Structure Assignment and Separation of Isomers of Palladium Oxide Cluster Anions Studied by Ion Mobility Mass Spectrometry

Geometric structures of palladium oxide cluster anions, Pd_nO_m ^- (n = 2-7, n ≥ m), were investigated by ion mobility mass spectrometry and quantum chemical calculations. Collision cross sections of the stable compositions, Pd_nO_n-1 ^- and Pd_nO_n ^-, were determined from the mobility measurement. Structures of these cluster ions were optimized by density functional theory calculations. The results commonly suggested that the plausible structures for Pd_nO_n-1 ^- were preferentially constructed by consecutive Pd-O-Pd bonds. For Pd_nO_n ^- clusters, structures with molecular oxygen (-O-O-) appeared at n ≥ 4 along with atomic oxygen. Moreover, bulky and compact isomers were found to coexist at n = 5 and 6 for Pd_nO_n-1 ^- and n = 4-6 for Pd_nO_n ^- from the experimental results. These findings showed a marked contrast with the corresponding cationic clusters in which the metal-core configurations were maintained as observed in the previous study.