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Photochemical Hydrogen Evolution at Metal Centers Probed with Hydrated Aluminium Cations, Al+(H2O)n, n=1–10

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molec...

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Published in:Chemistry : a European journal 2021-11, Vol.27 (66), p.16367-16376
Main Authors: Heller, Jakob, Pascher, Tobias F., Linde, Christian, Ončák, Milan, Beyer, Martin K.
Format: Article
Language:English
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Summary:Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s–3p excitations of Al+ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1. On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH+(H2O)n‐1 and are transparent in the investigated energy range. For n=4–8, a mixture of Al+(H2O)n and HAlOH+(H2O)n‐1 is present in the experiment. A complex sequence of events, including intersystem crossing to the triplet surface and electron transfer, is involved in photochemical hydrogen evolution in Al+(H2O)n. For two water molecules, high‐level calculations reveal a scenario for atomic hydrogen formation in which the system passes through a conical intersection between the two lowest‐lying triplet states, and the hydrogen atom is virtually ejected while the remaining hydrated aluminium hydroxide cation relaxes to its new equilibrium geometry.
ISSN:0947-6539
1521-3765
1521-3765
DOI:10.1002/chem.202103289