Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO₂(H₂O)_n ⁻, n=1–10, in the C−O Stretch Region

Abstract

We investigate anionic [Co,CO₂,nH₂O]⁻ clusters as model systems for the electrochemical activation of CO₂ by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250–2234 cm⁻¹ using an FT‐ICR mass spectrometer. We show that both CO₂ and H₂O are activated in a significant fraction of the [Co,CO₂,H₂O]⁻ clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C−O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C−O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)₂ ⁻. However, calculations find Co(HCOO)(OH)⁻ as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590–1730 cm⁻¹ are most likely due to higher lying isomers of the composition Co(HOCO)(OH)⁻. Upon additional hydration, all species [Co,CO₂,nH₂O]⁻, n≥2, undergo IRMPD through loss of H₂O molecules as a relatively weakly bound messenger. The main spectral features are the C−O stretching mode of the CO ligand around 1900 cm⁻¹, the water bending mode mixed with the antisymmetric C−O stretching mode of the HCOO⁻ ligand around 1580–1730 cm⁻¹, and the symmetric C−O stretching mode of the HCOO⁻ ligand around 1300 cm⁻¹. A weak feature above 2000 cm⁻¹ is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.

Unexpected structural variety: IR spectroscopy reveals different motifs of activated carbon dioxide in gas‐phase clusters of composition CoCO₂(H₂O)_n ⁻, n≤10. The dominant isomer class features a formate ligand, but a carbonyl‐dihydroxide structure with the metal inserted into the C−O bond of carbon dioxide is also abundant (see figure).

Introduction

Carbon dioxide as the most important greenhouse gas in the Earth's atmosphere is currently intensely investigated.1 The electrochemical route of activation involves the carbon dioxide radical anion CO₂ ⁻ as a short‐lived intermediate.2, 3 It is well known that CO₂ ⁻ is metastable and undergoes autodetachment with a measured lifetime of up to milliseconds.4, 5, 6, 7 This has been repeatedly confirmed by quantum chemical calculations.3, 7, 8, 9 In interaction with a rare gas matrix10 or a solvation shell such as (CO₂)_n ^{−[6, 11, 12]} or CO₂(H₂O)_n ⁻,13, 14 the radical anion is stabilized.4 The same is true in a salt environment where the interaction with positive charge centers is responsible for the stabilization.15, 16 In the interaction of CO₂ with metal ions, electron transfer from the metal to the electrophilic carbon atom can occur spontaneously, leading to complexes of the metal center with CO₂ ⁻.4, 17, 18, 19, 20 When a single bond is formed between the metal and the carbon atom, as observed for example, with the nickel group, coinage metal, or bismuth anions, the excess charge in this metalloformate η¹‐(C) complex, MCO₂ ⁻, is delocalized over the whole molecular ion.21, 22

Organometallic complexes of transition metals like cobalt can play an important role in catalytic reductions of CO₂,23 a key step in carbon capture and usage (CCU) processes. In the gas phase, the reverse reaction, CO oxidation leading to CO₂, has been observed with anionic cobalt oxide clusters.24 Decomposition reactions of copper formate revealed important elementary steps in the transformation of CO₂ to HCOOH.25 Schwarz has recently summarized the mechanistic insight into CO₂ activation derived from gas‐phase studies, combining experiment and theory.26

Vibrational spectroscopy is a powerful method for structural analysis in the gas phase.4 Vibrational spectra of Co_n ⁺(CH₃OH)₃ (n=1–3) were measured by IR photodissociation spectroscopy.27 Anionic cobalt clusters doped with methanol, ethanol, or propanol molecules were probed by IR spectroscopy in the O−H stretch region.28 Cobalt carbonyl cations Co(CO)_n ⁺ (n=1–9) were investigated in an Ar tagging experiment by the group of Duncan, finding one strong absorption for n=1 at 2156 cm⁻¹.29 Cationic metal–CO₂ complexes M⁺(CO₂)_n (M=Mg, Al, Si, V, Fe, Co, Ni, Rh, Ir) have been extensively investigated in the past decades,30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 and also anionic species M⁻(CO₂)_n (M=Ti, Mn, Fe, Co, Ni, Cu, Ag, Au, Sn, Bi) have received considerable attention, foremost by the group of Weber.41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51 Generally, the anionic CO₂ ⁻ stretching vibrations shift to the red compared to neutral CO₂ vibrations.4, 52 CO₂ as a ligand was also investigated as metal oxides, NbO₂ ⁺(CO₂)_n and TaO₂ ⁺(CO₂)_n by Mackenzie and co‐workers.53 Photoelectron spectroscopy by the Bowen group revealed CO₂ activation upon attachment to anionic cobalt pyridine complexes54 and provided a different look on anionic coinage metal complexes with CO₂.55

The above‐mentioned IR study of Co(CO₂)_n ⁻ showed that Co forms a core with two negatively charged CO₂ molecules attached via a bidentate motif, forming a twisted butterfly arrangement. Further CO₂ molecules surround this core.43 A very interesting study on cooperative effects which are operative during metal insertion into the C=O bond of CO₂ has been performed recently by the group of Weber with Ti⁻(CO₂)_n.51 Insertion of neutral Ti into the C=O bond of CO₂ had been predicted by quantum chemical calculations.56

In an environmentally benign chemical process, water is the ideal solvent. It is, therefore, important to understand cooperative effects during the activation of CO₂ in the presence of water molecules. We have recently demonstrated C−H,57 C−C,58, 59, 60 C−S61 bond formation and protonation reactions62 with CO₂ ⁻(H₂O)_n clusters in the gas phase. Nanocalorimetry revealed important details about the thermochemistry of the carbon dioxide radical anion, in particular, its hydration enthalpy.63, 64 Raman spectroscopy of CO₂ ⁻ in bulk aqueous solution65 places the symmetric stretching mode of hydrated CO₂ ⁻ at 1298 cm⁻¹. In our recent IR study on gas phase clusters CO₂ ⁻(H₂O)_n, we observed very similar values already around n=20.14 Some hydrated metal ions M⁺(H₂O)_n, M=Mg, Cr, Co, pick up exactly one CO₂ molecule, indicating that electron transfer from the metal to carbon dioxide takes place.66, 67, 68 In the case of magnesium, the electron is already present in the hydration shell, detached from the metal center, as recently confirmed by electronic spectroscopy of Mg⁺(H₂O)_n.69

For the structural analysis of hydrated metal ions M(H₂O)_n (M=Li⁺, Na⁺, Mg⁺, Mg²⁺, Al⁺, Ca²⁺, Co⁺, Co²⁺, Cu⁺, Ag⁺, Cs⁺, Ba²⁺, Tm³⁺, La³⁺), a series of infrared photodissociation studies are available.70, 71, 72, 73, 74, 75, 76, 77 Pure cationic cobalt clusters Co_n ⁺Ar were investigated spectroscopically by argon tagging.78 Herein, we report the first IR multiphoton dissociation (IRMPD) study investigating CO₂ attached to a metal anion solvated with water. The spectra of isolated CoCO₂(H₂O)_n ⁻, n=1–10, clusters along with quantum chemical calculations provide clear evidence of CO₂ and H₂O bond rearrangements already for the CoCO₂H₂O⁻ ion.

Experimental and Theoretical Methods

The experiments were performed on a modified 4.7 T FT‐ICR Bruker/Spectrospin CMS47X mass spectrometer64, 79, 80, 81, 82 equipped with a Bruker infinity cell.83 Ions are produced in an external laser vaporization source84, 85 with a 30 Hz pulsed frequency doubled Nd:YAG laser (Litron Nano S 60‐30). A gas mixture of He, H₂O, and CO₂ is expanded through a homebuilt piezoelectric valve. The laser is focused on a rotating Co target, producing a hot plasma, which is cooled by supersonic jet expansion. These ions are guided through a system of electrostatic lenses passing three differential pumping stages to the center of the ICR cell86 where they are stored and mass selected in a 4.7 T magnetic field87 under ultra‐high vacuum (≈10⁻¹⁰ mbar) conditions. A copper shield, which is cooled by liquid nitrogen to T ≈80 K, surrounds the cell88, 89 to minimize the amount of black body infrared radiative dissociation (BIRD).90, 91, 92, 93, 94, 95, 96, 97, 98, 99

From the rear side of the magnet, a tunable IR OPO laser system (EKSPLA NT273‐XIR) is coupled into the cell through a CaF₂ window.100 When absorption events lead to photodissociation,101 they are detected by the experiment. The measurements were performed in the range of 1250–2234 cm⁻¹ where characteristic C−O stretching modes are typically observed. Details on the experimental laser setup can be found elsewhere.14, 100 The present experiments are lacking information on the number of photons required for dissociation, thus we determine the IRMPD yield, which is total photofragment intensity divided by total ion intensity, irradiation time and laser power. In contrast to the usual definition of IRMPD yield,102 we also include the irradiation time, since we adjust it to avoid saturation effects and to increase the signal‐to‐noise ratio of weak bands. As already mentioned above, fragments like CO₂ ^.− and CO₂ ^.−(H₂O) cannot be detected,103 because the excess electron undergoes autodetachment. However, no signal loss was detected in the present experiment, implying that the decomposition into fragments like CO₂ ^.− and CO₂ ^.−(H₂O) does not take place to a significant extent.

Structure and properties of CoCO₂(H₂O)_n ⁻, n=1–10, were studied using methods of theoretical chemistry at the B3LYP/def2TZVPP level of theory. Benchmark calculations with respect to CCSD(T) results for the most stable isomers of n=1 can be found in Tables S1 and S2.

The CoCO₂ ⁻ ion exhibits either a metalloformate η¹‐(C) motif or the linear OCoCO⁻ inserted structure. Starting with those, we added a water molecule and constructed several isomers with both, an intact and an activated water molecule, resulting in 14 stable structures for the CoCO₂H₂O⁻ ion. By adding successive water molecules to various positions and optimizing the structures, we created structures for clusters with up to four water molecules. For seven selected structures, further solvation with up to a total of 10 water molecules was performed. Vibrational spectra are modeled by using Gaussian broadening with a full width at half maximum (FWHM) of 20 cm⁻¹ and scaled by a factor of 0.96. Wavefunction stabilization was performed for every calculation, with internal instability issues found in more than 20 % of calculated structures. All considered structures represent local minima. Transition states are verified through intrinsic reaction coordinate (IRC) calculations. For some transition states, starting points with a small offset along the normal vector of the corresponding imaginary frequency with subsequent steepest decent optimization had to be used to make the IRC calculations work. The Gaussian 16 software was employed for all calculations.104

Results and Discussion

Bare CoCO₂ ⁻

We start our discussion with the non‐hydrated ion, CoCO₂ ⁻. In the experiment, no fragments are observed in the investigated wavelength region even after irradiating for 20 s. This is in agreement with the results of Knurr et al.43 Dissociation to Co⁻ and CO₂, the lowest energy fragmentation pathway, requires 73 kJ mol⁻¹, calculated at the B3LYP/def2TZVPP level. IRMPD is inefficient in small systems with high binding energy, since the molecule undergoes radiative cooling before dissociation.

Monohydrated CoCO₂H₂O⁻

The absorption spectrum of the monohydrated ion, CoCO₂H₂O⁻, is shown in Figure 1 a. The only detected fragment is CoOH₂O⁻ formed in reaction (1), in which m is the number of photons:

$${\displaystyle \mathrm{CoCO}{}_2\mathrm{H}{}_2\mathrm{O}{}^{-}+mh\nu {}_{\mathrm{IR}}\to \mathrm{CoOH}{}_2\mathrm{O}{}^{-}+\mathrm{CO}}$$ (1)

Figure 1.

Comparison of a) measured IRMPD spectrum for CoCO₂H₂O⁻ resulting in CO loss with b–d) the calculated absorption cross section σ _theo for isomers Ia–h. The main band in the experiment was fitted with a Gaussian to determine the peak position. Geometry optimization and frequency calculation for each isomer was performed at the B3LYP/def2TZVPP level of theory. Relative energy of isomers is given in kJ mol⁻¹ including zero‐point correction.

In the measured IRMPD spectrum of CoCO₂H₂O⁻, the absorption maximum appears at 1881 cm⁻¹. A less intense broad band was observed in the 1570–1730 cm⁻¹ region. The absorption saturates upon longer irradiation at the maximum, but only 25 % of the precursor ions dissociate. Laser misalignment can be ruled out, since other ions could be almost fully depleted with the same laser alignment. This indicates that additional isomers are present with an abundance of ≈75 %, which do not absorb at this wavelength.

Quantum chemical calculations of CoCO₂H₂O⁻ reveal a rich structural diversity. The most stable structure is isomer Ia, with Co(OH)(HCO₂)⁻ structure, in which both H₂O and CO₂ are activated, see Figure 1. Isomer Ib with cobalt inserted in the C=O bond is less stable by 41 kJ mol⁻¹. Further isomers with activated H₂O, HCo(HCO₃)⁻ (Ic), HOCo(HOCO)⁻ (Id, Ie), and HCoOH(CO₂)⁻ (If) lie even higher in energy. Two isomers with intact H₂O (Ig) and both intact H₂O and CO₂ (Ih) lie about 180 kJ mol⁻¹ above Ia.

Figure 2 shows the potential energy surface of possible CO loss reactions for the CoCO₂H₂O⁻ ion. Figure 2 a reveals a low water activation energy on CoCO₂ ⁻ of 24 kJ mol⁻¹ relative to the entrance channel, transferring a hydrogen atom metal‐mediated to CO₂ and eventually creating the most stable Co(OH)(HCO₂)⁻ structure (Ia), with If as an intermediate. Another possible pathway can be seen in Figure 2 b, in which CO₂ activation in the absence of water requires 167 kJ mol⁻¹ relative to the entrance channel. As soon as water is added, the OCCoO(H₂O)⁻ structure (Ig) is formed. From there, water activation proceeds readily over a small barrier, and the path opens to form the OCCo(OH)₂ ⁻ ion (Ib). Water activation on bare Co⁻ requires 119 kJ mol⁻¹ (Figure 2 c). CO₂ can then be further activated over a barrier of about 80 kJ mol⁻¹ forming isomer Ic. A potential energy barrier of 243 kJ mol⁻¹ needs to be overcome for isomerization to Id with Co(OH)(HCO₂)₂ ⁻ structure.

Figure 2.

Potential energy surface for CoCO₂H₂O⁻, with relative energy including zero‐point correction in kJ mol⁻¹. Calculated at the B3LYP/def2TZVPP level of theory.

The most prominent spectral feature in the experiment at 1881 cm⁻¹ can be reproduced by the C=O vibration in both Ib and Ig isomers (Figure 1). However, isomerization of Ig to Ib faces a barrier of only 8 kJ mol⁻¹ (Figure 2 b) and is thus not expected to survive in the ICR cell. In the Ib structure, a CO group is present and will readily dissociate after absorption of 3–4 photons at 1881 cm⁻¹. We thus assign the 1881 cm⁻¹ band exclusively to isomer Ib. The experiment indicates that this isomer forms about 25 % of the total ion abundance, estimated from the IRMPD yield in saturation.

The weaker absorption band observed experimentally at 1570–1730 cm⁻¹ lies in the range of the H₂O bending mode and the antisymmetric stretching mode of CO₂ ⁻.14 The presence of an intact water molecule, isomers Ig and Ih, can be ruled out. According to Figure 2 these ions are expected to dissociate by loss of water, which is not observed in the experiment. The remaining calculated isomers Ia, Ic–f all exhibit vibrational modes in this region. The presence of the most stable isomer Ia is probable, also due to its vibration at ≈1300 cm⁻¹ observed for n>1 (see below). The CO loss energy is calculated to be 126 kJ mol⁻¹ with respect to isomer Ia, but it requires a rearrangement with a barrier of 291 kJ mol⁻¹. For that reason, it is not plausible that Ia contributes to the observed photodissociation spectrum, as approximately 15 photons would be required. Similarly, isomer Ic is topologically well separated from the CO loss pathway and CO₂ loss would be the most probable channel here.

Only isomers Id and Ie can thus account for the broad weak feature. These isomers feature a HOCO ligand, with absorptions in the relevant spectral region. Both face a barrier around 40 kJ mol⁻¹ against rearrangement to isomer Ib, and the barriers lie above the CO loss channel, Figure 2 b. Isomerization to Ib will, therefore, be immediately followed by CO loss. The barrier corresponds to the absorption of 2–3 photons. Depending on the orientation of the ligand in Id and Ie and dynamic effects, the spectrum may exhibit the observed broad structure, given the high conformational flexibility of the HOCO ligand. Since relatively few photons are required for dissociation of Id and Ie, a low abundance of these isomers is sufficient to cause the observed features.

We therefore conclude that from the calculated isomers, only Ib, Id and Ie contribute to the observed spectrum. Isomer Ia is very likely present, even as the most abundant isomer, but it does not lead to an IRMPD signal under the conditions of our experiment.

Dihydrated CoCO₂(H₂O)₂ ⁻

For clusters with two water molecules, water evaporation is exclusively observed, reaction (2) with n=2. The most intense absorption band shifts to the blue, and additional bands arise at both ends of the spectrum.

$${\displaystyle \mathrm{CoCO}{}_2\left(\mathrm{H}{}_2\mathrm{O}\right){}_n{}^{-}+mh\nu {}_{\mathrm{IR}}\to \mathrm{CoCO}{}_2\left(\mathrm{H}{}_2\mathrm{O}\right){}_{n-1}{}^{-}+\mathrm{H}{}_2\mathrm{O}}$$ (2)

The features from the monohydrated species are again observed, Figure 3. The absorption maximum in the IRMPD spectrum lies at 1898 cm⁻¹, shifted by about 18 cm⁻¹ to the blue, and roughly an order of magnitude more intense compared to the n=1 spectrum. The higher intensity is due to the fact that H₂O loss requires less energy than the loss of a CO molecule, that is, only about two photons. At longer irradiation times, CoOH₂O⁻ is formed by secondary fragmentation of CoCO₂H₂O⁻. To avoid saturation effects and secondary fragmentation, this strong band around 1900 cm⁻¹ is measured with shorter irradiation time than the rest of the spectrum.

Figure 3.

Comparison of a) measured IRMPD spectrum for CoCO₂(H₂O)₂ ⁻ with H₂O loss with b–d) calculated absorption cross section σ _theo for isomers IIa–h. The main band in the experiment was fitted with a Gaussian. Geometry optimization and frequency calculation for each isomer was performed at the B3LYP/def2TZVPP level of theory. Relative energy of isomers is given in kJ mol⁻¹ including zero‐point correction.

In the region of 1500–1700 cm⁻¹, two clearly visible bands at ≈1622 and ≈1665 cm⁻¹ are observed for n=2. Further, two new absorption bands are observed, a very weak transition between 1272 and 1314 cm⁻¹ and a band around 2060 cm⁻¹. The isomer absorbing in the former region seems to be present only in very little amount in our experiment. Even after irradiation times as long as 10 s, only ≈2 % of the ions dissociate due to laser irradiation. This band might arise due to the symmetric C−O stretching mode of an HCO₂ ⁻ ligand, which lies at 1314 cm⁻¹ in HCOO⁻(Ar).105, 106 In a recent study by Weber on [Ti(CO₂)_n]⁻, in which titanium inserts into a C=O bond, a small band observed at 2056 cm⁻¹ was assigned to oxalato ligands, which can be ruled out here.51

DFT calculations predict very similar structures compared to the case of one water molecule. The most stable isomer IIa has a (H₂O)(OH)Co(HCO₂)⁻ structure, that is, CO₂ and one H₂O are activated. Isomer IIb with an inserted metal in the C=O bond and an activated H₂O is less stable by only 31 kJ mol⁻¹. Further isomers lie at least ≈70 kJ mol⁻¹ higher in energy.

As seen in Figures 3, 4, and Figure S1, Supporting Information, calculated IR spectra do not change much when passing from one to two water molecules. The most intense band in the experiment at 1898 cm⁻¹ results from the C=O vibration in isomer IIb. The absorption at ≈1580–1700 cm⁻¹ is due to a mixture of the bending mode of the intact H₂O molecule and the antisymmetric C−O stretching mode in the HCO₂ ligand, with contributions from various isomers, for example, the C−O stretch in IIc‐e as well as the water bend in IIa or IIb. In isomers IIc and IIf, the frequencies corresponding to the Co−H vibration lie between 1650 and 1800 cm⁻¹.

Figure 4.

IR spectra of CoCO₂(H₂O)_n ⁻, 1≤n≤10, isomers a‐‐c (see Figures 1, Figure 2, and Figure S4, Supporting Information, for the respective structures) calculated at the B3LYP/def2TZVPP level of theory. Spectra of less stable isomers are shown in Figure S1, Supporting Information.

The small absorption at low energies can be assigned to either isomer IIa or IIc. Depending on the angle of the (HCO₂) complex in IIa, the absorption might shift even more to higher energies as seen in Figure S2, Supporting Information. The presence of an exotic Co(OH)(H₂O)⋅⋅⋅HCO₂ ⁻ complex with a relative energy of 24 kJ mol⁻¹ could also account for the observed band, see Figure S2, Supporting Information. However, formation of such an isomer does not correspond to the observed water loss within the IRMPD process.

With respect to the experimentally measured band at 2060 cm⁻¹, no calculated isomer features harmonic vibrational modes near this wavenumber. Our excited states calculations at the equation of motion‐coupled cluster singles and doubles (EOM‐CCSD) level show that there are also no low‐lying electronically excited states in the IR region. Such states are calculated in the OCoCO⁻ ion but disappear upon water activation. Most likely, the band origins from overtones and combination bands of lower‐lying transitions.

Larger hydrated species

Clusters with n>2 also evaporate a single water molecule upon resonant IR irradiation, reaction (2). Saturation effects become more evident with increasing cluster size, and the effect on the band shape of the absorption at 1900 cm⁻¹ is shown for n=3 with two different irradiation times t _IR in Figure S3, Supporting Information.

The spectra for n=1–10 are shown in Figure 5, with the spectra for n=1, 2 included for comparison. Generally, one can see a blueshift of the bands at ≈1300 and ≈1900 cm⁻¹, whereas the other two bands do not exhibit a systematic shift. These shifts are compared in Figure 6, in which the evolution of absorption maxima with respect to the cluster size is shown. For the band at ≈1300 cm⁻¹, experimental data shows an average shift of ≈4 cm⁻¹ per water molecule. This shift is reproduced in the calculations by the vibrations of isomers Ia‐Xa with an average shift of ≈3 cm⁻¹ per water molecule (Figure 6 a). The corresponding structures are shown in Figure S4, Supporting Information.

Figure 5.

Infrared multiple photon dissociation spectra of CoCO₂(H₂O)_n ⁻ for 1≤n≤10 in the 1250–2234 cm⁻¹ region. An irradiation time of 20 s is used for n=1. For n>1 ions are generally irradiated for 3 s, with exception of the main peak, which is measured with 1 s for n=2 and 0.5 s for n>2 to avoid saturation.

Figure 6.

The peak positions of the Gauss fits of each absorption band compared with the theoretical value for cluster sizes n=1–10 in the following regions: a) 1303–1337 cm⁻¹, b) 1881–1938 cm⁻¹. Starting from n=4, two Gaussians are required for the fit in b), see Figure 5. See Figure S5, Supporting Information, for further details of the evolution of the region 1881–1938 cm⁻¹.

The most intense absorption is found for all cluster sizes between 1860 and 1960 cm⁻¹ and shifts to the blue with increasing n. As mentioned above, this band corresponds to the C=O vibration in isomer b, and its shift can be well reproduced by our calculations, see Figure 6 b. For n≤6, a nearly linear blue shift is observed. As seen in Figure S5, Supporting Information, a shoulder arises on the low‐energy side for n=4, which becomes more and more dominant for higher n, and two data points are included for these cluster sizes in Figure 6 b. This band is also seen in CO adsorption experiments on a Co surface.107 It is also seen as a very weak feature for inserted isomers in Co(CO₂)_n ⁻ by the group of Weber.43 We interpret it here as the emergence of a new isomer, most likely involving a hydrated CO group.

The wavenumber region of 1550–1750 cm⁻¹ is composed mainly of water vibrations, with minor contributions from the antisymmetric stretching mode of formate. No clear trends can be identified due to several isomers contributing to the spectral envelope. Theoretical calculations do not show any clear trend with respect to the cluster size for any isomer, Figure 4.

The last feature at ≈2060 cm⁻¹ exhibits a pronounced band only for 2≤n≤5. It does not shift to the blue with increasing n. However, the band broadens with increasing n so that the band is smeared out for n≥6, resulting in a raised baseline as seen in Figure 5. For larger clusters, the band might be explained by a combination of H₂O bending ν₂, H₂O libration ν_L2, and bending of H₂O triplets ν_T2,108 as seen before in the spectra of CO₂ ⁻(H₂O)_n.14

Conclusions

We measured IR multiple photon dissociation spectra of the CoCO₂(H₂O)_n ⁻ systems. As reported before,43 the non‐hydrated species CoCO₂ ⁻ does not show an IRMPD signal in the wavelength region investigated. Already for n=1, the most prominent absorption is characteristic of a metal‐coordinated CO group, which shows that the Co atom has inserted into the C=O bond of CO₂. However, the spectra also show that multiple isomers are present, and those without a metal coordinated CO seem to prevail. Two isomers featuring a HOCO ligand are most likely responsible for the weak, broad transition around 1570–1730 cm⁻¹, since they have absorptions in that region and simple rearrangements allow for the release of CO.

For n≥2, all primary IRMPD signals are due to loss of one H₂O molecule. The probably most abundant isomer class that features a formate ligand is directly evidenced by a band the position of which shifts from 1303 to 1337 cm⁻¹ upon hydration with up to 10 H₂O molecules. The region, which could be indicative of a HOCO ligand, however, is now smeared out by overlapping absorptions due to the water bending and antisymmetric HOCO⁻ or HCOO⁻ stretching modes. The most intense absorption of the C−O stretching mode in the metal inserted isomer shifts to the blue with increasing n, from 1881 to 1938 cm⁻¹. A weak feature at roughly 2060 cm⁻¹, which is assigned to a combination band of low‐lying water modes, smears out with increasing solvation, leading to an elevated baseline for large clusters in this region.

We rationalize the presence of different isomers by the pronounced non‐equilibrium conditions in the ion source. Due to the specific nature of the potential energy surface of the and thus persist under the experimental conditions.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

E. Barwa, M. Ončák, T. F. Pascher, A. Herburger, C. van der Linde, M. K. Beyer, Chem. Eur. J. 2020, 26, 1074.