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

Abstract

Geometric structures of palladium oxide cluster anions, Pd n O 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 n O n–1 – and Pd n O 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 n O n–1 – were preferentially constructed by consecutive Pd–O–Pd bonds. For Pd n O 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 n O n–1 – and n = 4–6 for Pd n O 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.