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 [U^VIO₂(C₅H₄NS^-)]⁺ but as a neutral radical with unpaired electrons localized on the sulfur atom in [Np^VO₂(C₅H₄NS^•)]⁺ and [Pu^VO₂(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.

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.

Collision-induced dissociation of [UO2 (NO3 )(O2 )]- and reactions of product ions with H2 O and O2

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.

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)]+.

Collision-induced dissociation of [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- and reactions of product ions with H2 O and O2

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.

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)]. Rapid Commun Mass Spectrom 2018;32:1085-1091. [PMID: 29645301 DOI: 10.1002/rcm.8135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

RATIONALE

In a previous study [Rapid Commun Mass Spectrom. 2004;18:3028-3034], collision-induced dissociation (CID) of [U^VI 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 [U^VI 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 [U^VI O₂ (ClO₄ )]⁺ generates [U^VI O₂ (Cl)]⁺ , presumably by elimination of two O₂ molecules. Using low isolation/reaction times, we found that [U^VI O₂ (Cl)]⁺ will undergo an IMR with H₂ O to generate [U^VI O₂ (OH)]⁺ .

CONCLUSIONS

With lower levels of background H₂ O, CID experiments reveal that the intrinsic dissociation pathway for [U^VI O₂ (ClO₄ )]⁺ leads to [U^VI 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 [U^VI O₂ (Cl)]⁺ by CID, followed by a very rapid hydrolysis reaction to leave [U^VI O₂ (OH)]⁺ .

Influence of Background H2O on the Collision-Induced Dissociation Products Generated from [UO2NO3]<sup/>

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 H₂O in a 2-D, linear ion trap (LIT) make it possible to examine fragmentation pathways and reactions not observed in earlier studies conducted with 3-D ion traps (Van Stipdonk et al. J. Am. Soc. Mass Spectrom. 14, 1205-1214, 2003). In the present study, we revisited the dissociation of complexes composed of uranyl nitrate cation [U^VIO₂(NO₃)]⁺ coordinated by alcohol ligands (methanol and ethanol) using the 2-D LIT. With relatively low levels of background H₂O, collision-induced dissociation (CID) of [U^VIO₂(NO₃)]⁺ primarily creates [UO₂(O₂)]⁺ by the ejection of NO. However, CID (using He as collision gas) of [U^VIO₂(NO₃)]⁺ creates [UO₂(H₂O)]⁺ and UO₂⁺ when the 2-D LIT is used with higher levels of background H₂O. Based on the results presented here, we propose that product ion spectrum in the previous experiments was the result of a two-step process: initial formation of [U^VIO₂(O₂)]⁺ followed by rapid exchange of O₂ for H₂O by ion-molecule reaction. Our experiments illustrate the impact of residual H₂O in ion trap instruments on the product ions generated by CID and provide a more accurate description of the intrinsic dissociation pathway for [U^VIO₂(NO₃)]⁺. Graphical Abstract ᅟ.

Thermodynamics and Reaction Mechanisms of Decomposition of the Simplest Protonated Tripeptide, Triglycine: A Guided Ion Beam and Computational Study

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. Graphical Abstract ᅟ.

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 )](+) .

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.

Electronic structure and characterization of a uranyl di-15-crown-5 complex with an unprecedented sandwich structure

A uranyl–di-15-crown-5 complex with a unique slipped sandwich structure was synthesized and characterized by infrared spectroscopy and quantum-chemical methods.

Infrared multiple photon dissociation spectroscopy of group I and group II metal complexes with Boc-hydroxylamine

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.

Isomer-Specific IR-IR Double Resonance Spectroscopy of D2-Tagged Protonated Dipeptides Prepared in a Cryogenic Ion Trap

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.

Electron transfer dissociation of dipositive uranyl and plutonyl coordination complexes

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).

Gas-phase coordination complexes of U(VI)O2(2+), Np(VI)O2(2+), and Pu(VI)O2(2+) with dimethylformamide

Electrospray ionization of actinyl perchlorate solutions in H(2)O with 5% by volume of dimethylformamide (DMF) produced the isolatable gas-phase complexes, [An(VI)O(2)(DMF)(3)(H(2)O)](2+) and [An(VI)O(2)(DMF)(4)](2+), where An = U, Np, and Pu. Collision-induced dissociation confirmed the composition of the dipositive coordination complexes, and produced doubly- and singly-charged fragment ions. The fragmentation products reveal differences in underlying chemistries of uranyl, neptunyl, and plutonyl, including the lower stability of Np(VI) and Pu(VI) compared with U(VI).

Characterizing the intramolecular H-bond and secondary structure in methylated GlyGlyH+ with H2 predissociation spectroscopy

We report vibrational predissociation spectra of the four protonated dipeptides derived from glycine and sarcosine, GlyGlyH(+)•(H(2))(1,2), GlySarH(+)•(D(2))(2), SarGlyH(+)•(H(2))(2), and SarSarH(+)•(D(2))(2), generated in a cryogenic ion trap. Sharp bands were recovered by monitoring photoevaporation of the weakly bound H(2) (D(2)) molecules in a linear action regime throughout the 700-4200 cm(-1) range using a table-top laser system. The spectral patterns were analyzed in the context of the low energy structures obtained from electronic structure calculations. These results indicate that all four species are protonated on the N-terminus, and feature an intramolecular H-bond involving the amino group. The large blue-shift in the H-bonded N-H fundamental upon incorporation of a methyl group at the N-terminus indicates that this modification significantly lowers the strength of the intramolecular H-bond. Methylation at the amide nitrogen, on the other hand, induces a significant rotation (~110°) about the peptide backbone.

A study of fragmentation of protonated amides of some acylated amino acids by tandem mass spectrometry: observation of an unusual nitrilium ion

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.

Vibrational characterization of simple peptides using cryogenic infrared photodissociation of H2-tagged, mass-selected ions

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.

Apparent inhibition by arginine of macrocyclic b ion formation from singly charged protonated peptides

There is now strong evidence for the existence of macrocyclic isomers of b(n)(+) ions, the formation and subsequent opening of which can lead to loss of sequence information from protonated peptides in multiple-stage tandem mass spectrometry experiments. In this study, the fragmentation patterns of protonated YARFLG and permuted isomers of the model peptide were investigated by collision-induced dissociation. Of interest was the potential influence of the arginine residue, and its position in the peptide sequence, on formation of the presumed macrocyclic b(5) ion isomer and potential loss of sequence information. We find that regardless of the sequence position (either internal or at the N- or C-terminus), only direct sequence ions or ions directly related to fragmentation of the arginine side chain are observed.

Variable denticity in carboxylate binding to the uranyl coordination complexes

Tris-carboxylate complexes of uranyl [UO(2)](2+) with acetate and benzoate were generated using electrospray ionization mass spectrometry, and then isolated in a Fourier transform ion cyclotron resonance mass spectrometer. Wavelength-selective infrared multiple photon dissociation (IRMPD) of the tris-acetato uranyl anion resulted in a redox elimination of an acetate radical, which was used to generate an IR spectrum that consisted of six prominent absorption bands. These were interpreted with the aid of density functional theory calculations in terms of symmetric and antisymmetric -CO(2) stretches of the monodentate and bidentate acetate, CH(3) bending and umbrella vibrations, and a uranyl O-U-O asymmetric stretch. The comparison of the calculated and measured IR spectra indicated that the predominant conformer of the tris-acetate complex contained two acetate ligands bound in a bidentate fashion, while the third acetate was monodentate. In similar fashion, the tris-benzoate uranyl anion was formed and photodissociated by loss of a benzoate radical, enabling measurement of the infrared spectrum that was in close agreement with that calculated for a structure containing one monodentate and two bidentate benzoate ligands.

Infrared multiple photon dissociation spectroscopy of sodium and potassium chlorate anions

The structures of gas-phase, metal chlorate anions with the formula [M(ClO(3))(2)](-), M = Na and K, were determined using tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Structural assignments for both anions are based on comparisons of the experimental vibrational spectra for the two species with those predicted by density functional theory (DFT) and involve conformations that feature either bidentate or tridentate coordination of the cation by chlorate. Our results strongly suggest that a structure in which both chlorate anions are bidentate ligands is preferred for [Na(ClO(3))(2)](-). However, for [K(ClO(3))(2)](-) the best agreement between experimental and theoretical spectra is obtained from a composite of predicted spectra for which the chlorate anions are either both bidentate or both tridentate ligands. In general, we find that the overall accuracy of DFT calculations for prediction of IR spectra is dependent on both functional and basis set, with best agreement achieved using frequencies generated at the B3LYP/6-311+g(3df) level of theory.

IRMPD spectroscopy of anionic group II metal nitrate cluster ions

Anionic group II metal nitrate clusters of the formula [M(2)(NO(3))(5)](-), where M(2) = Mg(2), MgCa, Ca(2), and Sr(2), are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy to obtain vibrational spectra in the mid-IR region. The IR spectra are dominated by the symmetric and the antisymmetric nitrate stretches, with the latter split into high and low-frequency components due to the distortion of nitrate anion symmetry by interactions with the cation. Density functional theory (DFT) is used to predict geometries and vibrational spectra for comparison to the experimental spectra. Calculations yield two stable isomers: the first one contains two terminal nitrate anions on each cation and a single bridging nitrate ("mono-bridging"), while the second structure features a single terminal nitrate on each cation with three bridging nitrate ligands ("tri-bridging"). The tri-bridging isomer is calculated to be lower in energy than the mono-bridging one for all species. Theoretical spectra of the tri-bridging structure provide a better qualitative match to the experimental infrared spectra of [Mg(2)(NO(3))(5)](-) and [MgCa(NO(3))(5)](-). However, the profile of the low-frequency nu(3) band for the Mg(2) complex suggests a third possible isomer not predicted by theory. The IRMPD spectra of the Ca(2) and Sr(2) complexes are better reconciled by a weighted summation of the spectra of both isomers suggesting that a mixture of structures is present.

Iron and Cobalt Complexes of 2,6-Diacetylpyridine-bis(R-thiosemicarbazone) (R=H, phenyl) Showing Unprecedented Ligand Deviation from Planarity

The syntheses, characterization, and single-crystal X-ray crystal structures are reported for four complexes of iron and cobalt with the pentadentate ligands, 2,6-diacetylpyridinebis(thiosemicarbazone) (H(2)L(1)) and 2,6-diacetylpyridinebis-(phenylthiosemicarbazone) (H(2)L(2)), including a cobalt dimer displaying a deviation from planarity which is unprecedented for this class of ligands and allows the ligand to occupy five positions of a pseudo-octahedral coordination sphere. This dimer reacts with KCN to produce a mononuclear complex of relevance to the active site of cobalt nitrile hydratase.

Formation of [b3 - 1 + cat]+ ions from metal-cationized tetrapeptides containing beta-alanine, gamma-aminobutyric acid or epsilon-aminocaproic acid residues

The presence and position of a single beta-alanine (betaA), gamma-aminobutyric acid (gammaABu) or epsilon-aminocaproic acid (Cap) residue has been shown to have a significant influence on the formation of b(n)+ and y(n)+ product ions from a series of model, protonated peptides. In this study, we examined the effect of the same residues on the formation of analogous [b3 - 1 + cat]+ products from metal (Li+, Na+ and Ag+)-cationized peptides. The larger amino acids suppress formation of b3+ from protonated peptides with general sequence AAXG (where X = beta-alanine, gamma-aminobutyric acid or epsilon-aminocaproic acid), presumably because of the prohibitive effect of larger cyclic intermediates in the 'oxazolone' pathway. However, abundant [b3 - 1 + cat]+ products are generated from metal-cationized versions of AAXG. Using a group of deuterium-labeled and exchanged peptides, we found that formation of [b3 - 1 + cat]+ involves transfer of either amide or alpha-carbon position H atoms, and the tendency to transfer the atom from the alpha-carbon position increases with the size of the amino acid in position X. To account for the transfer of the H atom, a mechanism involving formation of a ketene product as [b3 - 1 + cat]+ is proposed.

Generation and collision-induced dissociation of ammonium tetrafluoroborate cluster ions

Singly and doubly charged cluster ions of ammonium tetrafluoroborate (NH4BF4) with general formula [(NH4BF4)nNH4]+ and [(NH4BF4)m(NH4)2]2+, respectively, were generated by electrospray ionization (ESI) and their fragmentation examined using collision-induced dissociation (CID) and ion-trap tandem mass spectrometry. CID of [(NH4BF4)nNH4]+ caused the loss of one or more neutral NH4BF4 units. The n = 2 cluster, [(NH4BF4)2NH4]+, was unique in that it also exhibited a dissociation pathway in which HBF4 was eliminated to create [(NH4BF4)(NH3)NH4]+. Dissociation of [(NH4BF4)m(NH4)2]2+ occurred through two general pathways: (a) 'fission' to produce singly charged cluster ions and (b) elimination of one or more neutral NH4BF4 units to leave doubly charged product ions. CID profiles, and measurements of changing precursor and product ion signal intensity as a function of applied collision voltage, were collected for [(NH4BF4)nNH4]+ and compared with those for analogous [(NaBF4)nNa]+ and [(KBF4)nK]+ ions to determine the influence of the cation on the relative stability of cluster ions. In general, the [(NH4BF4)nNH4]+ clusters were found to be easier to dissociate than both the sodium and potassium clusters of comparable size, with [(KBF4)nK]+ ions the most difficult to dissociate.

Sandwich Compounds of Cyanotrispyrazolylborates: Complexation-Induced Ligand Isomerization

Reaction of the new cyanoscorpionate ligand, hydrotris(4-cyano-3-phenyl)pyrazolylborate (Tp(Ph),(4CN)) with Co(II), Mn(II), and Fe(II) unexpectedly results in the isolation only of crystals containing sandwich complexes in which the ligands have been isomerized to produce the heterocyanoscorpionate hydrobis(4-cyano-3-phenylpyrazolyl)(4-cyano-5-phenylpyrazolyl)borate (Tp(Ph),(4CN*)). The three complexes have been characterized crystallographically and are isostructural, with each ligand acting in a tridentate manner toward the metal. The isomerization of the ligand appears to be more facile than that of the analogous non-cyano ligand, Tp(Ph), with which crystals of the unisomerized sandwich compound have been isolated for Mn(II) and Fe(II).

Investigation of the neutral loss of a full amino acid mass during collision-induced dissociation of the b(3)+ ion derived from a model peptide containing a 4-aminobutyric acid residue

In a previous study we found that a dominant fragmentation pathway observed for collision-induced dissociation (CID) of b(3)+ derived from peptides with sequence AXAG, where X is gamma-aminobutyric acid (gammaAbu) or epsilon-aminocaproic acid (Cap), involved the loss of 89 mass units (u). A neutral loss of 89 u corresponded to the free acid mass of an alanine (A) residue. This specific pathway was studied in greater detail here using a series of A(gammaAbu)AG peptides with strategic positioning of (15)N, (13)C and (2)H isotope labels. Based on the extensive labeling, several possible routes to the net elimination of 89 u are proposed. One is based on initial elimination of either aziridinone or imine and CO, followed by opening of an oxazolinone, tautomerization and elimination of H2O. Another involves formation of an aziridinone by cleavage of the N-terminal amide bond, and transfer of O and H atoms to this fragment via an H-bonded ion-molecule complex to complete the loss of 89 u. Both types of pathway include the transfer/migration of H atoms from the alpha-carbon position of gammaAbu or A residues.

Generation of Gas-Phase VO2+, VOOH+, and VO2+−Nitrile Complex Ions by Electrospray Ionization and Collision-Induced Dissociation

Cationic metal species normally function as Lewis acids, accepting electron density from bound electron-donating ligands, but they can be induced to function as electron donors relative to dioxygen by careful control of the oxidation state and ligand field. In this study, cationic vanadium(IV) oxohydroxy complexes were induced to function as Lewis bases, as demonstrated by addition of O2 to an undercoordinated metal center. Gas-phase complex ions containing the vanadyl (VO2+), vanadyl hydroxide (VOOH+), or vanadium(V) dioxo (VO2+) cation and nitrile (acetonitrile, propionitrile, butyronitrile, or benzonitrile) ligands were generated by electrospray ionization (ESI) for study by multiple-stage tandem mass spectrometry. The principal species generated by ESI were complexes with the formula [VO(L)n]2+, where L represents the respective nitrile ligands and n=4 and 5. Collision-induced dissociation (CID) of [VO(L)5]2+ eliminated a single nitrile ligand to produce [VO(L)4]2+. Two distinct fragmentation pathways were observed for the subsequent dissociation of [VO(L)4]2+. The first involved the elimination of a second nitrile ligand to generate [VO(L)3]2+, which then added neutral H2O via an association reaction that occurred for all undercoordinated vanadium complexes. The second [UO(L)4]2+ fragmentation pathway led instead to the formation of [VOOH(L)2]+ through collisions with gas-phase H2O and concomitant losses of L and [L+H]+. CID of [VOOH(L)2]+ caused the elimination of a single nitrile ligand to generate [VOOH(L)]+, which rapidly added O2 (in addition to H2O) by a gas-phase association reaction. CID of [VONO3(L)2]+, generated from spray solutions created by mixing VOSO4 and Ba(NO3)2 (and precipitation of BaSO4), caused elimination of NO2 to produce [VO2(L)2]+. CID of [VO2(L)2]+ produced elimination of a single nitrile ligand to form [VO2(L)]+, a V(V) analogue to the O2-reactive V(IV) species [VOOH(L)]+; however, this V(V) complex was unreactive with O2, which indicates the requirement for an unpaired electron in the metal valence shell for O2 addition. In general, the [VO2(L)2]+ species required higher collisions energies to liberate the nitrile ligand, suggesting that they are more strongly bound than the [VOOH(L)2]+ counterparts.