Geometrical Structures of Platinum Oxide Cluster Anions Studied by Ion Mobility-Mass Spectrometry

Geometrical structures of platinum oxide cluster anions were investigated using ion mobility-mass spectrometry and compared with their cationic counterparts obtained from our previous study. The measured collision cross sections (CCSs) with He under 170 K of Pt_nO_n^– anions were larger than those of Pt_nO_n⁺ cations, suggesting that the anions adopt more extended structures than their cationic analogues. In particular, planar Pt frameworks with bridging oxygen atoms were predicted for Pt_nO_n^– (n = 4–7), based on the agreement between the experimental CCS values and those in theoretical calculations. For n = 4 and 5, these structures are consistent with the lowest-energy geometries reported for the corresponding neutral clusters. In contrast, the cationic species adopt three-dimensional structural motifs for n ≥ 5. The assigned structures and natural bond orbital (NBO) charge analyses indicate that increased electron density on the Pt framework stabilizes the planar configurations in the anions, leading to larger CCSs.