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 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 S-1 to T-2 takes place, and photochemical hydrogen formation starts on the T-2 surface, which passes through a conical intersection, changing to T-1. 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.

Automated summary

Hydrated aluminium cations were investigated as a photochemical model system with up to ten water molecules using UV action spectroscopy. The intense photodissociation starts at 4.5 eV for two to eight water molecules, and quantum chemical calculations show that solvation shifts the excitations into the photon energy range. During photochemical relaxation, internal conversion occurs, leading to hydrogen formation and further reactions on different energy surfaces.