Reactions of Group 3 Metals with OF - American Chemical Society

Sep 20, 2012 - CCSD(T) frequencies for selected molecules were obtained by numerical differentiation of the energies. The MOLEKEL26 molecular graphics...
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Reactions of Group 3 Metals with OF2: Infrared Spectroscopic and Theoretical Investigations of the Group 3 Oxydifluoride OMF2 and Oxyfluoride OMF Molecules Yu Gong,† Lester Andrews,*,† and Charles W. Bauschlicher, Jr.‡ †

Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States Entry Systems and Technology Division, NASA Ames Research Center, Mail Stop 230-3, Moffett Field, California 94035-0001, United States

‡

ABSTRACT: The oxidifluoride molecules, OYF2 and OLaF2, are produced via the reactions of laser ablated metal atoms with OF2 in solid argon. The product structures are characterized using matrix isolation infrared spectroscopy as well as theoretical calculations. Similar to the very recently characterized OScF2 molecule, OYF2 is predicted to have a 2B2 ground state with C2v symmetry while the heavier OLaF2 has a 2A″ ground state with near C2v symmetry. The unpaired electron is mainly located on the terminal oxygen atom, suggesting radical character for the group 3 OMF2 molecules. In addition, the closed shell singlet OMF molecules with bent geometries are also observed, and they are found to have triple metal−oxygen bonds with higher stretching frequencies and shorter bond lengths than their OMF2 counterparts. α-Fluorine transfer from OF2 to metal centers is predicted to be highly exothermic, which is very favorable for the formation of new OMF2 and OMF species.

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INTRODUCTION As bridges between transition metal oxides and fluorides, the oxyfluoride compounds have attracted great interest due to their potential intermediate properties, which can be used to adjust the electrical and magnetic properties of either oxides or fluorides.1 A series of transition metal containing oxyfluoride compounds with different compositions and crystalline structures was synthesized and structurally characterized several decades ago, providing rich information on the properties of these oxyfluoride species in the solid state.1 Despite the progress made in solid state chemistry, little is known about these species at the molecular level, which contrasts the wellestablished binary transition metal oxide and fluoride molecules.2,3 Unlike the reactions between solid state oxides and fluorides at high temperature,1 it is quite efficient and specific to make oxyfluoride molecules in cryogenic matrixes via the reactions of metal atoms and OF2, which has been demonstrated by the formation of the thorium and uranium as well as the group 4 OMF2 molecules.4,5 Infrared spectroscopy and theoretical calculations revealed that these tetravalent oxyfluoride molecules possess terminal oxo ligands with triple bond character, and that the bifluoride transfer reactions are highly favorable thermodynamically. Very recently, two oxyfluoride molecules OScF and OScF2 were identified as reaction products of scandium and OF2.6 While the former species is structurally similar to those of the group 4 OMF2 molecules in spite of the trivalent character of scandium, a scandium−oxygen single bond with a terminal oxo radical was found in the OScF2 case. The OHgF radical was also characterized to have a radical oxygen ligand, which retains the II oxidation state of mercury.7 Compared to the well-known examples of charged metal oxide ions containing radical oxygen ligands in the gas phase,8−11 © 2012 American Chemical Society

information about the corresponding neutral species is quite limited. A few molecules such as group 6 and 7 MO4 as well as group 5 M2O6 clusters produced as photodetachment products of the corresponding anions were proposed to have radical oxygen centers.12−14 Evidence is also provided for the existence of transition metal containing oxygen species in the condensed phase.15,16 Finally, neutral radical species in the MOF2 form were predicted to be stable for the boron group elements.17 In this article, we report the formation and characterization of the heavier group 3 OMF and OMF2 (M = Y, La) molecules. These species are produced via the spontaneous reactions of laser ablated yttrium and lanthanum atoms and OF2. Similar to the scandium analogues, the heavier group 3 OMF molecules are also found to have triple MO bonds, whereas the OMF2 molecules possess a MO single bond with a terminal oxo radical.

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EXPERIMENTAL AND THEORETICAL METHODS The experimental apparatus and procedure for preparation and characterization of the product molecules in excess argon at 4 K have been described previously.18 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a freshly cleaned yttrium or lanthanum target mounted on a rotating stainless steel rod. These laser-ablated metal atoms were codeposited with 3−4 mmol of argon (Matheson, research) containing 1.0% OF2 (Ozark-Mahoning) onto a CsI cryogenic window for 60 min. The 18OF2 sample (91% 18O enriched) was synthesized and Received: August 9, 2012 Revised: September 19, 2012 Published: September 20, 2012 10115

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kindly provided by Arkell and co-workers.19 Both OF2 and 18 OF2 samples were used without further purification in a passivated stainless steel, vacuum manifold. FTIR spectra were recorded at 0.5 cm−1 resolution on a Nicolet 750 FTIR instrument with a HgCdTe range B detector. Matrix samples were annealed at different temperatures and cooled back to 4 K for spectral acquisition. Selected samples were subjected to broadband photolysis by a medium-pressure mercury arc street lamp (Philips, 175W) with the outer globe removed. Complementary density functional theory (DFT) calculations were performed using the Gaussian03 program.20 The hybrid B3LYP density functional was employed in our calculations.21 The 6-311+G(d) basis set was used for oxygen, fluorine, and scandium, and the 28 electron core SDD pseudopotential was used for yttrium and lanthanum.22,23 In one case where the B3LYP calculations and experiment showed an unexpected difference, we used the coupled cluster singles and doubles approach24 including the effect of connected triples determined using perturbation theory CCSD(T).25 Harmonic vibrational frequencies at the B3LYP level were obtained analytically at the optimized structures, while the CCSD(T) frequencies for selected molecules were obtained by numerical differentiation of the energies. The MOLEKEL26 molecular graphics program was used to view vibrational modes and the molecular orbitals (MOs). The bonding was analyzed by plotting the orbitals obtained using the natural bond order (NBO) technique.27 The NBOs were more localized than the DFT MOs, and for OScF2, the NBOs were found to be in much better agreement with the natural orbitals (NOs) obtained from a configuration interaction calculation including single and double excitations (CISD). Since the CISD NOs and DFT NBOs yield very similar descriptions of the bonding, we believe that the NBOs yield a more realistic picture of the bonding than do the delocalized DFT orbitals.

(not shown) is due to the vibrational fundamental of the OF radical, which is common in all the experiments involving OF2 as reactant.28,29 The OF radical is produced here during deposition by irradiation from the laser ablation plume on the target surface in front of the cold window.18a Other minor common absorptions such as CF4, CO, CO2, and FOO30 were also observed in the infrared spectra with very low intensities. In the infrared spectra of yttrium reaction products, absorptions due to YF2+, YF3, and YF4− were present in the spectra, the positions of which are almost the same as those reported in the reactions of yttrium and F2.31 Additionally, the weak absorption at 872.0 cm−1 assigned to the argon solvated YO+ cation was also observed right after sample deposition.32 New absorptions at 560.7, 557.9, and 532.2 cm−1 as well as 799.4 and 512.9 cm−1 (Figure 1, trace a) increased slightly on annealing. Broad band irradiation favored the first group, while no change was found for the absorptions in the second group. All of these bands decreased upon further sample annealing following irradiation. In the reactions of laser-ablated yttrium and 18OF2, the new product absorptions were observed at 560.7, 557.7, 507.1, 760.9, and 512.9 cm−1 (Figure 1, trace b), while no shifts were observed for the yttrium fluoride species. The behavior of the new absorptions during UV irradiation and subsequent annealing were almost the same as observed in the OF2 experiments. Similarly, new absorptions at 502.3, 478.3, and 453.2 cm−1 as well as at 750.2 and 446.5 cm−1 were observed when lanthanum reacted with OF2 in argon in addition to the previously reported LaF2+, LaF3, and LaO+ absorptions.31,33 All of the new product absorptions are listed in Table 1. Oxydifluoride OMF2. In the reactions of yttrium and OF2, the 560.7 and 557.9 cm−1 absorptions are assigned to the antisymmetric and symmetric F−Y−F stretching modes of the new product molecule based on the band positions as well as the very small 18O shifts. The symmetric F−Y−F stretch is expected to have more 18O shift depending on the coupling between this mode and metal−oxygen stretching mode, as observed in the OScF2 case.6 Note that the antisymmetric YF3 stretching mode absorption31 overlaps with the new 560.7 cm−1 absorption in the yttrium experiments. The third band at 532.2 cm−1 shifted to 507.1 cm−1 in the reactions with 18OF2 sample. The oxygen 16O/18O isotopic frequency ratio of 1.0495 is quite close to the value for the YO diatomic molecule (1.0505),32,33 suggesting a Y−O stretching mode for this new molecule. As a result, the OYF2 structure is proposed based on observations of the 560.7, 557.9, and 532.2 cm−1 absorptions. Following the yttrium case, the three absorptions at 502.3, 478.3, and 453.2 cm−1 in the lanthanum experiments can be assigned to the symmetric and antisymmetric F−La−F stretching modes, and the La−O stretching mode of the new OLaF2 molecule. Also note that the LaF3 absorption31 overlaps with the new 478.3 cm−1 absorption in the lanthanum experiments. The antisymmetric F−M−F stretching mode, which typically has no 18O shift, is usually more intense than the symmetric counterpart. The large 21.2 cm−1 18O shift and 16/18 isotopic frequency ratio of 1.0491 are appropriate for the La−O stretching mode although this 16/18 isotopic frequency ratio is lower than the value for the LaO diatomic molecule (1.0537)32,33 owing here to slight mode mixing with the symmetric F−M−F stretching mode, which results in a 0.9 cm−1 18O shift for the latter. For the OYF2 molecule, the computed frequencies are in reasonable agreement with experiment. However, the difference

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RESULTS AND DISCUSSION Selected infrared spectra obtained after reactions of laserablated yttrium and lanthanum atoms with 1.0% OF2 and 1.0% 18 OF2 in argon are shown in Figure 1. The band at 1028.1 cm−1

Figure 1. Infrared spectra of laser-ablated yttrium and lanthanum atoms and isotopically substituted OF2 reaction products in solid argon: (a) Y + 1.0% 16OF2, after annealing to 20 K; (b) Y + 1.0% 18 OF2, after annealing to 20 K; (c) La + 1.0% 16OF2, annealing to 20 K; and (d) La + 1.0% 18OF2, after annealing to 20 K. 10116

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Table 1. Vibrational Frequencies Observed in Solid Argon and Calculated Frequencies for Group 3 OMF2 and OMF Molecules Using 16OF2 and 18OF2 Reagentsa 18

OF2 OScF2b

OYF2e

OLaF2f

OScF OYF OLaF

mode

obsd

Sc−O str. modec antisym. F−Sc−F sym. F−Sc−F Y−O str. mode antisym. F−Y−F sym. F−Y−F La−O str. antisym. F−La−F sym. F−La−F Sc−O str. Sc−F str. Y−O str. Y−F str. La−O str. La−F str.

d 689.0 665.3 532.2 560.7 557.9 453.2 478.3 502.3 912.7 588.0 799.4 512.9 750.2 446.5

calcd 597.8 700.2 676.6 538.6 581.9 570.3 469.2 486.8 506.0 964.9 608.3 824.9 538.8 770.9 451.8

(1) (324) (248) (17) (243) (174) (185) (275) (37) (272) (197) (210) (155) (267) (180)

obsd d 688.5 655.4 507.1 560.7 557.7 432.0 478.3 501.4 874.8 587.7 760.9 512.9 711.9 446.5

OF2 calcd 580.9 700.1 666.8 516.5 581.9 566.1 446.8 486.8 504.4 924.4 608.0 785.1 538.8 731.5 451.7

(11) (324) (231) (47) (242) (137) (151) (275) (61) (257) (194) (192) (154) (240) (180)

Only frequencies above 400 cm−1 are listed, and the frequencies were not scaled. Numbers in parentheses are infrared intensities (km/mol). Frequencies of OScF2 and OScF are taken from ref 6. cThe Sc−O str. mode is strongly mixed with symmetric F−Sc−F str. mode. dToo weak to be observed. eCalculated frequencies are listed for the 2B2 ground state of OYF2. The frequencies calculated at the CCSD(T) level are 542.6 (a1), 594.5(b2), and 590.4 (a1); 18OYF2, 517.0 (a1), 594.5 (b2), and 589.9(a1) cm−1. fCalculated frequencies are given for the 2A″ ground state of OLaF2 a b

values as listed in Table 1. Such crossover in vibrational frequencies was also observed for the recently characterized group 3 difluoride molecules,31 which probably has a mechanical origin. Figure 2 shows the optimized structures of OYF2 and OLaF2 together with the structure for OScF2.6 The M−O bond length

in isotopic shifts for the symmetric F−Y−F and Y−O stretching vibrations shows a larger variation and mode mixing than would have been expected (Table 1). The 18O shifts for these modes at the B3LYP level are 4.1 and 22.2 cm−1 compared with the analogous experimental values of 0.2 and 25.1 cm−1. Changing the basis set, pseudopotential, or functional can reduce the symmetric F−Y−F mode isotopic shift somewhat, but it remains larger than experiment. However, switching to the more rigorous CCSD(T) approach yields isotopic shifts of 0.5 and 25.5 cm−1, which are in good agreement with experiment. Apparently the DFT approach is not describing the mixing of the symmetric F−Y−F and Y−O stretching vibrations as accurately for OYF2 as for the other molecules. It should be noted that we also found an excited 2A′ state with the unpaired electron in the O 2p out-of-plane orbital. This state is 4.9 kcal/mol higher in energy at B3LYP level with the frequencies very similar to those of the 2B2 state. However, the calculated isotopic shifts for the symmetric F−Y−F and Y− O stretches of the 2A′ state (0.2 and 25.1 cm−1) are closer to experiment than those of the 2B2 state. At the CCSD(T) level, this state is 6.3 kcal/mol above the 2B2 state, with isotopic shifts of 0.1 and 25.7 cm−1. On the basis of the B3LYP calculations one might be tempted to assign the ground state as 2A′; however, in light of the CCSD(T) calculation that predicts a 2 B2 state ground state with isotopic shifts in agreement with experiment, we feel that the ground state is more likely 2B2 than 2 A′. The OLaF2 molecule is predicted to have a 2A″ ground state, and the calculated frequencies as well as the isotopic shifts are in good agreement with the experimental values (Table 1). The 2 A″ state has the oxygen open shell orbital in the same relative orientation as the 2B2 state, i.e., if the molecule were planar, the open shell orbital would be in the molecular plane. Note that the symmetric F−M−F stretching vibrational modes for scandium and yttrium are 23.7, 2.8 cm−1 lower than the antisymmetric modes, while the symmetric F−La−F mode is 24.0 cm−1 higher. These shifts agree well with the calculated

Figure 2. Optimized structures (bond lengths in angstrom units and bond angles in degrees) for the group 3 OMF2 and OMF reaction product molecules computed at the B3LYP/6-311+G(d) level of theory. The structures of OScF2 and OScF were computed with results from ref 6.

is predicted to be 2.124 Å for yttrium, 2.221 Å for lanthanum, and 1.938 Å6 for scandium. All of these values are in the same region as the corresponding bond lengths calculated for the group 3 M(OH)x and CH3OMCH3 molecules.34,35 Compared with the recently proposed single metal−oxygen bond lengths from covalent radii for group 3 metals,36 the values of OYF2, OLaF2, and OScF2 are about 0.2 Å shorter. Figure 3 illustrates selected molecular orbitals of OScF2 to help understand the metal−oxygen bonding in the group 3 OMF2 molecules. The σ bond between metal and oxygen atoms is composed mostly of a Sc 3d4s hybrid and O 2p orbitals. The unpaired electron is found to reside on the O in-plane 2p orbital, which is mainly 10117

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The nature of the terminal oxo bonds in the group 3 OMF2 molecules is quite different from that of group 4 as well as uranium and thorium analogues.4,5 Since these metals have four or more valence electrons, it is possible to form metal oxygen multiple bonds in the OMF2 molecules. The limit of three valence electrons for group 3 metals cannot support a multiply bonded oxo ligand in addition to the two metal fluorine bonds. However, multiple bonding is still possible if only one fluorine atom is connected to the metal center, which will be described in detail next. Oxyfluoride OMF. The other new product in the reactions of yttrium and OF2 absorbs at 799.4 and 512.9 cm−1. For the first band, it should be due to the Y−O stretching vibration on the basis of the band position as well as the characteristic 16 O/18O isotopic frequency ratio (1.0506) given by the experiment with 18OF2, which is almost the same as that for the YO molecule (1.0505).33 The lower band at 512.9 cm−1 exhibited no 18O shift, indicating a Y−F stretch as in the binary yttrium fluorides, which absorb in the same region. Since both the Y−F and Y−O stretching modes are observed, the second new molecule observed when yttrium reacted with OF2 is assigned to the triatomic OYF molecule. The OLaF molecule absorbs at 750.2 and 446.5 cm−1, and its 18O counterpart bands were observed at 711.9 and 446.5 cm−1. This evidence identifies La−O and La−F stretching modes for the analogous new trivalent OLaF molecule. These assignments for the OMF molecules are supported by density functional calculations at B3LYP level of theory. All three of these molecules are predicted to have closed shell singlet ground states (1A′) with bent geometries (Figure 2). The Sc−O bond length is the shortest followed by Y−O and La−O. The latter two bond lengths only differ by about 0.01 Å. All of these values are close to the bond lengths for other group 3 complexes with terminal oxo ligands calculated at the same level of theory and basis set.38,39 A similar increase is also found for the M−F bond distances, but the difference between the Y− F and La−F lengths is larger than those for Y−O and La−O. Frequency calculations give three vibrational modes for these new OMF molecules. Since the bending mode is around 150− 200 cm−1, far below our spectral limit, we will only focus on the two stretching modes above 400 cm−1. In the OYF case, the 824.9 cm−1 frequency given by calculations together with the isotopic frequency ratio of 1.0507 is in line with the experimental observations (799.4 cm−1 and 1.0506 in Table 1). No 18O shift is computed for the Y−F stretch of the 18OYF molecule as observed in our experiments. Similar with the yttrium case, both the band positions and isotopic frequency ratios for the OLaF molecule are in line with the experimental results (Table 1). Note that a small 18O shift is found for the Sc−F stretch of OScF,6 while the M−F stretching frequencies of the two heavier analogues are not affected by 18O substitution. This is the result of the increase in metal atomic weight, which reduces the coupling between M−O and M−F stretching modes. The M−O bond lengths in all the group 3 OMF molecules are about 0.25 Å shorter than those in the OMF2 molecules, consistent with the higher M−O stretching frequencies. The calculated molecular orbitals show both σ and π interactions between the group 3 metal and the terminal oxygen atom, as exemplified in Figure 3 for OScF. The difference in the bonding between OScF2 and OScF is clearly visible. The σ orbital arises mainly from the overlap of metal ds hybrid and O 2p orbitals, which is similar to that found in OScF2. The out-of-plane 2p

Figure 3. Selected NBO orbitals for OScF2 and OScF (computed with results from ref 6) as well as spin density profile (bottom) for OScF2.

nonbonding in character. The third orbital plotted is essentially an O lone pair π orbital. Similar bonding interactions are found for the OYF2 and OLaF2 molecules. As a result, the M−O bonds in the group 3 OMF2 molecules are close to single bonds. Consistent with this notion, the M−O stretching frequencies of all the OMF2 molecules are about 300 cm−1 lower than typical terminal M−O stretching frequencies,32,33 which consequently fall into the regions of group 3 metal fluoride absorptions.31 The radical character is also illustrated by the calculated spin density profile, which is also shown for OScF2 in Figure 3. This nicely illustrates that the unpaired electron is located primarily on the oxygen atom for the OScF2 molecule. The structures of group 3 OMF2 molecules are quite similar to the corresponding trifluoride molecules due to the essentially single M−O bond character. Both ScF3 and YF3 molecules were found to have planar geometries, while the LaF3 molecule possessed a pyramidal structure.31 The change in geometries of the OMF2 molecules follows that of the trifluorides. It is common for the geometries of group 3 MX3 species to change from planar to nonplanar due to the increase in participations of metal d orbitals in forming the M−X bond.37 The M−O bond lengths are about 0.1 Å longer than the values of M−F bonds in the same molecule as well as in the trifluoride molecules, which can be considered as a result of the slightly larger radius of oxygen than fluorine. For the vibrational frequencies, the antisymmetric F−Y−F and F−La−F stretching frequencies of OYF2 and OLaF2 are almost the same as those of YF3 and LaF3. Mulliken charge calculations reveal that the metal centers in the OMF2 molecules are positively charged by 1.09, 1.28, and 1.26 down the group, about 0.5 lower than the values for the corresponding trifluoride molecules.31 This probably arises from the smaller electronegativity of oxygen compared to fluorine. 10118

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orbital of O interacts with the metal d orbital to give the π bond, in contrast to OScF2. As a result, a double metal−oxygen bond is formed, which is expected to be stronger than the single σ bond, as reflected by the change in bond lengths. In addition, a dative π bond mainly containing oxygen in-plane orbitals also contributes to bonding in the group 3 OMF molecules, as in the group 4 as well as uranium and thorium OMF2 cases.4,5 Thus, the OMF molecules also possess triple bond character. Interestingly, the M−O distances in the OMF molecules are close to the sum of proposed triple bond radii for group 3 metal and oxygen atoms.40 The crystal structures for group 3 oxyfluorides with supposed OMF stoichiometries were reported previously.41 In these early structural studies, the M−O distances in the solids range from about 2.4 to 2.2 Å, which are much longer (0.7 to 0.4 Å, respectively) than our computed values for the OMF molecules, but they are closer to our computed M−O bond lengths for the OMF2 molecules. This shows that M−O single bonds exist in the crystal structures. The M−F distances in OScF and OYF crystals were reported to be 0.5 and 0.4 Å longer than our computed structural parameters for both OMF and OMF2 molecules (M = Sc, Y), but the calculated La−F bond length is only about 0.1 Å shorter than that reported in 1941 for an OLaF crystal. Reactions in the Matrix. The experimental observations for the reactions of yttrium and lanthanum, as well as scandium6 and OF2, reveal that all of the OMF2 molecules are produced spontaneously without photoexcitation, and neither the M(OF2) complex nor the one fluorine transfer product FOMF is stabilized. Attempts to compute structures for the M(OF2) complexes failed owing to convergence to the experimentally characterized OMF2 molecules. Although the FOYF molecule is predicted to be stable as found for FOScF,6 it is not possible to locate the optimized structure for FOLaF at the B3LYP level of theory since it proceeds to the OLaF2 structure during optimization. The FOYF molecule with an end-on bonded OF group is predicted to be 90 kcal/mol higher in energy than the OYF2 molecule, and no infrared absorptions can be assigned to this insertion product on the basis of its calculated frequencies. Similar to the results for group 4 metals as well as uranium and thorium,4,5 the M−F bonds for group 3 metals are much stronger than the O−F bond.42 Our B3LYP calculations find that formation of the group 3 OMF2 molecules is highly exothermic by 310,6 304, and 333 kcal/mol for ground state scandium,6 yttrium, and lanthanum atoms (reaction 1).

M + OF2 → OMF2

instead. This reaction is calculated to be endothermic by 99 and 83 kcal/mol for yttrium and lanthanum respectively, close to the value for Sc (97 kcal/mol).6 No growth was observed for the OMF molecules upon λ > 220 nm irradiation, suggesting that the photons produced by the mercury arc lamp are probably not high enough to overcome the barriers for fluorine loss reactions. Hence, it is most likely for the decomposition of OMF2 to occur in the gas phase during sample deposition where the exothermicity of reaction 1 exceeds the endothermicity of reaction 3 by 238, 199, and 1496 kcal/mol for yttrium, lanthanum, and scandium, respectively. It is quite common for the initially formed molecules to decompose in the gas phase due to their energetic characters.43 In addition, the decomposition of OMF2 can also occur in the solid matrix via fluorine loss upon high energy irradiation. The vacuum UV irradiation produced by the laser ablating metal target is high enough in energy44 to initiate the dissociation of the species being trapped in the matrix, where F atoms can diffuse away from their site of photoproducton.45 M + OF → OMF

(2)

OMF2 → OMF + F

(3)

Previous studies on the group 3 metal atom reactions with water provide interesting contrasts. These reactions revealed that the HMOH molecules were produced spontaneously on annealing, which were also found to be the most stable isomer among the possible isomers considered.46 Since the O−H bond is much stronger than the O−F bond, while the case reverses for M−F and M−H bonds,42 the overall reaction pathway should be different when the fluorine atoms are replaced by hydrogen. As expected, our calculations on the OMH2 and HOMH molecules at the B3LYP/6-311++G(d,p)/SDD level of theory indicate that the former radical species are less stable than the latter insertion products by 30−50 kcal/mol, which is completely different from the case for the fluorine substituted analogues investigated here. Additionally, H2 elimination together with the formation of group 3 monoxide molecules were observed when the sample was irradiated by UV−visible photons in the H2O reactions, while no monoxide band was observed throughout the OF2 reactions. On the basis of our calculations, F2 elimination (reaction 4) is endothermic for the scandium, yttrium, and lanthanum reactions by 203, 199, and 193 kcal/mol, respectively, much higher than those for H2 elimination.46 However, binary group 3 metal fluoride absorptions increased together with the increase of the OF band upon broad band irradiation. OF2 is known to decompose under UV irradiation to give the OF radical and a fluorine atom.28 Such fluorine atoms can react with group 3 metal atoms to form the corresponding binary metal fluoride products. Although the neutral metal monoxide molecules were not observed in the OF2 experiments, the corresponding cations were present in the infrared spectra, which should be the result of fluorine in the system serving as electron trap, as found for the more widely used chlorocarbon molecules.18,47 The metal difluoride cations are observed for the same reason. Unlike the absence of OMH2 molecules in the experiment,46 both OMF and OMH species were identified in the respective reactions. These kinds of triatomic products are expected to be stable enough for group 3 metals due to the trivalent character of the metal atom.

(1)

For the triatomic OMF molecules, it is reasonable for them to be produced upon the spontaneous reactions of metal atoms and OF radicals formed via photoinduced decomposition of OF2 during sample deposition. These reactions (reaction 2) are predicted to be exothermic with 237 (Y) and 282 (La) kcal/ mol of energy released, comparable to the value of 246 kcal/ mol for Sc.6 However, the infrared intensity of the OF radical in our experiments is much lower than that of unreacted OF2 present in the same matrix. If the calculated relative oscillator strength of the antisymmetric F−O−F (OF2) and the O−F (OF) stretching modes (2:1) is taken into consideration, the amount of OF2 relative to OF in the spectra taken right after sample deposition will be 10:1 to 50:1, which depend on the laser energy used. As a result, it is less probable for reaction 2 to be the major channel of OMF formation in our experiment, and decomposition of OMF2 (reaction 3) should be considered

OMF2 → MO + F2 10119

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CONCLUSIONS Group 3 metal reactions with OF2 have been investigated using matrix isolation infrared spectroscopy and theoretical calculations. Ground state group 3 metal atoms react with OF2 to give the OMF2 oxidifluoride (M = Y, La, and Sc) molecules spontaneously. Our B3LYP calculations reveal that all three product molecules possess C2v (Sc, Y) or near C2v (La) structures with M−O single bonds, which makes the OMF2 molecule a terminal oxo radical species. In addition, the OMF oxyfluoride molecules are also confirmed by comparison between experimental and theoretical frequencies. Calculations at the B3LYP level indicate that the OMF molecules have bent geometries with closed shell singlet ground states. The high frequencies as well as the short bond distances reveal triple bond character for the M−O bonds in the OMF molecules, which is consistent with the results from calculated molecular orbitals. Formation of these group 3 oxyfluoride species is predicted to be highly exothermic due to weak O−F bonds in the precursor and strong M−F bonds in the products. This also influences the reaction mechanism of group 3 metal atoms with OF2, which is quite different from that of its structural analogue H2O.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from DOE Grant DE-SC0001034 and helpful discussions with our colleague Lin Pu.

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REFERENCES

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