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: b
2
+
, y
1
+ 2H
+
, b
3
+
, CO loss, y
2
+ 2H
+
, and a
1
+
; three secondary products: a
2
+
, a
3
+
, and y
2
+ 2H – CO
+
; and two tertiary products: high energy y
1
+ 2H
+
and a
2
– 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: y
1
+ 2H
+
, b
3
+
, CO loss, a
1
+
, and a
3
+
, and refines those for b
2
+
and y
2
+ 2H
+
previously measured.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
We present infrared photodissociation spectra of two protonated peptides that are cooled in a ∼10 K quadrupole ion trap and “tagged” with weakly bound H2 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 CO stretching bands are completely resolved. To address the site-specificity of the CO stretches near 1800 cm−1, we incorporated one 13C into the tripeptide. The labeling affects only one line in the complex spectrum, indicating that each CO 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 H2 tagging regime.
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IJS, KILJ, NUK, PNG, UL, UM
In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ionmolecule reactions were used to create and characterize ions derived from homogeneous precursors ...composed of a uranyl cation (U
VI
O
2
2+
) 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
, 19891998. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely UO
2
(O
2
CCH
3
)
2
(O
2
CH)
and UO
2
(O
2
CCH
3
)(O
2
CH)
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 UO
2
(H)(O
2
CCH
3
)(O
2
CH)
. We also discovered that MS
n
CID of the acetateformate complexes leads to generation of the oxyl-methide species, UO
2
(O)(CH
3
)
, which reacts with H
2
O to generate UO
2
(O)(OH)
. DFT calculations support the observation that formation of UO
2
(O)(OH)
by elimination of CH
4
is favored over H
2
O addition and rearrangement to create UO
2
(OH)
2
(CH
3
)
.
Experiment and theory show that loss of acetaldehyde is favored over formaldehyde during intramolecular hydride attack in uranyl complexes with accetate and formate ligands.
Multiple-stage collision-induced dissociation (CID) of a uranyl propiolate cation, UO2(O2C-C≡CH)+, can be used to prepare the U-methylidyne species O═U≡CH+ J. Am. Soc. Mass Spectrom. 2019, 30, ...796-805. Here, we report that CID of O═U≡CH+ causes elimination of CO to create UH+, followed by a loss of H• to generate U+. A feasible, multiple-step pathway for the generation of UH+ was identified using DFT calculations. These results provide the first demonstration that multiple-stage CID can be used to prepare both U+ and UH+ directly from a UO22+ precursor for the subsequent investigation of ion-molecule reactivity.
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IJS, KILJ, NUK, PNG, UL, UM, UPUK
Difficulty in the preparation of gas-phase ions that include U in middle oxidation states(III,IV) have hampered efforts to investigate intrinsic structure, bonding and reactivity of model species. ...Our group has used preparative tandem mass spectrometry (PTMS) to synthesize a gas-phase U-methylidyne species, OUCH
, by elimination of CO from UO
(CCH)
M. J. van Stipdonk, I. J. Tatosian, A. C. Iacovino, A. R. Bubas, L. Metzler, M. C. Sherman and A. Somogyi,
, 2019,
, 796-805, which has been used as an intermediate to create products such as OUN
and OUS
by ion-molecule reactions. Here, we investigated the reactions of OUCH
with a range of alkyl halides to determine whether the methylidyne is a also a useful intermediate for production and study of the oxy-halide ions OUX
, where X = Cl, Br and I, formally U(IV) species for which intrinsic reactivity data is relatively scarce. Our experiments demonstrate that OUX
is the dominant product ion generated by reaction OUCH
with neutral regents such as CH
Cl, CH
CH
Br and CH
CHCH
I.
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.
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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. 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.
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
2
O 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
VI
O
2
(NO
3
)
+
coordinated by alcohol ligands (methanol and ethanol) using the 2-D LIT. With relatively low levels of background H
2
O, collision-induced dissociation (CID) of U
VI
O
2
(NO
3
)
+
primarily creates UO
2
(O
2
)
+
by the ejection of NO. However, CID (using He as collision gas) of U
VI
O
2
(NO
3
)
+
creates UO
2
(H
2
O)
+
and UO
2
+
when the 2-D LIT is used with higher levels of background H
2
O. 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
VI
O
2
(O
2
)
+
followed by rapid exchange of O
2
for H
2
O by ion-molecule reaction. Our experiments illustrate the impact of residual H
2
O in ion trap instruments on the product ions generated by CID and provide a more accurate description of the intrinsic dissociation pathway for U
VI
O
2
(NO
3
)
+
.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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