Experimental and computational investigation of the intrinsic reactions of [U^IIIF₂]⁺: formation of [U^IIIF(O₂CR)]⁺ & [U^III(O₂CR)₂]⁺ by reactions with carboxylic acids

Abstract

A major barrier to understanding the intrinsic reactivity of U(III) and U(IV) species is the limited experimental data available, largely because generating well-defined ions in these oxidation states for gas-phase studies is challenging. Using tandem mass spectrometry, we can address this problem by preparing and isolating mid-valent uranium ions in a linear ion trap and examining their intrinsic chemistry through ion–molecule reactions. In this study, we focused on U(III) using the reagent ion [U^IIIF₂]⁺, which was generated by collisional activation of the precursor [UO₂(O₂C C₆H₃F₂)(H₂O)₂]⁺ within a linear ion trap mass spectrometer modified for ion–molecule reactivity. Building on our recent characterization of U(IV) carboxylate ions, [U^IVF_n(O₂CR)_m]⁺ (n = 0–2; m = 1–3), and the density-functional-theory energetics governing their formation, we now report two analogous U(III) derivatives, [U^IIIF(O₂CR)]⁺ and [U^III(O₂CR)₂]⁺, generated using formic, acetic, propionic, and acrylic acids. DFT calculations indicate that formation of U(III) carboxylates occurs by a metathesis mechanism, analogous to the previously reported U(IV) congeners, though slightly higher in energy. These newly accessible ions expand the range of uranium(III) species available for studies of intrinsic 5f-element reactivity.

---

Introduction

Ion-trap tandem mass spectrometry (MSⁿ) has been used effectively to explore the chemistry of highly reactive organometallic complexes.^1–4 Actinide chemistry is relatively understudied compared to main group and transition metal elements despite the fact that solution-phase studies continue to reveal rich and unique chemistry of the elements.^5–17 The application of MSⁿ techniques by several research groups has expanded access to gas-phase U species,^18–41 substantially improving the potential for (experimental) investigations of intrinsic actinide chemistry. However, one significant challenge to exploring intrinsic U reactivity has been the generation of gas-phase ions with the metal in the +3 or +4 oxidation states. Studies of representative species have shown these to contain 1 and 2 5f-electrons, respectively, making them relevant to understanding the role of the 5f-orbitals in bonding.³⁷ In the condensed phase, success in reducing the uranyl moiety, [U^VIO₂]^+/2+ has been achieved by using electron withdrawing ligands positioned so that they have a strong interaction with the metal, thus weakening the U–O bond.⁴² We have found that in a similar way, propiolic^43–48 or dihalogenated benzoic acids,^49–52 coordinated to UO₂²⁺ will form complexes that, when subjected to collisional activation, convert oxo ligands into free carbon monoxide and reduce the metal. Building on this reactivity, we have prepared a suite of mid-oxidation-state uranium ions in the gas phase and examined their mono- and bimolecular chemistry using preparative tandem mass spectrometry (PTMSⁿ).

In an earlier study, we demonstrated the first generation of gas-phase uranium species in the formal +III and +IV oxidation states that lack oxo ligands, obtained directly from uranyl precursors using electrospray ionization (ESI) and CID. Scheme 1 shows the fragmentation reactions that lead to the formation of a U(IV) ion, [UFX(C≡CH)]⁺. Upon further collision-induced dissociation (CID), this species fragments to generate the corresponding U(III) ion, [UFX]⁺.^49,51,53 The new U^IV species reacted spontaneously with trace H₂O present in the ion trap to release acetylene and form the metal hydroxide [U^IVXY(OH)]⁺. Based on this observation, we hypothesized that a more acidic species, such as a carboxylic acid, would similarly activate the U–C bond, generating a novel U^IV species [U^IVXYA]⁺. We recently reported the reactions of [U^IVF₂(C≡CH)]⁺ with four carboxylic acids, which generated [U^IVF₂(O₂CR)]⁺ by elimination of HC≡CH.⁵² The product ions continued to react with neutral acid to create the dicarboxylate and tricarboxylate species [U^IVF(O₂CR)₂]⁺ and [U^IV(O₂CR)₃]⁺, respectively. This work established the first robust experimental route to U^IV carboxylates in the gas phase from a well-defined precursor.

Scheme 1 Formation of the ion [UFX]⁺ as previously reported. The formation was first reported as a difluoro derivative (X=F).⁴⁹ Reactions of the difluoro derivative of the U^IV ion [UF₂(C₂H)]⁺ with carboxylic acids were investigated and published recently.⁵¹ This study focus on similar reaction the difluoro derivative of the U^III ion [UF₂]⁺, generated by homolytic cleavage of the U–C bond of the U^IV precursor, shown here as the last step in the scheme.

Our initial report was followed by a systematic DFT study which produced a detailed analysis of the formate-formation pathway in uranium complexes with different halogens and clear thermodynamic and kinetic trends.⁵⁴ The study shows that progressive weakening of metal–halogen interactions strongly influences proton-transfer and ligand-substitution steps along the reaction coordinate. Electronic-structure analyses further demonstrate that the bonding interactions governing these pathways are highly localized and dominated by strong attractive and repulsive forces rather than dispersive effects.⁵⁴

Because the acids readily displaced the halide ligands in the U^IV system, we next examined the U^III species [U^IIIF₂]⁺ with the same carboxylic acids. Here, we report the results of reactions with formic, acetic, propionic, and acrylic acids, supported by DFT calculations modelling the observed reactivity. Together, these results expand the set of experimentally accessible mid-valent, gas-phase uranium ions which provide new insight into the intrinsic chemistry of 5f-element species, thereby advancing the broader development of gas-phase organo-actinide chemistry.

Methods

Both experimental and computational methods follow are similar to those of the U^IV carboxylate study previously reported.⁵² A brief description is provided below.

Experimental

A ThermoScientific (San Jose, CA) LTQ-XL linear ion-trap (LIT) mass modified for ion–molecule reaction studies was used for this study. The reagent ion [U^IIIF₂]⁺ is generated by spraying a solution of [U^VIO₂(O₂CC₆H₃F₂)(H₂O)₂]⁺, and fragmenting the precursor ion and resulting fragment ions using MSⁿ. The notation MSⁿ has long been used as an abbreviation for tandem mass spectrometry as it refers to the number of times an ion has been isolated and fragmented to generate the resultant product ion displayed, denoted n. For example, [U^IVF₂(C≡CH)]⁺ was generated by isolating and fragmenting the precursor ion [U^VIO₂(O₂CC₆H₃F₂)(H₂O)₂]⁺, selecting and dissociating a specific fragment ion, and repeating the cycle six times in series.⁴⁹ Thus, [U^IVF₂(C≡CH)]⁺ was isolated at the MS⁶ stage. The fragmentation of [U^IVF₂(C≡CH)]⁺ at the MS⁶ stage generates the reagent ion of this study, [U^IIIF₂]⁺, which is then isolated at the MS⁷ stage for IMRs. Ion molecule reactions (IMRs) were carried out by resonantly isolating and storing the reagent ion [U^IIIF₂]⁺ at m/z 276 in the ion trap, and ejecting all other ions. The reagent ion was stored for 1–1000 ms to react with the respective neutral acids. Therefore, all peaks observed aside from the initial, stored ion are generated from spontaneous reactions that take place within the trap, which is known to be approximately 310 K.⁵⁵

The chemical compositions of the IMR product ions were supported using two complementary experimental approaches. First, the ion isolation time was varied to confirm that a given product ion originates from an ion–molecule reaction. For a true IMR product, the ion abundance is predominantly time-dependent: increasing the isolation time of the reagent ion in the presence of the neutral reactant results in a corresponding increase in the relative abundance of the product ion, as gas-phase IMRs are effectively irreversible under these conditions. Although competing or secondary reactions may occur, these processes are readily identified by the emergence of additional product ions. When secondary reactions become significant, the abundance of the primary product ion may initially increase with isolation time, but then decrease at longer times as it is consumed to form secondary products; this diagnostic behavior is easily observed experimentally.

Second, CID experiments provide additional support for product-ion assignments. Observed neutral losses can often be rationalized based on chemical plausibility; for example, a loss of 28 Da may correspond to CO, C₂H₄ rather than less likely alternatives. In cases where multiple assignments could explain a given mass loss, isotopic labeling can be employed to distinguish between them, although such labeling was not required for the systems examined in this study. The design and operation of the home-built manifold used to facilitate the IMRs have been described elsewhere.⁵²

Computational

Calculations were executed using the Gaussian 16 suite of programs.⁵⁶ DFT calculations were employed to map out the probable pathways by which [U^IIIF₂]⁺ reacts with the respective carboxylic acids and predict relative reaction energetics. The default temperature of the calculations is 298 K, which is slightly cooler than the estimated temperature of the experiment. Two hybrid density functionals, PBE0⁵⁷ and B3LYP,⁵⁸ were used based on success in prior studies modelling gas-phase U chemistry. M06L⁵⁹ has been used with good success to predict the energetics of middle oxidation state U reactions, but some disagreement has been seen with structures, so the Minnesota functional was only employed here to double check the formic acid data for redundancy.⁵² We continued to use the minimally augmented (maug-) correlation consistent Dunning basis sets, as they provide a good compromise between computational accuracy and cost.^60,61 On the light atoms (not U), we used the double ζ maug-cc-pVDZ for gross optimizations and then reoptimized with the triple ζ maug-cc-pVTZ for more accurate energy predictions.^60,62–65 As for the U, the Peterson correlation-consistent basis sets cc-pVDZ-PP and cc-pVTZ-PP were used, along with the fully relativistic ECP60MDF pseudopotential.⁶⁶ Only the triple ζ structures are reported. Transition states were primarily calculated using the QST3 method for initial gross optimization,^67,68 and reoptimized with the Berny method.⁶⁹ Intrinsic reaction coordinate (IRC) calculations were used to confirm that transition state structures bridge the appropriate minima.⁷⁰ The coordinates of the optimized structures and transition states are reported in the SI.

Results and discussion

Experimental investigation of the reactions between [U^IIIF₂]⁺ and carboxylic acids

As described above, the generation of the reagent ion [U^IIIF₂]⁺ (m/z 276) at the MS⁷ stage results from fragmentation of [UIVF₂(C≡CH)]⁺ (m/z 301), the formation of which has been detailed elsewhere.⁴⁹ The reactions investigated in this study are summarized in Table 1. In addition to these processes, the reagent ion [UF₂]⁺ was observed to undergo spontaneous oxidation, forming the species [OUF₂]⁺ (m/z 292) and [UO₂F]⁺ (m/z 289). The latter species further undergoes hydrolysis to produce [UO₂OH]⁺ (m/z 287). These ions are observed in the spectra because both O₂ and H₂O are present in the trap as background gases, as electrospray ionization (ESI) is an atmospheric-pressure ionization method. This behavior has been reported in previous studies^49,51 and was confirmed here by isolating [UF₂]⁺ in the ion trap for up to 1 ms in the absence of any carboxylic acid. The spectra corresponding to these control experiments are provided in the SI.

Table 1 The reactions discussed throughout the paper are displayed here. They are organized by the neutral reagent, so for example RXNs 1 and 2 both have formic acid as the neutral reagent. Along with the chemical change, m/z changes are displayed as well. For example, in RXN 1, the reagent ion [UF₂]⁺ has an m/z of 276, while the product ion, [UF(O₂CH)]⁺, has an m/z of 302. Fig. 1(a) displays both peaks, along with the product ions from other reactions

#	Reactions	m/z changes
	Formation of U^III formates
1	[UF2]^+ + HO2CH → [UF(O2CH)]^+ + HF	276 → 302
2	[UF(O2CH)]^+ + HO2CH → [U(O2CH)2]^+ + HF	302 → 328
	Formation of U^III acetates
3	[UF2]^+ + HO2CCH3 → [UF(O2CCH3)]^+ + HF	276 → 316
4	[UF(O2CCH3)]^+ + HO2CCH3 → [U(O2CCH3)2]^+ + HF	316 → 356
	Formation of U^III propionates
5	[UF2]^+ + HO2CC2H5 → [UF(O2CC2H5)]^+ + HF	276 → 330
6	[UF(O2CC2H5)]^+ + HO2CC2H5 → [U(O2CC2H5)2]^+ + HF	330 → 384
	Formation of U^III acrylates
7	[UF2]^+ + HO2CC2H3 → [UF(O2CC2H3)]^+ + HF	276 → 328
8	[UF(O2CC2H3)]^+ + HO2CC2H3 → [U(O2CC2H3)2]^+ + HF	328 → 380

---

The mass spectra obtained by isolating the [U^IIIF₂]⁺ reagent ion for reactions with formic, acetic, propionic, and acrylic acids are shown in Fig. 1. The isolation/reaction times used for the spectra in Fig. 1 range from 10 to 30 ms. Product ion spectra collected over a broader range of isolation times (up to 1 s) are provided in the SI. Aside from the peaks that arise from oxidation and hydrolysis reactions described above, the remaining peaks in the mass spectra are attributed to ion–molecule reactions with the respective carboxylic acid, as well as to secondary reactions involving primary product ions. Fig. 1A shows a representative mass spectrum obtained from reaction with formic acid (neutral mass 46 Da).

Fig. 1 Mass spectra generated by isolating the ion [U^IIIF₂]⁺ at m/z 276 in the presence of formic (A), acetic (B), propionic (C), and acrylic (D) acids. The neutral acids and their masses are displayed in the top right corner of each spectrum. The product ions of most interest here are of the form [UF(carboxylate)]⁺ and [U(carboxylate)₂]⁺, which are seen in all four spectra. In (A), these can be seen at m/z 302 and 328. In (B), they are seen at m/z 316 and 356. In (C), they are seen at m/z 330 and 384. In (D), they are seen at m/z 328 and 380. The spectra displayed are not meant to compare relative intensities, as they were generated with different partial pressures of both reagents.

The primary product ion [UF(O₂CH)]⁺ (m/z 302, reaction 1) is observed, together with a secondary product ion, [U(O₂CH)₂]⁺ (m/z 328, reaction 2). These species are consistent with sequential incorporation of two formate ligands accompanied by HF elimination, resulting in a net mass increase of 26 Da per reaction. In addition, the signal at m/z 315, assigned to [UO₂(O₂CH)]⁺, appears 26 Da higher than the precursor [UO₂F]⁺ (m/z 289), indicating that the U^VI species also undergoes reaction with formic acid. Similar reactivity patterns were observed for the other carboxylic acids examined.

Fig. 1B presents the mass spectrum obtained with acetic acid (neutral mass 60 Da), in which both [UF(O₂CCH₃)]⁺ (m/z 316, reaction 3) and [U(O₂CCH₃)₂]⁺ (m/z 356, reaction 4) are detected. Analogously, exposure to propionic acid (neutral mass 74 Da; Fig. 1C) yields [UF(O₂CCH₂CH₃)]⁺ (m/z 330, reaction 5) and [U(O₂CCH₂CH₃)₂]⁺ (m/z 384, reaction 6). Reactions with acrylic acid (neutral mass 72 Da; Fig. 1D) produce the corresponding ions [UF(O₂CCHCH₂)]⁺ (m/z 328, reaction 7) and [U(O₂CCHCH₂)₂]⁺ (m/z 380, reaction 8). In addition to these primary and secondary products, several higher-order coordination complexes were observed, including [U(O₂CH)₂(HO₂CH)]⁺ (m/z 374; Fig. 1A), [U(O₂CCH₃)₂(HO₂CCH₃)]⁺ (m/z 416; SI), [U(O₂CCH₂CH₃)₂(HO₂CCH₂CH₃)]⁺ (m/z 458; SI), and [U(O₂CCHCH₂)₂(HO₂CCHCH₂)]⁺ (m/z 452; SI), among others.

During the U^IV study,⁵² we found that neutral carboxylic acids may persist as trace contaminant in the ion trap instrument for several days to weeks after use in ion–molecule reaction studies. As a result, certain ions, such as [UF(O₂CH)]⁺ (m/z 302, reaction 1), appeared in spectra acquired during experiments nominally conducted with different acids. This persistence also led to the formation of mixed-ligand species of the general formula [U(O₂CR¹)(O₂CR²)]⁺ (R¹, R² = H, CH₃, CH₂CH₃, CH=CH₂), including [U(O₂CH)(O₂CCH₃)]⁺ (m/z 342; Fig. 1B), which incorporate two distinct carboxylate ligands. Despite their concurrent presence, these mixed-ligand ions are readily distinguished based on their nominal mass difference and characteristic CID behavior. A complete inventory of all observed mixed-ligand combinations is provided in Table S1 of the SI.

Computational investigation of ion–molecule reaction pathways

Fig. 2 shows the Gibbs free energy diagrams for the reactions of all four carboxylic acids calculated with B3LYP. Reactions with formic acid (RXNs 1–2) are shown in green; acetic acid (RXNs 3–4) in red; propionic acid (RXNs 5–6) in blue; and acrylic acid (RXNs 7–8) in purple. For clarity, only numerical values for formic acid are included in the figure; Gibbs free energy values for all acids calculated with B3LYP are provided in Table 2. Because the product ions from RXNs 1, 3, 5, and 7 (left) serve as the reagent ions for RXNs 2, 4, 6, and 8, (right) respectively, these pairs of reactions are presented adjacent to one another. However, it is important to note that the partial pressure of He is significantly higher than that of the neutral reagent, indicating that the intermediate ion [UF(O₂CR)]⁺ will most likely undergo collisional cooling before reacting with a second carboxylic acid molecule.

Fig. 2 Gibbs free energy reaction coordinate diagram for the reactions of [U^IIIF₂]⁺ with formic (RXNs 1–2, green), acetic (RXNs 3–4, red), propionic (RXNs 5–6, blue), and acrylic acids (RXNs 7–8, purple). The first set of reactions (RXNs 1, 3, 5 and 7) is shown on the left. The main result of these reactions is the formation of [U^IIIF(carboxylate)]⁺, seen at structure IV. The second set of reactions (RXNs 2, 4, 6 and 8) is shown on the right. The main result of these reactions is the formation of [U^III(carboxylate)₂]⁺ seen at structure VIII. The rxns are normalized to zero so their relative energies could be compared. The mechanisms predicted are in good agreement with each other, and with U^IV analogs previously investigated. For the sake of clarity, only structures and numerical values are shown for formic acid, while values for the other acids can be found in Table 2.

Table 2 Numerical values for the Gibbs free energy of the reactions examined, as calculated with the B3LYP functional, are displayed here in kJ mol⁻¹. Structures are labelled as they appear in Fig. 2. For example, reactions 1 and 2 are displayed in the first row labelled Formic. Structure I in this row of the table corresponds to the green box in Fig. 2 labelled by the roman numeral I

	I	II	TSI	III	IV	V	VI	TSII	VII	VIII
Formic	0.0	−167.3	−76.8	−116.6	−68.3	0.0	−101.9	−15.8	−51.5	−6.9
Acetic	0.0	−193.1	−104.2	−139.6	−96.3	0.0	−117.6	−37.3	−65.2	−26.1
Propionic	0.0	−200.4	−111.2	−134.8	−103.6	0.0	−121.9	−41.1	−69.3	−27.3
Acrylic	0.0	−200.3	−114.3	−148.7	−110.1	0.0	−115.3	−39.2	−62.9	−26.6

---

The relative free energy values are shown relative to the energy of the initial precursor. For the case of RXNs 1, 3, 5, & 7, shown on the left, this is structure I. Structure I includes the energies of the ion [U^IIIF₂]⁺ and two equivalents of the respective acid, structure II includes the energies of the encounter complex [U^IIIF₂(HO₂CR)]⁺ plus one equivalent of the same acid, and so on. In general, M06L, PBE0 and B3LYP were consistent with each other, so only B3LYP data is shown. Data from the M06L and PBE0 calculations can be found in the SI (Table S2). The quartet spin surface was found to be preferential over the doublet, as shown in the SI (Table S3).

The reaction mechanisms predicted by DFT were highly conserved between the four carboxylic acids. As stated above, structure II represents the encounter complex [U^IIIF₂(HO₂CR)]⁺. In RXN 1, for example, the addition of neutral formic acid to [U^IIIF₂]⁺ (m/z 276; structure I, green) creates the encounter complex [U^IIIF₂(HO₂CH)]⁺ (structure II, green). Note: the m/z value of this encounter complex ion would be 322, but no peak at m/z 322 is seen experimentally, neither is any derivative of this ion for the other acids (m/z 336 for acetic, m/z 350 for propionic, m/z 348 for acrylic). The absence of peaks corresponding to acid adducts to the precursor ions, which represent the respective encounter complexes, is consistent with the computational prediction that these reactions proceed without an activation barrier in the gas phase, resulting in rapid conversion to downstream products. Thus, the DFT-derived mechanisms are in good qualitative agreement with the experimental observations. All reactions were predicted to be single step, exhibiting one transition state, represented by TSI in the cases of RXNs 1, 3, 5, & 7, and by TSII in the cases of RXNs 2, 4, 6, & 8. TSI shows the acidic proton transferring directly to the fluoride, with no interaction between the metal and the proton observed. The result of TSI is [U^IIIF(O₂CR)(HF)]⁺ (structure III). The dissociation of free HF from the ion results in the species represented by structure IV, [U^IIIF(O₂CR)]⁺. This species corresponds to the ions at m/z 302 (formic acid, RXN 1, green), m/z 316 (acetic acid, RXN 3, red), m/z 330 (propionic acid, RXN 5, blue), and m/z 328 (acrylic acid, RXN 7, purple).

As stated above, the species represented by structure IV are the reagent ions for RXNs 2, 4, 6, & 8. This structure is represented on the right as structure V. The second encounter species is represented by the formula [U^IIIF(O₂CR)(HO₂CR)]⁺ at structure VI, which were also not observed in the mass spectra (m/z values of 348, 376, 404, and 400 respectively), indicating a fast, barrierless reaction. The reactions proceed through TSII, which again exhibits a proton transfer from the acid to the remaining fluoride, and creates the product complex [U^III(O₂CR)₂(HF)]⁺ (structure VI). Finally, free HF dissociates from the complex, leaving the species [U^III(O₂CR)₂]⁺ (structure VIII), which corresponds to the ions at m/z 328 (formic acid), m/z 356 (acetic acid), m/z 384 (propionic acid), and m/z 380 (acrylic acid).

All reactions observed experimentally were predicted to be spontaneous at 298 K. The length of the carbon chain does not affect the qualitative features of the reaction pathways shown in the Gibbs free energy diagrams. All functionals reproduce the same overall mechanistic picture, including the presence and ordering of intermediates and transition states. However, as shown in Table S2, the functionals differ in their predictions of the relative reactivity of the carboxylic acids. In the absence of quantitative experimental thermochemical data, it is not possible to assess the accuracy of these energetic differences. Consequently, the primary conclusion is that the functionals are in good agreement with respect to the qualitative aspects of the reaction mechanisms, which is the focus of this study.

The difference between the TS structures and their respective encounter complexes are approximately equal for both reactions observed for each acid (approximately 90 kJ mol⁻¹ for both RXNs 1 & 2, for example), indicating that the replacement of one fluoride with a carboxylate does not significantly affect the energetics of the second reaction. In our previous study,⁵² the U^IV carboxylate [UF₂(O₂CR)]⁺ quickly became the dominant, base peak in the spectra, however, the U^III carboxylates presented here never became the dominant ion in the spectra, indicating slower reaction rates. Instead, secondary or competing reactions dominate the spectra at long isolation times. This cannot be definitively ascribed to thermodynamic favourability, as the kinetic parameters, such as the partial pressure of the neutral reagent, are not finely controlled in either of these experiments. However, there is a significant difference in the average predicted ΔG values which supports this experimental observation. For example, the first step to forming the U^IV carboxylates generated HC≡CH, and had an average ΔG of −199 kJ mol⁻¹ (B3LYP). In contrast, the first step to forming the U^III carboxylates generates HF, and has an average ΔG of −95 kJ mol⁻¹ (B3LYP). Ultimately, exploring the relative rates of reactions are beyond the scope of this work, as the rates of formation of the U^IV & U^III carboxylates should not be compared without further experimental data.

Lastly, we note that an alternative binding motif for metal–carboxylate ions has been investigated in specific cases, such as molybdenum oxyanions.⁷¹ To examine this possibility, we explored formation mechanisms for both formic and acetic acids using DFT. The resulting reaction free energy diagrams, calculated using B3LYP, are shown in Fig. 3. Fig. 3a presents the alternative pathway for formic acid (RXN 1), while Fig. 3b shows the corresponding pathway for acetic acid (RXN 3). In both cases, the mechanism proposed in Fig. 2 is included for comparison.

Fig. 3 Comparison of the Gibbs free energy of an alternate mechanisms for the reactions of [U^IIIF₂]⁺ with formic (RXN 1, Figure a) and acetic (RXN 3, Figure b) acids. The alternative mechanism is proposed by suggesting that the allylic proton, rather than the acidic proton, is transferred to make HF. This mechanism is shown in red for both formic (Figure a) and acetic (Figure b) reactions, while the previously reported mechanism is shown in black. The encounter complexes are relatively close in energy, shown in structure II. The transition states differ significantly, as do the resulting product and dissociation structures, shown at structures III, IV, and V, respectively. The red pathway is endergonic, indicating this alternative pathway is unfavored.

In both pathways, the acid binds through the carbonyl oxygen atom in the initial encounter complex. The key difference is the orientation of the –OH group relative to the uranium center. In the pathway shown in Fig. 2, the –OH group is oriented toward the metal center, enabling transfer of the acidic proton to form HF. In the alternative pathway (shown in red), the –OH group is oriented away from the metal center, such that formation of HF would require transfer of the non-acidic hydrogen.

The encounter complexes (structure II) are similar in energy for both orientations. However, the transition states (structure III) differ substantially. The pathway involving transfer of the non-acidic hydrogen lies above the energy of the reactants, indicating the presence of a reaction barrier that would preclude reactivity under the experimental conditions without activation. Because no collisional activation was applied in these experiments, this pathway is unlikely to contribute. Instead, the results support the mechanism shown in Fig. 2, in which the –OH group is oriented toward the metal center and the acidic proton is transferred.

Conclusions

To the best of our knowledge, the data presented here represents the first generation by ion–molecule chemistry of gas-phase cationic carboxylates which contain a formal U^III metal center. The novel species generated include [U^IIIF(O₂CR)]⁺, [U^III(O₂CR)₂]⁺, and in some cases, [U^III(O₂CR)₂(HO₂CR)]⁺ (R=H, CH₃, CH₂CH₃, & CH=CH₂). DFT calculations supported the findings of experiment by showing formation of U carboxylates and neutral HF is exergonic. There is no significant difference in energy predicted between the formation of [U^IIIF(O₂CR)]⁺ and of [U^III(O₂CR)₂]⁺, but the secondary reaction requires more time. The functionals performed with good agreement on mechanism, but differed slightly on the relative energies with respect to carboxylic reagent. The formation of these ions advances what is known about the intrinsic chemistry of middle oxidation states and sets the precedent for the use of common DFT functionals to probe the mechanisms of U^III-containing reactions. Future work will focus on fragmentation of these species, as well as the analogous U(IV) carboxylate ions. Previous gas-phase studies of metal–carboxylate complexes have shown that such fragmentation pathways can yield novel organometallic species, providing a useful route for expanding the range of accessible mid-valent uranium ions in the gas phase.

Author contributions

S. J. L. conceptualization, investigation, writing – original draft. M. R. H., J. A. H., A. P. Z., A. I. investigation. T. A. C. conceptualization, methodology, instrumentation. M. V. S. conceptualization, methodology, writing – review and editing, project administration.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional mass spectra, tabulated computational numeric values, and optimized structure coordinates for minima and transition states. See DOI: https://doi.org/10.1039/d6dt01072e.

Acknowledgements

S. J. L., M. R. H. and A. I. acknowledge the School of Science and Engineering of Duquesne University for funding of graduate and undergraduate research. J. A. H. and A. P. Z. acknowledge the National Science Foundation (CHE-1659823 and CHE-2244151) for undergraduate research funding. A. P. Z. acknowledges support from the Frank and Patsy Deverse undergraduate fellowship for undergraduate research. Computational resources were provided by the Center for Computational Sciences at Duquesne University and the National Science Foundation MRI (CHE-1726824).

References

1. R. A. J. O'Hair, The 3D Quadrupole Ion Trap Mass Spectrometer as a Complete Chemical Laboratory for Fundamental Gas-Phase Studies of Metal Mediated Chemistry, Chem. Commun., 2006,(14), 1469–1481,  10.1039/B516348J.
2. S. Osburn and V. Ryzhov, Ion–Molecule Reactions: Analytical and Structural Tool, Anal. Chem., 2013, 85(2), 769–778,  DOI:10.1021/ac302920a.
3. R. A. J. O'Hair, Organometallic Gas-Phase Ion Chemistry and Catalysis: Insights into the Use of Metal Catalysts to Promote Selectivity in the Reactions of Carboxylic Acids and Their Derivatives, Mass Spectrom. Rev., 2021, 40(6), 782–810,  DOI:10.1002/mas.21654.
4. K. Parker, N. E. Bollis and V. Ryzhov, Ion-Molecule Reactions of Mass-Selected Ions, Mass Spectrom. Rev., 2024, 43(1), 47–89,  DOI:10.1002/mas.21819.
5. D. E. Hobart and J. R. Peterson, Berkelium, in The Chemistry of the Actinide and Transactinide Elements, ed. L. R. Morss, N. M. Edelstein and J. Fuger, Springer Netherlands, Dordrecht, 2006, pp. 1444–1498.  DOI:10.1007/1-4020-3598-5_10.
6. A. R. Fox, S. C. Bart, K. Meyer and C. C. Cummins, Towards Uranium Catalysts, Nature, 2008, 455(7211), 341–349,  DOI:10.1038/nature07372.
7. P. L. Arnold, D. Patel, C. Wilson and J. B. Love, Reduction and Selective Oxo Group Silylation of the Uranyl Dication, Nature, 2008, 451(7176), 315–317,  DOI:10.1038/nature06467.
8. P. L. Arnold, Now U=C It, Nat. Chem., 2009, 1, 29–30.
9. P. L. Arnold, A.-F. Pécharman, E. Hollis, A. Yahia, L. Maron, S. Parsons and J. B. Love, Uranyl Oxo Activation and Functionalization by Metal Cation Coordination, Nat. Chem., 2010, 2(12), 1056–1061,  DOI:10.1038/nchem.904.
10. S. T. Liddle, The Renaissance of Non-Aqueous Uranium Chemistry, Angew. Chem., Int. Ed., 2015, 54(30), 8604–8641,  DOI:10.1002/anie.201412168.
11. S. A. Johnson and S. C. Bart, Achievements in Uranium Alkyl Chemistry: Celebrating Sixty Years of Synthetic Pursuits, Dalton Trans., 2015, 44(17), 7710–7726,  10.1039/C4DT01621A.
12. P. L. Arnold, M. S. Dutkiewicz and O. Walter, Organometallic Neptunium Chemistry, Chem. Rev., 2017, 117(17), 11460–11475,  DOI:10.1021/acs.chemrev.7b00192.
13. B. M. Gardner, C. E. Kefalidis, E. Lu, D. Patel, E. J. L. McInnes, F. Tuna, A. J. Wooles, L. Maron and S. T. Liddle, Evidence for Single Metal Two Electron Oxidative Addition and Reductive Elimination at Uranium, Nat. Commun., 2017, 8(1), 1898,  DOI:10.1038/s41467-017-01363-0.
14. P. L. Arnold, C. J. V. Halliday, L. Puig-Urrea and G. S. Nichol, Instantaneous and Phosphine-Catalyzed Arene Binding and Reduction by U(III) Complexes, Inorg. Chem., 2021, 60(6), 4162–4170,  DOI:10.1021/acs.inorgchem.1c00327.
15. J. M. Purkis, P. L. Arnold, R. Rutkauskaite, J. B. Love and J. Austin, Towards Catalytic Uranyl Hydrocarbon C-H Bond Cleavage, 2021.
16. A. N. Price, V. Berryman, T. Ochiai, J. J. Shephard, S. Parsons, N. Kaltsoyannis and P. L. Arnold, Contrasting Behaviour under Pressure Reveals the Reasons for Pyramidalization in Tris(Amido)Uranium(III) and Tris(Arylthiolate) Uranium(III) Molecules, Nat. Commun., 2022, 13(1), 3931,  DOI:10.1038/s41467-022-31550-7.
17. D. R. Russo, A. N. Gaiser, A. N. Price, S. Dumitruo-Claudiu, J. N. Wacker, N. Katzer, A. A. Peterson, J. A. Branson, X. Yu, S. N. Kelly, E. T. Ouellette, J. Arnold, J. R. Long, W. W. Lukens Jr, S. J. Teat, R. J. Abergel, P. L. Arnold, J. Autschbach and S. G. Minasian, Berkelium–Carbon Bonding in a Tetravalent Berkelocene, Science, 2025, 387(6736), 974–978.
18. P. Armentrout, R. Hodges and J. L. Beauchamp, Metal Atoms as Superbases: The Gas Phase Proton Affinity of Uranium, J. Am. Chem. Soc., 1977, 99(9), 3162–3163,  DOI:10.1021/ja00451a051.
19. W.-D. Wang, A. Bakac and J. H. Espenson, Uranium(VI)-Catalyzed Photooxidation of Hydrocarbons with Molecular Oxygen, Inorg. Chem., 1995, 34(24), 6034–6039,  DOI:10.1021/ic00128a014.
20. J. Q. Wang, A. K. Dash, J. C. Berthet, M. Ephritikhine and M. S. Eisen, Selective Dimerization of Terminal Alkynes Promoted by the Cationic Actinide Compound [(Et₂N)₃U][BPh₄]. Formation of the Alkyne π-Complex [(Et₂N)₂ U(C⋮C, ^tBu)(η²-HC⋮C ^tBu)][BPh₄], Organometallics, 1999, 18(13), 2407–2409,  DOI:10.1021/om980973w.
21. G. L. Gresham, A. K. Gianotto, P. D. B. Harrington, L. Cao, J. R. Scott, J. E. Olson, A. D. Appelhans, M. J. van Stipdonk and G. S. Groenewold, Gas-Phase Hydration of U(IV), U(V), and U(VI) Dioxo Monocations, J. Phys. Chem. A, 2003, 107(41), 8530–8538,  DOI:10.1021/jp035443e.
22. M. J. van Stipdonk, W. Chien, V. Anbalagan, G. L. Gresham and G. S. Groenewold, Oxidation of 2-Propanol Ligands during Collision-Induced Dissociation of a Gas-Phase Uranyl Complex, Int. J. Mass Spectrom., 2004, 237(2), 175–183,  DOI:10.1016/j.ijms.2004.07.007.
23. E. D. Pillai, K. S. Molek and M. A. Duncan, Growth and Photodissociation of U+(C6H6)n (N=1–3) and UOm+(C6H6) (m = 1, 2) Complexes, Chem. Phys. Lett., 2005, 405(4), 247–251,  DOI:10.1016/j.cplett.2005.02.038.
24. G. S. Groenewold, K. C. Cossel, G. L. Gresham, A. K. Gianotto, A. D. Appelhans, J. E. Olson, M. J. Van Stipdonk and W. Chien, Binding of Molecular O2 to Di- and Triligated [UO2]+, J. Am. Chem. Soc., 2006, 128(9), 3075–3084,  DOI:10.1021/ja0573209.
25. G. S. Groenewold, A. K. Gianotto, M. E. McIlwain, M. J. van Stipdonk, M. Kullman, D. T. Moore, N. Polfer, J. Oomens, I. Infante, L. Visscher, B. Siboulet and W. A. de Jong, Infrared Spectroscopy of Discrete Uranyl Anion Complexes, J. Phys. Chem. A, 2008, 112(3), 508–521,  DOI:10.1021/jp077309q.
26. C. M. Leavitt, V. S. Bryantsev, W. A. de Jong, M. S. Diallo, W. A. Goddard, G. S. Groenewold and M. J. Van Stipdonk, Addition of H₂O and O2 to Acetone and Dimethylsulfoxide Ligated Uranyl(V) Dioxocations, J. Phys. Chem. A, 2009, 113(11), 2350–2358,  DOI:10.1021/jp807651c.
27. A. M. Ricks, L. Gagliardi and M. A. Duncan, Infrared Spectroscopy of Extreme Coordination: The Carbonyls of U+ and UO2+, J. Am. Chem. Soc., 2010, 132(45), 15905–15907,  DOI:10.1021/ja1077365.
28. G. L. Gresham, A. Dinescu, M. T. Benson, M. J. Van Stipdonk and G. S. Groenewold, Investigation of Uranyl Nitrate Ion Pairs Complexed with Amide Ligands Using Electrospray Ionization Ion Trap Mass Spectrometry and Density Functional Theory, J. Phys. Chem. A, 2011, 115(15), 3497–3508,  DOI:10.1021/jp109665a.
29. P. D. Dau, J. Su, H.-T. Liu, J.-B. Liu, D.-L. Huang, J. Li and L.-S. Wang, Observation and Investigation of the Uranyl Tetrafluoride Dianion (UO2F42−) and Its Solvation Complexes with Water and Acetonitrile, Chem. Sci., 2012, 3(4), 1137–1146,  10.1039/C2SC01052F.
30. J. Su, P. D. Dau, Y.-H. Qiu, H.-T. Liu, C.-F. Xu, D.-L. Huang, L.-S. Wang and J. Li, Probing the Electronic Structure and Chemical Bonding in Tricoordinate Uranyl Complexes UO2X3− (X = F, Cl, Br, I): Competition between Coulomb Repulsion and U–X Bonding, Inorg. Chem., 2013, 52(11), 6617–6626,  DOI:10.1021/ic4006482.
31. W.-L. Li, J. Su, T. Jian, G. V. Lopez, H.-S. Hu, G.-J. Cao, J. Li and L.-S. Wang, Strong Electron Correlation in UO2−: A Photoelectron Spectroscopy and Relativistic Quantum Chemistry Study, J. Chem. Phys., 2014, 140(9), 094306,  DOI:10.1063/1.4867278.
32. J. Su, P. D. Dau, H.-T. Liu, D.-L. Huang, F. Wei, W. H. E. Schwarz, J. Li and L.-S. Wang, Photoelectron Spectroscopy and Theoretical Studies of Gaseous Uranium Hexachlorides in Different Oxidation States: UCl6q− (q = 0–2), J. Chem. Phys., 2015, 142(13), 134308,  DOI:10.1063/1.4916399.
33. S.-X. Hu, J. K. Gibson, W.-L. Li, M. J. van Stipdonk, J. Martens, G. Berden, B. Redlich, J. Oomens and J. Li, Electronic Structure and Characterization of a Uranyl Di-15-Crown-5 Complex with an Unprecedented Sandwich Structure, Chem. Commun., 2016, 52(86), 12761–12764,  10.1039/C6CC07205D.
34. R. J. Abergel, W. A. de Jong, G. J.-P. Deblonde, P. D. Dau, I. Captain, T. M. Eaton, J. Jian, M. J. van Stipdonk, J. Martens, G. Berden, J. Oomens and J. K. Gibson, Cleaving Off Uranyl Oxygens through Chelation: A Mechanistic Study in the Gas Phase, Inorg. Chem., 2017, 56(21), 12930–12937,  DOI:10.1021/acs.inorgchem.7b01720.
35. M. J. Van Stipdonk, A. Iacovino and I. Tatosian, Influence of Background H₂O on the Collision-Induced Dissociation Products Generated from [UO2NO3]+, J. Am. Soc. Mass Spectrom., 2018, 29(7), 1416–1424,  DOI:10.1007/s13361-018-1947-5.
36. M. J. van Stipdonk, I. J. Tatosian, A. C. Iacovino, A. R. Bubas, L. J. Metzler, M. C. Sherman and A. Somogyi, Gas-Phase Deconstruction of UO22+: Mass Spectrometry Evidence for Generation of [OUVICH]+ by Collision-Induced Dissociation of [UVIO2(C≡CH)]+, J. Am. Soc. Mass Spectrom., 2019, 30(5), 796–805,  DOI:10.1007/s13361-019-02179-6.
37. P. B. Armentrout and K. A. Peterson, Guided Ion Beam and Quantum Chemical Investigation of the Thermochemistry of Thorium Dioxide Cations: Thermodynamic Evidence for Participation of f Orbitals in Bonding, Inorg. Chem., 2020, 59(5), 3118–3131,  DOI:10.1021/acs.inorgchem.9b03488.
38. A. R. Bubas, A. C. Iacovino and P. B. Armentrout, Reactions of Atomic Thorium and Uranium Cations with SF₆ Studied by Guided Ion Beam Tandem Mass Spectrometry, J. Phys. Chem. A, 2022, 126(20), 3239–3246,  DOI:10.1021/acs.jpca.2c02090.
39. P. B. Armentrout, Quantitative Aspects of Gas-Phase Metal Ion Chemistry: Conservation of Spin, Participation of f Orbitals, and C–H Activation and C–C Coupling, J. Phys. Chem. A, 2023, 127(46), 9641–9653,  DOI:10.1021/acs.jpca.3c06023.
40. S. Rockow, A. R. Bubas, S. P. Krauel, B. C. Stevenson and P. B. Armentrout, Thermochemistry of Uranium Sulfide Cations: Guided Ion Beam and Theoretical Studies of Reactions of U+ and US+ with CS2 and Collision-Induced Dissociation of US+, Mol. Phys., 2024, 122(1–2), e2175595,  DOI:10.1080/00268976.2023.2175595.
41. J. E. Colley, A. G. Batchelor, B. W. Stratton and M. A. Duncan, Cation-π Bonding in Actinides: UO _x ⁺ (Benzene) (x = 0, 1, 2) Complexes Studied with Threshold Photodissociation Spectroscopy and Theory, J. Phys. Chem. Lett., 2025, 16(6), 1515–1521,  DOI:10.1021/acs.jpclett.4c03603.
42. N. L. Bell, B. Shaw, P. L. Arnold and J. B. Love, Uranyl to Uranium(IV) Conversion through Manipulation of Axial and Equatorial Ligands, J. Am. Chem. Soc., 2018, 140(9), 3378–3384,  DOI:10.1021/jacs.7b13474.
43. M. J. van Stipdonk, I. J. Tatosian, A. C. Iacovino, A. R. Bubas, L. J. Metzler, M. C. Sherman and A. Somogyi, Gas-Phase Deconstruction of UO22+: Mass Spectrometry Evidence for Generation of [OUVICH]+ by Collision-Induced Dissociation of [UVIO2(C≡CH)]+, J. Am. Soc. Mass Spectrom., 2019, 30(5), 796–805,  DOI:10.1007/s13361-019-02179-6.
44. L. J. Metzler, C. T. Farmen, T. A. Corcovilos and M. J. van Stipdonk, Intrinsic Chemistry of [OUCH]+ : Reactions with H₂O, CH₃, C≡N and O2, Phys. Chem. Chem. Phys., 2021, 23(8), 4475–4479,  10.1039/D1CP00177A.
45. L. J. Metzler, C. T. Farmen, A. N. Fry, M. P. Seibert, K. A. Massari, T. A. Corcovilos and M. J. van Stipdonk, Intrinsic Reactivity of [OUCH]+: Apparent Synthesis of [OUS]+ by Reaction with CS2, Rapid Commun. Mass Spectrom., 2022, 36(8), e9260,  DOI:10.1002/rcm.9260.
46. J. G. Terhorst, T. A. Corcovilos and M. J. van Stipdonk, Conversion of a UO ₂ ²⁺ Precursor to UH ⁺ and U ⁺ Using Tandem Mass Spectrometry to Remove Both “Yl” Oxo Ligands, J. Am. Soc. Mass Spectrom., 2023, 34(11), 2439–2442,  DOI:10.1021/jasms.3c00260.
47. J. G. Terhorst, T. A. Corcovilos, S. J. Lenze and M. J. van Stipdonk, Synthesis of Organo-Uranium(II) Species in the Gas-Phase Using Reactions between [UH]+ and Nitriles, Dalton Trans., 2024, 54(1), 231–238,  10.1039/D4DT02508C.
48. J. G. Terhorst, S. J. Lenze, L. J. Metzler, A. N. Fry, A. Ihabi, T. A. Corcovilos and M. J. van Stipdonk, Gas-Phase Synthesis of [OU–X]+ (X = Cl, Br and I) from a UO22+ Precursor Using Ion-Molecule Reactions and an [OUCH]+ Intermediate, Dalton Trans., 2024, 53, 5478–5483,  10.1039/D3DT02811A.
49. M. J. V. Stipdonk, E. H. Perez, L. J. Metzler, A. R. Bubas, T. Corcovilos and A. Somogyi, Destruction and Reconstruction of UO22+ Using Gas-Phase Reactions, Phys. Chem. Chem. Phys., 2021, 23(20), 11844–11851,  10.1039/D1CP01520F.
50. E. Perez, I. Tatosian, A. Bubas, A. Iacovino, S. Kline, L. Metzler, A. Somogyi, T. Corcovilos and M. van Stipdonk, Creation of [OUF]+ Using Gas-Phase Reactions of [UO2(C6F5)]+, Int. J. Mass Spectrom., 2021, 469, 116664,  DOI:10.1016/j.ijms.2021.116664.
51. S. J. Lenze, J. Terhorst, A. Ihabi, T. Corcovilos and M. J. van Stipdonk, Creation of Gas-Phase Organo-Uranium Species by Removal of “Yl” Oxo Ligands From UO22+ Carboxylate Precursor Ions, Rapid Commun. Mass Spectrom., 2025, 39(4), e9954,  DOI:10.1002/rcm.9954.
52. S. J. Lenze, J. G. Terhorst, M. R. Handel, J. A. Hartman, A. P. Zeiss, A. Ihabi, T. A. Corcovilos and M. J. Van Stipdonk, Experimental and, Computational Investigation of the Intrinsic Reactions of [UIVF2(CCH)]+: Formation of [UIVF2(O2CR)]+, [UIVF(O2CR)2]+, & [UIV(O2CR)3]+ by Reactions with Carboxylic Acids, Dalton Trans., 2025, 54(48), 18038–18045,  10.1039/D5DT02372F.
53. S. J. Lenze, J. G. Terhorst, A. N. Fry, A. Ihabi, M. J. Van Stipdonk and T. Corcovilos, A. Computational Comparison of Hydrolysis of [UFXC2H]+ (X=F, Cl, Br):[UFXOH]+ vs [UF(OH)(C2H)]+, in Fundamentals: Ion Structure/Energetics, Houston, TX, 2023.
54. A. A. Khairbek, M. I. Al-Zaben, A. Y. A. Alzahrani, R. Puchta and R. Thomas, Mechanistic and Electronic Structure Insights into Formate Formation in Uranium and Related Actinide Complexes, Dalton Trans., 2026, 55, 6886–6896,  10.1039/D6DT00462H.
55. W. A. Donald, G. N. Khairallah and R. A. J. O'Hair, The Effective Temperature of Ions Stored in a Linear Quadrupole Ion Trap Mass Spectrometer, J. Am. Soc. Mass Spectrom., 2013, 24(6), 811–815,  DOI:10.1007/s13361-013-0625-x.
56. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalamani, V. Barone, G. A. Petersson, H. Nakatsugi, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman and D. J. Fox, Gaussian 16, 2016 , https://gaussian.com/citation/ (accessed 2025-03-25).
57. C. Adamo and V. Barone, Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model, J. Chem. Phys., 1999, 110(13), 6158–6170,  DOI:10.1063/1.478522.
58. P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields, J. Phys. Chem., 1994, 98(45), 11623–11627,  DOI:10.1021/j100096a001.
59. Y. Zhao and D. G. Truhlar, A New Local Density Functional for Main-Group Thermochemistry, Transition Metal Bonding, Thermochemical Kinetics, and Noncovalent Interactions, J. Chem. Phys., 2006, 125(19), 194101,  DOI:10.1063/1.2370993.
60. E. Papajak, J. Zheng, X. Xu, H. R. Leverentz and D. G. Truhlar, Perspectives on Basis Sets Beautiful: Seasonal Plantings of Diffuse Basis Functions, J. Chem. Theory Comput., 2011, 7(10), 3027–3034,  DOI:10.1021/ct200106a.
61. M. Bursch, J.-M. Mewes, A. Hansen and S. Grimme, Best-Practice DFT Protocols for Basic Molecular Computational Chemistry, Angew. Chem., Int. Ed., 2022, 61(42), e202205735,  DOI:10.1002/anie.202205735.
62. T. H. Dunning, Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen, J. Chem. Phys., 1989, 90(2), 1007–1023,  DOI:10.1063/1.456153.
63. B. P. Pritchard, D. Altarawy, B. Didier, T. D. Gibson and T. L. Windus, New Basis Set Exchange: An Open, Up-to-Date Resource for the Molecular Sciences Community, J. Chem. Inf. Model., 2019, 59(11), 4814–4820,  DOI:10.1021/acs.jcim.9b00725.
64. D. Feller, The Role of Databases in Support of Computational Chemistry Calculations, J. Comput. Chem., 1996, 17(13), 1497–1586.
65. K. L. Schuchardt, B. T. Didier, T. Elsethagen, L. Sun, V. Gurumoorthi, J. Chase, J. Li and T. L. Windus, Basis Set Exchange: A Community Database for Computational Sciences, J. Chem. Inf. Model., 2007, 47(3), 1045–1052.
66. K. A. Peterson, Correlation Consistent Basis Sets for Actinides. I. The Th and U Atoms, J. Chem. Phys., 2015, 142(7), 074105,  DOI:10.1063/1.4907596.
67. C. Peng and H. B. Schlegel, Combining Synchronous Transit and Quasi-Newton Methods for Finding Transition States, Isr. J. Chem., 1993, 33(4), 449–454,  DOI:10.1002/ijch.199300051.
68. C. Peng, P. Y. Ayala, H. B. Schlegel and M. J. Frisch, Using Redundant Internal Coordinates to Optimize Equilibrium Geometries and Transition States, J. Comput. Chem., 1996, 17(1), 49–56,  DOI:10.1002/(SICI)1096-987X(19960115)17:1%253C49::AID-JCC5%253E3.0.CO;2-0.
69. H. B. Schlegel, Optimization of Equilibrium Geometries and Transition Structures, J. Comput. Chem., 1982, 3(2), 214–218,  DOI:10.1002/jcc.540030212.
70. H. P. Hratchian and H. B. Schlegel, Accurate Reaction Paths Using a Hessian Based Predictor–Corrector Integrator, J. Chem. Phys., 2004, 120(21), 9918–9924,  DOI:10.1063/1.1724823.
71. H. Z. Ma, A. J. Canty and R. A. J. O'Hair, Liberation of Carbon Monoxide from Formic Acid Mediated by Molybdenum Oxyanions, Dalton Trans., 2023, 52(43), 15734–15746,  10.1039/D3DT01983G.

---