Article pubs.acs.org/JPCA
Structures of [CoO(CO2)n]− and [NiO(CO2)n]− Clusters Studied by Infrared Spectroscopy Benjamin J. Knurr and J. Mathias Weber* JILA and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: We present infrared spectra of [CoO(CO2)n]− and [NiO(CO2)n]− clusters and interpret them in the framework of computational results employing density functional theory. We find that both [CoO(CO2)n]− and [NiO(CO2)n]− clusters are generally composed of the same core isomers. The dominant isomers consist of an η2 CO2 ligand and a CO3 moiety that can be bound to the metal atom with monodentate (η1) or bidentate (η2) connectivity. Minor structural isomers observed are composed of a C2O4 moiety with a lone oxygen atom or a CO3 unit.
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INTRODUCTION
METHODS Experimental Section. The experimental setup has been described in detail elsewhere.33 Briefly, Co or Ni vapor was created by laser vaporization of a rotating metal (Co or Ni) disc with the third harmonic of a pulsed Nd:YAG laser (355 nm). The metal vapor is then entrained in a supersonic expansion of CO2 generated by an Even−Lavie pulsed valve (stagnation pressure 5.5 bar), resulting in the formation of a number of different cluster species. The anions generated in the ion source are accelerated perpendicularly to the expansion in a Wiley− McLaren time-of-flight mass spectrometer, where they are separated by their mass to charge ratio. A single cluster species is mass selected using a mass gate and subsequently irradiated with the tunable output of an optical parametric converter operating in the range from 600 to 4400 cm−1 (7 ns pulse duration, 2 cm−1 bandwidth). Absorption of a photon with sufficient energy results in the loss of one or more CO2 units, and the resulting fragment anions are detected using a reflectron secondary mass analyzer. By monitoring the generation of the fragment ions as a function of photon wavelength we acquire an IR spectrum of the parent cluster. Multiple spectra are taken over at least two different days to ensure reproducibility and improve the signal-to-noise ratio. The experiment operates at a repetition rate of 20 Hz. Computational. Density functional theory calculations were performed on [CoO(CO 2 ) n] − and [NiO(CO 2 ) n ] − clusters using the TURBOMOLE V. 6.2 suite of programs. All calculations were performed employing the B3-LYP
Metal−oxides are commonly found species in transition-metal chemistry. For example, many reactions are catalyzed on metal−oxide surfaces with different degrees of reactivity being observed at oxide sites and oxide vacancies.1,2 Understanding the binding motifs and molecular interactions at these active sites can aid rational catalyst design and improve our understanding of surface chemistry. Studies of the structure and reactivity of transition metal oxide species in vacuo can provide detailed structural information without the problems associated with the intrinsic heterogeneity of a condensedphase environment. A number of groups have investigated the structures and reactivities of various ionic metal−oxide species in the gas phase. Experiments range from mass spectrometry and reactivity studies3−16 to photoelectron spectroscopy9,17,18 and vibrational spectroscopy.16,19−27 We previously studied [Ni(CO2)n]− and [Co(CO2)n]− complexes and noticed similar structural behavior for the CO2 complexes based on these two metals.28,29 In both cases, [M(CO2)2]− core ions were observed as the dominant core structure where the CO2 units were both bound in a bidentate fashion (η2 complexes) to the metal atom. On the basis of these previous results, we would expect similar core structures to exist in both [CoO(CO2)n]− and [NiO(CO2)n]− clusters. Alternatively, the connectivity of CO2 units could be different, e.g., in η1 complexes with only the carbon atom forming a bond to a neighboring atom, similar to motifs found in coinage metal− CO2 interaction.30−32 In this work, we present infrared spectra of [CoO(CO2)n]− clusters (n = 3−8) and [NiO(CO2)n]− clusters (n = 2−6). We employ density functional theory to aid in the interpretation of the spectra, and we compare the results of the two cluster series. © 2015 American Chemical Society
Received: October 29, 2014 Revised: January 15, 2015 Published: January 15, 2015 843
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The Journal of Physical Chemistry A functional34,35 with dispersion correction.36 For all atoms, def2TZVPP basis sets were used.37 Harmonic frequencies were calculated using the AOFORCE program.38,39 The calculated antisymmetric stretching frequencies of CO2 units forming η1 complexes with the metal or CO3 subunits that are bound with only one O atom to the metal (see discussion of structures below) were scaled by 0.9398 to account for anharmonicity. This factor comes from a comparison of the antisymmetric stretching mode in η1 AuCO2− complexes, found by Boese et al. at a high level of theory, to calculations at the above level of theory on the same species.40 This same factor has been used before as a scaling factor for similar species.30,31 Note that the normal modes of the CO2 subunit in a CO3 moiety bound by a single O atom to the metal follow roughly the same pattern as in an η1 complex, which justifies using this scaling factor. All other harmonic frequencies were scaled by 0.9754. This factor is based on the comparison of the experimentally measured41 and our calculated vibrational frequency of the CO 2 antisymmetric stretching mode in free neutral CO2. Analysis of the charge distribution in the clusters under study was carried out using a natural population analysis.42
Figure 1. Experimental spectra of [CoO(CO2)n]− (n = 3−8). Numbers denote the number of CO2 molecules present in the cluster. The exploratory experimental spectrum for [CoO(CO2)8]− was only acquired between 1500 and 2180 cm−1 after determining that the spectrum was not changing appreciably as a function of cluster size. Left and right traces are individually normalized and not on the same scale.
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RESULTS AND DISCUSSION The spectra of both [CoO(CO2)n]− and [NiO(CO2)n]− show two distinct regions. Peaks in the higher energy region (from 2250 to 2400 cm−1) are due to solvent CO2 molecules. These molecules retain the structural nature of a free CO2, and their antisymmetric CO stretch deviates little from that of free CO2. The second region (between 1000 and 2150 cm−1) is attributed to CO2 molecules that are involved in the charge carrying species and are referred to as “core” CO2 units. These CO2 units are geometrically distorted from free CO2 molecules and exhibit very different CO stretching frequencies than the solvent CO2 molecules, encoding the structure and charge distribution of the core ion. In the following, we first discuss Co- and Ni-based spectra separately and then compare the two species. Dominant peaks and their calculated values for selected cluster sizes are summarized in the Supporting Information. [CoO(CO2)n]−. The [CoO(CO2)n]− spectra show a single narrow peak in the solvent region and four dominant features at 1655, 1705, 1815, and 1840 cm−1 in the core ion region, in addition to some weaker peaks (see Figure 1). The smallest cluster for which photodissociation signal is observed is [CoO(CO2)3]−. On the basis of previous studies,28,29 the binding energy of a core CO2 unit is on the order of 104 cm−1, well outside the photon energy range of this experiment. In contrast, the binding energy of a solvent CO2 molecule is predicted to be roughly 1600 cm−1, indicating that a solvent CO2 molecule must be present in order for photodissociation to be observed. Since no fragmentation is observed in [CoO(CO2)2]−, it is likely that the structure of the [CoO(CO2)3]− cluster is [CoO(CO2)2]−·CO2, with two core CO2 units and one solvent CO2 molecule. Performing DFT calculations on the [CoO(CO2)2]− core yields a number of possible core structures (see Figure 2). The predicted global minimum is composed of a CO3 moiety and a CO2 moiety, both bound in a bidentate fashion to the Co atom (see Figure 2 isomer family A). This structure is reminiscent of the η2 structure of a CO2 binding to a metal atom with a CO3 ligand also binding as an η2 ligand on the opposite side of the metal atom. Both singlet and triplet states are possible in cobalt oxide−CO2 anion complexes. For isomer family A, they give rise to distinctly different structural conformations. In a singlet
Figure 2. Lowest energy structures for [CoO(CO2)2]−. Capital letters denote the core structure, and superscripts 1 and 3 denote singlet and triplet structures, respectively. Relative energies are given in meV.
configuration the CO2 and CO3 moieties of isomer family A are nearly perpendicular to each other, while in a triplet configuration the planes of the two moieties are twisted against each other by about 40°. Generally, the triplet structures are predicted to be lower in energy than their singlet counterparts for all reported structures. The next higher lying core structure is isomer B where the CO2 moiety is bound by only the carbon atom to the Co atom 844
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The Journal of Physical Chemistry A as an η1 ligand (see Figure 2). In isomer B, the CO2 and CO3 moieties are nearly coplanar in both triplet and singlet configurations. The next higher energy structure has the CO2 moiety bound to the Co atom by both oxygen atoms, and both CO3 and CO2 moieties lie in the same plane (see Figure 2, isomer C). Isomer D contains an η2 CO2 ligand and an η1 CO3 unit, the latter forming only a single Co−O bond. Finally, isomer E features a lone oxygen atom and a C2O4 moiety around the cobalt atom. Figure 3 shows a comparison of the experimental spectrum of [CoO(CO2)3]− to the calculated spectra of isomers
dominant core structure and isomer D is a minor contributor. At first glance, it is surprising that isomer D should be stable against conversion to isomer A, given its high calculated energy. Exploratory calculations let us estimate the barrier for conversion of isomer D to A to be at least 8000 cm−1. Interestingly, we note that in our calculations, isomer 3D converts to isomer 3A without an apparent barrier if it is solvated at the η2-CO2 ligand. Because it is highly unlikely that larger clusters are only solvated at the η1-CO3 ligand, we therefore exclude isomer 3D from our considerations. None of the other calculated isomers recovered the experimental spectrum. As the number of CO2 molecules increases, the peaks observed in the experimental spectra do not appreciably shift, although their relative intensities change. Since isomers 1A, 3A, and 1D are the only structures to recover features in the experimental spectra, it is necessary to investigate the effect of solvation on these core structures. Figure 4 shows the various
Figure 4. Lowest energy structures of [CoO(CO2)3]−. Capital letters denote the core structure, superscripts 1 and 3 indicate singlet and triplet configurations, and number/letter combination in parentheses denotes the number of solvent molecules and their position. Relative energies are given in meV.
Figure 3. Comparison of core isomers for [CoO(CO2)2]− to the experimental spectrum of [CoO(CO2)3]−. Isomer labels and relative energies (in meV) are given for each structure.
solvation positions around these core isomers. In addition to initial solvation of the [CoO(CO2)2]− core isomers, an additional core structure (isomer F) is predicted to be energetically relevant (see Figure 4). Isomer F is a modification of isomer E where the third CO2 has been added to the lone oxygen atom resulting in an η2 CO3 moiety opposite the C2O4 unit. The triplet structure is planar, while the CO3 and C2O4 subgroups are perpendicular to each other in the singlet configuration (see Figure 4 isomers 1F and 3F). However, if structure F is populated in our experiment, it should not be observed for clusters smaller than [CoO(CO2)4]−, where a solvent CO2 could be present.
presented in Figure 2. Only isomers 1A, 3A, and 1D are consistent with features in the experimental spectrum (see Figure 3) with 1A and 3A recovering the peaks at 1705 and 1815 cm−1 and 1D displaying peaks at 1655 and 1840 cm−1. While isomer 3B does appear to recover the feature at 1655 cm−1, the second predicted feature at 1750 cm−1 is not present in the experimental spectrum with an intensity consistent with the 1655 cm−1 feature. If isomer 3B is present in the experiment it contributes only as a minor isomer. It is clear from the observed relative peak intensities that structure A is the 845
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assigned core ions are robust (see Table 1 for charge distributions of the core species), with little solvent-induced
The calculated difference in signatures between solvation isomers of the same core structure is of the same order as between singlet and triplet configurations (20−25 cm−1 for the lower energy feature, 5−10 cm−1 for the higher energy peak). In contrast, the splittings between the experimental signatures are 50 cm−1 for the lower energy features and 30 cm−1 for the higher energy peaks. The widths and splittings of the experimental features as well as their insensitivity to cluster size suggest that only one multiplicity is present in our experiment and that theory overestimates solvent-induced shifts. While this makes it impossible to definitively assign a multiplicity based on the infrared spectra, we can still distinguish different core structures. For the sake of simplicity, we will continue the discussion using only simulated spectra for the singlet configurations. Figure 5 shows a comparison of the singlet structures presented in Figure 4 to the experimental spectrum of
Table 1. Partial Charge Distributions for [CoO(CO2)n]− Cores A, D, and F in Singlet and Triplet Statesa isomer 3
A 1 A 3 D 1 D 3 F 1 F a
no. of CO2 units
Co
CO2
CO3
C2O4
2 2 2 2 3 3
+1.10 +0.79 +0.88 +0.68 +1.42 +1.27
−0.68 −0.61 −0.55 −0.57
−1.42 −1.18 −1.33 −1.11 −1.12 −1.06
−1.30 −1.21
Charges are reported in units of e. See Figures 2 and 4 for structures.
shift. This is consistent with the experimental observation that the peak positions do not shift appreciably with increasing solvation. The highest occupied molecular orbitals (HOMOs) of both dominant motifs are mainly localized on the metal and on the CO3 moiety of the complex (see Supporting Information). Finally, there are small features present between 1700 and 1800 cm−1 in [CoO(CO2)4]− and larger clusters. Since these features first appear in [CoO(CO2)4]−, the core species exhibiting these stretches is likely of the form [CoO(CO2)3]−. Isomer F is the most likely candidate and is predicted to exhibit vibrational frequencies in this region. Exploratory calculations show that for this core structure different solvation positions (i.e., solvation around the C2O4 moiety vs the CO3 moiety) give rise to slight spectral shifts that recover the minor peaks in this range. These shifts are expected since the C2O4 subunit is larger and likely to exhibit a more polarizable charge distribution than any of the other subunits observed. [NiO(CO2)n]−. The experimental spectra of [NiO(CO2)n]− (n = 2−6) are shown in Figure 6. Again, there are two regions in the spectra, a solvent region (2300−2400 cm−1) and a core region (1000−2150 cm−1). All cluster ions in this series have one unpaired electron, so we do not need to distinguish different spin multiplicities. Since many of the structural motifs
Figure 5. Comparison of isomer 1F and the various solvation conformers of isomers 1A and 1D to the experimental spectrum of [CoO(CO2)3]−. Isomer labels and relative energies are given to the right.
[CoO(CO2)3]−. It is also clear that isomer F does not recover any of the dominant features in the experimental spectrum, because their positions do not change with increasing cluster size. However, isomer F may account for some of the small peaks around 1770 cm−1 in larger clusters (n ≥ 4). We therefore assign the peaks at 1705 and 1815 cm−1 to isomer A in either a triplet or a singlet configuration and the peaks at 1655 and 1840 cm−1 to isomer 1D. As the number of CO2 molecules increases, the peaks assigned to isomer 1D grow in intensity relative to those assigned to 1A and 3A, suggesting that the population of isomer 1 D grows relative to that of 1A and 3A. For both structures 1,3A and 1D, calculations confirm that the charge distribution of the
Figure 6. Experimental spectra of [NiO(CO2)n]−. Numbers indicate the number of CO2 units present. Left and right traces are individually normalized and on different scales. 846
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Figure 7. Lowest energy structures of [NiO(CO2)n]−. Structures on the left side are bare core structures, and structures on the right are examples of singly solvated core ions. Letters denote the core structure, while number/letter combination indicates number of solvent molecules and their position. Relative energies for [NiO(CO2)]− complexes are shown in italics, [NiO(CO2)2]− in plain font and [NiO(CO2)3]− in bold. No relative energy is given for F(1) since it is the only calculated structure of the form [NiO(CO2)4]−.
are similar to the analogous cobalt-containing species, we will use the same labels with analogous structures for cluster sizes n ≥ 2. Unlike in [CoO(CO2)n]− clusters, dissociation is first observed at n = 2, and a solvent peak is present, indicating that clusters of the form [NiO(CO2)]−·CO2 exist. Calculations reveal two possible structures for a [NiOCO2]− core ion; isomer G is a CO3 moiety bound in an η2 motif to the nickel atom, and isomer H is a CO2 bound in an η2 fashion to the nickel atom opposite the lone oxygen atom (see Figure 7). Solvation around the CO3 and CO2 moieties is predicted to yield stable structures but not the lowest energy structures for n ≥ 2 (see Figure 7). When compared with the experimental spectra for [NiO(CO2)2]−, isomers G and H recover the two major features observed at 1670 and 1750 cm−1, respectively (see Figure 8). The signal-to-noise ratio of the experimental spectrum of [NiO(CO2)2]− is noticeably lower compared to the other clusters probed in this series, although the parent ion abundance was in fact much greater than for larger clusters. Isomers G and H are not the lowest energy structures (see Figure 7), but they are the only structures that should be experimentally observable at cluster size n = 2, since they contain a solvent CO2. While both isomers G and H are populated, they are likely less populated than isomer A at this cluster size. This results in a small amount of observable fragmentation in [NiO(CO2)2]− clusters and explains the relatively poor signal-to-noise ratio, since only a small part of the total population exist as isomers that can undergo dissociation at the photon energies used in the experiment. It
Figure 8. Comparison of structures G, H, and A to the experimental spectrum of [NiO(CO2)2]−. See Figure 7 for relative energies and the text for discussion.
should also be noted that similarly broad features were observed in [Ni(CO2)2]− clusters and attributed to multiphoton effects.29 It is likely that the broad feature centered at 1825 cm−1 is the result of multiphoton-induced dissociation of isomer A (see Figure 8), but due to the low signal-to-noise 847
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1840 cm−1 is recovered. In contrast, if this solvation position is vacant, the feature at 1880 cm−1 is recovered. The features at 1645 and 1880 cm−1 are consistent with core structure D(1a), where there is preferential solvation around the CO3 moiety. Interestingly, the predicted differences in the charge distributions of isomer A and D are exclusive to the CO3 moiety and the Ni, leaving the CO2 moiety nearly unchanged (see Table 2), even though the HOMOs of both structures are largely
ratio, it is difficult to confirm the presence of multiphoton effects for this species. For [NiO(CO2)3]− and larger clusters, new peaks are observed, suggesting that new solvated core structures now contribute to the spectrum. The main peaks for [NiO(CO2)3]− are at 1645, 1685, 1840, and 1880 cm−1. Figure 7 shows all the relevant core structures predicted (see left column) and various solvation positions around the different cores (see right columns). Similar to [CoO(CO2)n]− clusters, structure A is predicted to be the global minimum structure. Structure D, also found in [CoO(CO2)n]− clusters, consists of a bidentate CO2 moiety and a CO3 unit bound to the Ni atom by only one oxygen atom. Structures E and F are similar as they both contain a C2O4 subunit and either a lone oxygen atom or a CO3 moiety, respectively. Structures A, D, and E could in principle be observable at [NiO(CO2)3]−, while isomer F should not be found until [NiO(CO2)4]− where it can be solvated. Figure 9 shows a comparison of selected core isomers to the experimental spectrum of [NiO(CO2)3]−. Solvation isomer
Table 2. Charge Distribution for All Calculated [NiO(CO2)n]− Core Structuresa isomer
no. of CO2 units
Ni
G H A D E F
1 1 2 2 2 3
+0.56 +0.65 +0.93 +0.72 +1.17 +1.39
a
O
CO2
−0.94
−0.71 −0.52 −0.50
CO3
C2O4
−1.56 −1.41 −1.22
−0.76 −1.10
−1.41 −1.29
Charges are reported in units of e. See Figure 7 for structures.
delocalized over the whole core ion (see Supporting Information). This leads to an overlap of the 1880 cm−1 features predicted for both isomers A(1a) and D(1a). The situation is unlike in [CoO(CO2)n]−, where the corresponding isomers are predicted to have completely different charge distributions and spectra. In other words, the features between 1600 and 1700 cm−1 tell us more about the connectivity of the CO3 moiety to the metal center, i.e., they inform us about the core isomer, while the features around 1880 cm−1 report solvation conformations. Table 2 shows the calculated charge distribution for relevant core species. We note that features are both predicted and observed in the region of 1100−1200 cm−1. However, these features are below the predicted dissociation threshold for a solvent CO2 molecule, and they likely originate from the warmer part of the ensemble. We therefore do not use them to assign structures. The weak feature observed at 1745 cm−1 in [NiO(CO2)3]− can be assigned to structure E(1). For n ≥ 4, there are two minor features at 1745 and 1780 cm−1 in this wavelength region. Assuming that the peak at 1745 cm−1 is due to core E, we can assign the feature at 1780 cm−1 to core F, consistent with the fact that this core should first be observable at n = 4. On the basis of the relative intensities, it is likely that both E and F are only weakly populated in our experiment. The predicted transitions around 1320 cm−1 for both isomers are below the dissociation threshold for a solvent CO2 and therefore likely to be suppressed in the experimental spectrum. As the number of CO2 molecules increases, both core structures E and F persist as minor contributors. The size evolution of the assigned features is similar to those of [CoO(CO2)n]−. The features assigned to A(1a) remain dominant in the spectra as the number of solvent molecules increases. Like in [CoO(CO2)n]− clusters, the relative intensity of the feature at 1645 cm−1 increases with increasing cluster size, suggesting an increased stabilization of isomer D with increased solvation. Additionally, there are minor isomers present (structures E and F) similar to those in [CoO(CO2)n]−.
Figure 9. Comparison of the calculated spectra of [NiO(CO2)3]− to the experimental spectra of [NiO(CO2)3]−. See Figure 7 for relative energies.
A(1a) recovers the two major peaks at 1685 and 1880 cm−1, while A(1b) recovers the peaks at 1685 and 1840 cm−1. The two high-wavenumber peaks represent modes mostly localized on the CO2 moiety, and the difference between the two solvation conformers shows that the charge distribution in the CO2 moiety is affected by solvation. Conversely, the charge distribution of the CO3 moiety encoded in the peak at 1685 cm−1 is quite robust and unaffected by solvation. The peak at 1840 cm−1 is present for n = 3 and larger, showing that if the CO2 solvation position is filled (like in A(1b)), the feature at
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CONCLUSIONS The structural motifs in [CoO(CO2)n]− and [NiO(CO2)n]− clusters are very similar. Both series of clusters exhibit dominant features belonging to structures consisting of a 848
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CO2 and a CO3 subunit (see structures A and D in both series). For both CoO and NiO, core isomer D is predicted to be stabilized with increasing solvation. In both cases a minor contributing structure, consisting of a CO3 unit and a C2O4 moiety, is predicted and observed in the experiment. While the structures in both cluster series are similar, the spectra show small differences, owing to differences in the charge distributions of the core species (see Tables 1 and 2). In particular, the charge distributions on the CO2 moiety in [CoO(CO2)n]− clusters change depending on the orientation of the CO3 unit (i.e., structure A vs D in Figure 2 and Table 1), while in [NiO(CO2)n]− clusters, the charge distributions on the CO2 moiety are nearly unchanged for the same two structural motifs (structures A and D in Table 2). In both [CoO(CO2)n]− and [NiO(CO2)n]− other isomers are populated to a small extent. The observation of different spectroscopic signatures generated by similar structures is reminiscent of the behavior of [Co(CO2)n]− and [Ni(CO2)n]− clusters where similar differences were observed.28,29
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ASSOCIATED CONTENT
S Supporting Information *
Experimentally measured frequencies and their calculated values for [CoO(CO2)3]− and [NiO(CO2)n]− (n = 2, 3), coordinates of the calculated core structures shown in Figures 2, 4, and 7, and HOMOs of dominant calculated structures (A and D). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: ++1-303-492-7841. E-mail:
[email protected]. edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the National Science Foundation for funding through Grant CHE-0845618 (for graduate student support of B.J.K.) and for instrumentation maintenance through Grant PHY-1125844.
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