Infrared Spectra and Structures of the Neutral and Charged CrCO2

Article pubs.acs.org/JPCA

Infrared Spectra and Structures of the Neutral and Charged CrCO2 and Cr(CO2)2 Isomers in Solid Neon Qingnan Zhang, Mohua Chen, and Mingfei Zhou* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: The reactions from codeposition of laser-ablated chromium atoms with carbon dioxide in excess neon are studied by infrared absorption spectroscopy. The species formed are identified by the effects of isotopic substitution on their infrared spectra. Density functional calculations are performed to support the spectral assignments and to interpret the geometric and electronic structures of the experimentally observed species. Besides the previously reported insertion products OCrCO and O2Cr(CO)2, the one-to-one Cr(CO2) complex and the one-to-two Cr(CO2)2 complex as well as the CrOCrCO and OCCrCO3 complexes are also formed. The Cr(CO2) complex is characterized to be side-on η2-C,O-coordinated. The Cr(CO2)2 complex is identified to involve a sideon η2-C,O-coordinated CO2 and an end-on η1-O-coordinated CO2. OCCrCO3 is a carbonate carbonyl complex predicted to have a planar structure with a η2-O,O-coordinated carbonate ligand. The CrOCrCO complex is predicted to be linear with a high-spin ground state. Besides the neutral molecules, charged species are also produced. The Cr(CO2)+ and Cr(CO2)2+ cation complexes are characterized to have linear end-on η1-O-coordinated structures with blue-shifted antisymmetric CO2 stretching vibrational frequencies. The OCrCO− anion is bent with the Cr−O and CO stretching frequencies red-shifted from those of OCrCO neutral molecule.

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INTRODUCTION Carbon dioxide is a naturally abundant carbon source that has been implicated as a contributor to the greenhouse effect. Conversion of CO2 into useful chemical materials is an active field in catalytic chemistry.1−3 The interactions of transition metal centers with CO2 serve as the simplest model in understanding the intrinsic mechanism of catalytic CO2 activation processes. Experimentally, the reactivity and kinetics of the reactions of transition metal atoms, metals, and metal oxide cations with carbon dioxide have been studied in the gas phase.4−15 The complexation and activation of carbon dioxide by transition metal atoms and metal oxide molecules have been intensively investigated in cryogenic matrices.16−36 Various carbon dioxide complexes in different coordination modes as well as the insertion products were formed and identified spectroscopically.16−36 Some transition metal cation−carbon dioxide complexes have also been studied by infrared photodissociation spectroscopy in the gas phase.37−42 Besides the experimental studies, quantum chemical calculations have also been performed to understand the reaction mechanisms as well as the structural and bonding properties of the corresponding complexes.43−69 In the case of chromium, the interaction of thermally generated chromium atoms with CO2 has been investigated using matrix isolation FTIR spectroscopy in solid CO2.18,19 It was found that the chromium atoms insert spontaneously into a CO bond yielding oxocarbonyl species. The reactions of laserablated Cr atoms with CO2 have been investigated in solid argon.24−26 The inserted products OCrCO and O2Cr(CO)2 © 2014 American Chemical Society

were identified by the effects of isotopic substitution on their infrared spectra, and the assignments were supported by density functional theory calculations.24−26 In addition, an endon-coordinated CO2 complex Cr−η1-OCO was also reported, which upon irradiation with UV-light, photoisomerized to give OCrCO.24,25 Earlier theoretical calculations at the SCF-CI level predicted that the η1-OCO end-on and η2-(O,O) side-on coordination complexes lie lowest in energy.58 In this paper, the reactions of laser-ablated chromium atoms and carbon dioxide are reinvestigated in the more inert neon matrix. We will show that besides the previously reported OCrCO and O2Cr(CO)2 molecules, the Cr(CO2), Cr(CO2)2, OCCrCO3, and CrOCrCO neutral complexes as well as the Cr(OCO)+, Cr(OCO)2+, and OCrCO− charged species are also formed and characterized in solid neon.

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EXPERIMENTAL AND THEORETICAL METHODS The experiments were performed using pulsed laser ablationmatrix isolation infrared absorption spectroscopy as described in detail previously.70 The 1064 nm fundamental wavelength of a Nd:YAG laser (Continuum, Minilite II; 10 Hz repetition rate) was used for ablation. The laser beam is focused onto a rotating chromium metal target. The resulting chromium atoms, cations, and electrons were codeposited with CO2/Ne mixtures onto a CsI window, which was cooled normally to 4 K by means of a Received: June 10, 2014 Revised: July 16, 2014 Published: July 17, 2014 6009

dx.doi.org/10.1021/jp505740j | J. Phys. Chem. A 2014, 118, 6009−6017

The Journal of Physical Chemistry A

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closed-cycle helium refrigerator. In general, matrix samples were deposited for 30 min at a rate of approximately 6 mmol/h. The CO2/Ne mixtures were prepared in a stainless steel vacuum line using standard manometric technique. Isotopically labeled 13CO2 (Spectra Gases Inc., 99%), C18O2 (Cambridge Isotopic Laboratories, 95%), and C16O2 + C16O18O + C18O2 (Cambridge Isotopic Laboratories, 61% 18O) samples and mixtures were used without further purification in different experiments. After deposition, the samples were annealed to allow the trapped reactants to diffuse and react. Selected samples were also subjected to broad-band irradiation using a tungsten lamp or a high-pressure mercury arc lamp with glass filters to initiate further isomerization or dissociation reactions. The infrared absorption spectra of the samples in the middle infrared region (4000−450 cm−1) were recorded on a Bruker VERTEX 80 V spectrometer at 0.5 cm−1 resolution using a liquid nitrogen cooled HgCdTe (MCT) detector. Quantum chemical calculations were performed to determine the molecular structures and to support the assignment of vibrational frequencies of the observed reaction products. Calculations were performed using the three-parameter hybrid functional according to Becke with additional correlation corrections due to Lee, Yang, and Parr (B3LYP).71,72 The aug-cc-pVTZ basis sets were used.73 Calculations were performed for all possible spin states of the species involved. The geometries were fully optimized, and the harmonic vibrational frequencies were calculated with analytic second derivatives. The reported species are predicted to have highspin ground states. Because the ⟨S2⟩ of the wave functions are very close to the ideal values, the spin contamination of the wave functions is not severe and the results are thus reliable. All the calculations were performed using the Gaussian 09 program.74

Figure 1. Infrared spectra in the 2400−2360 and 2180−1740 cm−1 regions from codeposition of laser-ablated chromium with 0.05% CO2 in neon: (a) after 30 min of sample deposition at 4 K, (b) after 12 K annealing, (c) after 15 min of λ > 500 nm irradiation, (d) after 15 min of λ > 400 nm irradiation, and (e) after 15 min of λ > 250 nm irradiation.

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RESULTS AND DISCUSSION The spectra in selected regions from codeposition of laserablated chromium atoms with 0.05% CO2 in neon are shown in Figures 1 and 2, respectively. Besides the CO2, CO2− (1658.4, 1253.8, and 714.1 cm−1),75 CO2+ (1421.8 cm−1),75 (CO2)2− (1852.4 and 1189.4 cm−1), and (CO2)2+ (2130.8 and 1274.6 cm−1) absorptions,76 new product absorptions are produced either on sample deposition or on annealing and photolysis. These absorptions can be classified into several groups based on their annealing and photochemical behaviors (labeled as A− I in Figures 1 and 2). Species A involves three bands at 1769.2, 1009.3, and 714.9 cm−1. These bands are weak on sample deposition, markedly increase on sample annealing, but are destroyed upon broad-band visible irradiation (λ > 500 nm). Two modes are observed for species B. Each mode involves two site absorptions (2048.5/2046.2 and 880.5/877.6 cm−1). These bands are the most intense absorptions after sample deposition. They remain almost unchanged upon sample annealing and almost disappear on λ > 500 nm visible light irradiation but increase under λ > 400 nm visible light or UV− visible (250 nm 400

Figure 2. Infrared spectra in the 1100−700 cm−1 region from codeposition of laser-ablated chromium with 0.05% CO2 in neon: (a) after 30 min of sample deposition at 4 K, (b) after 12 K annealing, (c) after 15 min of λ > 500 nm irradiation, (d) after 15 min of λ > 400 nm irradiation, and (e) after 15 min of λ > 250 nm irradiation.

nm visible light irradiation. Four bands at 2374.2, 1758.0, 1025.8, and 717.0 cm−1 are observed for species E. These bands are barely observed on sample deposition, increase greatly on annealing, but are destroyed on λ > 500 nm visible light irradiation. Five modes are observed for species F (Table 1). This species is present on sample deposition, increases on sample annealing and visible light irradiation, but is destroyed upon UV−visible light (250 nm 500 nm irradiation): (a) 0.05% 12C16O2, (b) 0.05% 13C16O2, (c) 0.025% 12C16O2 + 0.025% 13 16 C O2, (d) 0.05% 12C18O2, (e) 0.025% 12C16O2 + 0.025% 12C18O2, and (f) 0.075% (12C16O2 + 12C16O18O + 12C18O2). 6011



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inequivalent. The terminal one has a bond length of 1.190 Å, and the coordinated one has a much longer bond length of 1.303 Å. It can be regarded as a charge transfer complex, in which the charge on the CO2 ligand is about 0.49 e. Molecular orbital analysis shows that there is a bonding interaction between the singly occupied (SOMO) orbital of Cr (primarily a hybrid of the Cr 4s and 3d orbitals) and one of the empty π* orbitals of CO2. The agreement in band positions and isotopic frequency ratios (Table 2) supports the assignment. Previous matrix isolation infrared spectroscopic investigations assigned two bands at 1735.6 and 721.0 cm−1 to an end-on η1-Ocoordinated Cr(CO2) complex in solid argon.25 We suggest that these two bands should be reassigned to the side-oncoordinated complex in solid argon. DFT calculations found that the end-on η1-O bonded complex is not a stable structure. Geometry optimization starting from the end-on structure converges to the side-on-coordinated structure. OCrCO (B). The group B bands are assigned to the inserted OCrCO complex in solid neon. The band position and isotopic frequency ratios (12CO2/13CO2 = 1.0226, C16O2/C18O2 = 1.0226) imply that the 2048.5 cm−1 band is due to the terminal CO stretching vibration. The 880.5 cm−1 band exhibits almost no shift with 13CO2. The isotopic 12C16O2/12C18O2 ratio of 1.0444 is very close to that of diatomic CrO, indicating that the 880.5 cm−1 band is a CrO stretching vibration. The doublet features observed in the mixed sample experiments confirm that only one CO subunit is involved in the upper mode and one CrO fragment is involved in the low mode. The band positions of the major site are blue-shifted 34.1 and 13.9 cm−1 from the argon matrix values.24,25 The OCrCO molecule is predicted to have a 5A′ ground state, in agreement with previous calculations. Our DFT calculations indicate that the inserted structure is 6.8 kcal/mol less stable than the side-on bonded Cr(CO2) complex. Cr(OCO)+ (C). Two modes were observed for species C. The 2388.2 cm−1 absorption shifted to 2321.6 cm−1 with 13CO2 and to 2352.4 cm−1 with C18O2. The band position and isotopic frequency shifts indicate that this absorption is due to an antisymmetric CO2 stretching vibration. This band is blueshifted by 40.4 cm−1 from that of free CO2 in solid neon and can be attributed to the CO2 molecule bound directly to a positively charged metal center by end-on coordination between an oxygen atom of the CO2 ligand and the metal ion. The much weaker 1382.4 cm−1 band is the symmetric stretching mode. Thus, species C is assigned to the Cr(OCO)+ cation complex. The observation of a blue-shifted antisymmetric CO2 stretching vibration has been reported in previous investigations on the M+(CO2)n complexes.37−42 As discussed before, the magnitude of the blue shift correlates with the strength of bonding in the complexes. The Cr(OCO)+ cation exhibits a blue shift similar to those of other transition metal ion complexes. The Cr(OCO)+ cation complex is predicted to have a 6Σ ground state with linear geometry (Figure 7). The CO2 ligand is end-on bonded to Cr+ via one O atom with a Cr−O distance of 2.106 Å. The antisymmetric and symmetric CO2 stretching vibrations are predicted at 2443.0 and 1384.5 cm−1 (Table 2). OCrCO− (D). The photosensitive group D bands at 1838.5 and 836.3 cm−1 are assigned to the OCrCO− anion complex. The band position and isotopic frequency ratios (Table 2) imply that the upper band is due to a CO stretching vibration and the low band is a CrO stretching mode. The doublet spectral features observed in the experiments with the 12CO2 +

Figure 5. Infrared spectra in the 2170−1950 and 1850−1740 cm−1 regions from codeposition of laser-ablated chromium atoms with isotopic labeled CO2 in excess neon (spectra taken after 15 min of λ > 500 nm irradiation): (a) 0.05% 12C16O2, (b) 0.05% 13C16O2, (c) 0.025% 12C16O2 + 0.025% 13C16O2, (d) 0.05% 12C18O2, (e) 0.025% 12 16 C O2 + 0.025% 12C18O2, and (f) 0.075% (12C16O2 + 12C16O18O + 12 18 C O2).

Figure 6. Infrared spectra in the 1100−700 cm−1 region from codeposition of laser-ablated chromium atoms with isotopic labeled CO2 in excess neon (spectra taken after 15 min of λ > 500 nm irradiation): (a) 0.05% 12C16O2, (b) 0.05% 13C16O2, (c) 0.025% 12 16 C O2 + 0.025% 13C16O2, (d) 0.05% 12C18O2, (e) 0.025% 12C16O2 + 0.025% 12C18O2, and (f) 0.075% (12C16O2 + 12C16O18O + 12C18O2).

that this mode retains some symmetric OCO stretching character. The 714.9 cm−1 band is attributed to a CO2 bending vibration on the basis of its isotopic frequency shifts. As shown in Figures 3 and 4, each mode splits into a doublet in the experiments with the 12CO2 + 13CO2 (1:1) and C16O2 + C18O2 (1:1) mixed samples and into a quartet with approximately 1:1:1:1 relative intensities in the experiment with the C16O2 + C16O18O + C18O2 (61% 18O) mixed sample. These spectral features confirm that species A involves one CO2 subunit with two inequivalent O atoms, indicating that the CO2 ligand is side-on coordinated to the chromium center. DFT calculations predicted that the side-on η2-C,O coordination mode is the most stable addition structure. The complex has a 5A′ ground state with planar Cs symmetry (Figure 7). The two CO bonds of the CO2 ligand are quite 6012



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Table 2. Comparison between the Experimentally Observed and Calculated Vibrational Frequencies and Isotopic Frequency Ratios of the Species Reported in Solid Neon obs

calcd

mode

frequency

R12/13

R16/18

frequency

R12/13

R16/18

OCrCO CO str CrO str OCrCO− CO str CrO str O2Cr(CO)2 CO sym str CO asym str OCrO asym str Cr(OCO)+ OCO asym str OCO sym str Cr(OCO)2+ OCO asym str OCO sym str Cr(CO2) CO str OCO sym str OCO bend. Cr(CO2) (OCO) OCO asym str CO str OCO sym str OCO bend. OCCrCO3 CO str CO str CO2 asym str CO2 sym str CrO2 sym str CrOCrCO CO str CrOCr asym str

2048.5 880.5 1838.5 836.3 2128.8 2066.9 988.7 2388.2 1382.4 2384.2 1380.9 1769.2 1009.3 714.9 2374.2 1758.0 1025.8 717.0 2163.4 1789.6 1080.1 935.5 792.9 2011.0 864.0

1.0226 1.0001 1.0235 1.0018 1.0233 1.0234 1.0000 1.0286

1.0226 1.0444 1.0226 1.0372 1.0231 1.0245 1.0384 1.0152

1.0287

1.0155

1.0237 1.0119 1.0093 1.0287 1.0219 1.0125 1.0100 1.0227 1.0240 1.0279 1.0042 1.0015 1.0230 1.0000

1.0227 1.0356 1.0427 1.0151 1.0218 1.0354 1.0425 1.0240 1.0206 1.0164 1.0552 1.0557 1.0234 1.0508

2107.6 922.0 1844.7 878.1 2189.6 2144.4 1044.2 2443.0 1384.5 2437.9 1388.7 1833.4 1053.0 720.4 2430.2 1815.8 1077.9 724.7 2190.6 1821.0 1071.2 947.8 788.3 2058.7 847.3

1.0232 1.0000 1.0240 1.0000 1.0236 1.0236 1.0000 1.0292 1.0001 1.0292 1.0001 1.0255 1.0119 1.0105 1.0293 1.0258 1.0123 1.0112 1.0232 1.0260 1.0274 1.0029 1.0012 1.0234 1.0000

1.0242 1.0450 1.0229 1.0444 1.0251 1.0241 1.0389 1.0156 1.0606 1.0156 1.0606 1.0205 1.0403 1.0428 1.0155 1.0202 1.0399 1.0423 1.0244 1.0198 1.0178 1.0564 1.0543 1.0238 1.0512

donation of the CrO electrons to the empty 2π antibonding orbitals of CO. The molecular orbital that correlates to CrO to CO back-donation interaction is singly occupied in the neutral OCrCO molecule. Addition of an electron to this orbital of the neutral OCrCO results in the 4A″ ground state OCrCO− anion. Electron addition significantly elongates the CO bond length, which results in a reduction of the CO stretching vibrational frequency of the OCrCO− anion. Cr(CO2)2 (E). The group E absorptions at 2374.2, 1758.0, 1025.8, and 717.0 cm−1 appear on sample annealing. The isotopic shifts and splittings of the 1758.0, 1025.8, and 717.0 cm−1 bands are very similar to those of the side-on η2-C,Ocoordinated Cr(CO2) complex, suggesting that species E also involves a CO2 ligand with the side-on η2-C,O coordination fashion. The 2374.2 cm−1 band is an antisymmetric CO2 stretching vibration. The doublet spectral features observed in the experiments with the 12CO2 + 13CO2 and C16O2 + C18O2 samples (Figure 3, trace c and e) and the quartet spectral feature in the experiment using the C16O2 + C16O18O + C18O2 mixture (Figure 3, trace f) indicate that species E involves another CO2 subunit with an end-on η1-O coordination fashion. Accordingly, absorber E is assigned to the one-to-two Cr(CO2)2 complex involving a side-on η2-C,O-coordinated CO2 and an end-on η1-O-coordinated CO2. The complex is predicted to have a 5A′ ground state with planar Cs symmetry (Figure 8). The side-on bonded CO2 ligand is bent with a bond angle of 135.9°, whereas the end-on CO2 ligand is nearly linear. In this complex, the end-on-coordinated CO2 ligand serves as a weak electron donor, whereas the side-on bonded CO2 ligand acts as an electron acceptor. Because the chromium center in side-on bonded Cr(CO2) complex is positively charged, the second CO2 ligand prefers to coordinated to the chromium

Figure 7. Optimized structures and relative stabilities of the charged and neutral CrCO2 isomers (relative energies in kcal/mol, bond lengths in angstrom and bond angles in degrees). 13

CO2 and C16O2 + C16O18O + C18O2 mixtures (Figure 5) confirm that only one CO subunit is involved. The band positions are about 210.0 and 44.2 cm−1 red-shifted from the corresponding modes of the OCrCO neutral. The OCrCO− anion is predicted to have a 4A″ ground state with planar Cs symmetry (Figure 7). The bonding interactions in OCrCO are dominated by the synergic donations of the filled 5σ orbital of CO into an empty acceptor orbital of CrO and the back6013



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O2Cr(CO)2 (G). The group G absorptions at 2128.8, 2066.9, and 988.7 cm−1 are attributed to the inserted O2Cr(CO)2 molecule in neon, which are blue-shifted by 5.6, 7.2, and 7.3 cm−1 compared to the argon matrix values.24,25 The molecule is predicted to have a 1A1 ground state with C2v symmetry, which can be viewed as being formed by the interaction between a singlet excited state CrO2 fragment (electron configuration of (core)(a1)2(b1)0) and the CO ligands. The CrO2 molecule is determined to have a 3B1 ground state with an electron configuration of (core)(b1)1(a1)1. Upon b1 to a1 excitation, the b1 orbital, which is largely the Cr 3d orbital in character, that is oriented in the plane perpendicular to the CrO2 plane is empty and is able to accept σ donation from CO. The a1 orbital, which is primarily a CrO2 nonbonding orbital, is doubly occupied and acts as an orbital for back-donation from CrO2 to CO. Thus, both the σ donation and π back-donation interactions are strengthened upon triplet to singlet excitation. In like fashion, the (H2)2CrO2 and (NN)2CrO2 complexes are characterized to have similar singlet ground states.77,78 Cr(OCO)2+ (H). The 2384.2 cm−1 band (H) increases on annealing at the expense of the Cr(OCO)+ absorptions. This band is favored in the experiments with relatively high CO2 concentrations. The band position and isotopic frequency ratios (12CO2/13CO2 = 1.0287, C16O2/C18O2 = 1.0155) indicate that this band is due to an antisymmetric CO2 stretching mode. The spectral features in the experiments with mixed samples are not well resolved due to isotopic dilution and band overlap. However, two intermediate bands at 2390.9 and 2323.1 cm−1 can be clearly observed in the experiment with the 12CO2 + 13 CO2 sample, implying that species H involves two equivalent central symmetric CO2 ligands. Therefore, the 2384.2 cm−1 band is assigned to the antisymmetric CO2 stretching vibration of the Cr(OCO)2+ cation complex, which is computed to have a 6Σ ground state with linear D∞h symmetry (Figure 8). The cation complex is predicted to have a very strong antisymmetric CO2 stretching mode at 2437.9 cm−1. CrOCrCO (I). Two bands are observed for species I. The 2011.0 cm−1 band shifts to 1965.7 cm−1 with 13CO2 and to 1965.0 cm−1 with C18O2. The band position and isotopic frequency ratios (12C/13C:1.0230, 16O/18O:1.0234) indicate that this band is due to a CO stretching vibration. No intermediate absorptions are observed in the experiments with the 12CO2 + 13CO2, C16O2 + C18O2, and C16O2 + C16O18O + C18O2 mixed samples, indicating that only one CO unit is involved in this mode. The 864.0 cm−1 band exhibits no shift with the 13CO2 sample and shifts to 822.2 cm−1 with C18O2. No intermediate absorptions are observed in the experiments with the C16O2 + C18O2 and C16O2 + C16O18O + C18O2 mixtures, which confirms that only one oxygen atom is involved in this mode. The 16O/18O isotopic frequency ratio of 1.0508 is quite higher than that of diatomic CrO, suggesting an antisymmetric CrOCr stretching mode. Accordingly, species I is assigned to the CrOCrCO molecule, which can be regarded as a complex formed between the CrOCr molecule and carbon monoxide. The CrOCrCO complex is predicted to have a 11Σ ground state with linear geometry. In this structure, the 4s electrons of Cr are used to form chemical bonds with the central O atom, and the Cr 3d electrons remain unpaired. The ten unpaired metal-based electrons occupy the nonbonding molecular orbitals that are primarily atomic metal d orbitals in character. The CO stretching and antisymmetric CrOCr stretching vibrations of the ground state CrOCrCO molecule were calculated at 2058.7 and 847.3 cm−1 with the calculated isotopic

Figure 8. Optimized structures and relative stabilities of the charged and neutral Cr(CO2)2 isomers (relative energies in kcal/mol, bond lengths in angstrom, and bond angles in degrees).

center in the end-on η1-O fashion. The electrostatic interaction increases the antisymmetric stretching frequency of the end-oncoordinated CO2 ligand, which is blue-shifted by 26.4 cm−1 from that of free CO2 in solid neon. OCCrCO3 (F). The group F absorptions are assigned to different vibrational modes of OCCrCO3 (Table 1), a carbonate carbonyl complex. The band position and isotopic frequency shifts clearly indicate that the 2163.4 cm−1 band is a terminal CO stretching vibration. The doublet spectral features with the mixed samples are clear for involvement of only one CO subunit. The band position is blue-shifted by 25.8 cm−1 from that of diatomic CO in solid neon. The remaining bands are characteristic for a carbonate complex. The isotopic shift (12C/13C:1.0240, 16O/18O:1.0227) and splitting indicate that the 1789.6 cm−1 absorption is due to a CO stretching vibration and only one CO subunit is involved. The 1080.1 cm−1 absorption exhibits a large 13C shift (12C/13C:1.0279) but a quite small oxygen isotopic shift (16O/18O:1.0164). This band is mainly due to the antisymmetric OCO stretching vibration. The 935.5 cm−1 absorption has isotopic frequency ratios of 12 C/13C:1.0042 and 16O/18O:1.0552, suggesting a symmetric OCO stretching vibration. The 792.9 cm−1 band is the symmetric CrO2 stretching mode. As shown in Figure 6, the spectral features in the experiments with the C16O2 + C18O2 (split into a quartet) and C16O2 + C16O18O + C18O2 (split into a sextet) mixed samples indicate the involvement of a CO3 fragment with two equivalent oxygen atoms. The OCCrCO3 complex is predicted to have a 5A′ ground state with planar Cs symmetry (Figure 8). The carbonate group coordinates to the chromium center in a η2-O, O fashion. The molecule can be regarded as OCCr2+(CO3)2−. The CO ligand coordinates on the positively charged Cr center, which causes a blue shift of the CO stretching frequency due to electrostatic interactions. 6014



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frequency ratios, in excellent agreement with the observed values (Table 2). Reaction Mechanism. The experimental observations indicate that the ground state chromium atoms react with carbon dioxide to form the one-to-one Cr(CO2) complex spontaneously on annealing. The inserted OCrCO molecule absorptions increase only on UV−visible light irradiation, indicating that the insertion reaction requires activation energy. The inserted OCrCO molecule is predicted to be 6.8 kcal/mol less stable than the Cr(CO2) complex. The one-to-two Cr(CO2)2 complex is formed on annealing, indicating that the Cr(CO2) complex is able to react with another carbon dioxide molecule to give the Cr(CO2)2 complex with negligible activation energy. This addition reaction is predicted to be exothermic by 11.8 kcal/mol. The Cr(CO2)2 absorptions disappear under λ > 500 nm irradiation, during which the OCCrCO3 absorptions greatly increase. This observation suggests that Cr(CO2)2 undergoes photoinduced isomerism to OCCrCO3, which is predicted to be exothermic by 12.5 kcal/mol. The absorptions due to OCCrCO3 are destroyed under UV−visible (λ>250 nm) light irradiation, during which the inserted O2Cr(CO)2 molecule absorptions are produced. It implies that UV−visible light initiates OCCrCO3 to O2Cr(CO)2 rearrangement reaction, which is predicted to be endothermic by 9.5 kcal/mol. It is well-known that laser ablation of metal target produces not only neutral metal atoms but also metal cations and electrons. Therefore, anions can be produced during the cocondensation process via electron capture by neutral molecules and cations can be formed by metal cation reactions. Besides the neutral molecules discussed above, the Cr(OCO)+ and Cr(OCO)2+ cation complexes and the OCrCO− anion are formed along with the common CO2+, (CO2)2+, CO2−, and (CO2)2− species.

AUTHOR INFORMATION

Corresponding Author

*M. Zhou. E-mail: [email protected]. Tel: +86-2165643532. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge financial support from National Natural Science Foundation (Grant No. 21173053), Ministry of Science and Technology of China (2013CB834603 and 2012YQ220113-3), and the Committee of Science and Technology of Shanghai (13XD1400800).

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CONCLUSIONS The reactions of laser-ablated chromium atoms and carbon dioxide are reinvestigated in the more inert neon matrix by infrared absorption spectroscopy. Besides the previously reported insertion products OCrCO and O2Cr(CO)2, the one-to-one Cr(CO2) complex and the one-to-two Cr(CO2)2 complex as well as the CrOCrCO and OCCrCO3 complexes are also formed. The Cr(CO2) complex is formed on annealing, which is characterized to be a side-on η2-C,O-coordinated complex. The Cr(CO2)2 complex is identified to involve a sideon η2-C,O-coordinated CO2 and an end-on η1-O-coordinated CO2. The Cr(CO2)2 complex rearranges to the more stable OCCrCO3 isomer under visible light excitation, which is determined to be a η2-O,O-coordinated carbonate carbonyl complex with a planar structure. The CrOCrCO complex is predicted to be linear with a high-spin ground state. Besides the neutral molecules, charged species are also produced. The Cr(OCO)+ and Cr(OCO)2+ cation complexes are characterized to have linear end-on η1-O-coordinated structures with blueshifted antisymmetric CO2 stretching vibrational frequencies. The OCrCO− anion is bent with the Cr−O and CO stretching frequencies red-shifted from those of OCrCO neutral molecule.

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Article

ASSOCIATED CONTENT

S Supporting Information *

Calculated geometries, vibrational frequencies, and intensities; IR spectra; complete refs 1 and 74. This material is available free of charge via the Internet at http://pubs.acs.org. 6015



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Infrared Spectra and Structures of the Neutral and Charged CrCO2

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