Valence and Structure Isomerism of Al₂FeO₄⁺: Synergy of Spectroscopy and Quantum Chemistry

Abstract

We provide spectroscopic and computational evidence for a substantial change in structure and gas phase reactivity of Al3O4 + upon Fe-substitution, which is correctly predicted by multireference (MR) wave function calculations. Al3O4 + exhibits a cone-like structure with a central trivalent O atom (C3v symmetry). The replacement of the Al- by an Fe atom leads to a planar bicyclic frame with a terminal Al–O•– radical site, accompanied by a change from the Fe+III/O–II to the Fe+II/O–I valence state. The gas phase vibrational spectrum of Al2FeO4 + is exclusively reproduced by the latter structure, which MR wave function calculations correctly identify as the most stable isomer. This isomer of Al2FeO4 + is predicted to be highly reactive with respect to C–H bond activation, very similar to Al8O12 + which also features the terminal Al–O•– radical site. Density functional theory, in contrast, predicts a less reactive Al3O4 +-like “isomorphous substitution” structure of Al2FeO4 + to be the most stable one, except for functionals with very high admixture of Fock exchange (50%, BHLYP).

Structural Characterization of Nickel-Doped Aluminum Oxide Cations by Cryogenic Ion Trap Vibrational Spectroscopy

“…DFT does a surprisingly good job for these challenging systems, which really should be treated with multireference approaches. Even though the DFT energies are not predictive for transition metal-containing systems as studied here, the lowest energy isomer is typically (but not always) found to be the structure that contributes predominantly to the IRPD spectrum. For most systems, we find satisfactory agreement with the spectra of clusters containing the Ni atom in a high-spin configuration.…”

“…With increasing cluster size, the number of fourfold (and higher fold) coordinated Al atoms increases and three-dimensional 58 For the moderate cluster sizes (n = 3) characterized here, we find that cone-like 3D structures, originally identified for the corresponding (Al 2 O 3 ) 1−4 (AlO) + cations, 20 DFT does a surprisingly good job for these challenging systems, which really should be treated with multireference approaches. Even though the DFT energies are not predictive for transition metal-containing systems as studied here, 27 the lowest energy isomer is typically (but not always) found to be the structure that contributes predominantly to the IRPD spectrum. For most systems, we find satisfactory agreement with the spectra of clusters containing the Ni atom in a highspin configuration.…”

“…The corresponding IR spectra are complex and start to converge to that of amorphous alumina for systems containing more than ∼25 atoms . Less is known concerning the spectroscopy of metal-doped aluminum clusters, which can represent challenging benchmarks even for advanced electronic structure methods. − Ni-doped aluminum oxide clusters have not been studied previously by theory or experiment.…”

The Chemical Nature of Ti4O10–: Vibrational Predissociation Spectroscopy Combined with Global Structure Optimization

“…All calculations made use of the resolution of the identity approximation for the evaluation of the Coulomb term (RI-J). 45,46 For a better comparison with the experimental findings, the calculated stick spectra were convoluted with Gaussian line shapes with a full width at half-maximum (fwhm) of 15 cm −1 , and the cosine similarity score, 25 S, for the agreement between a particular simulated spectrum and the IRPD result was calculated as in previous studies: 47,48…”

“…For a better comparison with the experimental findings, the calculated stick spectra were convoluted with Gaussian line shapes with a full width at half-maximum (fwhm) of 15 cm –1 , and the cosine similarity score, S , for the agreement between a particular simulated spectrum and the IRPD result was calculated as in previous studies: , The vectors A and B contain the normalized calculated and measured IR intensities, respectively, sampled on the same wavelength grid. S can be interpreted as the normalized cross-correlation of A and B with a displacement of zero, or as their normalized discrete overlap integral.…”