Stable compositions and geometrical structures of vanadium oxide cluster ions, VmOn(±), were investigated by ion mobility mass spectrometry (IM-MS). The most stable compositions of vanadium oxide cluster cations were (V2O4)(V2O5)(m-2)/2(+) and (VO2)(V2O5)(m-1)/2(+), depending on the clusters with even and odd numbers of vanadium atoms. Compositions one-oxygen richer than the cations, such as (V2O5)m/2(-) and (VO3)(V2O5)(m-1)/2(-), were predominantly observed for cluster anions. Assignments of these stable cluster ion compositions, which were determined as a result of collision-induced dissociations in IM-MS, can partly be explained with consideration of spin density distribution. By comparing the experimental collision cross sections (CCSs) obtained from ion mobility measurement with CCSs of the theoretically calculated structures, we confirmed the patterned growth of geometrical structures partially discussed in previous theoretical and spectroscopic studies. We showed that even sized (V2O5)m/2(±) where m = 6-12 had right polygonal prism structures except for the anionic V12O30(-), and for the clusters of odd numbers of vanadium m, cations and anions can either have bridged or pyramid structures. Both of the odd sized structures proposed were derivatives from the even sized right polygonal prism structures. The exception, V12O30(-), which had a CCS almost equal to that of the neighboring smaller V11O28(-), should have a structure of higher density than the right hexagonal prism, in which it was proposed to be a captured pyramid structure, derived from V11O28(-).

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.

Herein, the compositions and geometrical structures of niobium oxide cluster ions were studied and compared with those of the lighter Group 5 counterpart vanadium oxide cluster ions by ion-mobility mass spectrometry (IM-MS). As a result of collision-induced dissociation in IM-MS, the compositions were found to be dependent on an odd and even number of niobium atoms, whereby the ions with (NbO)(NbO) and (NbO)(NbO) were identified as stable compositions for an odd number of Nb atoms, whereas (NbO) and (NbO)(NbO) were identified as stable compositions for an even number of Nb atom clusters. Furthermore, structural transitions were observed between m = 8 and 9 for cluster cations and m = 7 and 8 for cluster anions for experimental collision cross-sections (CCSs), which were determined from the arrival times in the ion-mobility measurements. Quantum chemical calculations were conducted on several structural candidates of these compositions for m = 2-12. For cluster cations with the sizes between m = 2 and 8 and cluster anions with m = 2-7, the structures were found to be similar to those of vanadium oxide cluster ions upon comparing the experimental CCSs with the theoretical CCSs of optimized structures. As compared to the vanadium oxide cluster ions, niobium oxide cluster cations with m ≥ 9 and anions with m ≥ 8 consisted of structures where some niobium atoms had more than five oxygen-atom coordination; thus, compact structures could be achieved in the case of niobium oxide cluster ions.