Geometrical Structures of Gas-Phase Cerium Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Geometrical structures of cerium oxide cluster cations, Ce_nO_m ⁺ (n = 2-6, m ≤ 2n), were studied by ion mobility mass spectrometry (IMMS). The most plausible structure for each composition was determined by comparison of a collision cross section (CCS) measured by IMMS with simulated CCSs of several candidate structures obtained from density functional theory (DFT) calculations. For Ce₂O_2,3 ⁺, Ce₃O_4,5 ⁺, and Ce₅O₉ ⁺, the IMMS results were mostly consistent with those of the vibrational spectroscopic study, whereas a more compact structure was suggested for Ce₄O₇ ⁺. The geometrical structures of some species, Ce₄O_5,6 ⁺, Ce₅O_6-8 ⁺, and Ce₆O_8-12 ⁺, were studied for the first time. For Ce_nO_2n ⁺ clusters, the IMMS measurements suggested more compact structures than the reported DFT calculations. Structures with a peroxide ion (O₂ ^2-) were found to be the most reasonable for Ce_nO_2n ⁺ compositions because they are consistent with the experimental results and have the lowest energies among the examined candidate structures, whereas other structures without O₂ ^2- are also possible. These findings offer important insights into the reaction mechanisms involving those clusters.