Infrared Spectroscopy of Size‐Selected Hydrated Carbon Dioxide Radical Anions CO2 .−(H2O)n (n=2–61) in the C−O Stretch Region

Abstract Understanding the intrinsic properties of the hydrated carbon dioxide radical anions CO2 .−(H2O)n is relevant for electrochemical carbon dioxide functionalization. CO2 .−(H2O)n (n=2–61) is investigated by using infrared action spectroscopy in the 1150–2220 cm−1 region in an ICR (ion cyclotron resonance) cell cooled to T=80 K. The spectra show an absorption band around 1280 cm−1, which is assigned to the symmetric C−O stretching vibration ν s. It blueshifts with increasing cluster size, reaching the bulk value, within the experimental linewidth, for n=20. The antisymmetric C−O vibration ν as is strongly coupled with the water bending mode ν 2, causing a broad feature at approximately 1650 cm−1. For larger clusters, an additional broad and weak band appears above 1900 cm−1 similar to bulk water, which is assigned to a combination band of water bending and libration modes. Quantum chemical calculations provide insight into the interaction of CO2 .− with the hydrogen‐bonding network.


Introduction
The carbon dioxide radicala nion CO 2 C À is ak ey intermediate in the electrochemical as well as catalytic activation of carbon dioxide, whichi sr elevant for using CO 2 as aC 1 buildingb lock. [1] The activation of CO 2 by reductivee lectron transfer is well investigated, as reviewedr ecently by Weber. [2,3] With its charge, CO 2 C À can be readily studied by mass spectrometry or in ion beams. Bare CO 2 C À is am etastable species [4][5][6] with al ifetimeo f tens of mst om s. [3] Upon solvation with molecules such as neutral CO 2 , [7,8] H 2 O, [9] or both, [10] however,i tt urns into as table species. The carbon dioxide radicala nion is highly reactive and forms CÀCb onds in the reaction with allyl alcohol, [11] methyl acrylate, [12] and 3-butyn-1-ol. [13] Evidence for HOCOC formation was found in the reactions with 3-butyn-1-ol as well as with HNO 3. [14] The groups of Weber and Duncan have established a significant amount of data based on the infrared photodissociation spectroscopy of carbond ioxide solvated metal centers M + /À (CO 2 ) n (M = Au, Ag, Co, Ni, Cu, Mg, Fe, Si, V, Al, Bi, TiO), [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] where CO 2 is activatedb yc harget ransfer to form the carbon dioxide radicala nion CO 2 C À or the oxalate dianion C 2 O 4 2À .M ackenzie et al. studied the fundamental binding motifs and activation of CO 2 in cationicm etal-CO 2 complexes M + (CO 2 ) n (M = Co, Rh, Ir), metal oxide clusters MO 2 + (CO 2 ) n (M = Nb, Ta ), and in platinum clusters Pt n À (n = 4-7) by infrared spectroscopy. [31][32][33] Bowena nd co-workerss howed, by using photoelectron spectroscopy,t hat anionic complexes of coinage metals and CO 2 (MCO 2 ) À are present as chemisorbed( M= Ag, Au) or physisorbed isomers(M= Cu, Au). Sanov and co-workers reported evidencef or charget ransfer to solventt ransitions in photoelectron imaging studies of (CO 2 ) n (H 2 O) m À after excitation at 400 nm. The same group reported photodissociation of CO 2 C À in water clusters. [34,35] Compared with neutral CO 2 ,t he symmetric n s and antisymmetric n as CÀOs tretching vibrations of CO 2 C À are considerably redshifted. The weakening of the CÀOb onds, owing to the additional electron in an antibonding molecular orbital, leads to ar edshift of both stretching modes. [2] The excess electron at the carbon atom bends the molecule to am ean angle of 1358, decreasing the difference between n s and n as . [3] The two bands of CO 2 C À in solidneon and argon were observed with vibrational spectroscopy by the groups of Jacox and Andrews, respectively. [36,37] The Nagata group publishedv ibrationals pectroscopy data for small hydrated CO 2 C À (H 2 O) n clusters in the OÀH stretch region above 2800 cm À1 . [10,38] They found that CO 2 C À (H 2 O) forms ar ing structure with two equivalent hydro-gen bonds,a nd two H 2 Om olecules are independently bound to the oxygen atoms of the CO 2 C À in CO 2 C À (H 2 O) 2 .F or [(CO 2 ) n (H 2 O)]C À ,a dditional IR absorption bands appear at n = 4, which are assigned to the bending overtone and the hydrogen-bonded OÀHv ibration of H 2 Ob ound to CO 2 C À througha single OÀH···O linkage. [38] Liu et al. performed ad etailed theoretical study on localization and time evolution of the excess electroni nh eterogeneous CO 2 -H 2 Osystems. [39] Their calculations show that hydrogen bonds are not only formed with the oxygen atoms of CO 2 ,b ut also with the carbon atom. Furthermore, they suggest that CO 2 C À is localized inside the cluster with four to seven H 2 Om olecules coordinated to the carbon dioxide radicala nion.T he reactivity of this species is heavily influenced by the surrounding hydrogen-bond network, which calls for ad etailed vibrational spectroscopic analysiso fC O 2 C À (H 2 O) n clusters.
The details of the interaction between ions and water are ideally investigated through microhydration studies, adding water molecules one at at ime. Asmis and co-workerss tudied how different anions [40,41] and dianions [42,43] behave upon stepwise hydration by using infrared spectroscopy combined with quantum chemical calculations. Speciali nterest was devoted to the number of water molecules needed to saturate the first solvations hell. In the HCO 3 À (H 2 O) n system, am aximum of five water molecules interactd irectly with the bicarbonate anion, [41] whereas for the SO 4 2À (H 2 O) n dianion,t welve water molecules are needed to close the first solvations hell. [42] These results show that vibrational spectroscopy is ap owerful tool to elucidate the structure of the water network surrounding the ionic core. Williams and co-workers investigatedh ow different core ions influence the hydrogen-bond network in large water clusters up to n % 550, far beyond the first solvation shell. [44][45][46][47][48][49][50][51] They found that the free OÀHb and, resultingf rom the vibration of water moleculesw ith at least one hydrogen atom not involved in hydrogen bonding, redshifts and increasesi ni ntensity with increasing positive charge. [44] Their work demonstrates long-range solvation effects upon hydration of different ions. In ar ecent temperature-controlled experiment with SO 4 2À (H 2 O) n ,t hese ion-water interactions were seen at temperatures that are relevant to Earth's atmosphere. [48] Thermochemical properties [52][53][54][55] as well as infrared spectroscopic signatures [56][57][58] have been frequently investigated as af unctiono f cluster size to determine the transition to bulk-like behavioro f hydratedi ons. Along these lines, we investigate size-selected CO 2 C À (H 2 O) n clusters, n 61, by using infrared photodissociation spectroscopy in the range 1150-2220 cm À1 at T = 80 K. Figure 1s hows the measured IR spectra of CO 2 C À (H 2 O) n , n = 2-61, at T = 80 Kb yu sing infrared action spectroscopy,a ssuming single photon absorption for the calculation of relative cross sections. This is correct only for those clusters with ar elatively high internal energy,w hich decay upon absorption of as ingle photon.T he majority of clusters may require two or three IR photonsf or dissociation,e specially at the low-energye nd of the spectrum. However,t oa ccountf or the frequency-depen-dent laser power and the variable irradiation time, the onephoton cross sectioni ss till useful. To derive absolute IR absorptionc ross sections, master equation modeling is required, which goes beyondt he scopeofthe current work.

Results and Discussion
For am ore quantitative analysis,a symmetric peaks are fitted with Gaussian distributions.T he most prominent contributing Gaussiand istributions are given in Figure 2, with peak posi- Figure 1. Infrareds pectra of CO 2 C À (H 2 O) n for n = 2-61, alongwith the noise level showning ray.The two absorptionlines are the symmetric CÀO stretching vibration startinga t1 243 cm À1 for n = 2a nd the regiona round 1650 cm À1 where the antisymmetric CÀOs tretchinga nd water bending modes overlap.T he asterisk in the n = 2p anel indicatesabandt hatiso nly slightly above the noise level and not expected for the small cluster size. It could be caused by signal fluctuations between two reference measurements (seethe Experimental Section).T he two arrows in the n = 2p anel show the calculated positionsfor n s (1193 cm À1 )and n as (1720 cm À1 )o fg as-phaseC O 2 C À .T he dashed line showsthe IR spectrum of liquid water,scaled to the maximumi ntensity of the n = 61 spectrum. [60] Chem. For n = 2, the cluster decayst hrough electron detachment, [59] and its spectrum was derived from the signal depletion upon photon absorption;t ogether with the low intensity of CO 2 C À (H 2 O) 2 ,t his leads to as ignificantly lower signal/noiser atio than for clusters n > 2, which decay by loss of water molecules.
The low-energy part of the spectrum at 1150-1350 cm À1 shows the symmetricC O 2 C À stretching mode n s .T he band position is sensitive to the hydrogen-bond network surrounding the carbon dioxide radicala nion, featuring as trong dependence on cluster size, see Figure 2a and Figure 61 cluster, n s is centered at 1296 cm À1 ,o nly 2cm À1 away from the value of CO 2 C À in liquid water,w hich wasr ecently determined as 1298 cm À1 by pulse radiolysist ime-resolved resonance Raman spectroscopy. [61] However, already for n = 20, n s lies at 1293 cm À1 ,w ithin 5cm À1 of the bulk value. We can conclude that the bulk hydration environmento fC O 2 C À is largely developed for CO 2 C À (H 2 O) 20 .A lthought he ion is still located at the clusters urface,t he most important interactions with water molecules are present, which are responsible for the frequency shift.
For the particularly stable, magic cluster sizes n = 49 and n = 55, as mall redshift of approximately 4cm À1 is observed, indicating as light destabilization of the HOMO in exchange for an increased rigidity of the hydrogen-bond network. These are, however,small effects, well within the line width.For n = 2, evidence for as econd peak at 1310 cm À1 is found, albeit only slightly above noise level. For clusters n = 5-10, the asymmetry of the peaks indicates contributions of different isomerso r from combination bands.B oth features are discussed below with the aid of quantum chemical calculations. The irregularities in the 1200-1350 cm À1 region for some clusters, in particular n = 30, 49, and 61, could not be assigned to specific isomers or combination bands and may be noise or aw eak water absorption. Accidental spatial alignment of oscillators at aspecific cluster size may afford intense combinationb ands. However, at the low-frequency end of the spectralr ange of the laser system,t he powerd rops, and the noise level goes up substantially.T herefore, and owing to the lower intensity of the symmetric CÀOs tretching mode, irradiation times of 1-2 sw ere chosen below 1350 cm À1 ,c ompared with 0.5 sf or the major part of the spectrum.
The antisymmetric stretching mode of CO 2 C À is observed around1 650 cm À1 ,s trongly coupledt ot he water bending mode n 2 ,d enoted n as /n 2 .T he band is strongly asymmetric, with the higher energy componentsb eing more blueshifted with increasing clusters ize. The overall band position, however,i s only mildly affected by cluster size, with as mall shift of 10 cm À1 for the measured cluster size range. The n as /n 2 absorption band was fitted with Gaussianf unctions (see FigureSI-2 in the SupportingI nformation);i nF igure 2b,w es how the position and full width at half maximum (FWHM) of the two most prominentc ontributions to the fits. The individual Gaussian functions, however,c annotb ea ssigned to specific vibrational modes owing to the strong coupling of n as and n 2 .Inp articular, the normalm ode analysis of our quantum chemical calculations (see below) does not yield as ingle mode where only the CO 2 C À atoms are in motion. Instead, severaln ormal modes of the cluster with frequenciesa round1 650cm À1 have both CO 2 C À and several H 2 Om olecules oscillating. The strength of this band scales approximately linearly with clusters ize, as expectedw ith the increasing number of water bending modes ( Figure SI-3

in the Supporting Information).
For larger clusters, an additional broad,w eak band appears at approximately 2100 cm À1 ,w hichi sa ssigned to ac ombination band of H 2 Ob ending n 2 (1638 cm À1 ), H 2 Ol ibration n L2 (395 cm À1 ), and bending of H 2 Ot riplets n T2 (50 cm À1 )k nown from bulk liquid water. [62][63][64] Its intensity overall increasesw ith cluster size, with ap ronounced exception at n = 49, ac luster size with increased stability,s ee Figure SI-4 (in the Supporting Information). In this cluster size region, as trong even-odd os- as af unctiono fc lustersize. Relative to n = 61, the area reaches 75 %a lready at n = 30, andd rops to 31 %a tn = 49, lower than the n = 20 value of 42 %. This largely parallels the behavioro f the water bending/CÀOs tretching peak in Figure SI-3 (in the Supporting Information), which indicates that, owing to itsi ncreaseds tability,t he n = 49 cluster requires more photons for dissociation than typical. In Figure1,w ea lso compare the spectrumo ft he largest cluster (n = 61) with the IR spectrum of liquid water measured at room temperature. [60] Both the intense peak at approximately 1650cm À1 and the broad peak at around2 100 cm À1 converge to the bulk spectrum, whereas the width of the n as /n 2 band is still smaller than in the bulk.
To get further insight into the hydration of CO 2 C À ,w ec alculated variousC O 2 C À (H 2 O) n structures, n = 0-20, at the B3LYP/6-311 ++G** level of theory.I nt he CO 2 C À ion, the calculations predict two features at 1193 and 1720 cm À1 ,a ssigned to the symmetric n s and antisymmetric n as stretching vibrations, respectively.T he third CO 2 C À vibration at 661 cm À1 corresponds to bending of the ion. Note that for gas-phase CO 2 C À ,a na dditional structure with amore loosely bound electron was found, reflecting its metastability. [65] This structure is, however, not relevant here, as CO 2 C À is readily stabilized by water molecules (see above).
For the hydrated species, three hydration motifs were considered:( i) structures with extensively hydrated CO 2 C À created so as to maximize the number of CO 2 C À ···H 2 Oi nteractions, preferably including the C···H interaction (further denoted as "solvated"); (ii)structures with CO 2 C À added on the surfaceo fa compactw ater cluster ("surface");( iii)clusters in which CO 2 C À is built into the water cluster structure ("incorporated"), that is, with CO 2 C À assuming atapositiont hat would be otherwise reserved for water molecules in an eutralw ater cluster,w ithout necessarily the C···H interaction present.T he latter group can be expected to representu sually the lowest-energy structure at 0K.S elected cluster sizes are shown in Figure 3, and all results for n = 0-20 are available in the Supporting Information ( Figure SI-7).
Within experimental accuracy,t he calculated position of the n s vibration matches quantitatively with the measured one in the whole n = 2-20 range (Figure 2c). The "solvated" structures with more pronounced C···H 2 Oi nteractions eem to follow the experimentalv alues more closely comparedw ith the "incorporated" structures with CO 2 C À positioned on the cluster surface (the difference is, however,w ithin error limits). Within the precision of the computational method, mainly with respectt o the accuracy of the DFT approach, it cannot be judged which cluster motif is preferred at af inite temperature. The n s frequencyf or "surface" structures with CO 2 C À attachedt oa na lready formed water cluster,o nt he other hand, lies outside the error limits of the experimental values. This is more pronouncedw hen the CO 2 C À is incorporated into the water cluster only by one oxygen atom (see clusters with n = 5, 7i nF igures 2c and 3). Therefore, we can conclude that the IR spectra document as trong interaction between CO 2 C À and water.N ote that CO 2 C À is not fully solvated even for the largest cluster con-sidered computationally (n = 20) owing to the insufficient size of the cluster.H owever,C O 2 C À in the "solvated" isomer already interacts with ac onsiderable number of water molecules.
The n = 2f eature at 1310 cm À1 ,i fr eal, can be assigned to a combination band of the CO 2 C À bending mode at 712 cm À1 with the water libration at 650 cm À1 for the "solvated" structure type. For n = 5, 7-10, the minority contribution to the symmetric stretchingp eak is assigned to ad ifferent isomer of the solvated type, and candidatesf or these isomersa re present in the calculations. In line with this argument, the contribution is most pronouncedf or n = 9, where the calculated energy difference between the two lowest-lying isomers is only 1.1 kJ mol À1 .F or n = 6, the situation is different, the peak exhibits as houlder on its blue end, and we could not find candidate isomerst oe xplain this feature. As the lowest-lying structure is ar elatively strained cube, we suggestt hat this particular geometry facilitates coupling of modes, for example, again the CO 2 C À bending mode at around 700-730cm À1 with the water librations below 700 cm À1 . To investigate the effect of the C···H 2 Oi nteraction on the vibrational frequency,w ef urther optimized 27 differents tructures for the CO 2 C À (H 2 O) 10 cluster( sampled from am olecular dynamics run at 300 Ks tarting from the lowest-energy structure in Figure 3). The C···H radial distribution function shows a low-intensity peak at about 2.1 ,i ndicating ar ather weak interaction, compared with 1.9 for the O···H radial distribution function( Figure SI

Conclusion
We have measured the IR spectra of CO 2 C À (H 2 O) n , n = 2-61, clusters trapped at at emperature of 80 K. We have shown that the symmetric stretch of the CO 2 C À anion approaches the bulk value already for n = 20. Analysis of the antisymmetric stretch is hindered by its coupling with the water bending vibration. Its shift, however,s eems to be limited within the measured range. The IR spectrum of CO 2 C À (H 2 O) 61 approaches the spectrum of CO 2 C À in bulk aqueous solution. Quantum chemical calculationsr eproduce quantitatively the position of the symmetric CO 2 C À vibration and suggest that the presence of the C···H interaction hasarather limited effect on the IR spectrum in the studied region.

Experimental and Computational Methods
The experiments were performed with am odified Bruker/Spectrospin CMS47X FT-ICR (ion cyclotron resonance) mass spectrometer described in detail elsewhere, [55,66,67] see also the Supporting Information for further details. Hydrated carbon dioxide radical anions CO 2 C À (H 2 O) n were generated in al aser vaporization source. [68][69][70] A gas mixture of helium with traces of CO 2 and water vapor was pulsed into the source region through ap iezoelectric valve and expanded into the UHV region (p(UHV) < 3 10 À10 mbar) of the instrument. The cell was cooled by liquid nitrogen to T % 80 Kt om inimize blackbody infrared radiative dissociation (BIRD). [71,72] The beam of an EKSPLA NT273-XIR optical parametric oscillator laser system was coupled to the cell covering the 4500-12000 nm region (833-2222 cm À1 ,l inewidth < 1cm À1 ,1 000 Hz repetition rate, pulse duration < 10 ns). The wavelength was calibrated by a HighFinesse Laser Spectrum Analyzer IR-III. Spectra are recorded by action spectroscopy,reaction (1).
The number of photons that are neededt oe vaporate a water molecule ranges from one to three, as discussed in the Supporting Information (Figure SI-10). [73,74] For the presentation of arealistic spectrum that accounts for laser energy and irradiation time, the single photon cross-section s is calculated by a modifiedL ambert-Beer's law: where I 0 is the intensity of the precursor, I l is the intensity of the fragments, l is the wavelength, P is the laser power, t is the irradiation time, h is the Planck constant, A is the area of the laser beam, and k is an empirical factor,w hich corrects for the contribution of BIRD andc ell warminge ffects caused by the laser.
For optimizationa nd harmonic frequency calculations,t he B3LYP/6-311 ++G** method was used. The scaling factor of 0.977 was chosen as to match the experimentalf requency of the symmetric CO 2 C À stretchf or n = 3. Am olecular dynamics run was performed for CO 2 C À (H 2 O) 10 at ac onstant temperature of 300 Ko nt he revPBE potential energy surface, employing the NosØ-Hoover thermostat and the time step of 0.5 fs, with the total running time of 15 ps. In total, 30 geometriesw ere picked in the constant interval of 0.5 ps and optimized into 27 different structures. Room temperature,i .e. 300 Kw as chosen in the MD run to sample the potential energy surface efficiently.A ll geometry optimization were performed with the Gaussian suite of programs. [75] The MD simulation was performed with the Quickstep module of the CP2K suite of programs. [76] A triple-zeta Gaussianb asis set augmented with diffuse functions plus the Goedecker-Teter-Hutter pseudopotential (with charge density cutoff of 280 Ry) for an auxiliary planewave basis set (TZV2P-MOLOPT-GTH) were used. Dispersion interactions were corrected with the GrimmeD 3m ethod (with Becke-Johnson damping). The clusterion was placed at the center of as imulation box with the parameters of 16 16 16 3 ,c orrected with the Martyna and Tuckerman Poisson solver.