Oxygen Atom Stabilization by a Main-Group Lewis Acid: Observation and Characterization of an OBeF2 Complex with a Triplet Ground State

Terminal oxygen radicals involving p- and d-block atoms are quite common, but s-block compounds with an oxygen radical character remain rare. Here, we report that alkaline-earth metal beryllium atoms react with OF2 to form the oxygen beryllium fluorides OBeF and OBeF2. These species are characterized by matrix-isolation infrared spectroscopy with isotopic substitution and quantum-chemical calculations. The linear molecule OBeF has a 2Π ground state with an oxyl radical character. The 3A2 (C2v) ground state of OBeF2 represents the unusual case of a triplet oxygen atom stabilized by a relatively weak interaction by the Lewis acidic BeF2. The interaction involves both a donor component from oxygen to empty Be orbitals and a back-bonding contribution from fluorine substituents toward oxygen.


Experimental and Computational Methods
The experimental apparatus used for the laser ablation of beryllium atoms and their reaction with OF 2 in exceed neon gases, as well as their deposition at 5 K using a closed cycle helium cryostat (Sumitomo Heavy Industries, RDK-205D) inside a vacuum chamber has been described in more detail in our previous works. [1] 16/18 OF 2 was synthesized by a known procedure using elemental fluorine and 16/18 OH 2 dispersed in solid NaF. [2]The Nd:YAG laser fundamental (Continuum, Minilite II, 1064 nm, 10 Hz repetition rate with 10 ns pulse width) with a pulse energy of up to 55 mJ/cm² was focused onto the metal targets, which gave an energetic plasma beam reacting with OF 2 and spreading toward the cold rhodium-plated mirror.
IR spectra were recorded in reflection using a Bruker Vertex 80v vacuum FTIR spectrometer equipped with a KBr beam splitter and a liquid nitrogen cooled MCT detector were used in the region of 5000-450 cm −1 and FIR multilayer mylar beam splitter with a liquid helium cooled bolometer was used in the region of 600-150 cm −1 .For each spectrum 200 scans at a resolution of 0.5 cm −1 were coadded.The matrix samples were annealed at different temperatures, and cooled back to 5 K for spectral acquisition.The selected samples were subjected to irradiation with a medium-pressure mercury arc streetlamp (λ > 220 nm).
Kohn-Sham density functional theory (DFT) calculations were performed at the M06-2X/def2-TZVPP [3] level with the Gaussian 16 program. [4]Natural population and natural bond orbital analyses [5] (NPA and NBO) were performed at the same level.Additional calculations were performed with the TURBOMOLE program package, version 7.7.1. [6]Extended transition-state analyses with natural orbitals for chemical valence (ETS-NOCV) were performed at the BP86+D3(BJ)/TZ2P level [7] using the ADF engine of the AMS software package (Release 2023.1). [8]upled cluster calculations with single, double, and perturbative triple substitutions, CCSD(T), were carried out in the closed-shell (RHF-CCSD(T)) and partially spin-restricted open-shell (RHF-RCCSD(T)) formalism using default frozen core settings as implemented in the MOLPRO2022 software package. [9]To closely approach the basis set limit, explicitly correlated calculations were performed using the F12a approximation. [10]Explicit correlation effects on the perturbative triples were estimated by scaling the (T) contribution as: All calculations were performed using aug-cc-pVTZ-F12 basis sets [11] (cc-pVTZ-F12 for Be [12] ) as well as the respective auxiliary basis sets automatically assigned by the MOLPRO program.Anharmonic vibrational frequencies were calculated at the VCISDTQP6 level of theory allowing up to 5 excitations within one mode. [13]VCI calculations were performed on a polynomial fit of a multi-level surface using CCSD(T*)-F12a/aug-cc-pVDZ-F12 energies for the 2-body and CCSD-F12a/aug-cc-pVDZ-F12 energies for the 3-body terms.Intensities were computed using the dipole surfaces at the HF level of theory. [14]rtical excitation energies were calculated at the LR-SCS-CC2 [15] and LR-CCSD levels [16] using aug-cc-pVQZ basis sets [17] with the TURBOMOLE program package.Calculations at the LR-CCSDT level [18] used aug-cc-pVDZ basis sets and the MRCC program package. [19]citation energies at the LR-CCSDT/aug-cc-pVQZ level are estimated as: Band splittings according to the Renner-Teller effect were calculated using the expressions (I, 44), (I, 47) and (I,48) provided by Herzberg. [20]The expressions require three input parameters: The vibrational frequency of the bending mode (here taken from our high-level calculations described above), the Renner parameter ε and the spin-orbit coupling strength A. Both ε and A were calculated at the MRCI+Q-F12 level [21] using the basis sets described above.All calculations are based on SA-CASSCF(5,6) calculations using the first two doublet states (the degenerate components of the 2 Π state).The active space included the bonding and anti-bonding σ-and π-orbitals of the Be-O bond.Inclusion of orbitals on F resulted in rotations with lowerlying orbitals and to natural orbital occupation numbers very close to two.For the calculation of ε, the vibrational frequencies of the bending mode were calculated using either one of the two lowest doublet states at MRCI+Q level.The Renner parameter is then approximately obtained as ε ≈ (ν 1 2 -ν 2 2 )/( ν 1 2 + ν 2 2 ), where ν i denotes the vibrational frequency of the bending mode in the ith doublet state.Spin-orbit coupling (SOC) eigenstates were obtained by diagonalization of the sum of the electronic and Breit-Pauli SOC Hamiltonian in the basis of the MRCI wave functions obtained for the two doublet states. [22]The energy eigenvalues of the electronic Hamiltonian were adapted to include the Davidson size-consistency correction (+Q) [23] using a relaxed reference.The SOC constant A can then be found as two-thirds of the energy difference between the two eigenstates.S2.Experimentally observed and calculated vibrational frequencies (cm −1 ) and intensities (km mol −1 ) of OBeF (M06-2X/def2-TZVPP, CCSD(T)/aug-cc-pVTZ and CCSD(T*)-F12a/(aug-)cc-pVTZ-F12 levels).

Figure S1 . 2 Figure S2 .Figure S3 .Figure S4 .Figure S5 .
Figure S1.Infrared difference spectra in the 1600-1300 cm −1 region from co-deposition of laser-ablated Be atoms with 0.05% OF 2 in neon.(a) Spectrum taken after annealing to 10 K minus the spectrum taken after 30 min of sample deposition at 5 K, (b) Spectrum taken after 10 min of full arc (λ > 220 nm) irradiation minus the spectrum taken annealing to 10 K.The bands of unidentified species (*) are labeled.

Figure S6 .
Figure S6.Calculated spin-density for beryllium oxyfluorides at the M06-2X/def2-TZVPP level.Isosurfaces of the densities are shown at a value of 0.005 a.u.

Table S1 .
Experimentally Anharmonic vibrational frequencies obtained at VCISDTQP6 level.Intensities were obtained based on dipole surfaces at the HF level.

Table S4 .
Anharmonic vibrational frequencies obtained at VCISDTQP6 level.Intensities were obtained based on dipole surfaces at the HF level.Vertical excitation energies (in eV) and oscillator strengths of OBeF 2 at various coupled-cluster levels (see above for details).