Structures of stable oxide cluster ions of first-row late transition metals: An ion mobility-mass spectrometric study

Ion mobility mass spectrometry has been applied to unveil stable compositions and geometrical structures of oxide cluster ions of first-row late transition metals such as Fe, Co, Ni, Cu, and Zn. Geometrical structures were assigned by comparison of collision cross sections obtained by this experiment with those calculated for the structures predicted by quantum chemical calculations. Mass spectra of stable compositions of the metal oxide cluster ions resulting from collision-induced-dissociation in the ion drift cell have also been obtained. Oxides of the late transition metals except for Cu were found to have common stable compositions with the same numbers of metal and oxygen atoms, (FeO)_n⁺, (CoO)_n⁺, (NiO)_n⁺, and (ZnO)_n⁺, in addition to oxygen-rich Fe_nO_n+1⁺ ions or oxygen-deficient Co_nO_n-1⁺, Ni_nO_n-1⁺, and Zn_nO_n-1⁺ ions for n up to ≈10. As for the copper oxide clusters, Cu_nO_m⁺ ions with n:m ≈ 2:1 were predominant in the mass spectrum. For the stable ions, structural transitions were commonly observed from two-dimensional (2D) cyclic or sheet structures to three-dimensional (3D) compact structures at around the size n smaller than 8. Both types of isomers coexist at n = 6-8 for (FeO)_n⁺ and (ZnO)_n⁺, and at n = 5 for (NiO)_n⁺, whereas 3D structures were clearly observed from n = 6 for (CoO)_n⁺. These structural transitions were characteristic for the oxide cluster ions of the late transition metals, in marked contrast to those of early transition metals.