Photoelectron Spectroscopy of Free Polyoxoanions Mo₆O₁₉²⁻ and W₆O₁₉²⁻ in the Gas Phase

Two doubly charged polyoxoanions, Mo 6 O 19 2- and W 6 O 19 2-, were observed in the gas phase using electrospray ionization. Their electronic structures were investigated using photoelectron spectroscopy and density functional calculations with relativistic effective core potentials. Each dianion was found to be highly stable despite the presence of strong intramolecular Coulomb repulsion, estimated to be about 2 eV for each system. The valence detachment features were all shown to originate from electronic excitations involving oxygen lone-pair type orbitals. Their observed energies were in excellent agreement with the theoretical vertical detachment energies calculated using time-dependent density functional theory. Despite being multiply charged, polyoxometalate oxide clusters can be studied in the gas phase, providing the opportunity for detailed benchmark theoretical studies on the electronic structures of these important transition-metal oxide systems.