Structures and vibrational spectroscopy of partially reduced gas-phase cerium oxide clusters

Abstract

This work demonstrates that the most stable structures of even small gas-phase aggregates of cerium oxide with 2–5 cerium atoms show structural motifs reminiscent of the bulk ceria. This is different from main group and transition metal oxide clusters, which often display structural features that are distinctly different from the bulk structure. The structures of Ce₂O₂⁺, Ce₃O₄⁺, and (CeO₂)_mCeO⁺ clusters (m = 0–4) are unambiguously determined by a combination of global structure optimizations at the density functional theory level and infrared vibrational predissociation spectroscopy of the cluster–rare gas atom complexes. The structures of Ce₂O₂⁺ and Ce₂O₃⁺ exhibit a Ce–O–Ce–O four-membered ring with characteristic absorptions between 430 and 680 cm⁻¹. Larger clusters have common structural features containing fused Ce–O–Ce–O four-membered rings which lead to intense absorption bands at around 500 and 650 cm⁻¹. Clusters containing a terminal CeO bond show a characteristic absorption band between 800 and 840 cm⁻¹. For some cluster sizes multiple isomers are observed. Their individual infrared signatures are identified by tuning their relative population through the choice of He, Ne or Ar messenger atoms. The present results allow us to benchmark different density functionals which yield different degrees of localization of unpaired electrons in Ce 4f states.