Infrared Spectroscopic and Theoretical Studies of the 3d Transition

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Infrared Spectroscopic and Theoretical Studies of the 3d Transition Metal Oxyfluoride Molecules Rui Wei,†,‡ Zongtang Fang,§ Monica Vasiliu,§ David A. Dixon,*,§ Lester Andrews,∥ and Yu Gong*,†,∥ †

Department of Radiochemistry, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China School of Nuclear Science and Technology, University of Chinese Academy of Sciences, Beijing 100049, China § Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States ∥ Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319, United States

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‡

S Supporting Information *

ABSTRACT: A collection of 3d transition metal (V, Mn, Fe, Co, and Ni) oxyfluorides were prepared via the reactions of laser-ablated metal atoms and OF2 in an argon matrix, and the products were identified by infrared spectroscopy together with 18OF2 substitution. OMF2 is the major product from the reactions of metal atoms and OF2. The tetravalent metal center is coordinated to two fluorine atoms and one oxygen atom. Triatomic OMF molecules were observed in the reactions of V, Mn, Fe, and Co with OF2. In addition to OMF and OMF2, OMnF3 and OFeF3 were also formed presumably via the reactions of OMnF and OFeF with F2 resulting from photodecomposition of OF2. The seldom observed OF radical was produced in all of these experiments. Electronic structure calculations at the density functional theory and molecular orbital theory including electron correlation effects (CCSD(T) and CASPT2) levels are used to aid in the assignment of the structures. For OMF (M = Sc−Mn), the structures are bent and those for M = Fe−Cu are linear. The OMF2 molecules are optimized to be C2v structures. Both OMF and OMF2 have a high spin ground state, with the exception of OCoF2 in which the ground state quartet is the lower energy structure. The M−O stretching frequency is a sensitive measure of the computational method in terms of the bond angle, the coupling of the M−O and M−F stretches, and the amount of spin on the oxygen. A bonding analysis in terms of the CAS orbitals shows that a number of the structures have a multireference character after M = Cr. Oxidation states of the metal are given based on the CASPT2 results. Heats of formation for the OMF and OMF2 are reported.

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fluoride molecules as compared to the direct evaporation of bulk materials synthesized by high-temperature reactions of solid state oxides and fluorides. Molecules in the forms of OMF2 and OMF were the major products from the reactions of metal atoms and OF2.10−17 Multiple M−O bonds with trivalent metal centers are common for the known OMF molecules,10−13 except for OHgF which contains Hg(II) and radical oxygen forming a single Hg−O bond.15 The known structures of OMF2 molecules are more diverse depending on the nature of the metal atoms. The OMF2 molecules for M = U, Th, and group 4 and 6 metal atoms have been characterized to have terminal M−O bonds with the metal centers in the +IV oxidation state.13,16,17 The radical character on the oxygen was identified for group 3 OMF2 molecules where the trivalent metal center is coordinated by two fluorine atoms and one oxygen radical,11,12 as well as for the group 11 metals.14 For the lanthanide oxyfluoride molecules, the

INTRODUCTION Transition metal oxyfluorides are important in a number of areas including energy production and storage, microelectronics, and catalysis. In addition to extensive studies on the preparation, morphology, functionalization, and application of oxyfluorides,1−3 studies of their crystal structures have provided insights into the relevant chemical and physical properties of solid state oxyfluorides. Whereas the crystal structures of most transition metal oxyfluorides with different stereochemistry have been summarized,4,5 only limited information is available for the metal oxyfluorides at the molecular level,6,7 in sharp contrast to the well-characterized binary transition metal oxide and fluoride molecules.8,9 Our recent studies revealed that metal oxyfluoride molecules can be readily prepared by the reactions of laser-ablated metal atoms and OF2 in a cryogenic argon matrix, which is highly favorable thermodynamically due to the formation of strong metal−F and metal−O bonds. This matrix reaction route is significantly more convenient for making simple metal oxy© XXXX American Chemical Society

Received: March 21, 2019

A

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

five 3d orbitals and the 4s orbital on the metal, three 2p orbitals on the oxygen, and three 2p orbitals on the F on the OMF and OMF2 molecules. The CAS calculations showed that the F orbitals were essentially always doubly occupied. We used the Feller−Peterson−Dixon (FPD) method48−50 as implemented previously28 for the calculation of the total atomization energies (TAE, ΣD0,0K) and the heats of formation of the MOF2 and OMF species. The optimized geometries at the CCSD(T)/aD level were used in single-point energy calculations at the second-order DK level with the all-electron aug-cc-pwCVXZ-DK (X = D, T, and Q) basis sets.46,47 These basis sets are denoted as awX-DK. Both valence and core−valence electrons are correlated. These DK-CCSD(T) energies were extrapolated to the complete basis set (CBS) limit using a mixed Gaussian/exponential formula.51 Following our previous work on the thermochemistry of transition metal oxides,52,53 the PW91 orbitals and the Brückner orbitals for the singlet molecule are also used for the initial wave functions for the coupled cluster calculations. The CCSD(T) calculations were done with MOLPRO.54,55 Spin−orbit corrections for the atoms were taken from Moore’s tables.56 The calculations were performed on the local Xeon- and Opteronbased Penguin Computing clusters, the Xeon-based Dell Linux cluster at the University of Alabama, the Opteron- and Xeon-based Dense Memory Cluster (DMC) and Itanium 2 based SGI Altix systems at the Alabama Supercomputer Center, and the Opteron based HP Linux cluster at the Molecular Science Computing Facility at Pacific Northwest National Laboratory. Molecular visualization was done using the AGUI graphics program from the AMPAC program package.57

oxidation state is +IV for Ce and mixed +III/IV for Pr and Tb. The Ln−O bonds for OCeF2, OPrF2, and OTbF2 are shorter than those for the remainder of the lanthanide analogs which possess the +III oxidation states and single Ln−O bonds.10 Herein, we report a systematic matrix isolation infrared spectroscopic and computational study of the structures and vibrational frequencies of the remaining 3d transition metal oxyfluoride molecules (V, Mn, Fe, Co, and Ni). This series of new oxyfluoride molecules is prepared via the reactions of laserablated metal atoms and OF2 in an argon matrix. These products are identified based on the experimental M−O and M−F stretching vibrational frequencies as well as 18O isotopic shifts and are further supported by electronic structure calculations. A key issue is the nature of the formal oxidation state of the metal. For OMF, is M in the +III state or is it in the +II state with an additional electron spin on the O atom? For OMF2, is M in the +IV state or is it in the +III state with an additional electron spin on the O atom?

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EXPERIMENTAL AND COMPUTATIONAL METHODS

Experimental Section. The 3d transition metal oxyfluoride molecules were synthesized and characterized in excess argon at 4K using the experimental apparatus and procedure described previously.18,19 The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 10 ns pulse width) was focused onto a freshly cleaned metal target mounted on a rotating stainless steel rod. These laser-ablated metal atoms were codeposited with 3−4 mmol of argon (Matheson, research grade) containing 1.0 or 2.0% OF2 (Ozark−Mahoning) onto a CsI cryogenic window for 60 min. The 18OF2 sample (91% 18O enriched) was synthesized and kindly provided by Arkell and coworkers.20 Both OF2 and 18OF2 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 λ > 220 nm photolysis by a medium-pressure mercury arc street lamp (Philips, 175W) with the outer globe removed. Computational. The initial geometry optimizations and vibrational frequencies of the OMF2 and OMF molecules for the 3d transition metals were calculated at the density functional theory (DFT) level with three different functionals: (1) B3LYP hybrid functional,21,22 (2) BP86 functional,23,24 and (3) PW9125,26 functional. The aug-cc-pVDZ basis set for O and F27 and aug-cc-pVDZ-PP basis sets for the metals with effective core pseudopotential (ECP)28,29 were used. We denote this basis set as aD. Only the PW91 results are discussed below as they showed the most consistent agreement with experiment. A stability analysis30 was performed for the DFT calculations and showed that the lowest energy spin state was stable, but some of the higher energy states did show instabilities. The DFT calculations were performed using Gaussian 09.31 Geometry optimizations and frequency calculations were subsequently performed at the coupled cluster CCSD(T) level32−35 for the closed-shell molecules (OScF and OTiF2) and at the coupled cluster R/UCCSD(T) level36−38 for the remaining open-shell molecules with the same basis sets and ECPs. Only the valence electrons were correlated. In the R/UCCSD(T) approach, a restricted open-shell Hartree−Fock (ROHF) calculation is initially performed and the spin constraint was then relaxed in the coupled cluster calculation. Geometry optimizations and frequency calculations were also done at the CASSCF39,40 and CASPT241,42 levels with scalar relativity included at the second-order Douglas−Kroll−Hess (DK)43−45 level with the all-electron aug-cc-pVDZ-DK basis sets.46,47 The active space for the CAS calculations for the optimization and frequency calculations are complicated and discussed below. We included up to

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RESULTS AND DISCUSSION The OF free radical absorbing at 1028.1 cm−1 is the major metalindependent species formed during codeposition of OF2 and laser-ablated metal atoms (V, Mn, Fe, Co, and Ni),58,59as observed in other reactions with metal atoms and laser plume photolysis of OF2.10−17 Its infrared intensity varied as the metal and laser energy were changed. The OF absorption increased upon λ > 220 nm irradiation and decreased when the sample was annealed. Minor common absorptions such as CF4, O3, and FOO are also present in the infrared spectra.60 Vanadium Spectra. The infrared spectra from reactions of vanadium and OF2 are shown in Figure 1. Binary vanadium oxide and fluoride molecules such as VO2 and VF2 were observed right after sample deposition.61,62 A series of ternary oxyfluoride absorptions also appeared together with the

Figure 1. Infrared spectra of laser-ablated vanadium atoms codeposited with 1.0% OF2 in excess argon at 4 K: (a) after 60 min deposition; (b) after annealing to 20 K; (c) after λ > 220 nm irradiation; and (d) after annealing to 30 K. The asterisks denote unknown absorptions. B

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry formation of VO2 and VF2. The three bands at 1051.9, 797.5, and 716.2 cm−1 were previously assigned to OVF3, which is a stable compound in the condensed phase.63 In addition, new oxyfluoride product bands were observed at 1021.8, 986.6, 764.2, 652.2, and 619.5 cm−1. All of these new bands increased upon UV−vis irradiation and decreased when the sample was annealed to 30 K. The two bands at 986.6 and 619.5 cm−1 slightly increased upon sample annealing to 20 K while the intensities of the other three bands remained constant. To help identify the product structures, reactions of laserablated vanadium atoms and 18OF2 in solid argon were carried out (Figure 2). The 18O shift of VO2 is consistent with the value

Figure 3. Infrared spectra of laser-ablated metal atoms codeposited with OF2 in excess argon at 4 K. All the spectra were taken after annealing to 30 K following λ > 220 nm irradiation: (a) Mn + 1.0% OF2; (b) Fe + 1.0% OF2; (c) Co + 2.0% OF2; and (d) Ni + 2.0% OF2. The black dot denotes an unknown band. The asterisks denote impurity absorptions.

Figure 2. Infrared spectra of laser-ablated vanadium atoms codeposited with 1.0% 18OF2 in excess argon at 4 K: (a) after 60 min deposition; (b) after annealing to 20 K; (c) after λ > 220 nm irradiation; and (d) after annealing to 30 K. The asterisks denote unknown absorptions. C16,18O2 denotes C16O2, C18O2, and C16O18O.

reported previously, while no shift was observed for VF2.61,62 For the known OVF3 molecule,63 the V−O and V−F stretches at 1051.9 and 716.2 cm−1 were shifted by 42.0 and 1.7 cm−1, and the strongest V−F stretching mode at 797.5 cm−1 was covered by 18OF2 bands. The two new bands at 1021.8 and 986.6 cm−1 shifted to 982.5 and 944.9 cm−1 upon 18O substitution. No shift was observed for the 764.2 cm−1 band, and the 652.2 and 619.5 cm−1 bands were red-shifted by 1.5 and 1.3 cm−1, respectively. Manganese, Iron, Cobalt, and Nickel Spectra. Figure 3 shows the infrared spectra from the reactions of laser-ablated manganese, iron, cobalt, and nickel atoms and OF2 in an argon matrix. Weak absorptions due to MnO, FeO, MnO2, FeO2, and CoO2 were observed after sample deposition.64−66 Manganese, iron, cobalt, and nickel difluorides are the major binary fluoride products,61,67,68 and trifluoride absorptions are well-resolved in some cases such as CoF3.69 In addition to these known species, three sets of new products were formed during the reactions of manganese, iron and OF2. Reactions of laser-ablated cobalt atoms and OF2 resulted in the formation of two new products. The only product resulting from the reactions of nickel and OF2 absorbs at 744.8 and 740.2 cm−1 which increased on sample annealing and almost disappeared when the sample was exposed to λ > 220 nm irradiation. All these experiments were repeated using 18OF2 to help identify the products, and the infrared spectra are shown in Figure 4. New product absorptions above 850 cm−1 exhibit large 18 O shifts (30−50 cm−1), while the 18O shifts for the product

Figure 4. Infrared spectra of laser-ablated metal atoms codeposited with 18OF2 in excess argon at 4 K. All the spectra were taken after annealing to 30 K following λ > 220 nm irradiation: (a) Mn + 1.0% 18 OF2; (b) Fe + 1.0% 18OF2; (c) Co + 2.0% 18OF2; (d) Ni + 2.0% 18OF2. The asterisks denote impurity absorptions. C16,18O2 denotes C16O2, C18O2, and C16O18O.

bands below 800 cm−1 are very small ( 220 nm irradiation. The first band shifted to 944.9 cm−1 with an isotopic frequency ratio of 1.0441, suggesting it should be the V−O stretch of the new molecule whose isotopic ratio is very close to that of diatomic VO.62 The absorption at 619.5 cm−1 is located in a region where vanadium fluoride molecules absorb.61 This band is assigned to the V−F stretching vibrational mode coupled slightly with the V−O stretch as indicated by the 1.3 cm−1 18O shift. Since no other absorption was observed in this region, we assign the new product absorptions at 986.6 and 619.5 cm−1 to the triatomic OVF molecule. C

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

stretch.13 The ONiF molecule was not observed from the reactions of laser-ablated nickel atoms and OF2. OMF2 (M = V, Mn, Fe, Co, and Ni) Assignments. The relative intensities of the 1021.8, 764.2, and 652.2 cm−1 absorptions of the vanadium product remained constant throughout the experiment (Figure 1), suggesting they should be due to different vibrational modes of the same molecule. The 1021.8 cm−1 band shifted to 982.5 cm−1 in the experiment using 18 OF2 with an 16O/18O frequency ratio of 1.0400 which is the same as found for the antisymmetric O−V−O stretch.23 However, a very weak band at 1021.8 cm−1 was observed in the 18OF2 experiment, and the relative intensity of the 1021.8 and 982.5 cm−1 absorptions (1:10) is about the same as that of 16 OF and 18OF (Figure 2). This is in good agreement with the fact that 9% 16OF2 exists in the 18OF2 sample, again showing that only one oxygen atom participates in this mode. As a result, the 1021.8 cm−1 band is assigned to the V−O stretch of the new molecule. Similar deviations on the isotopic frequency ratios of M−O stretches between OMF2 and MO have been observed in the cases of titanium and chromium.13,17 For the two lower bands, their band positions indicate they should be due to two V−F stretches, which is consistent with their very small 18O shifts. The 1.5 cm−1 redshift for the 652.2 cm−1 band indicates that it is the symmetric F−V−F stretch of the new molecule, and this mode is slightly coupled with the V−O stretch. The 764.2 cm−1 band without an 18O shift arises from the antisymmetric F−V−F stretch, similar to the cases of OTiF2 and OCrF2.13,17 On the basis of these experimental results, the 1021.8, 764.2, and 652.2 cm−1 absorptions are assigned to the OVF2 molecule. Following the vanadium case, the species OMF2 for M = manganese, iron, cobalt, and nickel can be identified as shown in Figure 3. The 927.8, 757.0, and 649.2 cm−1 bands are assigned to the OMnF2 molecule. The 16O/18O frequency ratio of the Mn− O stretch at 927.8 cm−1 (1.0418) lies between the values for MnO (1.0457) and MnO2 (1.0389, ν3),25 but the weak 927.8 cm−1 16O band observed in the 18OF2 experiment confirms the involvement of a single oxygen atom in this mode. OFeF2 absorbs at 882.7, 761.3, and 634.2 cm−1. The first band exhibited an isotopic frequency ratio of 1.0438, close to the value of diatomic FeO (1.0459).65 It should be noted that the Fe−O stretches of OFeF and OFeF2 were observed at 878.1 and 882.7 cm−1, both of which shifted to 845.7 cm−1 in the experiment with 18OF2. This is consistent with the fact that the relative intensities of the 882.7 and 761.3 cm−1 absorptions for OFeF2 are different from those of the 845.7 and 761.1 cm−1 absorptions for 18OFeF2, whereas the relative intensities of the 761.3 and 634.2 cm−1 absorptions are similar to those of their 18O counterparts at 761.1 and 633.4 cm−1. The Co−O stretch of OCoF2 was observed at 863.5 cm−1, and its 18O counterpart would appear around 825.1 cm−1 if the 16 O/18O frequency ratio of this mode were similar to that of diatomic CoO (1.0465).66 However, the intense band centered at 825.4 cm−1 due to the antisymmetric F−O−F stretch of residual 16OF2 in the 18OF2 sample makes it impossible to detect the Co−18O stretch of 18OCoF2 (Figure 4, trace c). Antisymmetric F−M−F (M = Mn, Fe, and Co) stretching vibrational frequencies are barely affected by 18O substitution, and the 18O shifts for the symmetric F−Mn−F, F−Fe−F, and F−Co−F stretches are only 3.1, 0.8, and 1.0 cm−1, respectively (Table 1). No band in the region where diatomic NiO absorbs71 can be assigned to the Ni−O stretch of the ONiF2 molecule. The two

Table 1. Experimental Stretching Vibrational Frequencies (cm−1) and 16O/18O Isotopic Frequency Ratios (in Parentheses) for the 3d Transition Metal Oxyfluoride Molecules molecule OScF11 OTiF17 OVF OCrF13 OMnF OFeF OCoF OScF211 OTiF217

OVF2

OCrF213

OMnF2

OFeF2

OCoF2 ONiF2 OCuF214 OVF363,70

OMnF3 OFeF3

OF2 912.7 588.0 962.7 609.4 986.6 619.5 935.9 646.0 906.9 604.5 878.1 618.1 844.8 624.8 689.0 665.3 1013.6 740.2 632.3 1021.8 764.2 652.2 1017.3 735.3 662.1 927.8 757.1 649.2 882.7 761.3 634.2 863.5 770.8 639.1 744.8 740.2 751.1 746.5 1051.9 716.2 797.5 991.2 754.9 691.6 948.5

18

OF2

874.8 (1.0433) 587.7 (1.0005) 922.3 (1.0438) 608.6 (1.0013) 944.9 (1.0441) 618.2 (1.0021) 900.3 (1.0395) 641.5 (1.0070) 863.4 (1.0504) 602.2 (1.0038) 845.7 (1.0383) 611.8 (1.0103) 816.0 (1.0353) 616.4 (1.0136) 688.5 (1.0007) 655.4 (1.0151) 968.8 (1.0462) 739.8 (1.0005) 630.9 (1.0022) 982.5 (1.0400) 764.2 (1.0000) 650.7 (1.0023) 976.5 (1.0418) 735.3 (1.0000) 659.9 (1.0033) 890.6 (1.0418) 757.0 (1.0001) 646.1 (1.0048) 845.7 (1.0438) 761.0 (1.0004) 633.4 (1.0013) not observed 770.7 (1.0001) 638.1 (1.0016) 744.8 (1.0000) 740.2 (1.0000) 751.1 (1.0000) 746.5 (1.0000) 1009.9 (1.0416) 714.5 (1.0024) covered by 18OF2 951.5 (1.0417) 754.9 (1.0000) 688.9 (1.0039) 902.7 (1.0507)

stretching mode Sc−O Sc−F Ti−O Ti−F V−O V−F Cr−O Cr−F Mn−O Mn−F Fe−O Fe−F Co−O Co−F antisym. Sc−F sym. Sc−F Ti−O antisym. Ti−F sym. Ti−F V−O antisym. V−F sym. V−F Cr−O antisym. Cr−F sym. Cr−F Mn−O antisym. Mn−F sym. Mn−F Fe−O antisym. Fe−F sym. Fe−F Co−O antisym. Co−F sym. Co−F antisym. 58Ni−F antisym. 60Ni−F antisym. 63Cu−F antisym. 65Cu−F V−O V−F (a) V−F (e) Mn−O Mn−F Mn−F Fe−O

Similarly, triatomic OMnF, OFeF, and OCoF molecules are identified on the basis of the observed M−O and M−F stretching modes (Figure 3). Although the 16O/18O ratios for the M−O stretches of OMF are different from those of the corresponding diatomic MO molecules,64−66 the appearance of the weak M−16O stretching modes of 16OMF (one-tenth as intense as the 18O counterparts) in the experiments with 18OF2 samples containing 9% 16OF2 confirms the involvement of one oxygen atom in this vibrational mode. This is analogous to the case of OCrF where the isotopic frequency ratio of the Cr−O stretch is almost the same as that of the antisymmetric O−Cr−O D

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Calculated Results for the OMF Molecules with Frequencies (ν, cm−1) and Infrared Intensities (I, km/mol) OScF (1A′)

OTiF (2A′)

OVF (3A′′)

OCrF (4A′′)

ν(M−O) I(M−O) 16 O/18O ν(M−F) I(M−F)

930.2 215 1.0436 622.1 176

1013.0 224 1.0440 683.5 174

1029.1 177 1.0447 671.9 140

996.3 173 1.0424 669.8 91

ν(M−O) 16 O/18O ν(M−F)

913.7 1.0432 626.1

976.3 1.0435 635.3

989.1 1.0418 667.5

921.5 1.0370 658.8

ν(M−O) O/18O ν(M−F)

930.3 1.0426 629.8

978.9 1.0438 622.6

1041.8 1.0446 624.6

1017.4 1.0385 657.1

ν(M−O) O/18O ν(M−F)

912.711 1.0433 588.0

962.717 1.0438 609.4

986.6 1.0441 619.5

935.913 1.0395 646.0

16

16

OMnF (5A′)a DFT/PW91 943.9 131 1.0432 666.8 86 CCSD(T) 763.4 1.0315 572.1 CASPT2 925.8 1.0416 667.4 exptl. 906.9 1.0504 604.5

OFeF (6Σ)b

OCoF (5Σ)

ONiF (4Σ)

OCuF (3Σ)

902.6 110 1.0412 635.3 67

907.0 101 1.0394 639.7 58

906.1 77 1.0376 649.5 52

818.0 16 1.0285 617.9 32

883.6 1.0345 651.2

768.9 mix 1.0377 627.6 mix

788.9 mix 1.0166 634.0 mix

818.0 mixc 1.0238 651.4 mix

963.9 1.0406 609.6

916.3 1.0363 657.2 mix

899.9 1.0346 670.8 mix

800.3 mix

878.1 1.0383 618.1

844.8 1.0353 624.8

not observed

not observed

not observed

not observed

664.0 mix

CCSD(T) is 5Π. bDFT/PW91 is 6A′. cCCSD(T) from ref 14. ν (Cu−O) = 837.5 cm−1 and ν (Cu−F) = 659.1 cm−1.

a

absorptions at 744.8 and 740.2 cm−1 are assigned to the antisymmetric F−58Ni−F and F−60Ni−F stretches. The 4.6 cm−1 shift arising from nickel isotopes is about the same as that of binary NiF2 (5.0 cm−1),72 suggesting the involvement of a NiF2 moiety in this mode. No absorption below 700 cm−1 was found to track with the antisymmetric F−Ni−F stretch of ONiF2. OMF3 (M = Mn and Fe) Assignments. New product absorptions at 991.2, 754.9, and 691.6 cm−1 from the reactions of manganese and 1.0% OF2 are assigned to the OMnF3 molecule (Figure 3, trace a). All of these absorptions were significantly increased and broadened when the sample was subjected to λ > 220 nm irradiation. Subsequent sample annealing to 30 K allowed further growth of these new product bands. The 16O/18O frequency ratio for the 991.2 cm−1 band is 1.0417, indicating it is a terminal Mn−O stretch similar to that of OMnF2. No 18O shift was observed for the 754.9 cm−1 band, and the 691.6 cm−1 band exhibited a small shift of 2.7 cm−1. Both absorptions lie in a region where binary manganese fluorides absorb,28 and they can be assigned to Mn−F stretches of a new molecule. Since the 991.2, 754.9, and 691.6 cm−1 absorptions were not observed when manganese reacted with O2 or F2,64,67 we assign them to different vibrational modes of a new manganese oxyfluoride OMnFx. The intensities of the OMnFx and OMnF absorptions were reduced by about 1/2 and 2/3 when 2.0% OF2 was used, while the yields of MnF2 and OMnF2 remained almost unchanged. It is most likely that the new OMnFx molecule results from the reaction of OMnF and F2. Molecular F2 is present in the matrix as a result of photoinduced dissociation of OF2 during laser ablation of the manganese target,73,74 which also causes the formation of MnF2 as shown in the infrared spectrum (Figure 3, trace a). Therefore, OMnF3 is proposed as the absorber for the new bands at 991.2, 754.9, and 691.6 cm−1. Note that the 999.9 and 692.3 cm−1 absorptions which correspond to the 991.2 and 691.6 cm−1 absorptions in our experiments were previously assigned to the OMnF2 molecule prepared via codeposition of manganese fluoride and oxide vapors.6 On the basis of our results, these absorptions should arise from OMnF3, while OMnF2 absorbs at 927.8, 757.0, and 649.2 cm−1.

The 948.5 cm−1 absorption observed in the reactions of iron and OF2 is probably due to the Fe−O stretch of OFeF3 molecule following the OMnF3 case. This band shifted to 902.7 cm−1 with an 16O/18O ratio of 1.0507 which lies between the ratios for the antisymmetric and symmetric O−Fe−O stretching modes of the triatomic FeO2 molecule and is slightly higher than the value of diatomic FeO.67 There are some broad and weak bands in the region of 700−750 cm−1 which could arise from the Fe−F stretches of the OFeF3 molecule. OMF Structure and Frequency Calculations. To further support the experimental assignments and to provide insights into the geometric and electronic structures of the new products, electronic structure calculations were performed at the DFT and correlated molecular orbital theory levels. The frequency results for the OMF molecules are shown in Table 2. The BP86 and B3LYP DFT results are reported in the Supporting Information together with the CAS results. In addition to testing the basis set level, CCSD(T)/aVTZ-DK(O)/VDZ-DK(M) and CASPT2/ aVTZ-DK(O)/VDZ-DK(M) calculations were performed. The results are given in the Supporting Information and show that there is only a moderate basis set effect. For closed-shell 1OScF, the calculated values at the DFT/ PW91, CCSD(T), and CASPT2 levels are in good agreement with the observed results11 with the calculated values for the Sc− O stretch in somewhat better agreement than is found for the Sc−F stretch with the best agreement found at the CCSD(T) level. In all cases, the Sc−F stretch is predicted to be too large by up to 42 cm−1. The CCSD(T) results are within 1 cm−1 of experiment for the Sc−O stretch and within 38 cm−1 of experiment for the Sc−F stretch. The molecule is bent with an angle of ∼130°. The CASPT2 calculations included the 3d and 4s orbitals on the metal for Sc, Ti, V, and Cr and the 2p orbitals on the oxygen. The prior DFT/B3LYP results11 are consistent with the current higher level correlated computational results as well as our DFT results with other functionals. For 2OTiF, the Ti−O frequency is predicted to significantly increase over that in OScF with the Ti−F frequency also increasing except at the CASPT2 level. The CCSD(T) value for the Ti−O stretch is 14 cm−1 larger than experiment,17 and the Ti−F stretch is predicted to be too large by 26 cm−1. The E

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the experimental assignments, the CASPT2 and DFT/PW91 values are consistent with those of the experiment, confirming that the molecule is bent. The CASPT2 value for the Mn−O stretch is 19 cm−1 larger than that of the experiment, and the Mn−F stretch is predicted to be 63 cm−1 greater than that of the experiment. The quintet is predicted to be the ground state by all of the methods. For OMnF, only the four singly occupied 3d orbitals are included in the CAS and CASPT2 as orbital rotations among the doubly occupied orbitals prevented convergence in the numerical optimization procedure when they are included in the CAS. The calculated 16O/18O ratios for the M−O stretch are all too low as compared to those of the experiment with the experimental value being much higher than those for any of the other experimental 16O/18O ratios. The best agreement for OFeF is found with the CCSD(T) values overall. The ground state is predicted to be the sextet with the quartet being 16 kcal/mol higher in energy at both the CCSD(T) and CASPT2 levels. The CASPT2 calculations included the five singly occupied 3d orbitals in the CAS to avoid the orbital rotation issue described above. The molecule is predicted to be linear at the CCSD(T) and CASPT2 levels and bent at the DFT/PW91 level. This suggests that the potential energy surface for the bend is quite flat. The CCSD(T) value for the Fe−O stretch is about 6 cm−1 larger than that of the experiment, and the Fe−F stretch is predicted to be 33 cm−1 greater than that of the experiment. The PW91 functional predicts slightly better agreement for the Fe−F stretch. The calculated 16O/18O ratio for the Fe−O stretch at the CASPT2 level is higher than the experimental ratio, and the CCSD(T) value is lower than that obtained from the experiment by a comparable amount. Only the DFT/PW91 method predicts the correct ordering for the experimental M−O stretches from M = V to M = Fe. For OCoF, the CCSD(T) value is clearly too low for the M− O stretch although it follows the same trend as the experiment, and the CASPT2 value is larger than experiment consistent with the DFT/PW91 value. All of the methods are in reasonable agreement with experiment for the Co−F stretch showing that the M−O stretch is a very sensitive measure of the computational method in terms of the bond angle, the M−O bond length, and the amount of coupling between the M−O and M−F stretches. The ground state is predicted to be the quintet with the triplet state being 22 kcal/mol higher in energy at the CCSD(T) level. The molecule is predicted to be linear by all three methods. The CASPT2 calculations included the five 3d orbitals and the three O 2p orbitals in the CAS for Co and Ni. The calculated 16O/18O ratios for the Co−O stretch are all greater than experiment with the CCSD(T) value closer to experiment than the CASPT2 value. There are no experimental data for ONiF, and again the CCSD(T) value for the Ni−O stretch seems to be too low as compared to DFT/PW91 and CASPT2, which are reasonably consistent with each other. The quartet is predicted to be the ground state with the doublet state being 6 kcal/mol higher in energy at the CCSD(T) level and 11 kcal/mol higher in energy at the CASPT2 level. The molecule is predicted to be linear at all levels. The M−O stretch is approximately constant from Fe to Ni at the DFT/PW91 level and decreases from Fe to Ni at the CASPT2 level. The M−F stretch is predicted to increase from Fe to Ni at the DFT/PW91 and CASPT2 levels. For OCuF, our computational results are in reasonable agreement within about 20 cm−1 with previous reported CCSD(T) results14 using a larger basis set. The triplet is the

CASPT2 results display a comparable agreement with experiment. The molecule is bent with an angle of ∼137°. The calculated isotopic ratio is in excellent agreement with experiment at both the CCSD(T) and CASPT2 levels. For OVF, a triplet ground state with a bent geometry was obtained at all levels of theory (Table 3). The closed-shell singlet Table 3. Relative Energies (kcal/mol) for Different Electronic States for OMF molecule 3

OVF 4 OCrF 5 OMnF 6 OFeF 5 OCoF 4 ONiF 3 OCuF

type ΔEa

DFT/PW91

CCSD(T)

CASPT2

3−1 4−2 5−3 6−4 5−3 4−2 3−1

25.6 16.5 18.6 3.1 17.8 8.7 32.4

14.6 35.9 39.1 15.6 22.0 6.1 19.6

15.5 20.9 11.1 21.3

The first value is the lowest energy spin state.

a

is at least 15 kcal/mol higher in energy. The V−O stretching frequency is predicted to increase from that in OTiF; in contrast, experimentally, there is a decrease in the V−O frequency as compared to the Ti−O frequency. The best agreement with the experiment for the V−O stretching frequency is found at the CCSD(T) level with the calculated value being about 44 cm−1 too high. The predicted V−O frequencies with the other computational methods are about 100 cm−1 too high as compared to those of the experiment. For the V−F frequencies, the best agreement is found at the CASPT2 level where the value is less than 10 cm−1 too high. At the CCSD(T) level, the calculated V−F frequency is also about 50 cm−1 too high just as predicted for the V−O stretch. The calculated isotopic ratios for the V−O stretch are in good agreement with those of the experiment with the largest deviation found at the CCSD(T) level with the predicted value being too low. The calculated values confirm our assignments for OVF. The molecule is bent with an angle of ∼137°. The M−O stretch is observed13 to decrease from M = V to M = Cr for OMF, and the calculations predict the same trend. Although the best agreement with experiment for the M−O and M−F stretches is at the CCSD(T) level, the Cr−O stretch is too low by ∼15 cm−1, and the Cr−F stretch is about 13 cm−1 too high. OCrF is predicted to have a quartet ground state with the doublet state 36 kcal/mol higher in energy at the CCSD(T) level. The molecule is bent with an angle of ∼156° at the CCSD(T) level and 138° at the CASPT2 level. The calculated 16 O/18O ratios for the M−O stretch are in good agreement with experiment at the CASPT2 level, but the CCSD(T) predicted value is too low. For OMnF, CCSD(T) makes predictions different from those of DFT/PW91 and CASPT2. The CCSD(T) method predicts a linear structure which leads to a dramatic decrease in both the M−O and M−F frequencies. The amount of coupling between the M−O and M−F stretches is increased in linear OMnF leading to the lack of agreement with experiment. It is likely that the starting Hartree−Fock orbitals cannot be corrected enough at the CCSD(T) level to get the correct structure. In contrast, DFT/PW91 and CASPT2 predict a bent molecule with an angle of 126° at the CASPT2 level and a corresponding smaller decrease in the M−O and M−F frequencies. Although the CCSD(T) values for a linear structure are not consistent with F

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ground state and is ∼20 kcal/mol more stable than the closedshell singlet at the CCSD(T) and CASPT2 levels. The molecule is predicted to be linear at all three levels. The two unpaired d electrons were included in the CAS. The M−O frequency mixes with the M−F frequency. The M−O and M−F frequencies and the corresponding M− O and M−F bond distances at the CCSD(T) and CASPT2 levels are summarized in Figure 5. The M−O bond distances

closer to experiment. These higher level molecular orbital theory including electron correlation effects results are consistent with prior DFT calculations.11 For OMF2, for all M except for Mn, Fe, and Co, the CASPT2 calculations included the five 3d orbitals and the 4s orbital on the metal as well as the three 2p orbitals on the oxygen. For Mn, Fe, and Co, the 4s orbital was not included in the CASPT2 calculations due to the orbital rotation issues described above. The OTiF2 molecule is predicted to be planar with C2v symmetry. The Ti−O stretch is much higher than the Sc−O stretch in OScF2, consistent with the formation of a TiO bond. The CCSD(T) and CASPT2 values are in good agreement with experiment for the Ti−O (within 15 cm−1) and Ti−F stretches (within 40−50 cm−1) as well as prior calculations.17 The calculated 16O/18O ratios for the Ti−O stretch at the CCSD(T) and CASPT2 levels are in good agreement with experiment. We predict that the molecule OVF2 is planar with a ground state of 2A1. The CCSD(T) and CASPT2 values for the V−O and V−F stretches are within 20−30 cm−1 of experiment. The calculated 16O/18O ratios for the M−O stretch at the CCSD(T) and CASPT2 levels are in good agreement with experiment. The molecule OCrF2 is planar with a ground state of 3A2. The CCSD(T) values for the Cr−O and Cr−F stretches are within 10−35 cm−1 of experiment.13 The CASPT2 Cr−O stretch is about 20 cm−1 below that of experiment. The closed-shell singlet excited state is 23 kcal/mol higher in energy. The calculated 16 O/18O ratios for the Cr−O stretch at the CCSD(T) and CASPT2 levels are in good agreement with that of experiment. Molecular OMnF2 is planar with a ground state of 4B2. The Mn−O stretch is calculated to be much higher than that of experiment by 140 cm−1 at the CCSD(T) level. In contrast, at the CASPT2 level, the Mn−O stretch is predicted to be too low by 27 cm−1. The Mn−F stretches are in reasonable agreement with those of experiment, being too high by 35−45 cm−1 at the CCSD(T) level. The calculated 16O/18O ratio for the Mn−O stretch at the CCSD(T) level is too high as compared to that of experiment, but this ratio is in better agreement at the CASPT2 level. The OFeF2 molecule is also planar with a ground state of 5B2. In contrast to OMnF2, the Fe−O stretches at all three levels are in reasonable agreement with that of experiment, being too high, similar to most of the other OMF2. The calculated Fe−F frequencies show the typical differences from experimental results described above. The calculated 16O/18O ratios for the Fe−O stretch at the CCSD(T) and CASPT2 levels are in good agreement with experiment. The OCoF2 molecule is predicted to be a nonplanar molecule at the CCSD(T) level with a ground state of 4A′ as the corresponding planar structure has an imaginary frequency. The molecule is predicted to be planar at the CASPT2 level. The FCoF bond angle is much smaller for the nonplanar CCSD(T) structure than that for the planar CASPT2 structure. The Co−O stretch at the CCSD(T) level is predicted to be ∼85 cm−1 higher than experiment. At the CCSD(T) level, a Co−F stretch mixes with the Co−O stretch leading to a very high Co−F frequency that is not consistent with experiment. Although the Co−O stretch is also predicted to be too high at the DFT/PW91 level, the Co−F stretches at this level are in qualitative agreement with experiment but are smaller than that of experiment rather than larger as expected for harmonic frequencies. At the CASPT2 level, the Co−O frequency is predicted to be too low by ∼45

Figure 5. Variation in the M−O and M−F bond distances (Å, top) and frequencies (cm−1, bottom) as a function of the metal for OMF.

decrease from Sc to V as the first two d electrons are added and then exhibit some oscillatory behavior as additional d electrons are added. The M−O stretches do not really follow these oscillations and build back up to a peak when 6 d electrons are added to form OCoF. The M−O stretch then decreases with additional d electrons even though the bond distance does not really decrease. The frequency plot clearly shows the issue for the Fe−O stretch at the CCSD(T) level. The M−F bond distances generally decrease from Sc to Cu but the M−F frequencies show little variation. PW91 does predict the correct trend for the M−O stretches in comparison with the known experimental values but usually does not provide the best agreement with experiment. The bending potential energy surfaces can be quite flat and this will affect the coupling between the M−O and M−F stretches. The experimental data is critical for determining the best computational method even for these “simple” triatomic molecules. OMF2 Structure and Frequency Calculations. The frequency results for the OMF2 molecules are shown in Table 4. The relative energies of different spin states for each OMF2 are given in Table 5. The 2OScF2 molecule is predicted to have a planar C2v structure with a ground state of 2A1. The Sc−O frequency is predicted to be quite low, consistent with a Sc−O single bond with the excess spin density localized on the O. The Sc−F frequencies are predicted to be higher than those of Sc−O. The CCSD(T) M−F frequencies are predicted to be about 50 cm−1 higher than those of experiment. The CASPT2 values are G

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 4. Calculated Results for the OMF2Molecules with Frequencies (ν, cm−1) and Infrared Intensities (I, km/mol) OScF2 (2A1)

OTiF2 (1A1)

OVF2 (2A1)

OCrF2 (3A2)

ν(M−O) I(M−O) 16 O/18O ν(M−F) I(M−F) ν(M−F) I(M−F)

603.0 1 1.0280 692.9 196 668.3 165

1040.9 218 1.0428 760.5 218 645.7 59

1066.7 161 1.0436 776.6 198 660.3 54

1065.5 161 1.0434 736.2 198 662.1 54

ν(M−O) 16 O/18O ν(M−F) ν(M−F)

615.5 mix 1.0284 737.6 715.5 mix

1027.6 1.0424 788.7 672.3

1041.3 1.0433 795.3 676.2

1024.4 1.0425 767.8 688.3

ν(M−O) O/18O ν(M−F) ν(M−F)

589.0 mix 1.0252 672.6 662.7

1021.6 1.0428 776.6 663.6

1020.1 1.0435 784.2 666.1

994.1 1.0430 759.3 684.5

ν(M−O) 16 O/18O ν(M−F) ν(M−F)

not observed

1013.617 1.0462 740.2 632.3

1021.8 1.0400 764.2 652.2

1017.313 1.0418 735.3 662.1

16

689.011 665.3

OMnF2 (4B2) PW91 1006.8 108 1.0438 757.7 157 650.2 43 CCSD(T) 1068.0 1.0450 793.0 694.0 CASPT2 890.6 1.0406 773.6 660.7 exptl. 927.8 1.0418 757.1 649.2

OFeF2 (5A1)

OCoF2 (4A1)

ONiF2 (5A1)

OCuF2 (4A2)

933.3 38 1.0456 748.3 176 622.1 21

927.1 23 1.0464 752.7 140 624.3 17

741.6 2 1.0446 678.8 75 592.4 22

683.3 3 1.0405 627.2 60 565.2 42

928.8 1.0446 800.6 679.8

947.0 1.0329 935.3a 675.3

834.7 1.0453 774.4 675.0

568.9 1.0425 828.7 768.3

905.1 1.0444 787.1 663.3

818.4 1.0338 791.6 672.4

919.8 1.0423 796.0 736.3

588.8 1.0345 749.4 722.1

882.7 1.0438 761.3 634.2

863.5 O not observed 770.7 638.1

not observed

not observed

744.8 not observed

751.114 not observed

18

a

The M−O stretches mix with the M−F stretches.

calculations. The DFT values for this Cu−F stretch are not in good agreement with experiment. We calculate values of 465.0, 790.2, and 655.1 cm−1 for the Cu−O and the two Cu−F modes for the 2B2 state at the CCSD(T) level, which can be compared to values of 510.2, 788.8, and 641.5 cm−1 reported in a prior study at the CCSD(T) level with a larger basis set but for a different state.14 The Cu−O stretch is again very sensitive to the level of the calculation. The Cu−O stretch is predicted to be significantly lower, consistent with the presence of a Cu−O single bond and radical character on the O atom. The geometry for 4A2 OCuF2 is very sensitive to the computational level in terms of the Cu−O bond distance and the F−M−F bond angle. The CCSD(T) Cu−O bond distance is the longest value, almost 0.08 Å longer than the CASPT2 value. The F−M−F bond angle is almost 16° larger at the CASPT2 level than at the CCSD(T) level. The M−F bond distances differ by much smaller amounts. The M−O stretching modes computed at the CCSD(T) level track the M−O bond distances as summarized in Figure 6. There is a significant increase in the M−O frequency from OScF2 to OTiF2 consistent with a significant decrease in the M−O bond distance as the M−O bond transitions from a single bond with radical character on the O for OScF2 to a double bond for OTiF2. As electrons are added to the 3d orbitals from Ti to Co, there are only small variations in the M−O bond distance, and the M−O frequencies do not vary much until Fe where there is a modest decrease. As the sixth electron is added to generate the Ni species, the M−O distance increases, and the M−O frequency decreases. When the seventh electron is added to form the Cu species, the molecule dramatically changes its geometry with a longer M−O bond with radical character on the O. The M−F bond distances decrease from 1.90 Å for OScF2 to 1.71 Å for OCoF2 (Figure 6). The M−F bond distances then do not change much for the rest of the period, slightly increasing near the end of the period. The effect of the change in bond distance on the M−F frequencies is somewhat smaller than the

Table 5. Relative Energies for Different Electronic States (kcal/mol) for OMF2 molecule 3

OCrF2 4 OMnF2 5 OFeF2 5 OFeF2 4 OCoF2 4 OCoF2 5 ONiF2 5 ONiF2 4 OCuF2

type ΔEa

B3LYP

PW91

CCSD(T)

3−1 4−2 5−3 5−1 4−6 4−1 5−3 5−1 4−2

21.8 20.2 6.5 32.1 4.5 14.2 7.1 27.3 8.0

20.7 14.0 −0.1 20.6 17.3 8.8 −3.0 14.0 11.8

22.8 22.0 10.2 32.3 5.7 22.6 8.9 18.8 4.9

CASPT2

14.0 9.1 6.8 2.2

a

First value is the lowest energy spin state

cm−1, and the Co−F frequencies are in better agreement with experiment. The molecule ONiF2 is calculated to be planar with a ground state of 5A1. The CCSD(T) and CASPT2 geometries are in qualitative agreement with each other. The CCSD(T) values for the Ni−F stretch are above the experimental value as expected. The CASPT2 Ni−O stretch is higher than the CCSD(T) value by ∼85 cm−1. The PW91 value is almost 100 cm−1 below the CCSD(T) value. Additional experiments are needed to determine which method predicts the best value as the Ni−O stretch was not observed in the current experiments. The OCuF2 molecule has been previously studied at the CCSD(T) level for the doublet, and a Cu−F band has been reported from matrix isolation work.14 Our calculated value at the CCSD(T) level for the antisymmetric Cu−F stretch is in good agreement with experiment although our value is for the 4 A2 state. This state is found to be lower than the 2B2 state which was previously reported to be the ground state.14 The CASPT2 method also predicts the same ground state as our CCSD(T) H

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effects. The PW91 value for the Fe−O stretch is again larger than experiment by a comparable amount as for OMnF3. Population Analysis OMF. We first use the results from the CASPT2 calculations to analyze the change on the bonding as electrons are added to OMF (see the Supporting Information). OScF is a closed-shell molecule with the Sc in the formal +III oxidation state. In all of the OMF, the oxygen is not a purely ionic −2, and there is back-bonding (electron donation) from the O 2p orbitals into different metal 3d and 4p orbitals. (The molecules are oriented mostly along the z axis.) The addition of an electron to form OTiF goes into the dx2−y2 orbital with the molecule bent (see Table S5). The bond angle increases from Sc to Ti. The next electron goes into the dxy orbital for OVF, and the bond angle further increases. The third electron goes into dz2 for OCrF, and the bond angle decreases from that in OVF. In OCrF, the dz2 orbital has some contribution from the oxygen, so there is some spin delocalized onto the oxygen. A further decrease in the bond angle occurs from Cr to Mn, and the fourth electron goes into the dxz orbital which is out of the molecular plane. For OFeF, the bond becomes linear, and the fifth electron goes into the dyz orbital. For OCoF, the sixth electron pairs up in the dx2−y2 orbital, leaving the four unpaired electrons in four spatially different orbitals to minimize electron repulsion (see Table S3). For ONiF, the seventh electron goes into the dxy orbital, and the O atom back-bonds into the dxz and dyz orbitals which are degenerate in a linear molecule. This leads to some spin being delocalized onto the O. The same type of interactions occur for OCuF with the last electron going into the dz2 orbital. There is still back-bonding from the O to the Cu leading to spin delocalization onto the O. We examined the amount of multireference character in the CAS wave functions for the OMF as shown in Table S3. The CAS results for the OMF for Sc to Cr show that a single configuration dominates and that configuration is the one discussed above. OMnF is multiconfigurational with one of the two largest components of the wave function being the one discussed above with the second component coming from an a′′ pair where a closed-shell pair splits into an open-shell singlet pair in two different orbitals. OFeF is also a multireference species with the dominant component given above with the second component having an excitation in the pi space. There is less multireference character in OCoF which is dominated by a single configuration. As noted above, this is probably due to the fact that the four unpaired electrons are distributed into four spatially separated orbitals to minimize electron repulsion with only the dx2−y2 orbital doubly occupied. The dx2−y2 orbital is the first 3d orbital to be occupied (in OTiF, see Table S5), so this is the most energetically favored configuration. ONiF also has multireference character with the largest configuration given above. However, there are a number of contributing configurations involving excitations into the π space. The largest configuration for OCuF is given above, and there is a significant contribution from states involving excitations into the π space. The T1 diagnostics75 (Table S15), although a very qualitative measure especially for transition metals, mostly follow this trend. With the HF orbitals and the DK basis sets, the T1 values for OMF increase from Sc to Co and then essentially level out showing an increase in the multireference character as one proceeds across the period, just as found in the MRCI calculations just described. Atomic orbital occupancies and populations were calculated using NBO676,77 for the natural bond orbital (NBO)78−81 natural population analysis (NPA) at the DFT/PW91 level. The

Figure 6. Variation in the M−O and M−F bond distances (Å, top) and frequencies (cm−1, bottom) as a function of the metal for OMF2.

effect of the M−O bond distance on the M−O frequencies, with the largest effects near the end of the period. OMF3 Structure and Frequency Calculations. We also calculated the vibrational spectra (Table 6) of 1OVF3, 3OMnF3, Table 6. DFT/PW91 Calculated Results for Selected OMF3 Molecules with Frequencies (ν, cm−1) and Infrared Intensities (I, km/mol) molecule OVF3b

1

3

OMnF3

4

OFeF3

freq (ν)a

int (I)

assignment

699.3/716.3 786.1/804.0 1086.6 [1.043]/1087.8 [1.042] 685.7 742.8 747.9 1055.1 [1.045] 629.8 642.1 693.4 995.9 [1.043]

38 358 146 45 115 116 139 144 38 102 68

V−F (a) V−F (e) V−O Mn−F Mn−F Mn−F Mn−O Fe−F Fe−F Fe−F Fe−O

a16

O/18O ratios of the M−O stretches are in brackets. bCCSD(T)/ aD values are after the “/”.

and 4OFeF3 for comparison to our experiments. The vibrational frequencies of 1OVF3 were included to provide a benchmark against experimental values.70 For 1OVF3 with C3v symmetry, the CCSD(T) values are in good agreement with those of the experiment with the V−O stretch being predicted to be about 30 cm−1 too high. The DFT/PW91 values are also in good agreement with the CCSD(T) values and those of the experiment for OVF3. For 3OMnF3, there is a distortion from C3v symmetry due to Jahn−Teller effects. The calculated Mn−F stretches at the DFT levels are in good agreement with experiment. The DFT/PW91 value for the Mn−O stretch is larger than the experimental value by ∼65 cm−1. For 4OFeF3, there is also a distortion from C3v symmetry due to Jahn−Teller I

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Fe followed by a decrease to Co, followed by more modest increases to Cu. Thus, in terms of the sum of the first three ionization potentials and the hardness, there is a break at Fe. As noted above, it is at Fe that the amount of +III character starts to decrease and the amount of +II character starts to increase. The hardness of the Fe increases, but the sum of the ionization potentials decreases so the molecule falls between the +III and +II oxidation states. As the sum of the ionization potentials increases, it is harder to reach the formal +III oxidation state. At the same time, the hardness is increasing which will favor an interaction with a O− which is harder than O2−. Population Analysis OMF2. The results from the CASPT2 calculations were used to analyze the change in the bonding as electrons are added to the OMF2. OScF2 is a doublet with the Sc in the formal +III oxidation state and a spin on the O atom which is in the −1 oxidation state. In all of the OMF2, the oxygen orbitals are not purely ionic and back-bond into different metal d and p orbitals. OTiF2 is a closed-shell molecule with the Ti in the +IV oxidation state. Addition of an electron to form OVF2 goes into the dx2−y2 orbital with the M−O bond oriented along the z axis. The next electron goes into the dxy orbital for OCrF2. These first two additional d electrons behave just as for the corresponding two electrons in OMF, even though the metals are different. Back-bonding of the O to the metal beginning at Cr leads to spin delocalization onto the oxygen. The third electron goes into the dxz orbital for OMnF2, which differs from the order of addition to generate OCrF. For OFeF2, the fourth electron goes into the dyz orbital. For OCoF2, the fifth electron pairs up in the dx2−y2 orbital. Note that the dz2 orbital is still unoccupied. For ONiF2, the sixth electron goes into the dx2−y2 orbital. The last electron to form OCuF2 pairs in the dxy orbital. We also examined the amount of multireference character in the CAS wave functions for the OMF2 as shown in the Supporting Information. The CAS results for the OMF2 for Sc to Cr showed that a single configuration dominates just as found for the OMF, and that configuration is the one discussed above. OMnF2 is multiconfigurational with the largest component of the wave function being the one discussed above with the second component coming from a spin recoupling in the b2 orbital. OFeF2 has even more multireference character with the dominant component given above with the next components having spin recoupling and excitations in the b2 and b1 orbitals. There is less multireference character in OCoF2 which is dominated by the configuration given above and the next excitations involve spin recoupling in the a1 and excitations in the b1 and b2 orbitals. ONiF2 is dominated by the single configuration given above. The next largest contributions for ONiF2 are for excitations in the b1 and b2 space. OCuF2 has some multireference character with the largest configuration given above. The next most important contributions involve excitations in the a1 and b2 space. The NBOs for the OMF2 molecules are similar in some ways to the OMF results. The amount of spin-paired 3d character is 2.1−2.2 e for M = Ti to Mn. It then starts to significantly increase from M = Fe to Cu. Again, there is 0.2−0.3 e in the 4s orbital, and this is mostly spin-paired. The unpaired spin on the O atom for OScF2 is consistent with the +III oxidation state of scandium. Excess α spin on oxygen is found for Fe (0.7 e), Co (0.8 e), Ni (1.5 e), and Cu (1.5 e). We also plotted the d excess, O 2p α−β spins, and M NPA charges for the 3d metals for OMF2 in Figure 8. There is not much variation in the metal NPA charge for the OMF2 molecules, and the metal NPA charges are slightly higher than those in OMF. We can correlate the increase in the 3d

natural population analysis from the NBOs is given in Supporting Information. On the metal for OMF, except for M = Sc and Cu, there is always 0.3−0.4 e in the 4s orbital, and the electron in the 4s orbital is mostly spin unpaired. There is 0.54 e in the 4s for Cu, and it is spin-paired. For M = Ti to Mn, there are 1.5−1.7 spin-paired 3d electrons. The amount of spin-paired 3d electrons continues to increase from Fe with 2.4 3d electrons spin-paired to Co with 4.6 3d electrons spin-paired, Ni with 7 3d electrons spin-paired, and Cu with 8.7 3d electrons spin-paired. There is a small negative spin polarization on the O for M = Ti to Cr and a small positive spin polarization on the O for M = Mn. For M = Fe to Cu, there is 0.9−1.3 e spin polarization on the O s. We plotted the d excess, O 2p α−β spins, and M NPA charges for the 3d metals as shown in Figure 7. There is not much

Figure 7. Population analysis as a function of the metal for OMF.

variation in the metal NPA charge across the 3d OMF molecules. The excess 3d population peaks at Fe as the 3d electrons spin pair after Fe. The spin on the O atoms also starts to increase at Mn and take a large jump to Fe and continues to increase at a smaller rate across the period to Cu. The above results allow us to assign the formal oxidation state III to Sc to Mn for the OMF. The results are not as clean for Fe to Cu. Fe is best assigned a fractional oxidation state of +III/+II as is Co. There is more +III on the Fe than +II, and the amount of +III decreases for Co. For Ni and Cu, it is clear that the metals are in the +II oxidation state. These results are completely consistent with the changes in the M−O stretching and bond distances. It is important to note that the fluorines remain in the −1 oxidation state, and it is the oxidation state of the O atom that is changing from −2 to −1 with the spin on the O atom increasing as there is more −1 character on the O. There are also changes in the amount of back-bonding from the O to the metal which can also affect the frequencies. The lower frequency for Sc−O is due to the larger +III ionic radius82 for Sc of 0.83 Å as compared to 0.69 Å for Ti and 0.65 Å for V. The remaining +III ionic radii, excluding Cu, range from 0.62 Å for Ni to 0.70 Å for Mn, so there is not as much variation due to the size of the metal ion as long as the metal ion is in the +III oxidation state. Are there any underlying atomic properties that can correlate with our formal oxidation state assignments based on changes in the M−O frequencies? The experimental ionization potentials are given in Table S17.83 The sum of the first three ionization potentials increases from 44.12 eV for Sc to 56.74 eV for Mn, decreases for Fe, and then increases to 64.86 eV for Cu. We can also calculate the hardness84−87 of the +III oxidation state for the atoms as η = (IP − EA)/2, where EA is the electron affinity and IP is the ionization potential. For a +III oxidation state, this is (IP4 − IP3)/2. As we go across the first transition metal period, η increases to Cr, decreases to Mn and then rapidly increases for J

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 7. Extrapolated Total Electronic Total Atomization Energies (kcal/mol) for OMF and OMF2 OMFx

ΔECBS(V) ΔECBS(CV)

ΔECBS‑DK(CV)/ HF

ΔECBS‑DK(CV)/ PW91

OMF

Figure 8. Population analysis as a function of the metal for OMF2.

excess with the increase in MO stretches for OMF2, and we note that such a trend is not found for the OMF. In this case, the excess 3d orbital population peaks at Fe and Mn. Again, the O atom spin population starts to increase at Mn and continues to rise until it reaches Ni where there is a slight decrease to Cu. The Sc in OScF2 is in the +III oxidation state. The next three OMF2 with M = Ti, V, and Cr clearly have the metal in the +IV oxidation state. As spin delocalizes onto the oxygen, the M−O frequency decreases as found for Mn, but it is still appropriate to assign a +IV oxidation state to the Mn. The oxidation states for the Fe in OFeF2 and the Co in OCoF2 are probably best assigned as a mixed +IV/+III state. These results are consistent with the decrease in the M−O stretching frequencies as well. It is best to assign an oxidation state of +III to the Ni in ONiF2 and a +II oxidation state to the Cu in OCuF2. We note that the amount of spin on the O (near two electrons) in OCuF2 suggests that the O is participating in a dative type bond to the metal. This is consistent with the low M−O frequency in OMF2. We note that the PT2 correlation correction was needed to even get the O to bind in OCuF2. Heats of Formation. The total atomization energies and the corresponding heats of formation of the OMF and OMF2 molecules were calculated at the FPD level using four different approaches. The approaches are as follows: (1) valence electron correlation only extrapolated to the CBS limit, (2) valence plus core−valence extrapolated to the CBS limit, (3) valence plus core−valence extrapolated to the CBS limit with relativistic effects treated at the DKH level using the Hartree−Fock (HF) orbitals, and (4) valence plus core−valence extrapolated to the CBS limit with relativistic effects treated at the DKH level using PW91 orbitals (Table 7). The role of core−valence effects is not small as shown in Table 7 and cannot be ignored. There are no systematic periodic trends for the core−valence effects. The errors in the pseudopotential also can be non-negligible. The T1 diagnostics75 are in general less than 0.04 which is typical for these types of transition metal compounds. Using the HF orbitals, the T1 values are >0.04 for 4OMnF2, 5ONiF2, and 4 OCuF2 but the use of the PW91 orbitals lowers these values to below 0.04. Except for 2OScF2, the T1 values are lower using the PW91 orbitals, so we use these energies for the heat of formation calculations, which should help to address issues with multireference character. For 1OScF, the use of Brueckner orbitals88−94 gives ΔECBS‑DK(CV) = 299.15 kcal/mol in good agreement with the other core−valence values. For 1OTiF2, the use of Brueckner orbitals gives ΔECBS‑DK(CV) = 445.16 kcal/mol, again in good agreement with the other calculations. The ΔECBS‑DK(CV)/PW91 total atomization electronic energies were used to calculate the heats of formation (Table 8). There is a good agreement with the experimental value of

1

OScF 2 OTiF 3 OVF 4 OCrF 5 OMnFa 6 OFeF 5 OCoF 4 ONiF 3 OCuF

295.09 293.78 272.20 242.57 202.51 219.19 213.78 205.27 167.03

300.29 296.47 274.79 243.32 200.41 218.39 213.45 201.87 163.08

2

384.72 444.56 416.81 358.98 302.09 299.57 275.35 250.31 195.97

391.15 447.82 419.84 359.74 302.69 298.97 275.87 247.42 192.78

299.92 295.83 272.94 240.15 199.40 217.33 211.56 199.99 161.38

299.63 298.13 276.89 243.58 200.41 216.05 209.67 199.58 160.48

389.49 446.11 417.06 355.96 300.54 297.32 273.65 245.55 191.14

391.72 446.49 424.21 363.44 307.07 303.90 278.33 246.68 194.59

OMF2 OScF2 1 OTiF2 2 OVF2 3 OCrF2 4 OMnF2 5 OFeF2 4 OCoF2 5 ONiF2 4 OCuF2 a

Linear structure.

−103.5 kcal/mol for 2OTiF.95 The calculated heat of formation for 4OCrF falls within the error bars of the experimental value of −74 ± 12 kcal/mol.90 The calculated heat of formation differs by ∼12 kcal/mol from the experimental value of −221.0 kcal/ mol for OTiF2.90 This is surprising because the calculated values for the heats of formation of TiO228,52 and TiF496 are in very good agreement with experiment. The calculated heat of formation for 3OCrF2 is outside the error bars of the experimental value of 146 ± 12 kcal/mol.90

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CONCLUSIONS Reaction products of 3d transition metal atoms (V, Mn, Fe, Co, and Ni) and OF2 were investigated using matrix isolation infrared spectroscopy and electronic structure calculations. New molecules in the form of OMF2 were observed for all of the metals, and triatomic OMF molecules were identified for V, Mn, Fe, and Co. In addition, OMnF3 and OFeF3 were formed in the experiments with manganese and iron. Electronic structure calculations at the DFT, CCSD(T), and CASPT2 levels were performed to predict the spatial-spin states, molecular geometries, and vibrational frequencies. The calculations support the experimental assignments and complete the description of periodic behavior when experimental data is absent. For the OMF molecules, the M−O bond distances decrease, and the M−O stretching frequencies increase from M = Sc to V. As more d electrons are added across the periodic table, the M− O stretching frequencies do not follow any simple trend even though the M−O bond distances do not change significantly. The M−F bond distances decrease across 3d transitional metals without significant changes on the M−F stretching frequencies. The M−O stretching frequencies are very sensitive to the computational methods and the amount of M−O and M−F coupling, especially in terms of whether the molecule is linear or bent. There is a significant increase in the M−O frequencies from OScF2 to OTiF2, consistent with a significant decrease in K

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 8. Components for Calculating the Total Atomization Energies and Calculated Heats of Formation (kcal/mol) OMFx

ΔECBS‑DK(CV)/PW91a

ΔEZPEb

1

OScF 2 OTiF 3 OVF 4 OCrF 5 OMnFh 6 OFeF 5 OCoF 4 ONiF 3 OCuF

299.63 298.13 276.89 243.58 201.58 216.05 209.67 199.58 160.48

−2.50 −2.74 −2.71 −2.61 −2.56 −2.31 −2.53 −2.65 −2.29

2

391.72 446.49 424.21 363.44 307.07 303.90 278.33 246.68 194.59

−3.45 −4.15 −4.36 −4.36 −4.30 −4.09 −4.13 −3.50 −3.35

ΔESOc

ΣD0,0Kd

ΔHf,0Kf

ΔHf,298Kg

−0.90 −1.21 −1.51 −0.61 −0.61 −1.81 −2.91 −3.41 −0.61

296.23 294.18 272.67 240.36 198.4 211.93 204.23 193.52 157.58

−128.9 −104.3 −72.8 −68.4 −53.6 −35.8 −25.2 −13.8 0.3

−129.4 −104.8 −73.2 −68.6 −54.0 −36.0 −25.3 −14.0 −0.1

−1.29 −1.60 −1.90 −1.00 −1.00 −2.20 −3.30 −3.80 −1.00

386.98 440.74 417.95 358.08 301.77 297.61 270.90 239.38 190.24

−201.2 −232.4 −199.6 −167.7 −138.4 −103.0 −73.4 −41.2 −13.9

−200.7 −232.5 −199.2 −167.1 −138.1 −102.5 −73.0 −40.5 −13.4

OMF

OMF2 OScF2 1 OTiF2 2 OVF2 3 OCrF2 4 OMnF2 5 OFeF2 4 OCoF2 5 ONiF2 4 OCuF2

Extrapolated CCSD(T)/awCVnZ-DK/PW91 energies with n = D, T, and Q using the equation: E(n) = ECBS + A exp[−(n − 1)] + B exp[−(n − 1)2]. bCalculated zero point energies at PW91/aD. cSpin orbit corrections. dΣD0,0K= ΔECBS‑DK(CV)/PW91 + ΔEZPE + ΔESO. fThe heats of formation at 0 K are derived from their TAE and the experimental heats of formation of the atoms. gAtomic thermal correction were used to calculate heats of formation at 298 K. hFor the linear 5OMnF: ΔECBS‑DK(CV)/PW91 = 200.41 kcal/mol; ΣD0,0K= 197.24 kcal/mol; ΔHf,0K = −52.4 kcal/mol; ΔHf,298K= −52.8 kcal/mol. a

The Sc in OScF2 has in the +III oxidation state with O atom in the −1 oxidation state. OTiF2 is a closed-shell molecule with the Ti in the +IV oxidation state. The electrons go into the dx2−y2, dxy, dxz, and dyz orbitals from V to Fe. The fifth electron pairs in the dx2−y2 orbital for OCoF2 and the sixth electron goes into the dz2 orbital for ONiF2. The last electron to form OCuF2 leads to an electron pair in the dxy. The OMF2 from Sc to Cr are dominated by a single configuration as found for the OMF. OMnF2 and OFeF2 are multireference, but there is less multireference character in OCoF2 and ONiF2. OCuF2 has some multireference character. The V and Cr in OMF2 are in the +IV oxidation state as is Mn even though there is some s spin delocalized onto the oxygen for the latter. The Fe and Co in OMF2 are assigned to a mixed +IV/+III state. The Ni in ONiF2 is assigned to a +III oxidation state, and the Cu in OCuF2 is assigned to a +II oxidation state. These results are consistent with the changes in the M−O stretching frequencies. The total atomization energies (ΔECBS‑DK(CV)) of OMF and OMF2 were calculated at the FPD level using four different approaches, and ΔECBS‑DK(CV) obtained using DFT/PW91starting orbitals for the CCSD(T) calculations were employed to calculate the heats of formation as the core−valence effects are not small and the PW91 functional shows lower T1 values.

the M−O bond distance as the M−O bond transitions from a single bond with radical O character to a double bond. Only small variations in the M−O bond distances and stretching frequencies are predicted for the OMF2 from Ti to Co. A significant increase in the M−O distances and decrease in the M−O stretching frequencies is found from Ni to Cu. No significant change is predicted for the M−F distances and stretching frequencies. The CASPT2 calculations for the OMF show that the oxygen is not a purely ionic −2 and that the O 2p orbitals back-bond into different metal 3d and 4p orbitals. Starting from closed-shell Sc, the orbitals are singly occupied in the following order from Ti to Mn: dx2−y2, dxy, dz2, dxz, and dyz. Electron pairing of the d orbitals begins at Co in the same order as the first three orbitals. It is clear from the CASPT2 calculations that significant multireference character begins at OMnF and that the remaining OMF except for OCoF also have significant multireference character. Oxidation state III can be assigned for the metal in OMF from Sc to Mn. Fe and Co can be assigned a fractional oxidation state of +III/+II, and Ni and Cu are in the +II oxidation state. These results correlate with the M−O stretching frequencies. The fluorine remains in the −1 oxidation state, and the oxygen changes from a −2 oxidation state to −1 leading to more spin on the O atom. The atomic ionization potentials can be used to provide some insights into what is happening for the OMF. The sum of the first three ionization potentials and the hardness show a break at Fe consistent with the inclusion of the +II oxidation state. As the formal +III oxidation state gets harder to form in terms of sum of the ionization energies, the atomic ion also becomes harder, so it cannot interact as well with the soft O2− ligand as with the harder O− ligand leading to a dominance of the formal +II oxidation state for OMF for the heavier elements in the first transition metal period.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00822. Complete citations for refs 31 and 55, molecular geometries and spins for OMF and OMF2, complete frequencies and IR intensities tables, CI vectors and CAS orbitals, population analysis for OMF and OMF2, total L

DOI: 10.1021/acs.inorgchem.9b00822 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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atomization energies, T1 values, complete second-order perturbation theory energy analysis from NBOs analysis for OMF and OMF2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.A.D.). *E-mail: [email protected] (Y.G.). ORCID

Monica Vasiliu: 0000-0001-7573-4787 David A. Dixon: 0000-0002-9492-0056 Lester Andrews: 0000-0001-6306-0340 Yu Gong: 0000-0002-8847-1047 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experimental work at UVA was supported by the University of Virginia (L.A.). Y.G. thanks the NSFC (21771189), the Strategic Priority Research Program and Frontier Science Key Program of the Chinese Academy of Sciences (Grants XDA02030000 and QYZDYSSW-JSC016) and the Young Thousand Talented Program for support. Y.G. and L.A. also thank Prof. Bruce Ault for sharing the OVF3 spectrum in solid argon. The computational work at UA was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) under the DOE BES Catalysis Center Program by a subcontract from Pacific Northwest National Laboratory (KC0301050-47319). D.A.D. also thanks the Robert Ramsay Chair Fund of The University of Alabama for support.

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REFERENCES

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