Formation of Bare UO22+ and NUO+ by Fragmentation of Gas-Phase

Article pubs.acs.org/JPCA

Formation of Bare UO22+ and NUO+ by Fragmentation of Gas-Phase Uranyl−Acetonitrile Complexes Michael J. Van Stipdonk,*,† Maria del Carmen Michelini,*,‡ Alexandra Plaviak,† Dean Martin,† and John K. Gibson§ †

Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282, United States Dipartimento di Chimica, Università della Calabria, 87030 Arcavacata di Rende, Italy § Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

S Supporting Information *

ABSTRACT: In a prior study [Van Stipdonk; et al. J. Phys. Chem. A 2006, 110, 959−970], electrospray ionization (ESI) was used to generate doubly charged complex ions composed of the uranyl ion and acetonitrile (acn) ligands. The complexes, general formula [UO2(acn)n]2+, n = 0−5, were isolated in an 3-D quadrupole ion-trap mass spectrometer to probe intrinsic reactions with H2O. Two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-exchange reactions. For the former, the intrinsic tendency to add H2O was dependent on the number and type of nitrile ligand. For the latter, charge exchange involved primarily the formation of uranyl hydroxide, [UO2OH]+, presumably via a collision with gas-phase H2O and the elimination of a protonated nitrile ligand. Examination of general ion fragmentation patterns by collision-induced dissociation, however, was hindered by the pronounced tendency to generate hydrated species. In an update to this story, we have revisited the fragmentation of uranyl−acetonitrile complexes in a linear ion-trap (LIT) mass spectrometer. Lower partial pressures of adventitious H2O in the LIT (compared to the 3-D ion trap used in our previous study) minimized adduct formation and allowed access to lower uranyl coordination numbers than previously possible. We have now been able to investigate the fragmentation behavior of these complex ions completely, with a focus on tendency to undergo ligand elimination versus charge reduction reactions. CID can be used to drive ligand elimination to completion to furnish the bare uranyl dication, UO22+. In addition, fragmentation of [UO2(acn)]2+ generated [UO2(NC)]+, which subsequently fragmented to furnish NUO+. Formation of the nitrido by transfer of N from cyanide was confirmed using precursors labeled with 15N. The observed formation of [UO2(NC)]+ and NUO+ was modeled by density functional theory.

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INTRODUCTION

transform ion cyclotron resonance mass spectrometer to generate NpO22+ and PuO22+.14 Heinemann and Schwarz produced bare NUO+ by the reaction of UN+ with O2.15 The NUO+ ion is particularly significant because it is isoelectronic with UO22+. The first reported transfer of uranyl ions from solution to the gas phase using electrospray ionization (ESI/MS) was as monopositive pentavalent UVO2+, reported in 1992.16 Thereafter, advances in fundamental understanding of uranyl coordination chemistry have been achieved by using this experimental technique.17−24 Most importantly, it has been shown that gas-phase, doubly charged complexes containing the uranyl ion can be generated by ESI. For example, ESI using solutions composed of uranyl−nitrate dissolved in a mixture of H2O and acetone generated gas-phase complexes containing the uranyl ion coordinated by acetone ligands.20 Using mild

The desire to gain an understanding of intrinsic uranium cation reactivity has motivated several mass spectrometric studies, with many early investigations focusing on the reactions of uranium in low oxidation states (i.e., U+ and UO+) with organic compounds1−6 or oxidation of uranium ions by small molecules such as O2, CO, N2O, and ethylene oxide.7−10 To improve the understanding of intrinsic uranium chemistry, particularly for higher oxidation states, we expanded the study to include species-dependent reactivity of a range of monopositive uranylligand cations using ion-trap mass spectrometry.11−13 Exploration of intrinsic uranyl ion chemistry in the past was impeded because of a lack of effective methods for generating doubly charged complexes containing UO22+. Cornehl et al.8 reported formation of the “bare” uranyl dication by gas-phase oxidation; charge-exchange reactions yielded a value for the second ionization potential for UO2+ that was consistent with vertical ionization energies obtained by ab initio calculations. Later, Marçalo and co-workers extended the approach of laser desorption/ionization and in situ oxidation in a Fourier © 2014 American Chemical Society

Received: July 2, 2014 Revised: August 14, 2014 Published: August 14, 2014 7838

dx.doi.org/10.1021/jp5066067 | J. Phys. Chem. A 2014, 118, 7838−7846

The Journal of Physical Chemistry A

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conditions, the dominant species observed in the ESI mass spectrum contained the uranyl ion coordinated by five acetone ligands, consistent with proposed most-stable structures in the solution phase. However, chemical mass shift data, ion peak shapes, and a plot of fractional ion abundance versus ion desolvation temperature suggested that in the gas phase, complexes with five equatorial acetone ligands are less stable than those with four. The initial work focusing on gas-phase uranyl complexes has been recently extended to neptunyl and plutonyl species, with an emphasis on revealing variations in chemistry across the early actinide series.25−27 A major advancement in recent years has been the preparation by ESI of uranyl halide anions with the general formulas UO2X3− (X = F, Cl, Br, I)28 and UO2X42− (X = F, Cl).29,30 Wang and coworkers have probed the electronic structures of these species, as well as that of UO2−,31 by anion photoelectron spectroscopy. Throughout our past studies of the gas-phase dissociation behavior of actinyl complexes, multiple-stage collision-induced dissociation (CID) experiments have suggested that the doubly charged complexes containing, for example, acetone do not shed their full complement of coordinating ligands but instead generate hydrated product ions (H2O replacing acetone ligands eliminated in the CID reactions) or undergo charge reduction reactions to cations such as [UO2OH]+ and UO2+ coordinated by acetone and/or H2O.20 It appeared that at no point were gas-phase complex ions containing the uranyl ion with two or fewer ligands generated. In a later study ESI was used to generate doubly charged complex ions composed of the uranyl ion and nitrile (acetonitrile, acn; propionitrile, pn; benzonitrile, bzn) ligands.21 The complexes, with general formula [UO2(RCN)n]2+, n = 0−5 (where R = CH3, CH3CH2, or C6H5), were isolated in an iontrap mass spectrometer to probe intrinsic reactions with H2O. Two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-exchange reactions. For the former, the intrinsic tendency to add H2O was dependent on the number and type of nitrile ligand. For the latter, charge exchange involved primarily the formation of uranyl hydroxide, [UO2OH]+, presumably via a collision with gas-phase H2O, and the elimination of a protonated nitrile ligand. Regardless of the complexing ligands used, the ability to investigate the intrinsic fragmentation behavior uranyl complexes with low coordination number has been impeded by high apparent hydration rates in a 3-D ion trap. In the present study, we used a linear quadrupole ion trap (LIT) to reexamine the fragmentation of gas-phase uranyl−acn complex ions. As described below, the lower partial pressures of adventitious H2O in the LIT (compared to the 3-D ion trap used in our previous study) minimized adduct formation and allowed access to lower uranyl coordination numbers than previously possible. We can now report a clearer picture of the fragmentation behavior of complex ions with composition [UO2(acn)n]2+, n = 1−5, with a focus on the tendency to undergo ligand elimination versus charge reduction reactions. More importantly, CID experiments with native and 15Nlabeled acetonitrile clearly demonstrate that a nitrido complex is formed by exchange of an oxo ligand of the uranyl ion with N from cyanide, the latter generated by decomposition of an acetonitrile ligand. The experimental observations have been substantiated and modeled by density functional theory (DFT).

Article

EXPERIMENTAL METHODS

Mass Spectrometry. ESI and CID experiments were performed on a ThermoScientific (San Jose, CA) LTQ-XL LIT mass spectrometer equipped with an Ion Max ESI source. For the ESI experiments, a stock solution (approximately 0.001 M) of uranyl nitrate hexahydrate was prepared in water/acetonitrile in a 60:40 (v:v) ratio. The solution was infused into the ESI-MS instrument using the incorporated syringe pump at a flow rate of 10−15 μL/min. The atmospheric pressure ionization stack settings for the LTQ (lens voltages, quadrupole and octopole voltage offsets, etc.) were optimized for maximum transmission of the doubly charged ions [UO2(acn)4]2+ and [UO2(acn)5]2+ to the ion-trap mass analyzer by using the autotune routine within the LTQ Tune program. Helium was used as the bath/ buffer gas to improve trapping efficiency and as the collision gas for CID experiments. For CID, precursor ions were isolated using an isolation width of 1.0−2.5 mass-to-charge (m/z) units. The exact value was determined empirically to provide maximum isolated ion intensity while ensuring isolation of a single isotopic peak. The normalized collision energy (NCE, as defined by ThermoScientific) was set between 5 and 18%, which corresponds the application of roughly 0.55−0.68 V tickle voltage to the end-cap electrodes with the current instrument calibration. The activation Q, which defines the frequency of the applied radio frequency potential, was set at 0.30. In all cases, the activation time employed was 30 ms. Spectra displayed represent the accumulation and averaging of at least 30 isolation, dissociation and ejection/detection steps. Computational Methodology. DFT computations were performed using the Gaussian09 (revision B01) quantum chemistry package.32 Small-core relativistic effective core potentials of the Stuttgart−Cologne group, so-called SDD, were employed for uranium along with the segmented basis set, corresponding to the (14s13p10d8f6g)/[10s9p5d4f3g] contraction.33,34 Augmented triple-ζ with diffuse function basis sets, aug-cc-pVTZ, were used for the rest of the atoms.35 The structures of the various isomers, reaction intermediates, and transition states, were optimized using two different density functionals (B3LYP36,37 and M06-L38,39). The choice of the density functionals and basis sets were made on the basis of the good performance observed in previous computational studies involving the computation of ionization energies of actinide oxides.40 All the reported transition states were confirmed by means of intrinsic reaction coordinates calculations.41−43 Single-point CCSD(T) calculations were then performed for the geometries of reactants and products obtained at the B3LYP level, using the same basis sets. A comparison of the bond distances of optimized minima and transition states obtained with the two different density functionals differ by less than 0.03 Å. There is a reasonable agreement between the relative energies obtained at B3LYP and M06-L levels; the largest difference in relative energies is 48 kJ/mol. All the reported reaction energies include the zero point energy corrections (ΔEZPE). Singlet-state optimizations were performed within the restricted Kohn−Sham formalism, whereas triplet-state species were studied using the unrestricted approach. For all the studied species we have checked the ⟨S2⟩ values to evaluate whether spin contamination had influenced the quality of the results. No significant spin contamination issues were detected. It should be noted that spin−orbit corrections were not explicitly included in our 7839



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mix of nitrile and H2O ligands was considerably lower than in our earlier study.20 The CID spectra generated from [UO2(acn)n]2+, n = 1−4, are shown in Figure 2. CID (MS/MS stage) of [UO2(acn)5]2+

calculations. Although spin−orbit effects are not expected to significantly affect the energetics of processes in which there is no change in the formal oxidation state of the heavy metal, substantial changes can occur for reaction energies involving actinide atoms in different oxidation states. Previous computational studies have shown that spin−orbit corrections lower the energies of uranyl(VI)/uranyl(V) reactions by ca. 30 kJ/mol as a consequence of the stabilization of open-shell uranyl(V).44−48 Similarly, inclusion of spin−orbit effects for reactions involving the reduction of U(V) to U(IV) can make the reaction energies more exothermic by ca. 29 kJ/mol.44 Because the oxidation state is U(VI) in both the reactants and products, the ligand dissociation energies and total reaction energies computed here are not expected to be significantly affected by spin−orbit effects. However, it is feasible that the energies of the reaction intermediates and transition states (Figure 5) involving species having different uranium oxidation states could be affected by the inclusion of spin−orbit corrections; this caveat does not substantially affect the key interpretations. QTAIM analysis was performed on reactants and products, as well as in all the lowest-energy reaction intermediates and transition states, to analyze the evolution of the uranium−ligand bonding during the reaction.49 Appropriate wave function extended files (wfx) were obtained with Gaussian09 at the B3LYP level and analyzed using the AIMAll package.50

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RESULTS AND DISCUSSION CID of [UO2(acn)n]2+, n = 1−5. Figure 1 shows the ESI mass spectrum generated from uranyl nitrate hexahydrate in

Figure 2. Collision-induced dissociation spectra generated from uranyl−acetonitrile complex ions: (a) fragmentation of [UO2(acn)4]2+, (b) fragmentation of [UO2(acn)3]2+, (c) fragmentation of [UO2(acn)2]2+, and (d) fragmentation of [UO2(acn)]2+.

at m/z 237.5 generated only [UO2(acn)4]2+ at m/z 217 and is not shown in Figure 2. The dominant product ion generated by the subsequent CID of [UO2(acn)4]2+ (MS3 stage, Figure 2a) was [UO2(acn)3]2+ at m/z 196.5, through a (single) acn elimination reaction. This observation is in sharp contrast to our earlier study using the 3-D quadrupole ion trap, in which the product ion spectrum derived from [UO2(acn)4]2+ was dominated by the mono- and dihydrates, [UO2(acn)3(H2O)]2+ and [UO2(acn)3(H2O)2]2+, m/z 205.5 and 214.5, respectively.21 The relative intensity of [UO2(acn)3]2+ in our earlier study was only ∼5% relative to the base peak. However, in the present study using the LIT the intensity of the monohydrate is only 10% relative to [UO2(acn)3]2+. We attribute the difference in the product ion distributions to a lower abundance of adventitious H2O in the LIT, whether by change in instrument geometry and design or better vacuum conditions. Given that the base vacuum chamber pressure is comparable in both the LIT and 3-D quadrupole ion trap, a plausible explanation for the lower water pressure in the LIT is more efficient pumping of the inside of the trap due to greater conductance through the larger orifices in the LIT geometry. In any case, it is clear that the tendency to form H2O adducts is significantly diminished in the present experiments.

Figure 1. Electrospray ionization mass spectrum generated from uranyl nitrate hexahydate dissolved in 60:40 (v:v) H2O/acn.

60:40 (v:v) water/acetonitrile using the LIT mass spectrometer. As in our previous study,20 the principal doubly charged uranyl−nitrile complex ions generated were [UO2(acn)4]2+ and [UO2(acn)5]2+ at m/z 217 and 237.5, respectively. Insets in Figure 1 show expanded views of the two dominant uranyl− acetonitrile complexes that reveal the isotopic peak spacing and the +2 charge state of the ions. Because we used a relatively high fraction of acetonitrile in the spray solution, the abundance of doubly charged complexes composed of uranyl ion with a 7840



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Product ion spectra for [UO2(acn)2]2+ and [UO2(acn)]2+ had to be collected in our prior study by isolating the species directly from the ESI source rather than using multiple-stage CID, and the product ion spectrum generated from CID of [UO2(acn)2]2+ was dominated by [UO2OH]+, [UO2OH(acn)]+, and the mono and dihydrates of [UO2OH(acn)]+. In the present study, the high abundance of [UO2(acn)2]2+ resulting from dissociation of [UO2(acn)3]2+ allowed us to continue the serial fragmentation experiments. The CID spectrum (MS5 stage) generated from [UO2(acn)2]2+ is shown in Figure 2c. The dominant product ion is [UO2(acn)]2+ at m/z 155.5. Also observed were singly charged product ions at m/z 287 and 310 and [UO2OH(acn)]+ at m/z 328. The species at m/z 287 corresponds to [UO2OH]+. The ion at m/z 310 is attributed to formation of [UO2(acn-H)]+. Subsequent CID of this species (Figure 2 of the Supporting Information) generated UO2+ at m/z 270, presumably through reductive elimination of CH2CN. Formation of deprotonated acn has precedent, as it is known that metalated nitriles and nitrile anions are stable and can be generated readily in the condensed phase.51−53 The CID spectrum (MS6 stage) of [UO2(acn)]2+ is shown in Figure 2d. As in our earlier study, one major product ion generated was [UO2OH]+ at m/z 287. Also observed was UO2+ at m/z 270 and [UO2OCH3]+ at m/z 301. More significantly, an ion at m/z 135 was observed, which we attribute to formation of UO22+ by elimination of the sole remaining acn ligand. In our earlier study, the reactivity of UO22+ was investigated by first using relatively harsh electrospray conditions, and then isolating the ion directly.21 The composition assignment of UO22+ was then supported by the fact that reactions with adventitious H2O furnished peaks at m/ z 270 and 287, consistent with formation of UO2+ and [UO2OH]+ by direct reduction and charge-reduction/exchange reactions in the ion trap. In the present study, UO22+ is apparently generated by direct ligand elimination in the gas phase. Isolation and storage of the product ion at m/z 135 generated UO2+ and [UO2OH]+ at m/z 270 and 287, respectively (spectrum not shown), thus supporting this conclusion. To the best of our knowledge, this is the first report of the direct generation of bare uranyl ion in the gas phase via the serial reduction of coordination number by CID. Because IE[UO2+] = 14.6 ± 0.4 eV54 exceeds IE[acn] = 12.2 eV,55 charge separation [UO2(acn)]2+ to UO2+ and acn+ should be energetically favored over the elimination of neutral acn. The formation of bare UO22+ is attributed to a Coulomb barrier to charge separation to produce UO2+ and acn+.56 Since our earlier study of uranyl−nitrile complexes, the energetics for acn addition to UO22+ has been investigated.57 Optimized geometries were obtained for UO22+ complexes with up to six nitrile ligands. Examination of the relative binding energies suggests that ligand addition is exothermic up to the fifth ligand addition, in agreement with other experimental and computational results.46,58−60 Addition of a sixth acn ligand is endothermic. The formation by ESI of complexes with up to five ligands, and in particular the absence of ion signal consistent with generation of [UO2(acn)6]2+, is consistent with calculations. The calculated relative exothermicity for ligand addition decreases with increasing coordination number. Although the quadrupole ion-trap instrument is not as well suited for measurements of bond dissociation energies as, for example, threshold CID studies, we found in the present study that the amount of applied collision energy needed to induce

In our past experiments, the tendency to generate gas-phase H2O adducts impeded a detailed study of the influence of coordination number on fragmentation behavior for uranyl and related complexes. In fact, the H2O “concentration” in the ion trap was sufficiently high to allow studies of intrinsic hydration rates,13 in which species such as [UO2OH]+ and [UO2NO3]+ were isolated and stored within the ion trap for times ranging from 1 to 1000 ms and allowed to react with H2O present in the ion trap. Although the partial pressure of water in the 3-D quadrupole ion trap could not be quantitatively determined, [H2O] for rate constant measurements was estimated at ∼8 × 1010 molecules cm−3 (or half of the total pressure) on the basis of ambient conditions existing in the ion trap (298 K and 5 × 10−6 Torr). In the present study, single H2O and CH3OH adducts (m/z 205.5 and m/z 212.5, respectively) are produced following isolation and storage of [UO2(acn)3]2+, without imposed collisional activation, for 1 s (spectrum not shown). The CH3OH presumably is present in the vacuum system because of its use in spray systems employed by unrelated peptide dissociation experiments conducted on the instrument. The formation of a methanol adduct was confirmed by the appearance of acn-coordinated [UO2OCH3]+ following isolation and CID of the species at m/z 212.5. For a comparison to earlier work,20 we also isolated [UO2OH]+ and [UO2NO3]+, both of which are generated in small quantities by ESI using the H2O/acn solvent mixture. Significant adduct formation was only observed for isolation times greater than 1 s (spectra not shown), whereas the fractional abundance of the mono- and dihydrates [UO2OH(H2O)]+ and [UO2OH(H2O)2]+ were ∼0.2 at 500 ms isolation time in our earlier study, and the trihydrate was the dominant product (fractional abundance of 0.9) when [UO2NO3]+ was isolated. Determination of [H2O] and the study of hydration rates of the uranyl−nitrile complexes in the LIT are not the focus of this study. However, the observations outlined above from our preliminary comparison of ligand addition certainly demonstrate that the amount of adventitious H2O in the linear ion trap used in the present study is significantly lower than in the 3-D ion trap employed in our previous study. In our prior study,20 CID of [UO2(acn)3]2+ produced [UO2(acn)2]2+ at only 5% relative intensity21 and the majority of the [UO2(acn)3]2+ ion population was instead converted to the mono- and dihydrates by ligand-addition reactions during the isolation step prior to collisional activation. The remainder of the fragment ion distribution included [UO2OH]+ and [UO2OH(acn)]+, and mono and dihydrates of [UO2OH(acn)]+ at m/z 346 and 364, respectively, presumably again because of the high H2O levels in the vacuum system and ion trap. Using the LIT in the present study, CID (MS4 stage, Figure 2b) of [UO2 (acn) 3]2+ at m/z 196.5 furnished [UO2(acn)2]2+ at m/z 176, with no H2O adduct formation observed using the default 30 ms activation time. Also observed in the CID spectrum of [UO2(acn)3]2+ was [UO2OH(acn)]+ at m/z 328. As in our earlier study, we attribute formation of this ion to a reactive collision with adventitious H2O in the ion trap. The complementary product ion would be protonated acn at m/z 42, which lies below the low-mass cutoff imposed during ion-trap CID. However, using the low-mass waveform setting on the LTQ-XL, the product ion spectrum generated by CID of [UO2(acn)3]2+ was collected in the mass range 20−200 u (Figure 1 of the Supporting Information). In this spectrum the sole product ion is protonated acn at m/z 42. 7841



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dissociation, in general, increased as the coordination number decreased, in accord with the predicted trend in ligand binding energy as revealed by de Jong and co-workers.57 Mulliken charges derived from calculations provide some insight into bonding within the uranyl complex ions. In the DFT study noted above, the change in charge on uranium from the bare uranyl ion to a complexed ion with five to six ligands was found to be less than 0.5 e−, suggesting that U retains a formal +2 charge throughout ligand addition; the actual charge on U in UO22+ is ∼+3.61,62 The electron density donated from the ligands tends to reside on the axial oxygen atoms. More importantly, while the N atoms of the acn ligands transfer electron density to the uranyl dication, the Mulliken charges also appear to remain relatively constant. This suggests that the bulk of the donated electron density comes from the carbon backbone of the ligands.53 The charge-exchange reactions to produce ligated uranyl− hydroxide have also been modeled computationally.63 In general, the formation of hydroxide species was modeled with the assumption that uranyl must first have two ligands bound to it, with at least one being H2O. The reaction energies of the charge-exchange reactions were found by DFT to be on the order of 10−25 kcal/mol less exoergic than the alternative ligand addition reactions. During the modeling of charge exchange, it was noted that the reactions can occur in one of two pathways. One involves loss of protonated H2O whereas elimination of protonated acn occurs in the other. Of the two pathways, the thermodynamically favorable one involves the loss of a protonated nitrile, and the charge-exchange reaction via loss of (acn + H)+ is sufficiently exoergic for chargeexchange reactions to be competitive with ligand addition reactions. Formation of a Nitrido Species. The other significant product ion derived by CID of [UO2(acn)]2+ appeared at m/z 296 (Figure 2d). The species is assigned a composition of [UO2(NC)]+ and described as a UO22+ complex with a cyanide ligand. The cyanide is presumably generated by heterolytic bond cleavage and elimination of CH3+ from the bound acetonitrile ligand in [UO2(acn)]2+. CID of the ion at m/z 296 (MS7 stage, Figure 3a) produced UO2+ and [UO2OH]+ (m/z 270 and 287, respectively). Surprisingly, a product ion was also observed at m/z 268, a value consistent with formation of NUO+ by exchange of an oxo ligand for N from the bound cyanide ligand. The neutral species eliminated is presumably CO. Formation of a nitrido complex in the gas phase during CID has precedent. In an earlier investigation, 64 CID of UO2(NCO)Cl2− generated a product ion with m/z value consistent with the formula NUOCl 2 −, with apparent elimination of neutral CO2. The NUOCl2− species as computed by DFT to have Cs symmetry and a singlet ground state and computed bond length and order consistent with formation of a terminal UN bond. In addition, the endothermic reaction to produce NUOCl2− through loss of CO2 was computed to be thermodynamically favored over direct elimination of NCO.64 It has also been shown that uranyl(V) oxo bonds can be activated in the gas phase using ion-trap mass spectrometry and reaction of U16O2+ with H218O to produce U16O18O+ and U18O2+.65 To confirm generation of NUO+ from the cyanide complex in the present study, we examined the fragmentation of species that included 15N-labeled acn. The CID spectrum of [UO2(15N-acn)]+, generated by multiple-stage CID, is shown

Figure 3. CID spectra generated from (a) [UO2(NC)]+ and (b) [UO2(15NC)]+.

in Figure 3b. A shift of one mass unit was observed for the species attributed to formation of NUO+, consistent with the transfer of the 15N label. Thus, the labeling study provides strong supporting evidence that a species with terminal nitride group is created by elimination of CO from [UO2(NC)]+. Our computational efforts were concentrated in studying the reaction mechanisms for the formation of NUO+ from the [UO2(acn)]2+ complex. Geometry optimizations at the DFT level were performed for all the species involved in the reaction mechanisms; the lowest-energy structure of all the reaction intermediates and transition states are reported in Figure 4. Our computations for NUO+ (Figure 5 and Table 1) are in agreement with the previously reported conclusions regarding the bonding characteristics of this cation.15,66−68 Dissociation energies were computed at DFT and CCSD(T) levels of theory. The computed dissociation energies for the formation of [UO2(NC)]+ and NUO+ at the CCSD(T) [B3LYP/M06-L] levels of theory are as follows (spin states in parentheses): [UO2 (acn)]2 + (1) → [UO2 (NC)]+ (1) + CH3+(1) ΔEZPE = 201 [175/202] kJ mol−1

(I)

[UO2 (NC)]+ (1) → NUO+(1) + CO(1) ΔEZPE = 140 [110/131] kJ mol−1

(II)

We assume that in reaction I the elimination of CH3+ involves a simple elongation and breaking of the C−C bond of acn without further rearrangements of the remaining [UO2(NC)]+ moiety. At the levels of theory used here (B3LYP/M06-L) the cyanide isomer, [UO2(CN)]+, was found to be higher in energy by ca. 31 kJ mol−1, in agreement with previous computational results for the two-69 and fourcoordinated60,70 adducts that indicate that uranyl prefers 7842



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Figure 5. Schematic representation of the B3LYP [M06-L] potential energy profile for the formation of NUO+ from [UO2(NC)]+. The potential energy profile relative energies were calculated with respect to the initial [UO2(acn)]2+ complex. Spin multiplicities are given in parentheses.

lower than the dissociation energy for elimination of the acn ligand from [UO2(acn)]2+, which according to our calculations is 402(394/397) kJ mol−1 at CCSD(T)(B3LYP/M06-L) level. This dissociation energy is significantly higher than the [UO2(acn)2]2+ → [UO2(acn)]2+ + acn dissociation energy (303/301 kJ mol−1 at the B3LYP/M06-L level of theory). According to our calculations, formation of NUO+ from the initial [UO 2 (NC)] + ion proceeds through a two-step mechanism (Figure 5) and involves a double crossing between the singlet and the triplet spin potential energy surfaces of the system. The first crossing between the singlet and triplet spinstate surfaces occurs just before the formation of the first transition state (TS1), which was found to be more stable at the triplet spin state. TS1 involves a folding of the NC moiety, which in this structure interacts with one of the -yl oxygen atoms through the C end. TS1 leads to the formation of the UO(NCO)+ triplet-state insertion intermediate, which is the lowest-energy isomer in the potential energy profile of the reaction. It is rather remarkable that triplet UO(NCO)+, formally a U(IV) complex, should be substantially lower in energy than singlet [UO2(NC)]+, formally a U(VI) complex. We note that the structure(s) of the product ion at m/z 296 generated by CID of UO2(acn)+ is not known: it may be [UO2(NC)]+, as indicated in reaction I, lower-energy OU(NCO)+, or a mixture of the two isomers. After the formation of the UO(NCO)+ intermediate, the reaction overcomes a second spin crossing and proceeds toward the formation of the reaction products, NUO+ + CO, after surpassing the second transition state (TS2) that involves the breaking of the N−C bond. We were able to locate a final NUO+(CO) complex on the triplet-state surface, whereas on the singlet-state surface we were unable to locate such a complex. Our calculations indicate that the elimination of NCO from the triplet UO(NCO)+ intermediate to form UO+ is endothermic by ca. 400 (424) kJ mol−1 at the B3LYP (M06-L) level of theory and is, therefore, not competitive with the CO loss process. The QTAIM bonding analysis for all the species involved in the lowestenergy reaction pathway is summarized in Table 1. For comparison, we have also reported the same analysis for UO22+ and UO2(acn)2+. The electron density U−O (3, −1) bond critical point (bcp) and the bond order (BO) clearly show the weakening of the U−O bond in going from the initial UO2(NC)+ ion to the UO(NCO)+ intermediate, in which these values are already very close to those of the NUO+ product. We note that UO2(NC)+ shows already a noticeable change in charge density and BO with respect to UO2(acn)2+ and with respect to bare uranyl, UO22+. After an initial decrease during the formation of TS1, the charge density at the U−N (3, −1) bcp and the corresponding BO increase steadily after the formation of the UO(NCO)+ insertion intermediate, up to the highest values in NUO+. We note that the charge density and BO of the U−N bond in NUO+ are significantly higher than the corresponding values for the U−O bond, which explains the shorter U−N bond and has been previously reported for other ions containing this moiety.64

isocyanide over cyanide for low coordination complexes, whereas cyanide is preferred in five-coordinated complexes.60,70,71 As explained below, during the evolution of the reaction II to form NUO+ substantial rearrangements of atoms take place in the [UO2(NC)]+ moiety, which clearly show that the singlet-state [UO2(NC)]+ structure is not the lowest-energy isomer. The endothermicity of reactions I and II is notably

CONCLUSIONS To summarize, we have revisited the multiple-stage CID of uranyl−acetonitrile complexes using a linear quadrupole iontrap mass spectrometer. Unlike our earlier investigation, the gas-phase conditions of the ion trap appear to involve much less adventitious H2O and CH3OH, thus allowing better access to the intrinsic gas-phase behavior (unimolecular dissociation) of

Figure 4. B3LYP geometrical parameters of [UO 2(acn) 2] 2+, UO2(acn)]2+, and the lowest-energy minima and transition states involved in the [UO2(NC)]+ → NUO+ + CO dissociation pathway. Bond lengths are in angstroms and angles in degrees. Molecular states, molecular symmetries, and transition states imaginary frequencies are given in parentheses.

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Table 1. Electron Densities (ρ), Laplacian (∇2ρ), and Energy Densities (H) at Selected Bond Critical Points and Bond Order (BO) for the Ground-State Structures of the UO2(NC)+ → NUO+ + CO Reaction Pathwaya,b,c U−O

U−N

N−C

C−O

ρ H ∇2ρ BO ρ H ∇2ρ BO ρ H ∇2ρ BO ρ H ∇2ρ BO

UO22+(1)

UO2(acn)2+(1)

UO2(NC)+(1)

TS1(3)d

UO(NCO)+(3)

TS2(1)

NUO+(1)

0.370 −0.388 0.370 2.28

0.354 −0.358 0.352 2.18 0.086 −0.024 0.218 0.58 0.484 −0.911 −0.498 1.97

0.338 −0331 0.326 2.12 0.125 −0.053 0.267 0.93 0.454 −0.836 −0.843 1.80

0.320 0.228 −0.301−0.168 0.270 0.228 2.09 1.47 0.077 −0.021 0.187 0.55 0.464 −0.857 −0.663 2.03 0.126 −0.059 0.085 0.68

0.293 −0.261 0.344 2.05 0.116 −0.045 0.322 0.96 0.427 −0.716 −1.077 1.51 0.475 −0.869 0.157 1.44

0.318 −0.302 0.280 2.05 0.261 −0.212 0.280 2.01 0.179 −0.112 −0.132 0.89 0.481 −0.889 0.427 1.67

0.327 −0.315 0.416 2.05 0.409 −0.463 −0.101 2.85

a

For comparison, the corresponding values for UO22+ and UO2(acn)2+ are also reported. bIn atomic units. Spin states in parentheses. The bond order was calculated using the delocalization index, which accounts for the electrons delocalized or shared between the atomic basins and corresponds to the topological bond order defined by Á ngyán et al.72 cFree CO (1.126 Å): ρ = 0.507, H = −0.950, ∇2ρ = 0.441, BO = 1.78. dThe second U−O bond interacts with the C atom.

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ACKNOWLEDGMENTS M.V.S. acknowledges support for this work in the form of startup funds from the Bayer School of Natural and Environmental Sciences and Duquesne University. Laboratory space renovation was made possible through support by the National Science Foundation through grant CHE-0963450. A.P. acknowledges support from the NSF-REU program (CHE1263279). M.C.M. acknowledges support by the Università della Calabria. J.K.G. acknowledges support by the U.S. Department of Energy, Basic Energy Sciences, at LBNL under contract No. DE-AC02-05CH11231. This research used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

the complexes to be investigated. Two important observations were made in the present study. First, we found that the coordination number of the uranyl ion, UO22+, can be reduced to zero in the gas phase using multiple-stage CID. Fragmentation involves primarily ligand elimination rather than charge reduction reactions that involve formation of acetone-coordinated uranyl hydroxide that plagued our earlier study. Apparently, for the first time, the bare uranyl ion has been created by direct elimination of ligands by CID. Second, CID of [UO2(acn)]2+ caused formation of [UO2(NC)]+. Subsequent CID of this species furnished a nitride species, NUO+, presumably with concomitant elimination of CO. DFT reveals that the energetics associated with formation of both [UO2(NC)]+ and NUO+ should be accessible under the experimental CID conditions. The computed pathway for conversion of [UO2(NC)]+ to NUO+ reveals two spin crossings and an unanticipated low-energy [OU(NCO)]+ intermediate.

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ASSOCIATED CONTENT

S Supporting Information *

The product ion spectrum of [UO2(acn)3]2+ at m/z 196.5 generated by combining results from CID of the precursor when using the normal and low mass waveforms on the LTQXL, the CID of the product ion at m/z 310 (derived by fragmentation of [UO2(acn)]2+ at m/z 155.5) and the full citation for ref 32 are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*M. J. Van Stipdonk. E-mail: [email protected]. *M. C. Michelini. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7844



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Formation of Bare UO22+ and NUO+ by Fragmentation of Gas-Phase

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