Compositions and Isomer Separation of Palladium Oxide Cluster Cations Studied by Ion Mobility Mass Spectrometry

Abstract

Geometric structures of gas-phase palladium oxide cluster cations, Pd n O m + , were investigated for stable compositions by ion mobility mass spectrometry (IMMS) and quantum chemical calculations. Pure metallic (m = 0) and oxygen-deficient (m < n) cluster cations were preferentially obtained from the mass spectra as a result of collision-induced dissociation. Structures of cluster series, Pd, and Pd 5 O m + (m = 3−8), were determined by comparing experimental collision cross sections obtained by IMMS and theoretical collision cross sections of optimized structures by density functional theory calculations. As for the Pd 3 O m + cluster cations, structural transition was observed from one-dimensional chains to twodimensional (2D) branched/2D sheets and finally to three-dimensional (3D) compact structures with increasing m. These 2D and 3D isomers were found to retain their triangular metal-core configuration. 2D sheets and 3D compact isomers that maintain a tetrahedral metal-core configuration were assigned for the Pd 4 O m + cluster ion. Two structural isomers were assigned for Pd 5 O m + , one with a 3D square pyramidal metal-core configuration and another one with a 3D distorted pentagonal. Furthermore, the structures of oxygen-deficient cluster ions include atomic oxygen preferentially, whereas structures with molecular oxygen were commonly assigned for oxygen-rich (m > n) cluster ions.

Structure Assignment and Separation of Isomers of Palladium Oxide Cluster Anions Studied by Ion Mobility Mass Spectrometry

“…For the Pd 7 O 7 − cluster, Ω calc of the most stable bulky 3D isomer (84.5 Å 2 ) showed ∼5 Å 2 larger than Ω exp (79.2 ± 1.5 Å 2 ), whereas the Ω exp of Pd ) and oxygen-deficient (m < n) clusters were reported as stable compositions for palladium oxide cluster cations. 16 Second, the stability order for the isomers of palladium oxide cluster anions in the quantum chemical calculations also showed much difference from that of the palladium oxide cluster cations reported previously. For instance, stable structures (ΔE = 0) of Pd n O n + for n = 3−5 were commonly observed as the 3D structures that contain metal-core configurations, 16 although similar metal-core configurations for the same size clusters were rarely observed for the corresponding anionic species.…”

“…The most stable structure for Pd 4 O 3 − was selected as a reference to determine the scaling factor of ionic radii for calculating Ω calc , as it is mentioned in Section 3. A similar structure having the terminal Pd atom was also assigned for its corresponding cluster cation (Pd 4 O 3 + ), 16 and this selection of structure is similar to the structure of Ni 4 O 3 + , which is the oxide cluster ion of the same group nickel as well. 24 As observed in the ATDs (Figure 3a), the coexistence of at least two different structural isomers was suggested for each , Ω calc values of the most stable structure (65.3 Å 2 ) were found to be the closest to the higher Ω exp (65.9 ± 2.6 Å 2 ) and thus assigned as one of the isomers.…”

“…The cyclic structure with a terminal Pd atom was confirmed as the most stable among all the structures examined for the Pd 4 O 3 − composition, which was similar to the structure of Pd 4 O 3 + reported previously. 16 The ratios of the hard sphere radii of the palladium and oxygen atoms were kept constant to account for the radius ratio in crystalline palladium(II) oxide, Pd 2+ /O 2− = 1.00:1.26. 37 The subsequent hard-sphere radii for the atoms in the cluster anions were Pd = 2.49 Å and O = 2.84 Å, including 1.15 Å for neutral He atoms.…”

“…For example, Pd 3 O 2−4 − cluster anions were observed in the place of Pd 3 O 1−6 + cluster cations. 16 The difference in numbers of oxygen, m, between cations and anions is likely due to the stabilization of the excess electrons in anion oxide clusters. Nevertheless, the abundantly observed peaks for the anionic species under the 50 eV condition, for example, Pd 1a), were found to be similar to those of cationic species.…”

“…15 In our previous study, we discussed the structures of palladium oxide cluster cations by ion mobility mass spectrometry (IMMS), and the structures containing metal-core configuration, in which direct metal−metal bonding is preferred, were suggested for the various numbers of O atoms, m, as a characteristic feature. 16 To the best of our knowledge, the only work on palladium oxide cluster anions was a mass spectrometric study of oxygendeficient cluster species. 17 Therefore, the detailed information on stable compositions and structures of Pd n O m − clusters has not been revealed yet.…”