhich necessarily results in a reduction in charge state upon elec-tron transfer. It is likely that this dipositive product is insteadformed by an energetic collision during the ion isolation andreaction time, either with He or other background gas molecules,or possibly with the fln- reagent ion without concomitant elec-tron transfer. Singly-charged product ions generated by ETD of[UVIO2(acn)5]

2+ (Fig. S6a, Table 1) included the bare [UVO2]+ cat-

ion (m/z 270) along with [UVO2(acn)]+ (m/z 311), [UVO2(acn)2]

+

(m/z 352) and [UVO2(acn)3]+ (m/z 393), and hydrates of these

products; most prominent were those at m/z 329, 347, 270 and386: each of these product ions results from reduction to [UVO2]

+

species.Observed in the ETD spectrum of [UVIO2(acn)5]

2+ were minorpeaks at m/z 384 and 402 which are attributed to formation ofO2 adducts of [UVO2(acn)2]

+ and [UVO2(acn)2(H2O)]+. Previous

experimental and theoretical studies[14] suggested that theformation of O2 adducts to gas-phase [UVO2]

+ species is sensitiveto the number and type of coordinating ligands, and involves a

[UVO2]+

[UVO2(acn)]+

[UVIO2(acn)4]2+

[UVO2(acn)(H2O)]+

[UVO2(acn)(H2O)2]+

[UVO2(acn)2]+

[UVO2(acn)2(H2O)]+

[UVO2(acn)2(O2)]+

[UVO2(acn)3]+

[UVIO2(acn)4]2+

[UVIO2(acn)3(H2O)2]2+

[UVIO2(acn)3(H2O)]2+

[UVIO2(OH)]+

[UVIO2(OH)(acn)]+

[UVIO2(OH)(acn)2]+

[UVIO2(OH)(acn)2(H2O)]+

(a)

(b)

(a)

(b)

Figure 1. (a) ETD and (b) CID of [UO2(acn)4]2+.

[PuVIO2(acn)4]2+

[PuVO2]+

[PuVO2(acn)]+

[PuVO2(acn)(H2O)]+

[PuVO2(acn)(H2O)2]+

[PuVO2(acn)2(H2O)]+

[PuVIO2(acn)4]2+

[PuVIO2(acn)3(H2O)]2+

[PuVIO2(OH)]+

[PuVIO2(OH)(acn)]+

[PuVIO2(OH)(acn)2]+

[PuVO2(acn)2(H2O)]+

(a)

(b)

[PuVIO2(acn)3(H2O)2]2+

Figure 3. (a) ETD and (b) CID of [PuO2(acn)4]2+.

wileyonlinelibrary.com/journal/jms Copyright © 2011 John Wiley & Sons, Ltd. J. Mass. Spectrom. 2011, 1247–1254

[PuVIO2(aco)4]2+

[PuVIO2(aco)4]2+

[PuVIO2(aco)3(H2O)]2+[PuVIO2(mo)(aco)2]2+

[PuVO2(aco)]+

[PuVO2(aco)2]+

[PuVO2(aco)2(H2O)]+

[PuVO2(aco)]+

[PuVO2(aco)2(H2O)]+

[PuVO2(aco)(H2O)]+

[PuVO2(aco)(H2O)2]+

[PuVO2(aco)2]+

(a)

(b)

Figure 4. (a) ETD and (b) CID of [PuO2(aco)4]2+.

ETD of Uranyl/Plutonyl Complexes

125

two-electron three-center bond with side-on coordination of theO2 molecule, which results in oxidation to UVI. Density-functionaltheory calculations suggest that symmetric side-on binding ofthe superoxo species is favored because the 60� angle betweenadjacent lobes of the U 5fd orbital provides for efficient overlapwith lobes of the O2 p*xy orbital, unlike the case of d orbitals,where the 90� angle between adjacent lobes leads to a less favor-able overlap.[14]

The same reduced [UVO2]+ species were generated by ETD of

[UVIO2(acn)4]2+ (Fig. 1a, Table 1). However, for this precursor ion,

there was a shift in preference for singly-charged product ions withfewer coordinating ligands. For example, while [UVO2(acn)2]

+

and [UVO2(acn)2(H2O)]+ at m/z 352 and 370 were present at

~30 and ~50% relative intensity in ETD of [UVIO2(acn)5]2+,

the same species were observed at only ~20 and ~30% rela-tive intensity, respectively, in ETD of [UVIO2(acn)4]

2+ undersimilar experimental conditions. Instead, the dominant singly-charged product ions generated by ETD of [UVIO2(acn)4]

2+

were [UVO2(acn)]+, [UVO2(acn)(H2O)]

+ and [UVO2(acn)(H2O)2]+

at m/z 311, 329 and 347, respectively.ETD of [UVIO2(aco)4]

2+ (Fig. 2a, Table 1) produced a wide rangeof singly-charged product ions, all based on the [UVO2]

+ cationcoordinated by aco, or a combination of aco and H2O. As withacn, several product ions which include a bound O2 moleculewere observed. Ions generated by ETD of [UVIO2(aco)4]

2+ included[UVO2(aco)]

+, [UVO2(aco)2]+ and [UVO2(aco)3]

+ atm/z 328, 386 and444, respectively, and multiple hydrates of these primaryproducts. For the aco-coordinated UV dioxocation, the O2 adductsinclude [UVIO2(aco)(H2O)(O2)]

+ (m/z 378), [UVIO2(aco)2(O2)]+ (m/z

418) and [UVIO2(aco)2(H2O)(O2)]+ (m/z 436). ETD of the higher

coordination aco complex gave similar results except that themean number of ligands was greater: the dominant product

J. Mass. Spectrom. 2011, 1247–1254 Copyright © 2011 John W

shifted from [UVIO2(aco)2(H2O)(O2)]+ for [UVIO2(aco)4)]

2+ to[UVIO2(aco)3]

+ for [UVIO2(aco)5]2+.

In general, O2 adducts were more abundant for aco-coordi-nated [UVO2]

+ complexes than for the acn analogues, as isevident from comparison of Figs. 1a and 2a. This is consistent withearlier experimental and density-functional theory (DFT) studiesof aco-coordinated [UVO2]

+,[14] which showed that the bindingenergy of dioxygen to UV systems is sensitive to the presence ofelectron-donating ligands, which can increase the basicity of theuranium center and thereby facilitate electron transfer to O2. Alater study[15] involved gas-phase complexes of the formula[UVO2(aco)]

+ and [UVO2(dmso)]+ (dmso= dimethylsulfoxide) todetermine the general effect of ligand charge donation on thereactivity of [UVO2]

+ with respect to either H2O or O2. The fact thataddition of O2 is enhanced by strong s-donor ligands bound to[UVO2]

+ was supported by results from CID and ligand-additionreaction rate measurements, as well as DFT calculations of relativeenergies, which showed stronger bonds between [UVO2]

+ and O2

when dmso was the coordinating ligand, whereas bonds to H2Oare stronger for the aco complex.

For [PuVIO2(acn)4]2+ and [PuVO2(aco)4]

2+, the principal productions generated by ETD were [PuVO2]

+ species coordinated by 1 or2 acn or aco ligands, with and without hydration. For example,ETD of [PuVIO2(acn)4]

2+ (Fig. 3a, Table 2) furnished primarily[PuVO2(acn)]

+, [PuVO2(acn)(H2O)]+ and [PuVO2(acn)(H2O)2]

+ atm/z 315, 333 and 351, respectively. Likewise, ETD of [PuVIO2

(aco)4]2+ (Fig. 4a, Table 2) produced [PuVO2(aco)]

+, [PuVO2(aco)(H2O)]

+ and [PuVO2(aco)(H2O)2]+ at m/z 332, 350 and 368, respec-

tively. Formation of O2 adducts to the PuVO2+ species was not

observed, regardless of the number or type of ligands, acn oraco. In the case of O2 addition to uranyl(V) complexes, a two-electron three-centered bond is formed, with effective oxidationof UV to UVI. Although there are three non-bonding electronsassociated with the PuV metal center in plutonyl(V), and suchbonding should thus also be feasible there, this oxidation processis not observed for plutonyl. This result is consistent with thehigher stability of hexavalent uranium as compared with pluto-nium, as detailed above, and accordingly reflects an intrinsicdifference between the chemistry of [UVO2]

+ and [PuVO2]+.[4] It

would be desirable to employ theory to address the disparitybetween uranyl(V) and plutonyl(V) towards O2 addition, but suchcomputations are beyond the scope of this work.

CID of acn- and aco-coordinated UVIO22+ and PuVIO2

2+

Consistent with earlier studies,2e,3 CID of [UVIO2(acn)4,5]2+ and

[UVIO2(aco)4,5]2+ leads primarily to loss of a single coordinating

acn or aco ligand to furnish doubly-charged product ions, whichrapidly add one or more H2O molecules in the ion trap. The dom-inant product ion generated by CID of [UVIO2(acn)5]

2+ (m/z 237.5,Fig. S6b, Table 1) was [UVIO2(acn)4]

2+ (m/z 217), along with its H2Oadduct at m/z 226. All singly-charged product ions from CID of[UVIO2(acn)5]

2+ were less than 1% relative intensity; the mostintense of these were [UVIO2(OH)(acn)]

+ (m/z 328) and [UVIO2

(OH)(acn)2]+ (m/z 369). The most abundant product from CID of

[UVIO2(acn)4]2+ at m/z 217 (Fig. 1b, Table 1) was [UVIO2(acn)3

(H2O)]2+ at m/z 205.5, generated by elimination of an acn ligand

and addition of H2O; the second hydrate, [UVIO2(acn)3(H2O)2]2+at

m/z 214.5, was also observed. Less abundant monopositive productions generated by CID of [UVIO2(acn)4]

2+ included [UVIO2(OH)]+

(m/z 287), [UVIO2(OH)(acn)]+ (m/z 328), [UVIO2(OH)(acn)2]

+ (m/z 369)and [UVIO2(OH)(acn)2(H2O)]

+ (m/z 387).

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D. Rios et al.

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The most abundant product ion generated by CID of [UVIO2

(aco)5]2+ (m/z 280, Fig. S7b, Table 1) was [UVIO2(aco)4]

2+ at m/z251. Similar to the acn complexes, the primary product hydratedto [UVIO2(aco)4(H2O)]

2+ (m/z 260). Singly-charged product ionsfrom the five-aco complex were more abundant than from the five-acn counterpart and included species composed of [UVO2]

+, [UVIO2

(OH)]+ or [UVIO2(aco-H)]+ coordinated by aco, or aco and H2O. The

dominant product ion generated by CID of [UVIO2(aco)4]2+ at m/z

251 (Fig. 2b, Table 1) was formed by loss of one aco ligand and addi-tion of H2O ([UVIO2(aco)3(H2O)]

2+, m/z 231). CID of [UVIO2(aco)4]2+

and [UVIO2(aco)5]2+ (Figs. 2b and S7b, and Table 1) generated

significant amounts of [UVIO2(mo)(aco)2]2+ and [UVIO2(mo)(aco)3]

2+,respectively, which are tentatively postulated to comprise themesityl oxide (mo) ligand.3b

CID of [PuVIO2(acn)4]2+ (Fig. 3b, Table 2) generates [PuVIO2(acn)

3(H2O)]2+ (m/z 207.5) and [PuVIO2(acn)3(H2O)2]

2+ (m/z 216.5)through elimination of one acn ligand and addition of one ortwo H2O molecules. The singly-charged products from [PuVIO2

(acn)4]2+ included [PuVIO2(OH)]

+ (m/z 291), [PuVIO2(OH)(acn)]+

(m/z 332) and [PuVIO2(OH)(acn)2]+ (m/z 373). The minor peak at

m/z 375 has an unknown composition.As was the case for the PuVI-acn complex, the doubly-charged

product ions generated by CID of [PuVIO2(aco)4]2+, (Fig. 4b,

Table 2) involved elimination of an aco ligand and formation ofa monohydrate, [PuVIO2(aco)3(H2O)]

2+, at m/z 233. The speciestentatively assigned as [PuVIO2(mo)(aco)2]

2+ at m/z 244 was alsoobserved. Unlike in CID of [PuVIO2(acn)4]

2+ where the hexavalentoxidation state is retained in hydroxide products such as [PuVIO2

(OH)(acn)2]+, the monopositive products generated by CID of

[PuVIO2(aco)4]2+ arose via reduction to [PuVO2]

+ and eliminationof two or three aco ligands to furnish [PuVO2(aco)]

+ (m/z 332)and [PuVO2(aco)2]

+ (m/z 390). Although the monopositive CIDproducts from the [PuVIO2]

2+ complexes are minor, the greaterpropensity for the acn complexes to retain the PuVI oxidationstate by hydrolysis is intriguing. Among the relevant parametersare the comparative ionization energies (IEs) for the two ligands,9.7 eV for aco and 12.2 eV for acn, and their respective proton af-finities (PAs), 812 kJmol�1 (8.42 eV) for aco and 779 kJmol�1

(8.07 eV) for acn.[12] The greater PA[aco] directly implies moreelectron donation to the metal ion and therefore a greater stabil-ity of dipositive gas-phase aco complexes as compared with anal-ogous acn complexes. Interestingly, the dipole moment of acn(3.92 D) is greater than that of aco (2.88 D),[12] which might betaken to imply greater stabilization of cations by acn versus aco.However, the gas PA—and the directly related gas basicity[12]—provides the most direct and unequivocal measure of electron-donating capacity from a ligand to a cation in the gas phase,and thus of the ability of a ligand to stabilize cations in the gasphase. It should be emphasized that other factors, such as dipolemoment, are undoubtedly crucial for ion stabilization in solution,as well as during the various stages of the ESI process; disparitiesbetween cation coordination in solution and in the gas phasehave been discussed by Bühl et al.[16] It should also be remarkedthat the simple model of PA (i.e. gas basicity) is only valid forelementary monodentate ligands, such as acn and aco. Ligandswith a higher denticity, but a lower PA, can more effectively sta-bilize a metal cation without exhibiting a higher PA (basicity) forany individual coordinating site.Given that CID is a gas-phase process, the observations are

evaluated with consideration to gas-phase PA, neglecting otherproperties such as dipole moment which, as remarked above,may be crucial in formation of dipositive ions during ESI. As

wileyonlinelibrary.com/journal/jms Copyright © 2011 Jo

gas-phase cation complexes with aco and acn—e.g., [PuVIO2

(aco)4]2+ and [PuVIO2(acn)4]

2+—lose ligands during CID, the metal

center with acn ligands becomes more electron deficient thanthose with aco ligands, the latter ligand having a higher PA andthus greater intrinsic electron-donating capacity. Accordingly,for example, the effective positive charge on, and thus acidityof, the plutonium center in [PuVIO2(acn)2]

2+ should be greaterthan that in [PuVIO2(aco)2]

2+. This more acidic character evidentlyenables the [PuVIO2(acn)2]

2+ complex to abstract an electron-donating OH- ligand from a water molecule, producing [PuVIO2

(OH)(acn)2]+ and H+; [PuVIO2(OH)(aco)2]

+ is not observed.Another factor which may influence the observed processes is

the greater propensity of aco complexes to reduce by electrontransfer from a departing ligand—i.e. aco ➔ aco+ + e-—due tothe substantially lower IE[aco] (9.7 eV) as compared with IE[acn](12.2 eV)12. Due to this large disparity in IEs, elimination of aco+

from [PuO2(aco)3]2+ should be more facile than elimination of

acn+ from [PuO2(acn)3]2+; [PuO2(aco)2]

+ is observed whereas[PuO2(acn)2]

+ is not (see Figs. 3b and 4b). Charge reduction by elec-tron transfer from background water in the ion trap appears as aless likely process in view of the relatively high IE[H2O] = 12.6 eV.

[12]

In the case of CID of uranyl(VI) complexes, the higher stability of thehexavalent oxidation state evidently results in formation of UVI

hydroxides, rather than reduced [UVO2]+ complexes, for both aco

and acn complexes.

ETD versus CID

For uranyl, the UVI oxidation state is retained during CID, either bymaintaining the +2 charge or in a +1 product comprising either ahydroxide ligand from hydrolysis by background water, or anenolate ligand from deprotonation of a coordinating acetonemolecule. The primary CID pathway for plutonyl is loss of neutralligands with retention of PuVI. The more stable UVI oxidation stateevidently permits hydrolysis in conjunction with charge reduc-tion during CID. For both uranyl and plutonyl, the dominant ETDprocess is charge and oxidation state reduction, with multiple ligandelimination. In contrast to CID, ETD of uranyl(VI) results in reductionto uranyl(V) without hydrolysis; we thus infer that hydrolysis of uranylapparently only occurs via UVI, which process is precluded in ETD dueto charge and oxidation state reduction upon activation.

Under-coordinated uranyl(V) complexes produced by ETDreacted with di-oxygen to produce [UVIO2(L)2(O2)]

+ (L = aco, acn)adduct complexes. The assignment of the oxygen adduct as asuperoxo complex with an O2

� ligand is based on earlier DFTresults.[14] No such oxidation was observed for plutonyl(V), whichis attributed to the propensity of plutonium to exhibit loweroxidation states.[4] It is apparent that the prompt high-energyETD process results in low-stability uranyl(V) complexes whichcan be readily oxidized by dioxygen, whereas lower-energy CIDretains uranyl(VI) throughout. Reduced plutonyl(V) complexesfrom ETD were not similarly oxidized.

Hydrates are more prevalent in the ETD spectra of both the[UVIO2]

2+ and [PuVIO2]2+ complexes. The several intense hydrated

ETD product peaks for [PuO2(aco)4]2+, for example, contrast with

only a single minor hydrate peak in the corresponding CIDspectrum (Fig. 4). This effect can be largely attributed to a greaterabundance in ETD of low-coordinate [AnO2(L)n]

2+ (n= 0–3), whichare susceptible to hydration. Extensive ligand loss from monopo-sitive versus dipositive complexes is associated with weakerbinding of Lewis base ligands in the reduced charge complexes,which are inherently prevalent in ETD.

hn Wiley & Sons, Ltd. J. Mass. Spectrom. 2011, 1247–1254

ETD of Uranyl/Plutonyl Complexes

CONCLUSIONS

Reported here is the first comparison of ETD and CID of gas-phase,solvent-coordinated uranyl [UVIO2]

2+ and plutonyl [PuVIO2]2+ ions

generated by ESI, [AnVIO2(CH3COCH3)4]2+, [AnVIO2(CH3CN)4]

2+,[UVIO2(CH3COCH3)5]

2+ and [UVIO2(CH3CN)5]2+, where An=U or Pu.

Product ions generated by ETD involved reduction to UV and PuV,a direct result of the transfer of an electron during the activationstep. Ligand elimination concomitant with charge reduction inETD resulted in under-coordinated actinyl metal centers whichreadily hydrated in the ion trap. In contrast to ETD, CID of thecomplexes generated primarily doubly-charged fragments thatarise via loss of one or more coordinating ligands; these +2fragments then tend to undergo hydration. Formation of singly-charged species in CID was mostly dominated by charge reductionreactions in which hydroxides, or enolates in the case of acetonecoordination complexes, are formed. However, the singly-chargedspecies generated from CID of [PuVIO2(CH3COCH3)4]

2+ werereduced solvent-coordinated [PuVO2]

+ complexes, consistent withthe difference in stability of the UVI and PuVI species, and specificallythe more facile reduction of the latter.[4]

Singly-charged bis-ligated uranyl product ions generated fromETD undergo O2 addition to regain the hexavalent oxidation stateof the metal. In contrast, singly-charged uranyl CID product ionsretained the original UVI oxidation state and were not susceptibleto O2 addition. Unlike uranyl, singly-charged bis-ligated plutonyl(V) ions, generated from either ETD or CID, did not add O2, thisis another manifestation of the comparative stabilities of uranyl(V/VI) and plutonyl(V/VI).[4] The distinct appearance in relativelyelementary gas-phase complexes of such a key and well-established feature of condensed-phase actinide chemistry asredox stabilities may not appear as startling, or necessarily evenparticularly consequential. However, among the aspects of theseresults which can be considered as scientifically significant arethe following. Firstly, as gas-phase complexes such as thoseexamined in this work are unperturbed by bulk effects includingsolvation and crystal lattice interactions, they provide a demon-stration of ‘intrinsic chemistry’ and furthermore an opportunityto theoretically probe the origins and nature of oxidation statedifferences, such as those apparent between uranyl(VI/V)and plutonyl(VI/V). Secondly, demonstrations of direct relationshipsbetween gas-phase and condensed-phase chemistry presentopportunities to indirectly reveal condensed-phase chemistrywhich may not be directly accessible, such as in the cases of elusiveprotactinyl,[17] and presumed ‘seaborginyl’, SgVIO2

2+.[18]

Acknowledgements

This work was supported by the Director, Office of Science, Office ofBasic Energy Sciences, Division of Chemical Sciences, Geosciencesand Biosciences of the U.S. Department of Energy at LBNL, underContract No. DE-AC02-05CH11231. Work by MVS was supportedin part by a grant from the U. S. National Science Foundation (NSFgrant CAREER-0239800). Appreciation is due to Drs. JoaquimMarçalo, Paul O. Momoh, and Guoxin Tian for assistance andinsights.

125

Supporting Information

Supporting information may be found in the online version ofthis article.

J. Mass. Spectrom. 2011, 1247–1254 Copyright © 2011 John W

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