Potential energy surface for the reaction Sm+ + CO2 → SmO+ + CO: guided ion beam and theoretical studies

The importance of ion kinetic energy for interference removal in ICP-MS/MS

The effect of ion kinetic energy on gas phase ion reactivity with ICP-MS/MS was investigated in order to explore tuning strategies for interference removal. The collision/reaction gases CO2, N2O and O2 were used to observe the ion product distribution for 48 elements using an Agilent tandem ICP-MS (ICP-MS/MS) as a function of reaction gas flow rate (pressure) and ion kinetic energy. The kinetic energy of the incident ion was varied by adjusting the octopole bias (Voct). The three gases all form oxides (MO+) as the primary product with differing reaction enthalpies that result in distinct differences in the ion energies required for reaction with product ion distributions that vary with Voct. Consequently, by varying the ion kinetic energy (i.e., Voct), differences in interference reactivity can be used to achieve maximum separation. Three practical application examples were reported to demonstrate how the ion kinetic energy can be varied to achieve the ideal ion product distribution for interference resolution: CO2 for the removal of 238U in Pu analyses, CO2 for the removal of 40Ar16O vs. 56Fe, and O2 for the removal of Sm in Eu analyses, analogous to Pu/Am. The results demonstrate how the starting ion energy defined by Voct is an important factor to fully leverage the utility of any given reaction gas to remove interferences in the mass spectrum using ICP-MS/MS.

Assessing Gas-Phase Ion Reactivity of 50 Elements with NO and the Direct Application for 239Pu in Complex Matrices Using ICP-MS/MS

Understanding the reactivity of metal cations with various reaction gases in inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) is important to determine the best gas to use for a given analyte/interference pair. In this study, nitric oxide (NO) was investigated as the reaction gas following previous experimental designs. The reactions with 50 elements were investigated to examine periodic trends in reactivity, validate theoretical modeling of reaction enthalpies as a method to screen reactant gases, and provide a baseline data set for potential in-line gas separation methods. ICP-MS/MS studies involving actinides are typically limited to Th, U, and Pu, with analyses of Np and Am rarely reported in the literature. To date, only two previous methods have investigated the use of NO in ICP-MS/MS analyses. To showcase the utility of NO, a method was developed to measure 239Pu in the presence of environmental matrix constituent and other actinides, like what could be expected from postdetonation debris, with no chemical separation prior to analysis. 239Pu+ was reacted to form 239Pu16O+, eliminating interferences derived from the sample matrix by measuring the 239Pu+ intensity at m/z = 255 (239Pu16O+). To validate NO for 238U1H+ interference removal in environmental matrices, standard reference materials were diluted to 1 mg/g of solution and spiked to 0.05 pg/g of 239Pu and 1 μg/g 238U (Pu/U = 5 × 10-8). Measured 239Pu concentrations were within 6% of the spiked value. These results demonstrate that reliable 239Pu measurements can be made at levels relevant to nuclear forensics without the need for extensive chemical matrix separation prior to analysis.

A guided ion beam investigation of UO2+ thermodynamics and f orbital participation: Reactions of U+ + CO2, UO+ + O2, and UO+ + CO

A guided ion beam tandem mass spectrometer was employed to study the reactions of U+ + CO2, UO+ + O2, and the reverse of the former, UO+ + CO. Reaction cross sections as a function of kinetic energy over about a three order of magnitude range were studied for all systems. The reaction of U+ + CO2 proceeds to form UO+ + CO with an efficiency of 118% ± 24% as well as generating UO2+ + C and UCO+ + O. The reaction of UO+ + O2 forms UO2+ in an exothermic, barrierless process and also results in the collision-induced dissociation of UO+ to yield U+. In the UO+ + CO reaction, the formation of UO2+ in an endothermic process is the dominant reaction, but minor products of UCO+ + O and U+ + (O + CO) are also observed. Analysis of the kinetic energy dependences observed provides the bond energies, D0(U+-O) = 7.98 ± 0.22 and 8.05 ± 0.14 eV, D0(U+-CO) = 0.73 ± 0.13 eV, and D0(OU+-O) = 7.56 ± 0.12 eV. The values obtained for D0(U+-O) and D0(OU+-O) agree well with the previously reported literature values. To our knowledge, this is the first experimental measurement of D0(U+-CO). An analysis of the oxide bond energies shows that participation of 5f orbitals leads to a substantial increase in the thermodynamic stability of UO2+ relative to ThO2+ and especially transition metal dioxide cations.

f-Block reactions of metal cations with carbon dioxide studied by inductively coupled plasma tandem mass spectrometry

f-Block chemistry offers an opportunity to test current knowledge of chemical reactivity. The energy dependence of lanthanide cation (Ln+ = Ce+, Pr+, Nd+-Eu+) and actinide cation (An+ = Th+, U+-Am+) oxidation reactions by CO2, was observed by inductively coupled plasma tandem mass spectrometry. This reaction is commonly spin-unallowed because the neutral reactant (CO2, 1Σ+g) and product (CO, 1Σ+) require the metal and metal oxide cations to have the same spin state. Correlation of the promotion energy (Ep) to the first state with two free d-electrons with the reaction efficiency indicates that spin conservation is not a primary factor in the reaction rate. The Ep likely influences the reaction rate by partially setting the crossing between the ground and reactive states. Comparison of Ln+ and An+ congener reactivity indicates that the 5f-orbitals play a small role in the An+ reactions.

Quantitative Aspects of Gas-Phase Metal Ion Chemistry: Conservation of Spin, Participation of f Orbitals, and C-H Activation and C-C Coupling

In this Featured Article, I reflect on over 40 years of guided ion beam tandem mass spectrometry (GIBMS) studies involving atomic metal cations and their clusters throughout the periodic table. Studies that have considered the role of spin conservation (or lack thereof) are a primary focus with a quantitative assessment of the effects examined. A need for state-specific studies of heavier elements is noted, as is a more quantitative assessment of spin-orbit interactions in reactivity. Because GIBMS experiments explicitly evaluate the kinetic energy dependence of reactions over a wide range, several interesting and unusual observations are highlighted. More detailed studies of such unusual reaction events would be welcome. Activation of C-H bonds and ensuing C-C coupling events are reviewed, with future work encouraged. Finally, studies of lanthanides and actinides are examined with an eye on understanding the role of f orbitals in the chemistry, both as participants (or not) in the bonding and as sources/sinks of electron density. This area seems to be ripe for more quantitative experiments.

Guided Ion Beam Studies of the Dy + O → DyO+ + e- Chemi-ionization Reaction Thermochemistry and Dysprosium Oxide, Carbide, Sulfide, Dioxide, and Sulfoxide Cation Bond Energies

Guided ion beam tandem mass spectrometry (GIBMS) was used to measure the kinetic energy dependent product ion cross sections for reactions of the lanthanide metal dysprosium cation (Dy⁺) with O₂, SO₂, and CO and reactions of DyO⁺ with CO, O₂, and Xe. DyO⁺ is formed through an exothermic process when Dy⁺ reacts with O₂, whereas all other processes observed are found to be endothermic. The kinetic energy dependences of these cross sections were analyzed to yield 0 K bond dissociation energies (BDEs) for DyO⁺, DyC⁺, DyS⁺, DyO₂⁺, and DySO⁺. The 0 K BDE for DyO⁺ is determined to be 5.60 ± 0.02 eV from the weighted average of six independent thresholds, which are dominated by the slightly endothermic reaction of Dy⁺ with SO₂. Combined with the well-established Dy ionization energy (IE), this value indicates that the chemi-ionization reaction, Dy + O → DyO⁺ + e^-, is endothermic by 0.33 ± 0.02 eV. Theoretical BDEs for Dy⁺-O, Dy⁺-C, Dy⁺-S, ODy⁺-O, and Dy⁺-SO were calculated at several levels of theory and basis sets for comparison with experiment with reasonable agreement achieved.

Gas-Phase Reactivity of Ozone with Lanthanide Ions (Sm+, Nd+) and Their Higher Oxides

The kinetics of SmO_n⁺ (n = 0-2) and NdO_n⁺ (n = 0-2) with O₃ are measured using a selected-ion flow tube. Reaction of Nd⁺ to yield NdO⁺ + O₂ occurs rapidly, with a rate constant near the capture-controlled limit of ∼8 × 10^-10 cm³ s^-1. NdO⁺ reacts at ∼40% of the capture limit to yield NdO₂⁺ with little temperature dependence from 200 to 400 K. NdO₂⁺ likely reacts very slowly (k ∼ 10^-13 cm³ s^-1) to yield NdO⁺ + 2O₂, does not react to yield NdO₃⁺, and associates slowly (k ∼ 10^-12 cm³ s^-1) to yield NdO₂⁺(O₃)_1-3. Reaction of Sm⁺ also yields SmO⁺ at near the capture limit at all temperatures, but a significant fraction (∼50%) of the SmO⁺ is produced in excited states that are long-lived compared to the millisecond time scale of the experiment. These states are evidently resistant to both radiative and collisional relaxation. The excited-state production is likely due to a spin-conservation constraint on the reaction, despite the large spin-orbit coupling typical for lanthanide-containing species. Ground-state SmO⁺ reacts inefficiently (k = 2 × 10^-11 (T/300)^-2.5 cm³ s^-1) to yield SmO₂⁺ + O₂, while the excited-state SmO^+* reacts at the capture limit, with branching to yield Sm⁺ + 2O₂ (ΔH_r,0K = 148.7 ± 0.4 kJ mol^-1 for ground-state SmO⁺) approximately 60% of the time, the remainder forming SmO₂⁺, which further reacts with O₃ to yield SmO⁺ at about 1% of the collisional value.

Activation of CO2 by Actinide Cations (Th+, U+, Pu+, and Am+) as Studied by Guided Ion Beam and Triple Quadrupole Mass Spectrometry

Reactions of CO₂ with Th⁺ have been studied using guided ion beam tandem mass spectrometry (GIBMS) and with An⁺ (An⁺ = Th⁺, U⁺, Pu⁺, and Am⁺) using triple quadrupole inductively coupled plasma mass spectrometry (QQQ-ICP-MS). Additionally, the reactions ThO⁺ + CO and ThO⁺ + CO₂ were examined using GIBMS. Modeling the kinetic energy-dependent GIBMS data allowed the determination of bond dissociation energies (BDEs) for D₀(Th⁺-O) and D₀(OTh⁺-O) that are in reasonable agreement with previous GIBMS measurements. The QQQ-ICP-MS reactions were studied at higher pressures where multiple collisions between An⁺ and the neutral CO₂ occur. As a consequence, both AnO⁺ and AnO₂⁺ products were observed for all An⁺ except Am⁺, where only AmO⁺ was observed. The relative abundances of the observed monoxides compared to the dioxides are consistent with previous reports of the AnO_n⁺ (n = 1, 2) BDEs. A comparison of the periodic trends of the group 4 transition metal, lanthanide (Ln), and actinide atomic cations in reactions with CO₂ (a formally spin-forbidden reaction for most M⁺ ground states) and O₂ (a spin-unrestricted reaction) indicates that spin conservation plays a minor role, if any, for the heavier Ln⁺ and An⁺ metals. Further correlation of Ln⁺ and An⁺ + CO₂ reaction efficiencies with the promotion energy (E_p) to the first electronic state with two valence d-electrons (E_p(5d²) for Ln⁺ and E_p(6d²) for An⁺) indicates that the primary limitation in the activation of CO₂ is the energetic cost to promote from the electronic ground state of the atomic metal ion to a reactive state.

Determination of the SmO+ bond energy by threshold photodissociation of the cryogenically cooled ion

The SmO⁺ bond energy has been measured by monitoring the threshold for photodissociation of the cryogenically cooled ion. The action spectrum features a very sharp onset, indicating a bond energy of 5.596 ± 0.004 eV. This value, when combined with the literature value of the samarium ionization energy, indicates that the chemi-ionization reaction of atomic Sm with atomic oxygen is endothermic by 0.048 ± 0.004 eV, which has important implications on the reactivity of Sm atoms released into the upper atmosphere. The SmO⁺ ion was prepared by electrospray ionization followed by collisional breakup of two different precursors and characterized by the vibrational spectrum of the He-tagged ion. The UV photodissociation threshold is similar for the 10 K bare ion and the He tagged ion, which rules out the possible role of metastable electronically excited states. Reanalysis and remeasurement of previous reaction kinetics experiments that are dependent on D₀(SmO⁺) are included, bringing all experimental results in accord.

Samarium cation (Sm+) reactions with H2, D2, and HD: SmH+ bond energy and mechanistic insights from guided ion beam and theoretical studies

Guided ion beam tandem mass spectrometry is used to study the reaction of the lanthanide samarium cation (Sm⁺) with H₂ and its isotopologues (HD and D₂) as a function of collision energy. Modeling the resulting energy dependent product ion cross sections from these endothermic reactions yields 2.03 ± 0.06 eV (two standard deviations) for the 0 K bond dissociation energy of SmH⁺. Quantum chemical calculations are performed to determine stabilities of the ground and low-energy states of SmH⁺ for comparison with the experimentally measured thermochemistry. The calculations generally overestimate the SmH⁺ bond energy, but a better agreement between experiment and theory is achieved after correcting for spin-orbit energy contributions, with coupled-cluster with single, double and perturbative triple excitations/complete basis set [CCSD(T)/CBS] results reproducing the experiment well. In the HD reaction, the SmH⁺ product is observed to be favored over the SmD⁺ by about a factor of three, indicating that the reaction proceeds via a direct mechanism with short-lived intermediates. This is consistent with quantum chemical calculations of relaxed potential energy surface scans of SmH₂ ⁺, which show that there is no strongly bound dihydride intermediate. The reactivity and hydride bond energy of Sm⁺, which has a valence electron configuration typical of most lanthanides, are compared with previous results for the lanthanide cations La⁺, Gd⁺, and Lu⁺, which exhibit configurations more closely related to the group 3 metal cations, Sc⁺ and Y⁺. Periodic trends across the lanthanide series and insights into the role of the electronic configurations on hydride bond strength and reactivity with H₂ are discussed.

Lanthanides as Catalysts: Guided Ion Beam and Theoretical Studies of Sm+ + COS

Reactions of samarium cations with carbonyl sulfide are examined using a guided ion beam tandem mass spectrometer and a variable temperature selected ion flow tube apparatus. Formation of SmS⁺ + CO is observed in both instruments with a kinetic energy and temperature dependence demonstrating a barrierless reaction occurring with an efficiency of 26 ± 9%. Formation of SmO⁺ + CS is also observed at high kinetic energies and exhibits a threshold determined as 2.81 ± 0.32 eV, substantially higher than expected from known thermochemistry. The potential energy surfaces for these reactions along sextet and octet spin surfaces are also examined theoretically at the MP2 and CCSD(T) levels. The observed barrier for oxidation is shown to likely correspond to the energy of the crossing between surfaces corresponding to the ground state electronic configuration of Sm⁺ (⁸F,4f⁶6s¹) and an excited surface having two electrons in the valence space (excluding 4f), which are needed to form the strong SmO⁺ bond. In contrast, the S-CO bond is activated much more readily because this crossing occurs at much lower energies. This result is attributed to the much weaker S-CO bond energy as well as the ability of sulfur to bind effectively at different angles. Although both reactions are spin-forbidden, evidence for a more efficient spin-allowed process is also observed in the SmS⁺ + CO cross section.

Design and Application of a High-Temperature Linear Ion Trap Reactor

A high-temperature linear ion trap reactor with hexapole design was homemade to study ion-molecule reactions at variable temperatures. The highest temperature for the trapped ions is up to 773 K, which is much higher than those in available reports. The reaction between V₂O₆^- cluster anions and CO at different temperatures was investigated to evaluate the performance of this reactor. The apparent activation energy was determined to be 0.10 ± 0.02 eV, which is consistent with the barrier of 0.12 eV calculated by density functional theory. This indicates that the current experimental apparatus is prospective to study ion-molecule reactions at variable temperatures, and more kinetic details can be obtained to have a better understanding of chemical reactions that have overall barriers. Graphical Abstract.

Guided ion beam and theoretical studies of the bond energy of SmS. J Chem Phys 2017;147:214307. [PMID: 29221388 DOI: 10.1063/1.5009916]

Previous work has shown that atomic samarium cations react with carbonyl sulfide to form SmS⁺ + CO in an exothermic and barrierless process. To characterize this reaction further, the bond energy of SmS⁺ is determined in the present study using guided ion beam tandem mass spectrometry. Reactions of SmS⁺ with Xe, CO, and O₂ are examined. Results for collision-induced dissociation processes with all three molecules along with the endothermicity of the SmS⁺ + CO → Sm⁺ + COS exchange reaction are combined to yield D₀(Sm⁺-S) = 3.37 ± 0.20 eV. The CO and O₂ reactions also yield a SmSO⁺ product, with measured endothermicities that indicate D₀(SSm⁺-O) = 3.73 ± 0.16 eV and D₀(OSm⁺-S) = 1.38 ± 0.27 eV. The SmS⁺ bond energy is compared with theoretical values characterized at several levels of theory, including CCSD(T) complete basis set extrapolations using all-electron basis sets. Multireference configuration interaction calculations with explicit spin-orbit calculations along with composite thermochemistry using the Feller-Peterson-Dixon method and all-electron basis sets were also explored for SmS⁺, and for comparison, SmO, SmO⁺, and EuO.