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We report the first experimental study of the intrinsic chemistry of a U-methylidyne species, focusing on reaction of [OUCH]+ with H2O, O2 and CH3CN in the gas phase. DFT was also used to determine reaction pathways, and establish the mechanism by which [OUCH]+ is formed through collision-induced dissociation of [UO2(CCH)]+.

While the strong axial UO bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C–O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in “top-down” CID reactions and replacement in “bottom-up” ion–molecule and dissociation steps.

Building on our report that collision-induced dissociation (CID) can be used to create the highly reactive U-alkylidyne species [O=U≡CH]Cationic uranyl-propiolate precursor ions were generated by electrospray ionization, and multiple-stage CID in a linear trap instrument was used to prepare [O=U≡CH]The [O=U≡CH][O=U≡CH]

The goals of the present study were (a) to create positively charged organo-uranyl complexes with general formula [UO2 (R)]+ (eg, R═CH3 and CH2 CH3 ) by decarboxylation of [UO2 (O2 C─R)]+ precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas-phase H2 O. Collision-induced dissociation (CID) of both [UO2 (O2 C─CH3 )]+ and [UO2 (O2 C─CH2 CH3 )]+ causes H+ transfer and elimination of a ketene to leave [UO2 (OH)]+ . However, CID of the alkoxides [UO2 (OCH2 CH3 )]+ and [UO2 (OCH2 CH2 CH3 )]+ produced [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ , respectively. Isolation of [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ for reaction with H2 O caused formation of [UO2 (H2 O)]+ by elimination of ·CH3 and ·CH2 CH3 : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO2 (O2 C─CH═CH2 )]+ and [UO2 (O2 C─C6 H5 )]+ , caused decarboxylation to leave [UO2 (CH═CH2 )]+ and [UO2 (C6 H5 )]+ , respectively. These organometallic species do react with H2 O to produce [UO2 (OH)]+ , and loss of the respective radicals to leave [UO2 (H2 O)]+ was not detected. Density functional theory calculations suggest that formation of [UO2 (OH)]+ , rather than the hydrated UV O2+ , cation is energetically favored regardless of the precursor ion. However, for the [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ precursors, the transition state energy for proton transfer to generate [UO2 (OH)]+ and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO2 (H2 O)]+ . The situation is reversed for the [UO2 (CH═CH2 )]+ and [UO2 (C6 H5 )]+ precursors: The transition state for proton transfer is lower than the energy required for creation of [UO2 (H2 O)]+ by elimination of CH═CH2 or C6 H5 radical.

Because of the high stability and inertness of the U=O bonds, activation and/or functionalization of UO22+ and UO2+ remain challenging tasks. We show here that collision-induced dissociation (CID) of the uranyl-propiolate cation, [UVIO2(O2C-C≡CH)]+, can be used to prepare [UVIO2(C≡CH)]+ in the gas phase by decarboxylation. Remarkably, CID of [UVIO2(C≡CH)]+ caused elimination of CO to create [OUVICH]+, thus providing a new example of a well-defined substitution of an “yl” oxo ligand of UVIO22+ in a unimolecular reaction. Relative energies for candidate structures based on density functional theory calculations suggest that the [OUVICH]+ ion is a uranium-methylidyne product, with a U≡C triple bond composed of one σ-bond with contributions from the U df and C sp hybrid orbitals, and two π-bonds with contributions from the U df and C p orbitals. Upon isolation, without imposed collisional activation, [OUVICH]+ appears to react spontaneously with O2 to produce [UVO2]+.

Electrospray ionization and multiple-stage tandem mass spectrometry were used to study the collision-induced dissociation of methanol-coordinated copper-acetate cations, and the ion-molecule reactions of specific product ions. Our experiments led to the discovery of unusual gas-phase ions with compositions such as [Cu(CO2)(CH3OH)]+ and [Cu(N2)(CH3OH)]+. The latter is generated by spontaneous exchange of CO2 for N2 in an ion-molecule reaction. Isotopic labeling studies and high mass-resolution measurements provide data to support the product ion composition assignments. Density functional theory calculations corroborate the experimental observations, both with respect to the preferential decarboxylation over methanol ligand elimination, and the spontaneous nature of the ion-molecule reactions.

Electrospray ionization was used to generate species such as [ZnNO3(CH3OH)2]+ from Zn(NO3)2•XH2O dissolved in a mixture of CH3OH and H2O. Collision-induced dissociation of [ZnNO3(CH3OH)2]+ causes elimination of CH3OH to form [ZnNO3(CH3OH)]+. Subsequent collision-induced dissociation of [ZnNO3(CH3OH)]+ causes elimination of 47 mass units (u), consistent with ejection of HNO2. The neutral loss shifts to 48 u for collision-induced dissociation of [ZnNO3(CD3OH)]+, demonstrating the ejection of HNO2 involves intra-complex transfer of H from the methyl group methanol ligand. Subsequent collision-induced dissociation causes the elimination of 30 u (32 u for the complex with CD3OH), suggesting the elimination of formaldehyde (CH2 = O). The product ion is [ZnOH]+. Collision-induced dissociation of a precursor complex created using CH3-18OH shows the isotope label is retained in CH2 = O. Density functional theory calculations suggested that the “rearranged” product, ZnOH with bound HNO2 and formaldehyde is significantly lower in energy than ZnNO3 with bound methanol. We therefore used infrared multiple-photon photodissociation spectroscopy to determine the structures of both [ZnNO3(CH3OH)2]+ and [ZnNO3(CH3OH)]+. The infrared spectra clearly show that both ions contain intact nitrate and methanol ligands, which suggests that rearrangement occurs during collision-induced dissociation of [ZnNO3(CH3OH)]+. Based on the density functional theory calculations, we propose that transfer of H, from the methyl group of the CH3OH ligand to nitrate, occurs in concert with the formation of a Zn–C bond. After dissociation to release HNO2, the product rearranges with the insertion of the remaining O atom into the Zn–C bond. Subsequent C–O bond cleavage, with H transfer, produces an ion–molecule complex composed of [ZnOH]+ and O = CH2.

Uranium trioxide, UO

While the strong axial U[double bond, length as m-dash]O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C-O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in "top-down" CID reactions and replacement in "bottom-up" ion-molecule and dissociation steps.

We recently reported a detailed investigation of the collision-induced dissociation (CID) of [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- in a linear ion trap mass spectrometer (J. Mass Spectrom. DOI:10.1002/jms.4705). Here, we describe the CID of [UO2 (NO3 )(O2 )]- which is created directly by ESI, or indirectly by simple elimination of O2 from [UO2 (NO3 )(O2 )2 ]- . CID of [UO2 (NO3 )(O2 )]- creates product ions as at m/z 332 and m/z 318. The former may be formed directly by elimination of O2 , while the latter required decomposition of a nitrate ligand and elimination of NO2 . DFT calculations identify a pathway by which both product ions can be generated, which involves initial isomerization of [UO2 (NO3 )(O2 )]- to create [UO2 (O)(NO2 )(O2 )]- , from which elimination of NO2 or O2 will leave [UO2 (O)(O2 )]- or [UO2 (O)(NO2 )]- , respectively. For the latter product ion, the composition assignment of [UO2 (O)(NO2 )]- rather than [UO2 (NO3 )]- is supported by ion-molecule reaction behavior, and in particular, the fact that spontaneous addition of O2 , which is predicted to be the dominant reaction pathway for [UO2 (NO3 )]- is not observed. Instead, the species reacts with H2 O, which is predicted to be the favored pathway for [UO2 (O)(NO2 )]- . This result in particular demonstrates the utility of ion-molecule reactions to assist the determination of ion composition. As in our earlier study, we find that ions such as [UO2 (O)(NO2 )]- and [UO2 (O)(O2 )]- form H2 O adducts, and calculations suggest these species spontaneously rearrange to create dihydroxides.

Electrospray ionization (ESI) can produce a wide range of gas-phase uranyl (UO2 2+ ) complexes for tandem mass spectrometry studies of intrinsic structure and reactivity. We describe here the formation and collision-induced dissociation (CID) of [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- . Multiple-stage CID experiments reveal that the complexes dissociate in reactions that involve elimination of O2 , NO2 , or NO3 , and subsequent reactions of interesting uranyl-oxo product ions with (neutral) H2 O and/or O2 were investigated. Density functional theory (DFT) calculations reproduce experimental results and show that dissociation of nitrate ligands, with ejection of neutral NO2 , is favored for both [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- . DFT calculations also suggest that H2 O adducts to products such as [UO2 (O)(NO3 )]- spontaneously rearrange to create dihydroxides and that addition of O2 is favored over addition of H2 O to formally U(V) species.

Uranium trioxide, UO3, has a T-shaped structure with bent uranyl, UO22+, coordinated by an equatorial oxo, O2-. The structure of cation UO3+ is similar but with an equatorial oxyl, O center dot-. Neutral and cationic uranium trioxide coordinated by nitrates were characterized by collision induced dissociation (CID), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory. CID of uranyl nitrate, [UO2 (NO3)3]- (complex A1), eliminates NO2 to produce nitrate-coordinated UO3+, [UO2 (O. )(NO3)2]-(B1), which ejects NO3 to yield UO3 in [UO2 (O)(NO3)]- (C1). Finally, C1 associates with H2O to afford uranyl hydroxide in [UO2(OH)2 (NO3)]- (D1). IRMPD of B1, C1, and D1 confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: (B1) radical oxyl O.-; (C1) oxo O2-; and (D1) two hydroxyls, OH- . As the nitrates are bidentate, the equatorial coordination is six in A1, five in B1, four in D1, and three in C1. Ligand congestion in low-coordinate C1 suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1 epitomizes the inverse trans influence in UO3, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl v3 IR frequencies indicate the following donor ordering: O2- [best donor] >> O.- > OH-> NO3-.

Bupropion is an atypical antidepressant and smoking cessation aid associated with wide intersubject variability. This study compared the formation kinetics of three phase I metabolites (hydroxybupropion, threohydrobupropion, and erythrohydrobupropion) in human, marmoset, rat, and mouse liver microsomes. The objective was to establish suitability and limitations for subsequent use of nonclinical species to model bupropion central nervous system (CNS) disposition in humans. Hepatic microsomal incubations were conducted separately for the R- and S-bupropion enantiomers, and the formation of enantiomer-specific metabolites was determined using LC-MS/MS. Intrinsic formation clearance (CLint) of metabolites across the four species was determined from the formation rate versus substrate concentration relationship. The total clearance of S-bupropion was higher than that of R-bupropion in monkey and human liver microsomes. The contribution of hydroxybupropion to the total racemic bupropion clearance was 38%, 62%, 17%, and 96% in human, monkey, rat, and mouse, respectively. In the same species order, threohydrobupropion contributed 53%, 23%, 17%, and 3%, and erythrohydrobupropion contributed 9%, 14%, 66%, and 1.3%, respectively, to racemic bupropion clearance. The results demonstrate that phase I metabolism in monkeys best approximates that observed in humans, and support the preferred use of this species to investigate possible pharmacokinetic factors that influence the CNS disposition of bupropion and contribute to its high intersubject variability.

RATIONALE In a previous study [Rapid Commun Mass Spectrom. 2004;18:3028-3034], collision-induced dissociation (CID) of [UVI O2 (ClO4 )]+ appeared to be influenced by the high levels of background H2 O in a quadrupole ion trap. The CID of the same species was re-examined here with the goal of determining whether additional, previously obscured dissociation pathways would be revealed under conditions in which the level of background H2 O was lower. METHODS Water- and methanol-coordinated [UVI O2 (ClO4 )]+ precursor ions were generated by electrospray ionization. Multiple-stage tandem mass spectrometry (MSn ) for CID and ion-molecule reaction (IMR) studies was performed using a linear ion trap mass spectrometer. RESULTS Under conditions of low background H2 O, CID of [UVI O2 (ClO4 )]+ generates [UVI O2 (Cl)]+ , presumably by elimination of two O2 molecules. Using low isolation/reaction times, we found that [UVI O2 (Cl)]+ will undergo an IMR with H2 O to generate [UVI O2 (OH)]+ . CONCLUSIONS With lower levels of background H2 O, CID experiments reveal that the intrinsic dissociation pathway for [UVI O2 (ClO4 )]+ leads to [UVI O2 (Cl)]+ , apparently by loss of two O2 molecules. We propose that the results reported in the earlier CID study reflected a two-step process: initial formation of [UVI O2 (Cl)]+ by CID, followed by a very rapid hydrolysis reaction to leave [UVI O2 (OH)]+ .

While the strong axial U O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. In this study, CID of the pentafluorobenzoate precursor ion [UO2(O2C–C6F5)]+ was used to produce the organo-uranyl ion [UO2(C6F5)]+ by decarboxylation. Subsequent CID of [UO2(C6F5)]+ created [UO2(F)]+ by fluoride transfer and elimination of C6F4, rather than UO2+ by elimination of pentafluorophenyl radical (as has been observed for similar species). Moreover, upon reaction of [UO2(C6F5)]+ with H2O, apparent substitution of OH for F to create [UO3HC6F4]+ is favored over hydrolysis to produce [UO2(OH)]+ and (neutral) C6F5H. Subsequent CID of [UO3HC6F4]+ generates [UO2(F)]+ and [OUF]+. When [OUF]+ is isolated to react with H2O and O2 in the ion trap, the principal product ions observed are [UO2(F)]+ and [UO2(OH)]+. Experiments conducted with labeled reagent suggest that reaction with H218O leads to exchange of the oxo ligand and incorporation of the 18O label into [OUF]+ while reaction with O2 likely creates [UO2(F)]+. Formation of [UO2(OH)]+ from [OUF]+ is the result of a cascade of reactions, with initial formation of [UO2(F)]+ by reaction with O2, followed by hydrolysis to create the hydroxide species.

The following gas-phase uranyl/12-crown-4 (12C4) complexes were synthesized by electrospray ionization: [UO2(12C4)2]2+ and [UO2(12C4)2(OH)]+. Collision-induced dissociation (CID) of the dication resulted in [UO2(12C4-H)]+ (12C4-H is a 12C4 that has lost one H), which spontaneously adds water to yield [UO2(12C4-H)(H2O)]+. The latter has the same composition as complex [UO2(12C4)(OH)]+ produced by CID of [UO2(12C4)2(OH)]+ but exhibits different reactivity with water. The postulated structures as isomeric [UO2(12C4-H)(H2O)]+ and [UO2(12C4)(OH)]+ were confirmed by comparison of infrared multiphoton dissociation (IRMPD) spectra with computed spectra. The structure of [UO2(12C4-H)]+ corresponds to cleavage of a C–O bond in the 12C4 ring, with formation of a discrete U–Oeq bond and equatorial coordination by three intact ether moieties. Comparison of IRMPD and computed IR spectra furthermore enabled assignment of the structures of the other complexes. Theoretical studies of the chemical bonding features of the complexes provide an understanding of their stabilities and reactivities. The results reveal bonding and structures of the uranyl/12C4 complexes and demonstrate the synthesis and identification of two different isomers of gas-phase uranyl coordination complexes.

Developing a comprehensive understanding of the reactivity of uranium species remains an important goal in areas ranging from the development of nuclear fuel processing methods to studies of the migration and fate of the element in the environment. Electrospray ionization (ESI) can provide relatively easy access to gas-phase complexes containing uranium in high oxidation states for subsequent studies of intrinsic structure and reactivity. We report here the formation of a superoxo- complex, [UVIO2(O2)(O2C-CF3)2]−, which is created by ESI using “gentle” conditions (low sheath gas flow rate and low desolvation temperature). CID of [UVIO2(O2)(O2C-CF3)2]− causes elimination of O2, presumably with concomitant reduction of UVIO22+ to UVO2+. Remarkably, subsequent CID of [UVO2(O2C-CF3)2]− creates a species at m/z 330, which is attributed to formation of [UVO(F)4]−. A similar species is generated by multiple-stage CID in a linear ion trap, and collision-cell CID in a Fourier-transform ion-cyclotron resonance (FT-ICR) mass spectrometer, when initiated with the tris-trifluoroacetato complex [UVIO2(O2C-CF3)3]−. High accuracy mass measurement using the FT-ICR instrument confirms the composition assignment for the species at m/z 330.

We present a thorough characterization of fragmentations observed in threshold collision-induced dissociation (TCID) experiments of protonated triglycine (H+GGG) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Kinetic energy-dependent cross-sections for 10 ionic products are observed and analyzed to provide 0 K barriers for six primary products: [b2]+, [y1 + 2H]+, [b3]+, CO loss, [y2 + 2H]+, and [a1]+; three secondary products: [a2]+, [a3]+, and [y2 + 2H – CO]+; and two tertiary products: high energy [y1 + 2H]+ and [a2 – CO]+ after accounting for multiple ion-molecule collisions, internal energy of reactant ions, unimolecular decay rates, competition between channels, and sequential dissociations. Relaxed potential energy surface scans performed at the B3LYP-D3/6-311+G(d,p) level of theory are used to identify transition states (TSs) and intermediates of the six primary and one secondary products. Geometry optimizations and single point energy calculations were performed at several levels of theory. These theoretical energies are compared with experimental energies and are found to give reasonably good agreement, in particular for the M06-2X level of theory. This good agreement between experiment and theory validates the reaction mechanisms explored computationally here and elsewhere and allows identification of the product structures formed at threshold energies. The present work presents the first measurement of absolute experimental threshold energies of important sequence ions and non-sequence ions: [y1 + 2H]+, [b3]+, CO loss, [a1]+, and [a3]+, and refines those for [b2]+ and [y2 + 2H]+ previously measured.

In a previous study [Int. J. Mass Spectrom. 2010; 297: 67–75], the asymmetric O=U=O stretch (ν3) was measured for anionic uranyl complexes with composition [UO2(X)3]-, X = Cl-, Br- and I-. Within this group of complexes, the ν3 frequency red-shifts following the trend I > Br > Cl, suggesting concomitant weakening of the U=O bonds. However, a value for [UO2(F)3]- was not measured, which prevented a comprehensive comparison of measured ν3 positions to computed frequencies from density functional theory (DFT) calculations. Because the shift in ν3 is predicted to be most dramatic when X = F, we revisited these species using infrared multiple-photon photodissociation spectroscopy. As in our earlier study, a modest red-shift to the ν3 vibration of ∼ 6 cm-1 was observed for X = I-, Br-, and Cl-, and the position of the frequency follows the trend I- > Br- > Cl-. The value measured for [UO2(F)3]- is ∼43 cm-1 lower than the one measured for [UO2(Cl)3]-. Overall, the trend with respect to ν3 position is reproduced well by computed frequencies from DFT.

Electrospray ionization was used to generate species such as [ZnNO

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, AnVO2(C2O4)−, where An = Pa or U, are essentially actinyl ions, AnVO2+, coordinated by an oxalate dianion. Both react with water to yield the pentavalent hydroxides, AnVO(OH)2(C2O4)−. The chemistry of Pa and U becomes divergent for reactions that result in oxidation: whereas PaVI is inaccessible, UVI is very stable. The UVO2(C2O4)− complex exhibits a remarkable spontaneous exothermic replacement of the oxalate ligand by O2 to yield UO4– and two CO2 molecules. The structure of the uranium tetroxide anion is computed to correspond to distorted uranyl, UVIO22+, coordinated in the equatorial plane by two equivalent O atoms each having formal charges of −1.5 and U–O bond orders intermediate between single and double. The unreactive nature of PaVO2(C2O4)− toward O2 is a manifestation of the resistance toward oxidation of PaV, and clearly reveals the disparate chemistries of Pa and U. The uranium tetroxide anion, UO4–, reacts with water to yield UO5H2–. Infrared spectra obtained for UO5H2– confirm the computed lowest-energy structure, UO3(OH)2–.

© The Royal Society of Chemistry 2016. Understanding of the nature and extent of chemical bonding in uranyl coordination complexes is crucial for the design of new ligands for nuclear waste separation, uranium extraction from seawater, and other applications. We report here the synthesis, infrared spectroscopic characterization, and quantum chemical studies of a molecular uranyl-di-15-crown-5 complex. The structure and bonding of this unique complex featuring a distinctive 6-fold coplanar coordination staggered sandwich structure and an unusual non-perpendicular orientation of the uranyl moiety are evaluated using density functional theory and chemical bonding analyses. The results provide fundamental understanding of the coordination interaction of uranyl with oxygen-donor ligands.

The speciation and reactivity of uranium are topics of sustained interest because of their importance to the development of nuclear fuel processing methods, and a more complete understanding of the factors that govern the mobility and fate of the element in the environment. Tandem mass spectrometry can be used to examine the intrinsic reactivity (i.e., free from influence of solvent and other condensed phase effects) of a wide range of metal ion complexes in a species-specific fashion. Here, electrospray ionization, collision-induced dissociation, and gas-phase ion-molecule reactions were used to create and characterize ions derived from precursors composed of uranyl cation (UVIO22+) coordinated by formate or acetate ligands. Anionic complexes containing UVIO22+ and formate ligands fragment by decarboxylation and elimination of CH2=O, ultimately to produce an oxo-hydride species [UVIO2(O)(H)]-. Cationic species ultimately dissociate to make [UVIO2(OH)]+. Anionic complexes containing acetate ligands exhibit an initial loss of acetyloxyl radical, CH3CO2•, with associated reduction of uranyl to UVO2+. Subsequent CID steps cause elimination of CO2 and CH4, ultimately to produce [UVO2(O)]-. Loss of CH4 occurs by an intra-complex H+ transfer process that leaves UVO2+ coordinated by acetate and acetate enolate ligands. A subsequent dissociation step causes elimination of CH2=C=O to leave [UVO2(O)]-. Elimination of CH4 is also observed as a result of hydrolysis caused by ion-molecule reaction with H2O. The reactions of other anionic species with gas-phase H2O create hydroxyl products, presumably through the elimination of H2. Graphical Abstract ᅟ.

RATIONALE The lower levels of adventitious H2 O in a linear ion trap allow the fragmentation reactions of [UO2 OCH3 ](+) and [UO2 OCH2 CH3 ](+) to be examined in detail. METHODS Methanol- and ethanol-coordinated UO2 (2+) -alkoxide precursors were generated by electrospray ionization (ESI). Multiple-stage tandem mass spectrometry (MS(n) ) and collision-induced dissociation (CID) were performed using a linear ion trap mass spectrometer. RESULTS CID of [UO2 OCH3 (CH3 OH)n ](+) and [UO2 OCH2 CH3 (CH3 CH2 OH)n ](+) , n = 3 and 2, causes loss of neutral alcohol ligands, leading ultimately to bare uranyl-alkoxide species. Comparison of 'native' to deuterium-labeled precursors reveals dissociation pathways not previously observed in 3-D ion trap experiments. CONCLUSIONS UO2 H(+) is generated from [UO2 OCH3 ](+) by transfer of H from the methyl group. Variable-energy and variable-time CID experiments suggest that the apparent threshold for production of UO2 H(+) is lower than for UO2 (+) , but the pathway is kinetically less favored for the former than for the latter. CID experiments reveal that [UO2 OCH2 CH3 ](+) dissociates to generate [UO2 CH3 ](+) , a relatively rare species with a U-C bond, and [UO2 (O = CH2 )](+) .

Past studies of fragmentation reactions of doubly-charged uranyl (UO22+) complexes have been impeded by very rapid water addition reactions that cause H2O adducts to dominate product ion spectra. The fragmentation of uranyl-acetone (aco) complexes ([UO2(aco)n]2+, n = 1–5), generated by electrospray ionization, is revisited here using: (a) collisional activation in a linear ion trap (LIT) mass spectrometer in which the level of background H2O is significantly lower, and (b) infrared multiple-photon photodissociation (IRMPD, 10.6 μm) in the LIT and a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. Lower levels of adventitious H2O in the LIT provided access to fragmentation of [UO2(aco)n]2+, n = 1–5. For n = 3–5, direct elimination of aco ligands is the favored fragmentation pathway. For n = 1 and 2, charge reduction reactions are dominant. For [UO2(aco)2]2+, the most abundant product ion is [UO2(aco)]+, while UO2+ is observed following collision-induced dissociation (CID) of [UO2(aco)]2+. Minor peaks corresponding to ligated [UO2OH]+ are also observed. The IRMPD experiments in the FT-ICR yielded highly accurate mass measurements that confirm composition assignments, and shed light on dissociation reactions in a gas-phase environment that is entirely free of adventitious H2O. For [UO2(aco)n]2+, n = 3–5, the primary photodissociation channel is direct aco elimination, along with charge-reduction pathways that involve intra-complex proton transfer and formation of species that contain enolate ligands. Similar pathways are observed for IRMPD measurements in the LIT.

Because of the high stability and inertness of the U=O bonds, activation and/or functionalization of UO

Developing a comprehensive understanding of the reactivity of uranium-containing species remains an important goal in areas ranging from the development of nuclear fuel processing methods to studies of the migration and fate of the element in the environment. Electrospray ionization (ESI) is an effective way to generate gas-phase complexes containing uranium for subsequent studies of intrinsic structure and reactivity. Recent experiments by our group have demonstrated that the relatively low levels of residual H

Rationale Experiments were performed to probe the creation of apparent even-electron, [M–H]+ ions by CID of Ag-cationized peptides with N-terminal imine groups (Schiff bases). Methods Imine-modified peptides were prepared using condensation reactions with aldehydes. Ag+-cationized precursors were generated by electrospray ionization (ESI). Tandem mass spectrometry (MSn) and collision-induced dissociation (CID) were performed using a linear ion trap mass spectrometer. Results Loss of AgH from peptide [M + Ag]+ ions, at the MS/MS stage, creates closed-shell [M–H]+ ions from imine-modified peptides. Isotope labeling unambiguously identifies the imine C-H group as the source of H eliminated in AgH. Subsequent CID of the [M–H]+ ions generated sequence ions that are analogous to those produced from [M + H]+ ions of the imine-modified peptides. Conclusions Experiments show (a) formation of novel even-electron peptide cations by CID and (b) the extent to which sequence ions (conventional b, a and y ions) are generated from peptides with fixed charge site and thus lacking a conventional mobile proton. Copyright © 2015 John Wiley & Sons, Ltd.

Radical migration, both intramolecular and intermolecular, from the tyrosine phenoxyl radical Tyr(O•) to the cysteine radical Cys(S•) in model peptide systems was observed in the gas phase. Ion–molecule reactions (IMRs) between the radical cation of homotyrosine and propyl thiol resulted in a fast hydrogen atom transfer. In addition, radical cations of the peptide LysTyrCys were formed via two different methods, affording regiospecific production of Tyr(O•) or Cys(S•) radicals. Collision-induced dissociation of these isomeric species displayed evidence of radical migration from the oxygen to sulfur, but not for the reverse process. This was supported by theoretical calculations, which showed the Cys(S•) radical slightly lower in energy than the Tyr(O•) isomer. IMRs of the LysTyrCys radical cation with allyl iodide further confirmed these findings. A mechanism for radical migration involving a proton shuttle by the C-terminal carboxylic group is proposed.

In this study, peptides were derivatized by reaction with salicylaldehyde to create N-terminal imines (Schiff bases). Collision-induced dissociation of the imine-modified peptides produces a complete series of b and a ions (which reveal sequence). However, an unusual pathway is also observed, one that leads to elimination of the residue mass of the N-terminal amino acid despite the chemical modification to create the imine. This pathway was investigated further using infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory with alanine-glycine-glycine (AGG) as the test peptide. The IRMPD spectrum for the product generated by loss of 71 from modified AGG (Sal-AGG) matches one predicted for protonated Sal-GG, as well as the IRMPD spectrum of glycine-glycine derivatized independently to produce a Schiff base. We conclude that the residue mass of the N-terminal amino acid is likely eliminated through a pathway that involves nucleophilic attack by an amide N atom and possible formation of an imidazole-4-one intermediate.

The synthesis, reactivity, structures, and bonding in gas-phase binary and complex oxide anion molecules of protactinium and uranium have been studied by experiment and theory. The oxalate ions, An

RATIONALE Hydroxynicotinic acids (2-, 4-, 5- and 6-hydroxy) are widely used in the manufacture of industrial products, and hydroxypyridines are important model systems for study of the tautomerization of N-heterocyclic compounds. Here we determined the gas-phase structure of deprotonated 6-hydroxynicotinic acid (6OHNic). METHODS Anions were generated by electrospray ionization, and isolated and stored in a Fourier transform ion cyclotron resonance mass spectrometer. Infrared (action) spectra were collected by monitoring photodissociation yield versus photon energy. Experimental spectra were then compared with those predicted by density functional theory (DFT) and second-order Moller-Plesset (MP2) perturbation theory calculations. RESULTS For neutral 6OHNic, DFT and MP2 calculations strongly suggest that the 6-pyridone tautomer is favored when solvent effects are included. The lowest energy isomer of deprotonated 6OHNic, in the aqueous or gas phase, is predicted to be the 6-pyridone structure deprotonated by the carboxylic acid group. CONCLUSIONS The deprotonated, 6-pyridone structure is confirmed by comparison of the infrared multiple-photon photodissociation (IRMPD) spectrum in the region of 1100–1900 cm–1 with those predicted using DFT and MP2 calculations. Copyright © 2014 John Wiley & Sons, Ltd.

Survey vibrational predissociation spectra of several representative protonated peptides and model compounds reveal very diffuse absorptions near 2500 cm–1 that are traced to pentagonal cyclic ionic hydrogen bonds (C5 interactions) involving the excess charge centers. This broadening occurs despite the fact that the ions are cooled close to their vibrational zero-point levels and their spectra are obtained by predissociation of weakly bound adducts (H2, N2, CO2) prepared in a cryogenic ion trap. The C5 band assignments are based on H/D isotopic substitution, chemical derivatization, solvation behavior, and calculated spectra. We evaluate the extent to which this broadening is caused by anharmonic coupling in the isolated molecules by including cubic coupling terms in the normal mode expansion of the potential energy surface. This analysis indicates that the harmonic H-bonded stretching vibration is mixed with dark background states over much of the energy range covered by the observed features. The diffic...

RATIONALE: Hydroxamates are essential growth factors for some microbes, acting primarily as siderophores that solubilize iron for transport into a cell. Here we determined the intrinsic structure of 1:1 complexes between Boc-protected hydroxylamine and group I ([M(L)](+)) and group II ([M(L-H)](+)) cations, where M and L are the cation and ligand, respectively, which are convenient models for the functional unit of hydroxamate siderphores. METHODS: The relevant complex ions were generated by electrospray ionization (ESI) and isolated and stored in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Infrared spectra of the isolated complexes were collected by monitoring (infrared) photodissociation yield as a function of photon energy. Experimental spectra were then compared to those predicted by density functional theory (DFT) calculations. RESULTS: The infrared multiple photon dissociation (IRMPD) spectra collected are in good agreement with those predicted to be lowest-energy by DFT. The spectra for the group I complexes contain six resolved absorptions that can be attributed to amide I and II type and hydroxylamine N-OH vibrations. Similar absorptions are observed for the group II cation complexes, with shifts of the amide I and amide II vibrations due to the change in structure with deprotonation of the hydroxylamine group. CONCLUSIONS: IRMPD spectroscopy unequivocally shows that the intrinsic binding mode for the group I cations involves the O atoms of the amide carbonyl and hydroxylamine groups of Boc-hydroxylamine. A similar binding mode is preferred for the group II cations, except that in this case the metal ion is coordinated by the O atom of the deprotonated hydroxylamine group. Copyright (c) 2013 John Wiley & Sons, Ltd.

© 2017 American Chemical Society. Recent efforts to activate the strong uranium-oxygen bonds in the dioxo uranyl cation have been limited to single oxo-group activation through either uranyl reduction and functionalization in solution, or by collision induced dissociation (CID) in the gas-phase, using mass spectrometry (MS). Here, we report and investigate the surprising double activation of uranyl by an organic ligand, 3,4,3-LI(CAM), leading to the formation of a formal U6+chelate in the gas-phase. The cleavage of both uranyl oxo bonds was experimentally evidenced by CID, using deuterium and18O isotopic substitutions, and by infrared multiple photon dissociation (IRMPD) spectroscopy. Density functional theory (DFT) computations predict that the overall reaction requires only 132 kJ/mol, with the first oxygen activation entailing about 107 kJ/mol. Combined with analysis of similar, but unreactive ligands, these results shed light on the chelation-driven mechanism of uranyl oxo bond cleavage, demonstrating its dependence on the presence of ligand hydroxyl protons available for direct interactions with the uranyl oxygens.

Neutral complexes of zinc with N,N′-diisopropylpiperazine-2,3-dithione (iPr2Dt0) and N,N′-dimethylpiperazine-2,3-dithione (Me2Dt0) with chloride or maleonitriledithiolate (mnt2−) as coligands have been synthesized and characterized. The molecular structures of these zinc complexes have been determined using single crystal X-ray diffractometry. Complexes recrystallize in monoclinic P type systems with zinc adopting a distorted tetrahedral geometry. Two zinc complexes with mixed-valence dithiolene ligands exhibit ligand-to-ligand charge transfer bands. Optimized geometries, molecular vibrations and electronic structures of charge-transfer complexes were calculated using density functional theory (B3LYP/6-311G+(d,p) level). Redox orbitals are shown to be almost exclusively ligand in nature, with a HOMO based heavily on the electron-rich maleonitriledithiolate ligand, and a LUMO comprised mostly of the electron-deficient dithione ligand. Charge transfer is thus believed to proceed from dithiolate HOMO to dithione LUMO, showing ligand-to-ligand redox interplay across a d10 metal.

Combined collision-induced dissociation mass spectrometry experiments with DFT and MP2 calculations were employed to elucidate the molecular structures and energetics of dissociation reactions of uranyl species containing acetone and diacetone alcohol ligands. It is shown that solutions containing diacetone alcohol ligands can produce species with more than five oxygen atoms available for coordination. Calculations confirm that complexes with up to four diacetone alcohol ligands can be energetically stable but that the effective number of atoms coordinating with uranium in the equatorial plane does not exceed five. Water elimination reactions of diacetone alcohol ligands are shown to have two coordination-dependent reaction channels, through formation of mesityl oxide ligands or formation of alkoxide and protonated mesityl oxide species. The present results provide an explanation for the implausible observation of "[UO(2)(ACO)(6,7,8)](2+)" in and observed water-elimination reactions from purportedly uranyl-acetone complexes (Rios, D.; Rutkowski, P. X.; Van Stipdonk, M. J.; Gibson, J. K. Inorg. Chem. 2011, 50, 4781).

Isomer-specific vibrational predissociation spectra are reported for the gas-phase GlySarH(+) and SarSarH(+) [Gly = glycine; Sar = sarcosine] ions prepared by electrospray ionization and tagged with weakly bound D2 adducts using a cryogenic ion trap. The contributions of individual isomers to the overlapping vibrational band patterns are completely isolated using a pump-probe photochemical hole-burning scheme involving two tunable infrared lasers and two stages of mass selection (hence IR(2)MS(2)). These patterns are then assigned by comparison with harmonic (MP2/6-311+G(d,p)) spectra for various possible conformers. Both systems occur in two conformations based on cis and trans configurations with respect to the amide bond. In addition to the usual single intramolecular hydrogen bond motif between the protonated amine and the nearby amide oxygen atom, cis-SarSarH(+) adopts a previous unreported conformation in which both amino NH's act as H-bond donors. The correlated red shifts in the NH donor and C═O acceptor components of the NH···O═C linkage to the acid group are unambiguously assigned in the double H-bonded conformer.

Collision-induced dissociation (CID) of the peptide alanine-4-aminomethylbenzoic acid-glycine, A(AMBz)G generates a prominent b(2) ion despite a previous report [ER. Talaty, T.J. Cooper, S.M. Osburn, M.J. Van Stipdonk, Collision-induced dissociation of protonated tetrapeptides containing beta-alanine, gamma-aminobutyric acid, e-aminocaproic acid or 4-aminomethylbenzoic acid residues, Rapid Commun. Mass Spectrom. 20 (2006) 3443-3455] which showed that incorporation of the aromatic amino acid into a peptide sequence inhibits generation of b(n) ions formed by cleavage to the immediate C-terminal side of the residue. Infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations suggest that the b(2) ion generated from A(AMBz)G has an acylium structure. The b2 ion generated from (AMBz)AG, in which the aromatic residue is situated at the amino-terminus, is instead a conventional oxazolone. (c) 2012 Elsevier B.V. All rights reserved.

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. Reduction of both charge and oxidation state, which is inherent in ETD activation of [An(VI) O(2) (CH(3) COCH(3) )(4) ](2+) , [An(VI) O(2) (CH(3) CN)(4) ](2) , [U(VI) O(2) (CH(3) COCH(3) )(5) ](2+) and [U(VI) O(2) (CH(3) CN)(5) ](2+) (An = U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V) complexes add O(2) , whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantly doubly-charged products through loss of coordinating ligands. Singly-charged CID products of [U(VI) O(2) (CH(3) COCH(3) )(4,5) ](2+) , [U(VI) O(2) (CH(3) CN)(4,5) ](2+) and [Pu(VI) O(2) (CH(3) CN)(4) ](2+) retain the hexavalent metal oxidation state with the addition of hydroxide or acetone enolate anion ligands. However, CID of [Pu(VI) O(2) (CH(3) COCH(3) )(4) ](2+) generates monopositive plutonyl(V) complexes, reflecting relatively more facile reduction of Pu(VI) to Pu(V).

Nickel Superoxide Dismutase (NiSOD) and the A-cluster of Carbon Monoxide Dehydrogenase/Acetyl Coenzyme A Synthase (CODH/ACS) both feature active sites with Ni coordinated by thiolate and amide donors. It is likely that the particular set of donors is important in tuning the redox potential of the Ni center(s). We report herein an expansion of our efforts involving the use of 2,2′-dithiodibenzaldehyde (DTDB) as a synthon for metal–thiolate complexes to reactions with Ni complexes of N,N-dimethylethylenediamine (dmen). In the presence of coordinating counterions, these reactions result in monomeric square-planar complexes of the tridentate N2S donor ligand derived from the Schiff-base condensation of dmen and DTDB. In the absence of a coordinating counterion, we have isolated a Ni(II) complex with an asymmetric N2S2 donor set involving one amine and one imine N donor in addition to two thiolate donors. This latter complex is discussed with respect to its relevance to the active site of NiSOD.

Ion trap tandem mass spectrometry with collision-induced dissociation, and the combination of infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations, were used to characterize singly charged, 1: 1 complexes of Ca2+, Sr2+ and Ba2+ with salicylate. For each metal-salicylate complex, the CID pathways are: (a) elimination of CO2 and (b) formation of [MOH](+) where M = Ca2+, Sr2+ or Ba2+. DFT calculations predict three minima for the cation-salicylate complexes which differ in the mode of metal binding. In the first, the metal ion is coordinated by O atoms of the (neutral) phenol and carboxylate groups of salicylate. In the second, the cation is coordinated by phenoxide and (neutral) carboxylic acid groups. The third mode involves coordination by the carboxylate group alone. The infrared spectrum for the metal-salicylate complexes contains a number of absorptions between 1000 and 1650 cm(-1), and the best correlation between theoretical and experimental spectra is found for the structure that features coordination of the metal ion by phenoxide and the carbonyl O of the carboxylic acid group, consistent with the calculated energies for the respective species. Copyright (C) 2011 John Wiley & Sons, Ltd.

We report the first transmission of solvent-coordinated dipositive plutonyl ion, Pu(VI)O(2)(2+), from solution to the gas phase by electrospray ionization (ESI) of plutonyl solutions in water/acetone and water/acetonitrile. ESI of plutonyl and uranyl solutions produced the isolable gas-phase complexes, [An(VI)O(2)(CH(3)COCH(3))(4,5,6)](2+), [An(VI)O(2)(CH(3)COCH(3))(3)(H(2)O)](2+), and [An(VI)O(2)(CH(3)CN)(4)](2+); additional complex compositions were observed for uranyl. In accord with relative actinyl stabilities, U(VI)O(2)(2+)Pu(VI)O(2)(2+)Np(VI)O(2)(2+), the yields of plutonyl complexes were about an order of magnitude less than those of uranyl, and dipositive neptunyl complexes were not observed. Collision-induced dissociation (CID) of the dipositive coordination complexes in a quadrupole ion trap produced doubly- and singly-charged fragment ions; the fragmentation products reveal differences in underlying chemistries of plutonyl and uranyl, including the lower stability of Pu(VI) as compared with U(VI). Particularly notable was the distinctive CID fragment ion, [Pu(IV)(OH)(3)](+) from [Pu(VI)O(2)(CH(3)COCH(3))(6)](2+), where the plutonyl structure has been disrupted and the tetravalent plutonium hydroxide produced; this process was not observed for uranyl.

A tandem mass spectrometric study of a series of secondary amides of acetylglycine and hippuric acid utilizing electrospray ionization (ESI) was conducted. Among the fragment ions observed was an unusual one, which we have determined to be a nitrilium ion having the structure CH3-C≡N⊕-Ph or Ph-C≡N⊕-Ph by loss of the full mass of glycine as a neutral fragment. A mechanism that we propose involves an initial protonation of the oxygen atom at the N-terminus, followed by cyclization to a five-membered imidazolium ring, and its subsequent collapse to the nitrilium ion. This mechanism is supported by extensive isotopic labels and considerable variation of substituents. A similar study of the amides of acyl β-alanine and acyl γ-aminobutyric acid revealed that the former furnishes the same nitrilium ion, but not the latter. Thus, a six-membered intermediate is also possible and capable of losing the full mass of β-alanine as a neutral fragment. When the size of the ring is forced to be seven-membered, this pathway is blocked. When this study was expanded to include a variety of N-acylproline amides, the nitrilium ion was observed in 100% abundance only when the acyl group was acetyl. Thus a proline effect (involvement of a strained bicyclic [3.3.0] structure) is being observed.

We present infrared photodissociation spectra of two protonated peptides that are cooled in a ~10 K quadrupole ion trap and "tagged" with weakly bound H(2) molecules. Spectra are recorded over the range of 600-4300 cm(-1) using a table-top laser source, and are shown to result from one-photon absorption events. This arrangement is demonstrated to recover sharp (Δν ~6 cm(-1)) transitions throughout the fingerprint region, despite the very high density of vibrational states in this energy range. The fundamentals associated with all of the signature N-H and C=O stretching bands are completely resolved. To address the site-specificity of the C=O stretches near 1800 cm(-1), we incorporated one (13)C into the tripeptide. The labeling affects only one line in the complex spectrum, indicating that each C=O oscillator contributes a single distinct band, effectively "reporting" its local chemical environment. For both peptides, analysis of the resulting band patterns indicates that only one isomeric form is generated upon cooling the ions initially at room temperature into the H(2) tagging regime.

The infrared spectrum of the bis-uranyl nitrate complex [(UO(2))(2)(NO(3))(5)](-) was measured in the gas phase using multiple photon dissociation (IRMPD). Intense absorptions corresponding to the nitrate symmetric and asymmetric vibrations, and the uranyl asymmetric vibration were observed. The nitrate nu(3) vibrations indicate the presence of nitrate in a bridging configuration bound to both uranyl cations, and probably two distinct pendant nitrates in the complex. The coordination environment of the nitrate ligands and the uranyl cations were compared to those in the mono-uranyl complex. Overall, the uranyl cation is more loosely coordinated in the bis-uranyl complex [(UO(2))(2)(NO(3))(5)](-) compared to the mono-complex [UO(2)(NO(3))(3)](-), as indicated by a higher O-U-O asymmetric stretching (nu(3)) frequency. However, the pendant nitrate ligands are more strongly bound in the bis-complex than they are in the mono-uranyl complex, as indicated by the nu(3) frequencies of the pendant nitrate, which are split into nitrosyl and O-N-O vibrations as a result of bidentate coordination. These phenomena are consistent with lower electron density donation per uranyl by the nitrate bridging two uranyl centers compared to that of a pendant nitrate in the mono-uranyl complex. The lowest energy structure predicted by density functional theory (B3LYP functional) calculations was one in which the two uranyl molecules bridged by a single nitrate coordinated in a bis-bidentate fashion. Each uranyl molecule was coordinated by two pendant nitrate ligands. The corresponding vibrational spectrum was in excellent agreement with the IRMPD measurement, confirming the structural assignment. (C) 2011 Elsevier B.V. All rights reserved.

The intrinsic binding of halide ions to the metal center in the uranyl molecule is a topic of ongoing research interest in both the actinide separations and theoretical communities. Investigations of structure in the condensed phases is frequently obfuscated by solvent interactions, that can alter ligand binding and spectroscopic properties. The approach taken in this study is to move the uranyl halide complexes into the gas phase where they are free from solvent interactions, and then interrogate their vibrational spectroscopy using infrared multiple photon dissociation (IRMPD). The spectra of cationic coordination complexes having the composition [UO2(X)(ACO)3]+ (X = F, Cl, Br and I; ACO = acetone) were acquired using electrospray for ion formation, and monitoring the ion signal from the photoelimination of ACO ligands. The studies showed that the asymmetric v3 UO2 frequency was insensitive to halide identity as X was varied from Cl to I, suggesting that in these pseudo octahedral complexes, changing the nucleophilicity of the halide did not appreciably alter the binding in the complex. The v3 peak in the spectrum of the F-containing complex was ~ 10 cm-1 lower indicating stronger coordination in this complex. Similarly the ACO carbonyl stretches showed that the C=O frequency wasmore » relatively insensitive to the identity of the halide, although a modest shift to the blue was seen for the complexes with the more nucleophilic anions, consistent with the idea that they loosen solvent binding. Surprisingly, the v1 stretch was activated when the softer anions Cl, Br and I were present in the complexes. IR studies of the anionic complexes were conducted by measuring the v3 UO2 frequencies of [UO2X3]-, where X = Cl-, Br- and I-. The trifluoro complex could not be photodissociated. In these negatively charged complexes, the UO2 v3 values decreased with increasing anion nucleophilicity. This observation was consistent with DFT calculations that indicated that dissociation energy decreased on the order F > Cl > Br > I.« less

Multiple-stage tandem mass spectrometry and collision-induced dissociation were used to investigate loss of H(2)O or CH(3)OH from protonated versions of GGGX (where X = G, A, and V), GGGGG, and the methyl esters of these peptides. In addition, wavelength-selective infrared multiple photon dissociation was used to characterize the [M + H - H(2)O](+) product derived from protonated GGGG and the major MS(3) fragment, [M + H - H(2)O - 29](+) of this peak. Consistent with the earlier work [ Ballard , K. D. ; Gaskell , S. J. J. Am. Soc. Mass Spectrom. 1993 , 4 , 477 - 481 ; Reid , G. E. ; Simpson , R. J. ; O'Hair , R. A. J. Int. J. Mass Spectrom. 1999 , 190/191 , 209 -230 ], CID experiments show that [M + H - H(2)O](+) is the dominant peak generated from both protonated GGGG and protonated GGGG-OMe. This strongly suggests that the loss of the H(2)O molecule occurs from a position other than the C-terminal free acid and that the product does not correspond to formation of the b(4) ion. Subsequent CID of [M + H - H(2)O](+) supports this proposal by resulting in a major product that is 29 mass units less than the precursor ion. This is consistent with loss of HN horizontal lineCH(2) rather than loss of carbon monoxide (28 mass units), which is characteristic of oxazolone-type b(n) ions. Comparison between experimental and theoretical infrared spectra for a group of possible structures confirms that the [M + H - H(2)O](+) peak is not a substituted oxazolone but instead suggests formation of an ion that features a five-membered ring along the peptide backbone, close to the amino terminus. Additionally, transition structure calculations and comparison of theoretical and experimental spectra of the [M + H - H(2)O - 29](+) peak also support this proposal.