Compositions and Isomer Separation of Palladium Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Geometric structures of gas-phase palladium oxide cluster cations, Pd_<i>n</i>O_<i>m</i>⁺, were investigated for stable compositions by ion mobility mass spectrometry (IMMS) and quantum chemical calculations. Pure metallic (<i>m</i> = 0) and oxygen-deficient (<i>m</i> < <i>n</i>) cluster cations were preferentially obtained from the mass spectra as a result of collision-induced dissociation. Structures of cluster series, Pd₃O_<i>m</i>⁺ (<i>m</i> = 1–6), Pd₄O_<i>m</i>⁺ (<i>m</i> = 2–8), and Pd₅O_<i>m</i>⁺ (<i>m</i> = 3–8), were determined by comparing experimental collision cross sections obtained by IMMS and theoretical collision cross sections of optimized structures by density functional theory calculations. As for the Pd₃O_<i>m</i>⁺ cluster cations, structural transition was observed from one-dimensional chains to two-dimensional (2D) branched/2D sheets and finally to three-dimensional (3D) compact structures with increasing <i>m</i>. These 2D and 3D isomers were found to retain their triangular metal-core configuration. 2D sheets and 3D compact isomers that maintain a tetrahedral metal-core configuration were assigned for the Pd₄O_<i>m</i>⁺ cluster ion. Two structural isomers were assigned for Pd₅O_<i>m</i>⁺, one with a 3D square pyramidal metal-core configuration and another one with a 3D distorted pentagonal. Furthermore, the structures of oxygen-deficient cluster ions include atomic oxygen preferentially, whereas structures with molecular oxygen were commonly assigned for oxygen-rich (<i>m</i> > <i>n</i>) cluster ions.