Reactions of gas-phase uranyl formate/acetate anions: reduction of carboxylate ligands to aldehydes by intra-complex hydride attack.

In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ion-molecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (UVIO22+) coordinated by either formate or acetate ligands [E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom., 2016, 27, 1989-1998]. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely [UO2(O2C-CH3)2(O2C-H)]- and [UO2(O2C-CH3)(O2C-H)2]-. Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO2(H)(O2C-CH3)(O2C-H)]-. We also discovered that MSn CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO2(O)(CH3)]-, which reacts with H2O to generate [UO2(O)(OH)]-. DFT calculations support the observation that formation of [UO2(O)(OH)]- by elimination of CH4 is favored over H2O addition and rearrangement to create [UO2(OH)2(CH3)]-.

Unusual Actinyl Complexes with a Redox-Active N,S-Donor Ligand.

Understanding the fundamental chemistry of soft N,S-donor ligands with actinides across the series is critical for separation science toward sustainable nuclear energy. This task is particularly challenging when the ligands are redox active. We herein report a series of actinyl complexes with a N,S-donor redox-active ligand that stabilizes different oxidation states across the actinide series. These complexes are isolated and characterized in the gas phase, along with high-level electronic structure studies. The redox-active N,S-donor ligand in the products, C5H4NS, acts as a monoanion in [UVIO2(C5H4NS-)]+ but as a neutral radical with unpaired electrons localized on the sulfur atom in [NpVO2(C5H4NS•)]+ and [PuVO2(C5H4NS•)]+, resulting in different oxidation states for uranium and transuranic elements. This is rationalized by considering the relative energy levels of actinyl(VI) 5f orbitals and S 3p lone pair orbitals of the C5H4NS- ligand and the cooperativity between An-N and An-S bonds that provides additional stability for the transuranic elements.

Intrinsic reactivity of [OUCH]+ : Apparent synthesis of [OUS]+ by reaction with CS2.

RATIONALE: Building on our report that collision-induced dissociation (CID) can be used to create the highly reactive U-alkylidyne species [O=U≡CH]+ , our goal was to determine whether the species could be as an intermediate for synthesis of [OUS]+ by reaction with carbon disulfide (CS2 ). METHODS: 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]+ . Neutral CS2 was admitted into the trap through a modified He inlet and precision leak valves. RESULTS: The [O=U≡CH]+ ion reacts with CS2 to generate [OUS]+ . CID of [OUS]+ causes elimination of the axial sulfide ligand to generate [OU]+ . Using isotopically labeled reagent, we found that [OUS]+ reacts with O2 to create [UO2 ]+ . CONCLUSIONS: [O=U≡CH]+ proves to be a useful reagent ion for synthesis of [OUS]+ , a species that to date has only been created by gas-phase reactions of U+ and U2+ . Dissociation of [OUS]+ to create [OU]+ , but not [US]+ , and the efficient conversion of the species into [UO2 ]+ , is consistent with the relative differences in U-O and U-S bond energies.

Intrinsic chemistry of [OUCH]+: reactions with H2O, CH3C[triple bond, length as m-dash]N and O2.

We report the first experimental study of the intrinsic chemistry of a U-methylidyne species, focusing on reaction of [OUCH]+ with H2O, O2 and CH3C[triple bond, length as m-dash]N 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(C[triple bond, length as m-dash]CH)]+.

Destruction and reconstruction of UO22+ using gas-phase reactions.

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.

Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3 versus Oxyl in UO3.

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•-. 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 ν3 IR frequencies indicate the following donor ordering: O2-[best donor] ≫ O•-> OH-> NO3-.

Isotope labeling and infrared multiple-photon photodissociation investigation of product ions generated by dissociation of [ZnNO3(CH3OH)2]+: Conversion of methanol to formaldehyde.

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.

Collision-induced dissociation of [UVI O2 (ClO4 )]+ revisited: Production of [UVI O2 (Cl)]+ and subsequent hydrolysis to create [UVI O2 (OH)].

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)]+ .

Uranyl/12-crown-4 Ether Complexes and Derivatives: Structural Characterization and Isomeric Differentiation.

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.

Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO4.

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-.

Cleaving Off Uranyl Oxygens through Chelation: A Mechanistic Study in the Gas Phase.

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 and 18O 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.

Even-electron [M-H](+) ions generated by loss of AgH from argentinated peptides with N-terminal imine groups.

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 (MS(n) ) 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.

Apparent activation of H2O and elimination of H2 from gas-phase mixed-metal complexes containing silver, calcium and deprotonated glycine.

RATIONALE: Ion trap mass spectrometry was used to study the reactivity of species derived from gas-phase, mixed-metal complexes, [Ag2 Xx(Gly-H)3 ](+) , where Xx = Ca, Mg, Sr and Ag, and in particular the apparent activation of an H2 O ligand added during an ion-molecule reaction. METHODS: Precursor [Ag2 Xx(Gly-H)3 ](+) complexes were formed by electrospray ionization (ESI) using spray solutions in which AgNO3 , XxNO3 and glycine were mixed in a 1:1:3 molar ratio. Specific species for study of ion-molecule reactions were created in a "top down" fashion using collision-induced dissociation (CID). Ion-molecule reactions were performed by selective isolation and storage in a linear ion trap, where reactions with adventitious H2 O can occur. RESULTS: Multiple stages of CID of [Ag2 Ca(Gly-H)3 ](+) resulted in the formation of [AgHCa(Gly-H)](+) . An ion-molecule reaction of this ion produced a peak 16 mass units higher which is hypothesized to be a result of addition of H2 O followed by loss of H2 . This reaction was studied further by replacing Ca with Mg, Sr and Ag; as well as by incorporating deuterium-labelled glycine into the complex. CONCLUSIONS: The experimental results showed the following pattern for the apparent rates of reaction: Mg > Sr > Ca. When silver is the only metal present there is an addition of water but no loss of H2 . DFT and MP2 calculations help identify plausible pathways for decomposition of H2 O and formation of H2.

Collision-induced dissociation of uranyl-methoxide and uranyl-ethoxide cations: Formation of UO2 H(+) and uranyl-alkyl product ions.

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 )](+) .

A mixed valence zinc dithiolene system with spectator metal and reactor ligands.

Neutral complexes of zinc with N,N'-diisopropylpiperazine-2,3-dithione ( i Pr2Dt0) 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-valent 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.

Solution, Solid, and Gas Phase Studies on a Nickel Dithiolene System: Spectator Metal and Reactor Ligand.

The syntheses of cationic nickel complexes using N,N'-dimethyl piperazine 2,3-dithione (Me2Dt(0)) and N,N'-diisopropyl piperazine 2,3-dithione ((i)Pr2Dt(0)) ligands are reported. These ligands were used in synthesizing bis and tris(dithione)Ni(II) complexes as tetrafluoroborate or hexafluorophosphate salts, i.e., [Ni((i)Pr2Dt(0))2][BF4]2 ([1a][BF4]2), [Ni((i)Pr2Dt(0))2][PF6]2 ([1a][PF6]2), [Ni(Me2Dt(0))2][BF4]2 ([1b][BF4]2), [Ni((i)Pr2Dt(0))3][BF4]2 ([2a][BF4]2), and [Ni((i)Pr2Dt(0))3][PF6]2 ([2a][PF6]2), respectively. Complex [2a][PF6]2 was isolated from a methanolic solution of [1a][PF6]2. Compound [1a][BF4]2 crystallizes in a trigonal crystal system (space group, P31/c) and exhibits unique packing features, whereas [2a][BF4]2 crystallizes in a monoclinic (P21/n) space group. Cyclic voltammograms of [1a][BF4]2 and [1b][BF4]2 are indicative of four reduction processes associated with stepwise single-electron reduction of the ligands. Spectroelectrochemical experiments on [1a][BF4]2 exhibit an intervalence charge transfer (IVCT) transition as a spectroscopic signature of the mixed-valence [Ni((i)Pr2Dt(0))((i)Pr2Dt(1-))](-) species. Analysis of this IVCT band suggests that this ligand based mixed valence complex, [Ni((i)Pr2Dt(0))((i)Pr2Dt(1-))](-), behaves more like a traditional class II/III metal based mixed-valence complex. The density functional theory (DFT) and time dependent DFT calculations provide a theoretical framework for understanding the electronic structures and the nature of excited states of the target compounds that are consistent with their spectroscopic and redox properties. Vibrational spectra of [1a](2+) and [2a](2+) were investigated as discrete species in the gas phase using infrared multiple photon dissociation (IRMPD) spectroscopy.

Infrared multiphoton dissociation spectroscopy of a gas-phase complex of uranyl and 3-oxa-glutaramide: an extreme red-shift of the [O═U═O](2+) asymmetric stretch.

The gas-phase complex UO2(TMOGA)2(2+) (TMOGA = tetramethyl-3-oxa-glutaramide) prepared by electrospray ionization was characterized by infrared multiphoton dissociation (IRMPD) spectroscopy. The IRMPD spectrum from 700-1800 cm(-1) was interpreted using a computational study based on density functional theory. The predicted vibrational frequencies are in good agreement with the measured values, with an average deviation of only 8 cm(-1) (<1%) and a maximum deviation of 21 cm(-1) (<2%). The only IR peak assigned to the linear uranyl moiety was the asymmetric ν3 mode, which appeared at 965 cm(-1) and was predicted by DFT as 953 cm(-1). This ν3 frequency is red-shifted relative to bare uranyl, UO2(2+), by ca. 150 cm(-1) due to electron donation from the TMOGA ligands. Based on the degree of red-shifting, it is inferred that two TMOGA oxygen-donor ligands have a greater effective gas basicity than the four monodentate acetone ligands in UO2(acetone)4(2+). The uranyl ν3 frequency was also computed for uranyl coordinated by two TMGA ligands, in which the central Oether of TMOGA has been replaced by CH2. The computed ν3 for UO2(TMGA)2(2+), 950 cm(-1), is essentially the same as that for UO2(TMOGA)2(2+), suggesting that electron donation to uranyl from the Oether of TMOGA is minor. The computed ν3 asymmetric stretching frequencies for the three actinyl complexes, UO2(TMOGA)2(2+), NpO2(TMOGA)2(2+) and PuO2(TMOGA)2(2+), are comparable. This similarity is discussed in the context of the relationship between ν3 and intrinsic actinide-oxygen bond energies in actinyl complexes.

Gas-phase tyrosine-to-cysteine radical migration in model systems.

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.

Infrared multiple-photon dissociation spectroscopy of deprotonated 6-hydroxynicotinic acid.

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 Møller-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.

Formation of bare UO2(2+) and NUO(+) by fragmentation of gas-phase uranyl-acetonitrile complexes.

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, UO2(2+). 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 (15)N. The observed formation of [UO2(NC)](+) and NUO(+) was modeled by density functional theory.