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

Geometric structures of palladium oxide cluster anions, PdnOm- (n = 2-7, n >= m), were investigated by ion mobility mass spectrometry and quantum chemical calculations. Collision cross sections of the stable compositions, PdnOn-1- and PdnOn-, 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 PdnOn-1- were preferentially constructed by consecutive Pd-O-Pd bonds. For PdnOn- 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 PdnOn-1-, and n = 4-6 for Pd(n)On(-) 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.