Formation and Characterization of the Uranyl–SO2 Complex, UO2

Article pubs.acs.org/JPCA

Formation and Characterization of the Uranyl−SO2 Complex, UO2(CH3SO2)(SO2)− Yu Gong and John K. Gibson* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: The uranyl−SO2 adduct, UO2(CH3SO2)(SO2)−, was prepared and characterized by mass spectrometric studies as well as by density functional theory. Collision induced dissociation of UO2(CH3SO2)2− in an ion trap resulted in the formation of UO2(CH3SO2)(SO2)−, which spontaneously reacted with O2 to give UO2(CH3SO2)(O2)−, with SO2 released. The UO2(CH3SO2)(SO2)− complex is computed to have a triplet ground state at the B3LYP level, and the SO2 ligand is coordinated to uranium through two oxygen atoms, similar to the coordination mode of SO2 in its complexes with hard metals. On the basis of the calculated geometric parameters and vibrational frequencies of the SO2 ligand, the UO2(CH3SO2)(SO2)− complex can be considered as a UVO2+ cation coordinated by SO2− and CH3SO2− anions. The UO2(CH3SO2)(O2)− complex is computed to have a peroxo ligand, suggesting that UV in UO2(CH3SO2)(SO2)− is oxidized to the UVI state upon O2 substitution for SO2.

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INTRODUCTION As a major component of air pollution, SO2 is known to cause serious environmental hazards such as acid rain,1 such that effective catalysts must be developed to minimize or mitigate its production. A number of experimental and theoretical studies have been carried out on the absorption, dissociation, and oxidation of SO2 on single crystal metal surfaces, as well as on pure and doped metal oxide surfaces.2 Although different SOx (x = 0−4) products have been proposed and observed, depending on the properties of the surface, the initial step is always SO2 adsorption. Studies of simple SO2−metal adducts can provide information on interactions between metal centers and sulfur dioxide, which is key to the catalytic degradation of SO2. A series of metal−SO2 adducts have been synthesized, and the coordination modes of SO2 in these complexes are wellestablished.3−5 For noble metal−SO2 adducts, the sulfur atom was found to bind the metal center in most cases, with oxygen participation observed only in the η2-S,O coordination mode.3,4 However, the monodentate and bidentate oxygen-coordinated structures were reported for complexes of hard metals such as alkali and first row transition metals with SO2.5 In contrast to main group and transition metal−SO2 adducts, actinide analogues are scarce. Applications of actinides, especially uranium, in catalysis have attracted attention in recent years.6 For uranium, it has been demonstrated that uranium dioxide can enable SO2 degradation, in a similar manner to transition metal systems.7 Accordingly, molecular UO2/SO2 complexes are of interest as simplified models for probing uranium−ligand interactions. Mass spectrometric studies coupled with collision-induced dissociation (CID) have been demonstrated as a useful method to produce reactive organic and inorganic intermediates.8,9 As a © 2013 American Chemical Society

soft ionization technique, electrospray ionization (ESI) is capable of transferring solution species into the gas phase, which presents an approach for elucidating mechanistic processes in solution using gas phase methods.10,11 Recently, the combination of ESI and CID has been used to generate a series of gas phase metal complexes, the structures and reactivities of which were investigated, with the support of theoretical calculations in many cases.12,13 Although H3C−SO2 bond cleavage is well-known in the gas phase,14 this type of reaction has not been reported in the formation of metal−SO2 adducts. A recent comparative study of the CID behavior of Cu(CH3XO)(CH3YO)− (X = CO, SO and Y = SO, SO2) species revealed that SO2 loss occurs most easily, followed by loss of CO2 and SO3, in accord with calculated reaction potential energy profiles.15 Only the Cu(CH3SO3)(CH3)− and Cu(CH3CO2)(CH3)− products with copper−carbon bonds were observed; no Cu−SO2 adducts via CH3 loss were reported. We here report the formation of the uranyl−SO2 complex UO2(CH3SO2)(SO2)− in a quadrupole ion trap (QIT) mass spectrometer by CID of UO2(CH3SO2)2− via C−S bond cleavage and CH3 elimination.

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EXPERIMENTAL AND THEORETICAL METHODS The experiments reported here were performed using an Agilent 6340 quadrupole ion trap mass spectrometer (QIT/ MS) with the electrospray ionization (ESI) source inside a radiological contaminant glovebox, as described in detail elsewhere.16 The UO2(CH3SO2)x− anions were produced by Received: November 7, 2012 Revised: January 11, 2013 Published: January 11, 2013 783

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ESI of methanol solutions of UVIO2(ClO4)2 and CH3SO2Na mixtures (UVIO2(ClO4)2:CH3SO2Na = 1:10−1:15, 200 μM UVIO2(ClO4)2) . Excess CH3SO2Na (Aldrich) is necessary since the UVIO2(ClO4)2 stock solution (8.36 mM, pH = 2) at Lawrence Berkeley National Laboratory is highly acidic. The MSn CID capabilities of the QIT/MS allow isolation of ions with a particular mass-to-charge ratio, m/z, and subsequent insertion of an ion−molecule reaction time without applying ion excitation. The reactions inside the ion trap occur at the temperature around 300 K.17 In high-resolution mode, the instrument has a detection range of 20−2200 m/z with a mass width (fwhm) of ∼0.3 m/z. Mass spectra were recorded in the negative ion accumulation and detection mode. Spectra were taken with the following instrumental parameters: solution flow rate, 60 μL hr−1; nebulizer gas pressure, 15 psi; capillary voltage and current, 4000 V, 26.9 nA; end plate voltage offset and current, −500 V, 47.5 nA; dry gas flow rate, 5 L min−1; dry gas temperature, 325 °C; capillary exit, −147.3 V; skimmer, −40 V; octopole 1 and 2 DC, −12 and −3.67 V; octopole RF amplitude, 200.0 Vpp; lens 1 and 2, 5.0, and 60.0 V; trap drive (i.e., RF voltage), 190.6 V. The trap drive was optimized, and the yield of the UO2(CH3SO2)2− anion was improved when a high trap drive of 190.6 V was used. High-purity nitrogen gas for nebulization and drying in the ion transfer capillary was supplied from the boil-off of a liquid nitrogen Dewar. As has been discussed elsewhere,18,19 the background water and O2 pressure in the ion trap is estimated to be on the order of 10−6 Torr. The helium buffer gas pressure in the trap is constant at ∼10−4 Torr. Quantum chemical calculations were performed using the Gaussian09 program.20 The hybrid B3LYP density functional was employed.21 The 6-311++G(d,p) basis set was used for H, C, O, and S, and the 60 electron core SDD pseudopotential was used for uranium.22,23 All of the geometrical parameters were optimized by starting with different structures and spin states, and the harmonic vibrational frequencies were obtained analytically at the optimized structures. Singlet state calculations were performed using the restricted formalism, whereas the open shell (doublet and triplet) structures were optimized using the unrestricted approach. Spin contamination was found to be minor, in all cases, less than 1%. All of the reported reaction energies, ΔE, are at 298 K, the approximate temperature of the experiments,17 and include the zero-point vibrational energy (ZPVE) correction at 0 K and the thermal correction to 298 K. The natural charge distribution was obtained using the natural bond order (NBO) analysis implemented in Gaussian09.24

was subjected to CID, the major product peak at m/z 413 appeared (Figure 1). This peak at m/z 15 below

Figure 1. CID spectrum of UO2(CH3SO2)2−. In the inset is the spectrum that results when UO2(CH3SO2)(SO2)− (A) produced by CID is isolated; it is apparent that UO2(CH3SO2)(O2)− (B) is produced as a result of spontaneous reaction of A with background O2 in the ion trap.

UO2(CH3SO2)2− indicates CH3 loss and the formation of UO2(CH3SO2)(SO2)− in which uranyl is coordinated by a SO2 ligand. Further evidence for the formation of this species is evident in the inset of Figure 1, where isolation of UO2(CH3SO2)(SO2)− was accompanied by a peak at m/z 381; this peak corresponds to the formation of UO2(CH3SO2)(O2)− from O2 substitution in UO2(CH3SO2)(SO2)− due to trace O2 in the ion trap (see below).19 The uranyl−SO2 adduct UO2(CH3SO2)(SO2)− is formed via the C−S bond cleavage in the UO 2 (CH 3 SO 2 ) 2 − anion. In contrast, CID of UVIO2(CH3SO2)3− primarily resulted in ligand loss to give U V O 2 (CH 3 SO 2 ) 2 − , together with its oxygen adduct UO2(CH3SO2)2(O2)−; a very weak peak corresponding to SO2 loss (m/z 443) also appeared. To obtain insights into the structure and bonding of the UO2(CH3SO2)(SO2)− complex, density functional theory (DFT) calculations at the B3LYP level were carried out. As shown in Figure 2, UO2(CH3SO2)(SO2)− exhibits a triplet ground state with approximately nonplanar Cs symmetry. Both the SO2 and CH3SO2 ligands are bound to the uranium center with bidentate oxygen coordination. This structure is calculated to be 33 kJ/mol lower in energy than the closed shell singlet state, and 23 kJ/mol lower in energy than the triplet isomer with monodentate coordination of SO2 (Figure S3, Supporting Information). For ground-state UO2(CH3SO2)(SO2)−, the bond length in the SO2 ligand is calculated as 1.555 Å, about the same as for the S−O bond in the CH3SO2 ligand (1.558 Å). Geometry optimizations at the same level of theory on neutral SO2 result in a bond length of 1.458 Å, while the computed bond length for the SO2− anion (1.542 Å) is quite close to that calculated for the SO2 ligand in UO2(CH3SO2)(SO2)−. The calculated geometries of both neutral and anionic SO2 agree well with previous DFT and ab initio calculations.25 Frequency calculations reveal that the symmetric and antisymmetric OSO stretching vibrational modes for SO2 in the UO2(CH3SO2)(SO2)− complex should be 936 and 960 cm−1, close to those of SO2− anion (937 and 1018 cm−1) but 195 and 350 cm−1 lower

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RESULTS AND DISCUSSION The mass spectrum from ESI of mixtures of CH3SO2Na and UO2(ClO4)2 shows a series of (NaClO4)x(ClO4)− cluster peaks (Figure S1, Supporting Information). Products due to the UO 2 (CH 3 SO 2 ) 3 − , UO 2 (CH 3 SO 2 ) 3 (CH 3 SO 2 Na) − , and UO2(CH3SO2)3‑y(ClO4)y− (y = 1, 2) anion complexes were observed at m/z 507, 609, 527, and 547. In addition to these uranyl (VI) species, a less intense peak at m/z 428 corresponding to UO2(CH3SO2)2− with a uranium(V) center appeared; this species is of particular interest here. The yield of UO2(CH3SO2)2− in the parent mass spectrum was optimized (Figure S2, Supporting Information) so that isolation and CID studies could be performed. Concurrent with isolation of UO2(CH3SO2)2− was the appearance of UO2(CH3SO2)2(O2)−, as discussed below. When the isolated UO2(CH3SO2)2− anion 784

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Figure 2. Optimized geometries of the UO2(CH3SO2)(SO2)− (3A) and UO2(CH3SO2)(O2)− complexes (1A) at B3LYP level of theory (bond lengths in Å and bond angles in deg). The bond lengths of uranyl are in red; the O−U−O bond angles are 179.3 and 171.9°, respectively.

than those of neutral SO2 (1131 and 1310 cm−1). Accordingly, the SO2 ligand in UO2(CH3SO2)(SO2)− can be considered as SO2−, such that the oxidation state is UV. Adducts between SO2 and group IV metals have been identified in recent neon matrix isolation infrared spectroscopic studies, with the vibrational frequencies for the SO2 ligands observed around 970 cm−1,26 quite close to the frequencies calculated here for the new uranyl−SO2 adduct with a ligand exhibiting SO2− character. Spin density in the present calculations indicates one unpaired electron is mainly localized on uranium (1.1 e), while the other one is delocalized on the SO2 ligand (1.0 e), consistent with the negative charge on the SO2 ligand in UO2(CH3SO2)(SO2)−. Notably, the SO2− species with sulfur in the +3 oxidation state was previously identified upon adsorption of SO2 on oxygen vacancies in cerium oxide systems.27,28 According to recent DFT+U calculations, adsorbed SO2− was found to be oxygen bound to cerium atoms.29 The calculated S−O bond lengths were between 1.51 and 1.59 Å with bond angles around 107− 110°, which depend on the Ce−OSO distance. The symmetric and antisymmetric O−S−O stretches are about 110 and 240 cm−1 red-shifted from those of gas phase SO2. These structural parameters and vibrational frequencies are similar to the values obtained here for the SO2 ligand in UO2(CH3SO2)(SO2)−. Studies of SO2 adducts of main group and transition metals have revealed that the coordination mode of SO2 depends on the nature of the metal center.3−5 Soft donors such as noble metals tend to form complexes with SO2 through η1-S or η2S,O coordination,3,4 while the end-on bonded structure with a metal−oxygen bond is dominant for hard metals like alkali and first row transition metals.5 Consistent with this notion, the coordination mode of SO2 with the hard uranyl cation30 is similar to those involving other hard metal centers. Natural charge calculations suggest that the uranyl subunit, UO2, in UO2(CH3SO2)(SO2)− is positively charged by 0.29 e, while −0.63 and −0.66 e negative charges are found on the CH3SO2 and SO2 ligands, respectively. Compared with the natural charges on the alkali metals (0.98−0.99 e) in complexes comprising [M(SO2)]+ units (M = Li, Na, K, Rb, and Cs),31 the uranyl subunit is much less positively charged, suggesting that charge transfer occurs from the CH3SO2 and SO2 ligands to UO2. This is in accord with the longer S−O bond lengths in UO2(CH3SO2)(SO2)− than in alkali metal−SO2 adducts, where the metal−ligand bonding interactions are mainly ionic.31 As shown in Figure 1, the appearance of the m/z 381 peak due to UO2(CH3SO2)(O2)− when UO2(CH3SO2)(SO2)− (m/ z 413) was isolated suggests that reaction 1 with background O2 in the ion trap proceeds spontaneously during the time associated with the isolation process.

UVO2 (CH3SO2 )(SO2 )− + O2 → UVIO2 (CH3SO2 )(O2 )− + SO2

ΔE = −18 kJ/mol

(1)

On the basis of geometry optimization, the UO2(CH3SO2)(O2)− complex is computed to have a closed shell singlet ground state with uranyl coordinated by bidentate O2 and CH3SO2 ligands (Figure 2), which exhibits a nonplanar structure with approximately Cs symmetry. The calculated O−O bond length of 1.439 Å for UO2(CH3SO2)(O2)− is consistent with that for a peroxo complex,32 suggesting the uranium center in UO2(CH3SO2)(SO2)− is oxidized from UV to UVI upon O2 substitution. In addition, the O−U−O stretching vibrational frequencies are also sensitive to the oxidation state of uranium.30 The antisymmetric O−U−O stretch for the UVO2(CH3SO2)(SO2)− complex is computed to be 857 cm−1, 25 cm−1 red-shifted from the value for UVIO2(CH3SO2)(O2)− (882 cm−1), although the stretching mode in the peroxo complex involves more CH 3 SO 2 participation. This is analogous to the lower stretching frequency for UVO2+ compared with UVIO22+.30 Geometry optimizations on the triplet states of UO2(CH3SO2)(O2)− converge to superoxo isomers with shorter O−O bond lengths (Figure S4, Supporting Information). The bidentate superoxo isomer is calculated to be 49 kJ/mol less stable than the ground state peroxo structure, and the end-on bonded isomer is 105 kJ/mol less stable (Figure S4, Supporting Information). Frequency calculations reveal that the antisymmetric O−U− O stretching frequencies for both bidentate and end-on bonded isomers are 844 and 845 cm−1 respectively, in line with the UV character for these high energy superoxo isomers. In contrast to UO2(CH3SO2)(O2)− formed by reaction 1, no indication was found for the formation of UO2(SO2)(O2)− via reaction 2. UVO2 (CH3SO2 )(SO2 )− + O2 → UVIO2 (SO2 )(O2 )− + CH3SO2

ΔE = 88 kJ/mol

(2)

Calculations on the reaction energetics indicate that reaction 1 is slightly exothermic, by 18 kJ/mol, while reaction 2 is endothermic by 88 kJ/mol such that spontaneous substitution of CH3SO2 by O2 should not be observed. The difference in energetics between reactions 1 and 2 can be traced to the large electron affinity for neutral CH3SO2 (2.59 eV at B3LYP level). In contrast, the electron affinity for SO2 is calculated to be 1.67 eV; the experimental value is 1.107 ± 0.008 eV.33 Previous calculations revealed that the CH3OSO isomer is slightly more stable than CH3SO2.14,34 A similar result is obtained in our calculations, which show a lower endothermicity of 32 kJ/mol for reaction 2 if CH3SO2 is replaced by CH3OSO. However, the high energy barrier for isomerization given by those 785

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results in ligand loss associated with reduction to give UO2(CH3SO2)2−; the yield of UO2(CH3SO2)2(SO2)− was minuscule. The CID behavior of UO2(CH3SO2)2− differs markedly from that of Cu(CH3SO2)2−, which exclusively results in the formation of Cu(CH3SO2)(CH3)− via SO2 loss.

calculations makes it unlikely that CH3OSO is the neutral leaving molecule. Regardless of the structure of the eliminated neutral, reaction 2 is endothermic and should not be observed. Formation of the UO2(CH3SO2)(SO2)− product via C−S bond cleavage of the UO2(CH3SO2)2− complex (reaction 3) is calculated to be endothermic by 162 kJ/mol.

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UVO2 (CH3SO2 )2− → UVO2 (CH3SO2 )(SO2 )− + CH3 ΔE = 162 kJ/mol

S Supporting Information *

Complete author list of ref 20, ESI/MS of the CH3SO2Na and UO2(ClO4)2 mixtures (Figures S1 and S2), and all the computed structures (Figures S3−S6) at DFT/B3LYP level of theory. This material is available free of charge via the Internet at http://pubs.acs.org.

(3) V

O2(CH3SO2)2−,

In contrast to the CID results for U for UVIO2(CH3SO2)3−, the dominant CID process was CH3SO2 ligand loss with reduction to give UVO2(CH3SO2)2−. Very weak UO2(CH3SO2)2(CH3)− and UO2(CH3SO2)2(OH)− peaks were also apparent, but the peak due to UO2(CH3SO2)2(SO2)− was barely detectable. Other uranyl (VI) complexes are known to undergo reduction via neutral ligand loss during CID.35 The observation of UO2(CH3SO2)2(O2)− indicates that uranyl (V) coordinated by CH3SO2 ligands is oxidized by O2 to form the superoxo complex, which exhibits the particularly stable UVI oxidation state. Such gas-phase oxidation of UV to UVI has been reported36 and was observed in a recent study of O2 addition to uranyl (V) hydrates.19 The CID behavior of UO2(CH3SO2)2− observed here is quite different from that of Cu(CH3SO3)(CH3SO2)− and Cu(CH3CO2)(CH3SO2)−, where only the SO2 loss products, Cu(CH3SO3)(CH3)− and Cu(CH3CO2)(CH3)−, were produced.15 Similarly, we found that CID of Cu(CH3SO2)2− gave exclusively Cu(CH3SO2)(CH3)− via SO2 loss. It should be noted that a Cu(CH3CO2)(CH3)(SO2)− intermediate with a Cu−S bond was computed to be stable by density functional calculations but was not detected.15 Geometry optimization of the UO2(CH3SO2)(SO2)− complex with a U−S interaction converges to the structure with U−O bonding as shown in Figure 2. The SO2 loss reaction for the UO2(CH3SO2)2− complex (reaction 4) is computed to be endothermic by 254 kJ/mol from our B3LYP calculations, 92 kJ/mol higher than the observed CH3 loss (reaction 3).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was fully supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Heavy Element Chemistry at LBNL, under Contract No. DE-AC0205CH11231. Computations were done using 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-AC0205CH11231.

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REFERENCES

(1) Stern, A. C.; Boubel, R. W.; Tourner, D. B.; Fox, D. L. Fundamentals of Air Pollution, 2nd ed.; Academic Press: Orlando, FL, 1984. (2) See for examples: (a) Haase, J. Structural Studies of SO2 Adsorption on Metal Surfaces. J. Phys. 1997, 9, 3647−3670. (b) Smirnov, M. Y.; Kalinkin, A. V.; Pashis, A. V.; Sorokin, A. M.; Noskov, A. S.; Kharas, K. C.; Bukhtiyarov, V. I. Interaction of Al2O3 and CeO2 Surfaces with SO2 and SO2 + O2 Studied by X-ray Photoelectron Spectroscopy. J. Phys. Chem. B 2005, 109, 11712− 11719. (c) Luckas, N.; Vines, F.; Happel, M.; Desikusumastuti, A.; Libuda, J.; Goerling, A. Density Functional Calculations and IR Reflection Absorption Spectroscopy on the Interaction of SO2 with Oxide-Supported Pd Nanoparticles. J. Phys. Chem. C 2010, 114, 13813−13824. (3) Schenk, W. A. The Coordination Chemistry of Small SulfurContaining Molecules: A Personal Perspective. Dalton Trans. 2011, 40, 1209−1219. (4) Amgoune, A.; Bourissou, D. σ-Acceptor, Z-type Ligands for Transition Metals. Chem. Commun. 2011, 47, 859−871. (5) Mews, R.; Lork, E.; Watson, P. G.; Görtler, B. Coordination Chemistry in and of Sulfur Dioxide. Coord. Chem. Rev. 2000, 197, 277−320. (6) Fox, A. R.; Bart, S. C.; Meyer, K.; Cummins, C. C. Towards Uranium Catalysts. Nature 2008, 455, 341−349. (7) Schlereth, T. W.; Hedhili, M. N.; Yakshinskiy, B. V.; Gouder, T.; Madey, T. E. Adsorption and Reaction of SO2 with a Polycrystalline UO2 Film: Promotion of S−O Bond Cleavage by Creation of ODefects and Na or Ca Coadsorption. J. Phys. Chem. B 2005, 109, 20895−20905. (8) Squires, R. R. Gas-Phase Carbanion Chemistry. Acc. Chem. Res. 1992, 25, 461−467. (9) Eller, K.; Schwarz, H. Organometallic Chemistry in the Gas Phases. Chem. Rev. 1991, 91, 1121−1177.

UVO2 (CH3SO2 )2− → UVO2 (CH3SO2 )(CH3)− + SO2 ΔE = 254 kJ/mol

ASSOCIATED CONTENT

(4)

The preference for the loss of CH3 over SO2 upon CID of the UO2(CH3SO2)2− complex results from the stronger interactions between uranium and SO2, which contrasts with the case of Cu(CH3SO2)2−.

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CONCLUSIONS Both UO2(CH3SO2)2− and UO2(CH3SO2)3− were produced by ESI of mixtures of UO2(ClO4)2 and CH3SO2Na, and UO 2 (CH 3 SO 2 )(SO 2 ) − was produced upon CID of UO2(CH3SO2)2− via C−S bond cleavage. Calculations at the B3LYP level reveal that the UO2(CH3SO2)(SO2)− complex exhibits a triplet ground state with the SO2 ligand oxygen bound to the uranium center in a bidentate fashion, which is similar to the coordination mode of SO2 in its adducts with hard metals. The SO2 ligand can be considered as an SO2− anion, consistent with the longer S−O bond length, as well as the lower vibrational frequencies compared with those of the SO2 molecule. The UO2(CH3SO2)(SO2)− complex reacts spontaneously with O2 to form the peroxo UO2(CH3SO2)(O2)− complex, with uranium oxidized from UV to UVI. In contrast to UO2(CH3SO2)2−, CID of UO2(CH3SO2)3− mainly 786

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(28) Mashchenko, A. I.; Pariiskii, G. B.; Kazanskii, V. B. Radical Formed by Chemisorption of Electron Acceptor Molecules on Surfaces of n-Semiconductors. III. Formation of Radicals in the Chemisorption of Sulfur Dioxide on the Surface of Reduced Titanium Dioxide and the Reactions of these Radicals with Oxygen. Kinet. Katal. 1968, 9, 151−157. (29) Lu, Z.; Mueller, C.; Yang, Z.; Hermansson, K.; Kullgren, J. SOx on Ceria from Adsorbed SO2. J. Chem. Phys. 2011, 134, 184703. (30) Denning, R. G. Electronic Structure and Bonding in Actinyl Ions and their Analogs. J. Phys. Chem. A 2007, 111, 4125−4143. (31) Derendorf, J.; Keßler, M.; Knapp, C.; Rühle, M.; Schulz, C. Alkali Metal-Sulfur Dioxide Complexes Stabilized by Halogenated closo-Dodecaborate Anions. Dalton Trans. 2010, 39, 8671−8678. (32) Gong, Y.; Zhou, M. F.; Andrews, L. Spectroscopic and Theoretical Studies of Transition Metal Oxides and Dioxygen Complexes. Chem. Rev. 2009, 109, 6765−6808. (33) Nimlos, M. R.; Ellison, G. B. Photoelectron Spectroscopy of SO2−, S3−, and S2O−. J. Phys. Chem. 1986, 90, 2574−2580. (34) Chu, L. K.; Lee, Y. P. Infrared Absorption of CH3SO2 Detected with Time-Resolved Fourier-Transform Spectroscopy. J. Chem. Phys. 2006, 124, 244301. (35) Sokalska, M.; Prussakowska, M.; Hoffmann, M.; Gierczyk, B.; Frański, R. Unusual Ion UO4− Formed upon Collision Induced Dissociation of UO2(NO3)3−, UO2(ClO4)3−, UO2(CH3COO3)3− Ions. J. Am. Soc. Mass Spectrom. 2010, 21, 1789−1794. (36) Leavitt, C. M.; Bryantsev, V. S.; de Jong, W. A.; Diallo, M. S.; Goddard, W. A., III; Groenewold, G. S.; Van Stipdonk, M. J. Addition of H2O and O2 to Acetone and Dimethylsulfoxide Ligated Uranyl(V) Dioxocations. J. Phys. Chem. A 2009, 113, 2350−2358.

(10) Schröder, D. Applications of Electrospray Ionization Mass Spectrometry in Mechanistic Studies and Catalysis Research. Acc. Chem. Res. 2012, 45, 1521−1532. (11) Agrawal, D.; Schröder, D. Insight into Solution Chemistry from Gas-Phase Experiments. Organometallics 2011, 30, 32−35. (12) O’Hair, R. A. J. The 3D Quadrupole Ion Trap Mass Spectrometer as a Complete Chemical Laboratory for Fundamental Gas-Phase Studies of Metal Mediated Chemistry. Chem. Commun. 2006, 1469−1481. (13) O’Hair, R. A. J. Gas Phase Ligand Fragmentation to Unmask Reactive Metallic Species. In Reactive Intermediates; Santos, L. S., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2010. (14) Ratliff, B. J.; Tang, X.; Butler, L. J.; Szpunar, D. E.; Lau, K. C. Determining the CH3SO2 → CH3 + SO2 Barrier from Methylsulfonyl Chloride Photodissociation at 193 nm Using Velocity Map Imaging. J. Chem. Phys. 2009, 131, 044304. (15) O’Connor Sraj, L.; Khairallah, G. N.; da Silva, G.; O’Hair, R. A. J. Who Wins: Pesci, Peters, or Deacon? Intrinsic Reactivity Orders for Organocuprate Formation via Ligand Decomposition. Organometallics 2012, 31, 1801−1807. (16) Rios, D.; Rutkowski, P. X.; Shuh, D. K.; Bray, H. T.; Gibson, J. K.; Van Stipdonk, M. J. Electron Transfer Dissociation of Dipositive Uranyl and Plutonyl Coordination Complexes. J. Mass Spectrom. 2011, 46, 1247−1254. (17) Gronert, S. Estimation of Effective Ion Temperatures in a Quadrupole Ion Trap. J. Am. Soc. Mass Spectrom. 1998, 9, 845−848. (18) (a) Rutkowski, P. X.; Michelini, M. C.; Bray, T. H.; Russo, N.; Marçalo, J.; Gibson, J. K. Hydration of Gas-Phase Ytterbium Ion Complexes Studied by Experiment and Theory. Theor. Chem. Acc. 2011, 129, 575−592. (b) Rutkowski, P. X.; Michelini, M. C.; Gibson, J. K. Proton Transfer in Th(IV) Hydrate Clusters: A Link to Hydrolysis of Th(OH)22+ to Th(OH)3+ in Aqueous Solution. J. Phys. Chem. A 2013, 117, 451−459. (19) Rios, D.; Michelini, M. C.; Lucena, A. F.; Marçalo, J.; Bray, T. H.; Gibson, J. K. Gas-Phase Uranyl, Neptunyl, and Plutonyl: Hydration and Oxidation Studied by Experiment and Theory. Inorg. Chem. 2012, 51, 6603−6614. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (21) (a) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (22) (a) McLean, A. D.; Chandler, G. S. Contracted Gaussian Basis Sets for Molecular Calculations. I. Second Row Atoms, Z = 11−18. J. Chem. Phys. 1980, 72, 5639−5648. (b) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650−654. (23) Kuchle, W.; Dolg, M.; Stoll, H.; Preuss, H. Energy-Adjusted Pseudopotentials for the Actinides. Parameter Sets and Test Calculations for Thorium and Thorium Monoxide. J. Chem. Phys. 1994, 100, 7535−7542. (24) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, version 3.1. (25) Liang, J.; Li, H. Franck−Condon Simulation of the Photoelectron Spectrum of SO2 Including Duschinsky Effects. Chem. Phys. 2005, 314, 317−322. (26) Liu, X.; Wang, X. F.; Wang, Q.; Andrews, L. OMS, OM(η2-SO), and OM(η2-SO)(η2-SO2) Molecules (M = Ti, Zr, Hf): Infrared Spectra and Density Functional Calculations. Inorg. Chem. 2012, 51, 7415−7424. (27) Schoonheydt, R. A.; Lunsford, J. H. Electron Paramagnetic Resonance of SO2-on Magnesium Oxide. J. Phys. Chem. 1972, 76, 323−328. 787

dx.doi.org/10.1021/jp311034x | J. Phys. Chem. A 2013, 117, 783−787