Compositions and Isomer Separation of Palladium Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry Abdul LatifM. WuJenna W. J. NagataToshiaki NakanoMotoyoshi OhshimoKeijiro MisaizuFuminori 2019 Geometric structures of gas-phase palladium oxide cluster cations, Pd<sub><i>n</i></sub>O<sub><i>m</i></sub><sup>+</sup>, 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, Pd<sub>3</sub>O<sub><i>m</i></sub><sup>+</sup> (<i>m</i> = 1–6), Pd<sub>4</sub>O<sub><i>m</i></sub><sup>+</sup> (<i>m</i> = 2–8), and Pd<sub>5</sub>O<sub><i>m</i></sub><sup>+</sup> (<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 Pd<sub>3</sub>O<sub><i>m</i></sub><sup>+</sup> 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 Pd<sub>4</sub>O<sub><i>m</i></sub><sup>+</sup> cluster ion. Two structural isomers were assigned for Pd<sub>5</sub>O<sub><i>m</i></sub><sup>+</sup>, 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.