Observation and the Origin of Magic Compositions of Co_<i>n</i>O_<i>m</i>^– Formed in Oxidation of Cobalt Cluster Anions

To obtain atomistic insights into the early stage of the oxidation process of free cobalt cluster anions Co_<i>n</i>^–, the reaction of Co_<i>n</i>^– (<i>n</i> ≤ 10) with varied pressure of O₂ was studied experimentally and theoretically. Population analysis of the oxidation products Co_<i>n</i>O_<i>m</i>^– as a function of <i>m</i> revealed two types of magic compositions: the population decreases abruptly upon addition of a single O atom to and removal of a single O atom from the magic compositions. Magic compositions of the former type were further divided into oxygen-rich (<i>n</i>:<i>m</i> ∼ 3:4) and oxygen-poor (<i>n</i>:<i>m</i> ∼ 1:1) series. The oxygen-rich compositions most likely correspond to fully oxidized states, since the compositions are comparable to those of Co₃O₄ in the bulk. Their appearance is ascribed to the significant reduction of binding energies of O atoms to fully oxidized clusters. In contrast, oxygen-poor compositions correspond to the intermediates of the full oxidation states in which only the surface is oxidized on the basis of theoretical prediction that oxidation proceeds by bonding O atoms sequentially on the surface of Co_<i>n</i>^– while retaining its morphology. Their appearance is ascribed to the kinetic bottleneck against internal oxidation owing to significant structural change of the Co_<i>n</i> moiety. In contrast, magic compositions of the latter type are associated with the abrupt increase of survival probability as anionic states during the relaxation of internally hot Co oxide clusters based on the <i>m</i>-dependent behaviors of adiabatic electron affinities determined by photoelectron spectroscopy

Collision-Induced Dissociation of Undecagold Clusters Protected by Mixed Ligands [Au₁₁(PPh₃)₈X₂]⁺ (X = Cl, CCPh)

We herein investigated collision-induced dissociation (CID) processes of undecagold clusters protected by mixed ligands [Au₁₁(PPh₃)₈X₂]⁺ (X = Cl, CCPh) using mass spectrometry and density functional theory calculations. The results showed that the CID produced fragment ions [Au_<i>x</i>(PPh₃)_<i>y</i>X_<i>z</i>]⁺ with a formal electron count of eight via sequential loss of PPh₃ ligands and AuX­(PPh₃) units in a competitive manner, indicating that the CID channels are governed by the electronic stability of the fragments. Interestingly, the branching fraction of the loss of the AuX­(PPh₃) units was significantly smaller for X = CCPh than that for X = Cl. We ascribed the effect of X on the branching fractions of dissociations of PPh₃ and AuX­(PPh₃) to the steric difference