Infrared photodissociation spectroscopy of mass-selected [TaO 3 (CO 2 ) n ] + ( n = 2-5) complexes in the gas phase.

We report a joint experimental and theoretical study on the structures of gas-phase [TaO 3 (CO 2 ) n ] + ( n = 2-5) ion-molecule complexes. Infrared photodissociation spectra of mass-selected [TaO 3 (CO 2 ) n ] + complexes were recorded in the frequency region from 2200 to 2450 cm -1 and assigned through comparing with the simulated infrared spectra of energetically low-lying structures derived from quantum chemical calculations. With the increasing number of attached CO 2 molecules, the larger clusters show significantly enhanced fragmentation efficiency and a strong band appears at around 2350 cm -1 near the free CO 2 antisymmetric stretching vibration band, indicating only a small perturbation of CO 2 molecules on the secondary solvation sphere while higher frequency bands corresponding to the core structure remain largely unaffected. A core structure [TaO 3 (CO 2 ) 3 ] + is identified to which subsequent CO 2 ligands are weakly attached and the most favorable cluster growth path is verified to proceed on the triplet potential energy surface higher in energy than that of ground states. Theoretical exploration reveals a two-state reactivity (TSR) scenario in which the energetically favored triplet transition state crosses over the singlet ground state to form a TaO 3 + core ion, providing new information on the cluster formation correlated with the reactivity of tantalum metal oxides towards CO 2 .