Al⁺ Activates Acetone to Form Pinacolate

The interaction between aluminum cations and acetone is studied in the gas phase via photodissociation vibrational spectroscopy from 1100 to 2000 cm^-1. Spectra of Al⁺(acetone)(N₂) and ions with the stoichiometry of Al⁺(acetone) (n = 2-5) were measured. The experimental results are compared to DFT calculated vibrational spectra to determine the structures of the complexes. The spectra show a red shift of the C=O stretch and a blue shift of the CCC stretch, which decrease as the size of the clusters increases. The calculations predict that the most stable isomer for n ≥ 3 is a pinacolate, in which oxidation of the Al⁺ enables reductive C-C coupling between two acetone ligands. Experimentally, pinacolate formation is observed for n = 5, as evidenced by a new peak observed at 1185 cm^-1 characteristic of the pinacolate C-O stretch.