Article pubs.acs.org/JPCA
Carbon Dioxide Activation by Scandium Atoms and Scandium Monoxide Molecules: Formation and Spectroscopic Characterization of ScCO3 and OCScCO3 in Solid Neon Qingnan Zhang, Hui Qu, Mohua Chen, and Mingfei Zhou* Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: The reactions of carbon dioxide with scandium monoxide molecules and scandium atoms are investigated using matrix isolation infrared spectroscopy in solid neon. The species formed are identified by the effects of isotopic substitution on their infrared spectra as well as density functional calculations. The results show that the ground state ScO molecule reacts with carbon dioxide to form the carbonate complex ScCO3 spontaneously on annealing. The ground state Sc atom reacts with two carbon dioxide molecules to give the carbonate carbonyl complex OCScCO3 via the previously reported OScCO insertion intermediate on annealing. The observation of these spontaneous reactions is consistent with theoretical predictions that both the Sc + 2CO2 → OCScCO3 and ScO + CO2 → ScCO3 reactions are thermodynamically exothermic and are kinetically facile, requiring little or no activation energy.
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OScCO and OSc+CO compounds.32−35 The gas phase kinetic experiment indicates that the scandium atom reaction exists a nonnegligible activation barrier.8 The insertion reaction was confirmed by a matrix isolation study, which found that laserablated scandium atoms react with carbon dioxide molecules to give primarily the OScCO insertion molecule in solid argon.12 The OScCO complex isomerizes to the less stable side-on bonded OSc(η2-CO) isomer under visible light excitation and ionizes to the inserted OScCO+ and OScOC+ cations upon UV light irradiation. The potential energy surfaces of the Sc + CO2 → ScO + CO reaction were theoretical investigated.34,35 The results indicate that the lowest energy path corresponds to the initial formation of a η2-C,O coordination complex followed by the insertion of Sc into a C−O bond to give the OScCO insertion isomer. The overall insertion reaction is predicted to be exothermic and almost barrierless.34,35 In this paper, the reactions of scandium atoms as well as scandium monoxide molecules with carbon dioxide are investigated in the neon matrix. We will show that the ground state ScO molecule reacts with carbon dioxide to form the carbonate complex ScCO3 spontaneously on annealing. The ground state Sc atom can react with two carbon dioxide
INTRODUCTION Carbon dioxide is the richest carbon source in the atmosphere. The conversion of CO2 into useful chemicals 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.4,5 Complexes formed between transition metal atoms, cations, and anions with carbon dioxide can act as both structural and functional models for surface-bound intermediates in catalytic conversion processes.5 The reactions of transition metal atoms and simple oxide molecules with carbon dioxide have been intensively studied both in the gas phase6−10 and in solid noble gas matrices.11−30 These investigations indicate that carbon dioxide not only can form complexes with transition metal centers but also can be reduced to carbon monoxide via insertion reactions. The insertion intermediates and various coordination complexes in different coordination modes including η1-O, η1-C, η2-C,O, and η2-O,O fashions were characterized spectroscopically.11−30 Besides the experimental studies, the reaction mechanisms as well as the structural and bonding properties of the reaction intermediates and products were investigated by various quantum chemical calculations.31−52 The reactions of scandium atoms with carbon dioxide is one of the most studied systems. Theoretical calculations anticipated that the ground state scandium atoms are able to cleavage the CO bond of carbon dioxide to form the inserted © XXXX American Chemical Society
Received: December 2, 2015 Revised: January 6, 2016
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ited with CO2 in excess neon. The spectra in selected regions are shown in Figure 1. Besides the ScO and CO2 absorptions
molecules to give the carbonate carbonyl complex OCScCO3 via the previously reported OScCO insertion intermediate.
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EXPERIMENTAL AND THEORETICAL METHODS The reactions were studied by infrared absorption spectroscopy in solid neon. The scandium atoms and scandium monoxide molecules were prepared via pulsed laser evaporation. The experimental setup for pulsed laser evaporation and matrix isolation Fourier transform infrared (FTIR) spectroscopic investigation has been described in detail previously.53 Briefly, the 1064 nm Nd: YAG laser fundamental (Spectra Physics, DCR 150, 10 Hz repetition rate and 8 ns pulse width) was used for evaporation. The laser beam is focused onto a rotating scandium metal or bulk Sc2O3 metal oxide targets. The Sc2O3 target was prepared by sintered Sc2O3 metal oxide powders. The laser-evaporated species were codeposited with carbon dioxide in excess neon onto a CsI window cooled normally to 4 K by means of a 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. The infrared absorption spectra of the samples in the middle infrared region (4000−450 cm −1 ) were recorded with a Bruker VERTEX 80 V spectrometer at 0.5 cm−1 resolution using a liquid nitrogen cooled HgCdTe (MCT) detector. The samples were annealed to different temperatures and cooled back to 4 K for spectral measurement. Quantum chemical calculations were performed to bolster the spectral assignments and to interpret the geometric and electronic structures of the experimentally observed species. Geometry optimizations were performed with the GAUSSIAN 09 program applying the three parameter hybrid functional according to Becke with additional correlation corrections by Lee, Yang and Parr (B3LYP).54−56The AUG-cc-pVTZ basis set was used for all atoms.57,58 The harmonic vibrational frequencies were calculated with analytic second derivatives. Transition states were optimized applying the synchronous transit-guided quasi-Newton (STQN) method and were verified by intrinsic reaction coordinate (IRC) calculations.59 Single-point energy calculations were also performed at the CCSD(T) level on the basis of the B3LYP optimized structures with the same basis set.60 The gradient corrected BP86 functional in conjunction with uncontracted Slater-type orbitals (STOs) as basis functions was used for the bonding analyses.61−63 The latter basis sets for all elements have triple-ζ quality augmented by two sets of polarization functions (ADF-basis set TZ2P). The BP86/TZ2P calculations were performed using the B3LYP/AUG-cc-pVTZ optimized geometries with the program package ADF2014.10.64
Figure 1. Infrared spectra in the 1860−1740 and 1100−740 cm−1 regions from codeposition of laser-evaporated scandium monoxide with 0.1% CO2 in neon: (a) after 30 min of sample deposition at 4 K; (b) after annealing to 9 K; (c) after annealing to 11 K.
(2347.7 and 668.0 cm−1), a group of absorptions at 1798.7, 1038.9, 911.0, 823.3, and 787.1 cm−1 is produced on sample deposition. These absorptions increase together on annealing at the expense of the ScO absorption. The experiments are repeated using the isotopic-substituted 13CO2 and C18O2 samples and mixtures. The spectra in selected regions using different isotopic samples are shown in Figures 2 and 3, respectively. The band positions of the observed new product are listed in Table 1. Experiments have also been performed using a metallic scandium target. The infrared spectra in selected regions from codeposition of laser-evaporated scandium atoms and 0.075% CO2 in neon are shown in Figure 4. The ScO, CO2 (2347.7 and 668.0 cm−1), CO (2140.8 cm−1), CO2− (1658.4, 1253.8, and 714.1 cm−1), CO2+ (1421.8 cm−1), (CO2)2− (1852.4 and
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RESULTS AND DISCUSSION Pulsed laser evaporation of bulk Sc2O3 target under controlled laser energy followed by condensation with pure neon forms ScO (962.5 cm−1) as the only product in the infrared spectrum.65 New product absorptions are observed when the laser-evaporated scandium monoxide molecules are codepos-
Figure 2. Infrared spectra in the 1820−1730 and 1050−1000 cm−1 regions from codeposition of laser-evaporated scandium monoxide molecules with isotopic-labeled CO2 in excess neon: (a) 0.1% CO2; (b) 0.1% 13CO2; (c) 0.05% 12CO2 + 0.05% 13CO2; (d) 0.1% C18O2; (e) 0.1% (C16O2 + C16O18O + C18O2). Spectra were taken after 30 min of sample deposition at 4 K followed by 11 K annealing. B
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Figure 3. Infrared spectra in the 940−760 cm−1 region from codeposition of laser-evaporated scandium monoxide molecules with isotopic-labeled CO2 in excess neon: (a) 0.1% CO2; (b) 0.1% 13CO2; (c) 0.05% 12CO2 + 0.05% 13CO2; (d) 0.1% C18O2; (e) 0.1% (C16O2 + C16O18O + C18O2). Spectra were taken after 30 min of sample deposition at 4 K followed by 11 K annealing. −1
Figure 4. Infrared spectra in 1950−1750 and 1100−750 cm−1 regions from codeposition of laser-evaporated scandium atoms with 0.075% CO2 in excess neon: (a) 30 min of sample deposition at 4 K; (b) after 12 K annealing; (c) after 15 min of visible light (500 < λ < 580 nm) irradiation.
and C18O2 samples, and the 12CO2 + 13CO2 (1:1), C16O2 + C18O2 (1:1), and C16O2 + C16O18O + C18O2 (61% 18O) mixtures are shown in Figures 5 and 6, respectively. ScCO3. The absorptions at 1798.7, 1038.9, 911.0, 823.3, and 787.1 cm−1 observed in the ScO + CO2 experiments are assigned to different vibrational modes of ScCO3 (Table 1), a carbonate complex with the CO3 unit coordinated to Sc in a bidentate fashion formed by the reaction between ScO and CO2. The 1798.7 cm−1 band shifts to 1756.3 cm−1 with 13CO2 and to 1774.5 cm−1 with C18O2. The band position and isotopic shifts indicate that the 1798.7 cm−1 absorption is a bridged carbonyl stretching vibration. No intermediate absorption is observed in the spectrum using the 12CO2 + 13CO2 mixed sample, suggesting the existence of only one carbon atom in this mode. When the C16O2 + C16O18O + C18O2 mixed sample is used, this mode splits into four absorptions at 1804.9, 1798.7, 1774.5, and 1769.4 cm−1. Among them, the absorptions at 1798.7 and 1774.5 cm−1 are due to the ScC16O3 and Sc(16O,18O)C18O isotopomers formed from the Sc16O + C16O2 and Sc16O + C18O2 reactions, respectively. The 1804.9 and 1769.4 cm−1 absorptions are attributed to the Sc(16O,18O)C16O and Sc(16O,16O)C18O isotopomers originated from the Sc16O + C16O18O reactions. Note that the band position of Sc(16O,18O)C16O is even higher than that of ScC16O3, whereas
−1
+
1189.4 cm ), and (CO2)2 (2130.8 and 1274.6 cm ) absorptions are observed on sample deposition.66,67 In addition, new product absorptions are observed either on sample deposition or on annealing. The 2175.4 and 933.3 cm−1 absorptions are observed on sample deposition and decrease on annealing. The upper band is due to a CO stretching vibration, which is 34.6 cm−1 blue-shifted from that of free CO. The low band is a ScO stretching mode, which is red-shifted by only 29.2 cm−1 from that of diatomic ScO in solid neon. These two bands can be attributed to a weakly bound OSc-CO complex. The 1887.2 and 918.4 cm−1 bands correspond to the absorptions reported at 1873.4 and 894.1 cm−1 that were previously assigned to the inserted OScCO molecule in solid argon.12 The observed red shift from neon to argon for the ScO stretching mode is slightly larger than usual matrix effect.68 This suggests that OScCO might be weakly coordinated by argon atom(s) in solid argon matrix as reported previously for many transition metal oxide species.69 These two absorptions are observed on sample deposition and increase on annealing. Besides the above-mentioned species, a group of absorptions at 1926.2, 1792.7, 1042.6, 906.8, and 797.7 cm−1 appears on annealing after the OScCO absorptions. The spectra in selected regions from the experiments using the isotopic-labeled 13CO2
Table 1. Observed Neon Matrix and Calculated (B3LYP/AUG-cc-pVTZ) Vibrational Frequencies (cm−1) for ScCO3 Sc13CO3
ScCO3 mode υ1 υ2 υ3 υ4 υ5 υ6 υ7 υ8 υ9
(CO str) (CO2 sym str) (ScO2 sym str + CO2 scissor) (ScO2 scissor) (CO3 out-of-plane deform.) (ScO2 in-plane bend) (CO2 asym str) (CO3 in-plane deform.) (ScO2 asym str)
obs
a
1798.7 (1.0) 911.0 (0.2) 823.3 (0.2) 787.1 (0.1) 1038.9 (0.3)
calcd
b
1831.2(1006) 934.4(120) 814.7(241) 472.0(58) 795.6(16) 146.3(5) 1049.8(222) 643.2(0) 394.5(4)
ScC16O18O18O
obs
calcd
obs
calcd
1756.3 906.9 820.8
1784.9(939) 931.0(121) 814.0(249) 469.5(56) 771.2(16) 146.1(5) 1023.3(214) 643.3(0) 391.7(3)
1774.5 882.1 802.1
1795.7(993) 903.3(105) 793.2(232) 463.2(55) 789.1 (15) 141.7(5) 1040.4(215) 619.8(1) 384.9(4)
783.7 1014.4
767.7 1029.0
a
The values in parentheses are the integrated intensities normalized to the most intense absorption. bThe calculated IR intensities are listed in parentheses in km/mol. C
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the band position of Sc(16O,16O)C18O is even lower than that of Sc(16O,18O)C18O, which suggests the existence of Fermi resonance with the overtone or combination of low modes (most likely with the overtone of the symmetric CO2 stretching mode). The 1038.9 cm−1 absorption shifts to 1014.4 cm−1 with 13 CO2 and to 1029.0 cm−1 with C18O2. The band position and isotopic shifts are indicative of an antisymmetric CO2 stretching vibration of the CO3 moiety. The 911.0 cm−1 absorption with smaller carbon-13 isotopic shift and larger oxygen-18 isotopic shift than those of the 1038.9 cm−1 absorption is appropriate for the symmetric CO2 stretching mode. The 823.3 and 787.1 cm −1 absorptions are assigned to the scissoring and deformation modes of the CO3 moiety. The mixed isotopic spectral features shown in Figures 2 and 3 confirm the involvement of a CO3 fragment with two equivalent oxygen atoms. We carried out quantum chemical calculations using the DFT method to investigate the structure and vibrational spectrum of the ScCO3 molecule. DFT calculations predict that ScCO3 has a 2A1 ground state with planar C2v symmetry (Figure 7). The CO3 moiety coordinates to the Sc center in a bidentate fashion. The calculated vibrational frequencies of ScCO3 are also listed in Table 1. The calculated vibrational frequencies are slightly higher than the experimental data as expected, which is a common and well-known result of the harmonic approximation and approximate exchange−correlation functional used. The noble-gas matrix effect, which is not considered by the calculations, is another factor that contributes to the difference between the matrix experimental and computed values. The calculated relative IR intensities and isotopic frequency shifts are in good agreement with the experimental values except the ν5 mode. The disagreement might be caused by Femi resonance with the combination or overtone of low modes. OCScCO3. The absorptions at 1926.2, 1792.7, 1042.6, 906.8, and 797.7 cm−1 observed in the Sc + CO2 experiments are assigned to different vibrational modes of OCScCO3, a carbonate carbonyl complex of scandium. The 1926.2 cm−1 absorption shifts to 1882.9 cm−1 with 13CO2 and to 1881.8 cm−1 with C18O2. The band position and isotopic frequency shifts clearly show that this absorption belongs to a terminal CO stretching vibration. The doublet spectral features presented in the experiments with the mixed samples are clear for the involvement of only one CO subunit in this mode. The band positions of the other absorptions are very close to those of ScCO3, suggesting the involvement of a ScCO3 moiety.
Figure 5. Difference infrared spectra in the 1950−1700 cm−1 region from codeposition of laser-evaporated scandium atoms with isotopiclabeled CO2 in excess neon: (a) 0.075% CO2; (b) 0.075% 13CO2; (c) 0.05% 12CO2 + 0.05% 13CO2; (d) 0.075% C18O2; (e) 0.1% (C16O2 + C16O18O + C18O2). Each spectrum was taken after 15 min of visible light (λ > 500 nm) irradiation minus spectrum taken after 12 K annealing.
Figure 6. Difference infrared spectra in the 1070−740 cm−1 region from codeposition of laser-evaporated scandium atoms with isotopiclabeled CO2 in excess neon: (a) 0.075% CO2; (b) 0.075% 13CO2; (c) 0.05% 12CO2 + 0.05% 13CO2; (d) 0.075% C18O2; (e) 0.1% (C16O2 + C16O18O + C18O2). Each spectrum was taken after 15 min of visible light (λ > 500 nm) irradiation minus spectrum taken after 12 K annealing.
Figure 7. Optimized structures of the OScCO, ScCO3, and OCScCO3 complexes formed from the Sc + CO2 and ScO + CO2 reactions (bond lengths in angstroms and bond angles in degrees). D
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Table 2. Observed Neon Matrix and Calculated (B3LYP/AUG-cc-pVTZ) Vibrational Frequencies (cm−1) for OCScCO3a O13CSc13CO3
OCScCO3 b
mode
obs
υ1 (CO str) υ2 (CO str) υ3 (CO2 sym str) υ4 (ScO2 sym str + CO2 scissor) υ5 (CO3 out-of-plane deform.) υ6 (ScO2 scissor + Sc−C str) υ11 (CO2 asym str) υ12 (CO3 in-plane deform.) υ13 (ScO2 asym str + CO3 in-plane deform.)
1926.2(0.94) 1792.7(1.00) 908.3(0.03) 797.7(0.11)
1042.6(0.37)
calcd
c
2015.2(1195) 1821.7(1189) 924.9(128) 805.4(176) 803.0(88) 487.9(53) 1040.4(309) 641.6(3) 419.1(4)
18
OCScC18O3
obs
calcd
obs
calcd
1882.9 1746.0 904.3 795.9 776.7
1969.6(1145) 1775.8(1105) 921.3(129) 803.9(252) 779.2(20) 485.6(51) 1014.1(297) 641.6(3) 415.4(4)
1881.8 1761.6 862.7 757.6 791.2
1967.7(1132) 1786.1(1175) 876.6(111) 763.8(219) 794.2(20) 478.1(49) 1021.1(294) 605.3(3) 406.3(5)
1017.9
1022.9
Only the frequecies above 400 cm−1 are listed. bThe values in parentheses are the integrated intensities normalized to the most intense absorption. The calculated IR intensities are listed in parentheses in km/mol.
a c
asymptote at the B3LYP level. The calculations at the CCSD(T) level give a slightly higher value of 36.5 kcal/mol. The OScCO (2A″) complex is bound by 39.8 or 43.3 kcal/mol with respect to the CO + ScO (2Δ) fragments at the B3LYP and CCSD(T) levels of theory. We analyze the nature of the donor−acceptor interactions in OScCO and OCScCO3 with the EDA (energy decomposition analysis) in conjunction with the NOCV (natural orbitals for chemical valence) method,71 which gives a detailed insight into the bonding situation. The numerical results of the OScCO and OCScCO3 interactions at the BP86/TZ2P level are listed in Table 3. The calculated values for the intrinsic interactions
The spectral features observed in the experiments with the 12 CO2 + 13CO2 and C16O2 + C16O18O + C18O2 mixed samples confirm the involvement of a CO3 fragment with two equivalent oxygen atoms. Taking the antisymmetric CO2 stretching mode observed at 1042.6 cm−1; for example, this mode splits into a doublet in the 12CO2 + 13CO2 spectrum, whereas a sextet is observed in the experiment with the C16O2 + C16O18O + C18O2 sample (Figure 6), indicating the major involvement of two equivalent O atoms that is coupled by a third inequivalent O atom. The 797.7 cm−1 absorption splits into two separated absorptions at 795.9 and 776.7 cm−1 with 13 CO2 and at 791.2 and 757.6 cm−1 with C18O2, indicating that the 797.7 cm−1 absorption includes two unresolved vibrational modes. The one with large carbon-13 and small oxygen-18 isotopic frequency shifts can be attributed to the out-of-plane CO3 deformation mode, whereas another one with small carbon-13 and large oxygen-18 isotopic frequency shifts is assigned to the symmetric ScO2 stretching mode. As shown in Figure 7, the OCScCO3 molecule is predicted to have a 2A′ ground state with nonplanar Cs symmetry. The carbonate group coordinates to the scandium center in a η2O,O fashion. The carbonyl ligand coordinates to the scandium center terminally via the carbon atom with a predicted Sc−C bond length of 2.249 Å. The observed and calculated vibrational frequencies are compared in Table 2. The good agreement between the experimental and calculated frequency shifts for all isotopomers leaves no doubt that the assignment of the vibrational frequencies to OCScCO3 is correct. Bonding Analysis. As discussed above, the ScCO 3 molecule has a 2A1 ground state. The unpaired electron occupies an a1 orbital, which is primarily a nonbonding hybrid of the Sc 4s and 3dσ orbitals. According to the natural bond orbital (NBO) analysis,70 the scandium atom possesses a rather large positive charge of +1.25 e. Thus, ScCO3 can be viewed as a charge transfer complex Sc2+(CO3)2− with the Sc center in a formal oxidation state of +II. The electronic ground state of OCScCO3 is 2A′ with the unpaired electron occupying an a′ orbital. This a′ orbital is primarily a nonbonding dπ orbital of the ScCO3 moiety that comprises significant Sc to CO 2π* back-donation. Therefore, the ground state OCScCO 3 correlates to a 2B1 excited state of ScCO3, which is predicted to lie 18.2 (B3LYP) or 22.6 kcal/mol (CCSD(T)) higher in energy than the 2A1 ground state of ScCO3. For the analogue OScCO molecule, it is characterized to have a 2A″ ground state that correlates to an excited 2Δstate of ScO.12,33−35 The dissociation energy of OCScCO3 is calculated to be 35.0 kcal/ mol with respect to the CO + ScCO3 (2B1) dissociation
Table 3. EDA-NOCV Results of the Chemical Bonding in OC−ScY (Y = O, CO3) at BP86/TZ2P (Energy Values in kcal/mol) OC−ScO
OC−ScCO3
fragment
CO + ScO (2Δ)
CO + ScCO3 (2B1)
ΔEint ΔEPauli ΔEelstat ΔEorb ΔEorb σ ΔEorb π ΔEorb rest
−54.1 70.8 −37.4 −87.6 −44.9 −32.3 −10.3
−45.0 99.5 −55.4 −89.1 −33.1 −41.1 −14.9
ΔEint exhibit the same trend as the bond dissociation energies calculated at the B3LYP and CCSD(T) levels. The data show that the two species have very similar orbital interactions. For both species, the OC → ScY (Y = O, CO3) σ donation and the OC ← ScY π back-donation interactions both are quite strong. The σ donation is slightly stronger than the π back-donation in OScCO, whereas the π back-donation is slightly stronger than the σ donation in OCScCO3. It is well-known that metal to CO π back-donation has a much stronger impact on the C−O stretching frequency than CO to metal σ donation.72−74 The CO stretching frequencies of OScCO and OCScCO3 are redshifted by more than 200 cm−1 with respect to free CO due to distinct π back-donation. This is quite different with the recently reported OCBeCO3 and OCCrCO3 complexes, which exhibit blue-shifted CO stretching frequencies.21,75 It has been shown that the OCBeCO3 carbonyl complex exhibits very little Be to CO π back-donation and that the bonding interactions come mainly from CO to Be σ donation.75 Reaction Mechanisms. The spectra in Figure 1 clearly demonstrate that the ground state scandium monoxide E
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without any barrier. This process is predicted to be exothermic by 29.8 (B3LYP) or 35.7 kcal/mol (CCSD(T)).
molecules react with carbon dioxide in solid neon to form the ScCO3 molecules. The ScCO3 absorptions increase on annealing, suggesting that the reaction is spontaneous with negligible activation energy. The potential energy profile along the ScO + CO2 → ScCO3 reaction path calculated at the B3LYP level is shown in Figure 8. The reaction proceeds with
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CONCLUSIONS Carbon dioxide fixation and activation by scandium atom and scandium monoxide molecule have been investigated by matrix isolation infrared absorption spectroscopy and density functional theory calculations. The scandium monoxide molecules prepared by pulsed laser evaporation of bulk metal oxide target react with carbon dioxide in solid neon to form the scandium carbonate ScCO3 complex spontaneously on annealing. The Sc atom reacts with carbon dioxide to form the previously reported OScCO insertion molecule, which further reacts with additional CO2 molecule in forming the OCScCO3 complex on annealing. The OScCO and OCScCO3 complexes exhibit redshifted CO stretching vibrations with respect to free CO. Quantum chemical calculations show that both the OC → ScY σ donation and the OC ← ScY π back-donation interactions are quite strong in these donor−acceptor complexes OC-ScY (Y = O, CO3). The observation of the spontaneous formation of ScCO3 and OCScCO3 on annealing is consistent with theoretical predictions that both the Sc + 2CO2 → OCScCO3 and ScO + CO2 → ScCO3 reactions are thermodynamically exothermic and each elementary reaction steps are kinetically facile, requiring little or no activation energy.
Figure 8. Potential-energy profile (after ZPE correction, in kcal/mol) for the ScO + CO2 reaction. The relative energies were calculated at the B3LYP/AUG-cc-pVTZ and CCSD(T)//B3LYP (in parentheses) levels.
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ASSOCIATED CONTENT
* Supporting Information
the initial formation of a 1:1 OSc(η1-OCO) complex without any barrier. The complex is predicted to have a 2A′ ground state with a planar Cs structure, in which the CO2 ligand is η1-O endon coordinated to the Sc center of ScO. This complex has a binding energy of 5.7 (B3LYP) or 5.3 kcal/mol (CCSD(T)) with respect to the ground state reactants: ScO (2Σ) and CO2. The OSc(η1-OCO) complex rearranges to the ScCO3 isomer via a transition state (TS1). This isomerization process is exothermic by 15.5 (B3LYP) or 17.1 kcal/mol (CCSD(T)) with a very low barrier of 0.7 kcal/mol. The overall reaction is predicted to be exothermic by 21.2 (B3LYP) or 22.4 kcal/mol (CCSD(T)), and proceeds via a transition state lying 5.0 (B3LYP) or 4.6 kcal/mol (CCSD(T)) lower in energy than the ground state reactants. Owing to the low barrier, the OSc(η1OCO) complex intermediate cannot be trapped in solid neon matrix. This result is quite different from the TiO + CO2 and NbO + CO2 reactions,29,30 in which the OTi(η1-OCO) and ONb(η1-OCO) complexes are formed, which rearrange to the O2Ti(η1-CO) and O2Nb(η1-CO) isomers instead of carbonate complexes under photoexcitation. Both the inserted OScCO and OCScCO3 species are observed in the Sc + CO2 experiments. The Sc + CO2 reaction has been studied previously. Gas phase kinetic experiment implies that the reaction has an activation barrier of 2.9 kcal/ mol.8 Theoretical calculations indicate that the lowest energy path corresponds to the initial formation of a η2-C,O coordination complex followed by the insertion of Sc into a C−O bond to give the OScCO insertion intermediate.34,35 The insertion reaction is predicted to be exothermic and barrierless,35 in agreement with experimental observation that the OScCO species is formed spontaneously on annealing in solid argon and neon matrices.12 The OCScCO3 absorptions increase on annealing after the OScCO absorptions, implying that OCScCO3 is formed via the inserted OScCO intermediate. Our calculations indicate that the OScCO molecule can react with CO2 to form the OCScCO3 complex spontaneously
S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b11809. Calculated geometries, vibrational frequencies, and intensities; complete refs 1 and 54 (PDF)
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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 The work was supported by the Ministry of Science and Technology of China (2013CB834603 and 2012YQ220113-3) and the National Natural Science Foundation of China (grant nos. 21173053 and 21433005).
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