Conversion of a UO22+ Precursor to UH+ and U+ Using Tandem Mass Spectrometry to Remove Both “yl” Oxo 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.


Mass Spectrometry Experiments
Methanol (CH 3 OH), propiolic acid and 18 O-labeled O 2 were purchased from Sigma-Aldrich Chemical (St.Louis, MO) and used as received.A sample of uranyl propiolate was prepared in house by digestion of solid UO 3 (Strem Chemicals, Newburyport MA) with aqueous propiolic acid.Caution: uranium oxide is radioactive (α-and γ-emitter), and proper shielding, waste disposal, and personal protective gear should be used when handling the material.

Generation of the [O=U≡CH]
+ intermediate by PTMS n of uranyl propioloate precursors was performed using a previously established procedure [1].Briefly, electrospray ionization (ESI) and CID experiments were performed on a ThermoScientific (San Jose, CA) LTQ-XL linear ion trap mass spectrometer that has been modified to allow study of ion-molecule reactions [2][3][4].The atmospheric pressure ionization stack settings were optimized for maximum transmission of [UO 2 (O 2 C-C≡CH)(CH 3 OH) 2 ] + (m/z 403) to the ion trap using the auto-tune routine via the LTQ Tune program.
For CID experiments, helium was used as the bath/buffer gas.Target ions were isolated using a width of 1.0 to 1.5 m/z units.The exact value was determined empirically to provide maximum ion intensity while ensuring isolation of a single isotopic peak.Values for the (mass) normalized collision energy (NCE, as defined by ThermoScientific) and activation Q were chosen empirically to enhance dissociation efficiency.For these experiments, the NCE and activation Q settings for CID of [UO 2 (C≡CH)] + (Figure 1a) were 20% and 0.35, respectively.For CID of [O=U≡CH] + (Figure 1b), the settings for NCE and activation Q were 18% and 0.33, respectively.For CID of [UH] + (Figure 1c), the NCE and activation Q settings were 22% and 0.75, respectively.

Density Functional Theory Calculations
DFT calculations were used to determine a feasible fragmentation pathway for [O=U≡CH] + .Geometry optimizations for potential precursor, intermediate and product ion structures were performed using the B3LYP [5][6][7] and PBE0 [8,9] functionals.Peterson's cc-pVTZ-PP correlation-consistent triple-zeta basis set [10] was used with the ECP60MDF Stuttgart/Koeln relativistic pseudopotential [11], and the cc-pVTZ basis set was used on was used on C, O, and H.An ultrafine integration grid was employed.Vibrational frequency calculations were used to determine whether optimized structures were true minima (no imaginary frequencies) and for thermal corrected (298.150K)energies.Transition state calculations were performed by the QST2 and QST3 methods [12].Intrinsic reaction coordinate (IRC) calculations were used to confirm that the transition states bridged the appropriate minima.Species were modelled in the singlet, triplet and quintet spin states.All calculations were performed using the Gaussian 16 group of programs [13].

2 Figure
Scheme S2.Proposed pathways for reaction of U + with (neutral) O 2 and H 2 O.