Geometrical structures of iron oxide cluster cations have been analyzed by ion mobility mass spectrometry. The series of (FeO)n(+) and FenOn + 1(+) cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into a drift cell. Arrival time distributions in the ion mobility spectrometry indicate that two structural isomers coexist for the (FeO)n(+) clusters at n ≥ 5. By comparison of experimental collision cross sections determined from the arrival times with theoretical ones, two-dimensional ring and sheet structures were assignable for (FeO)n(+) (n = 3-8). In addition to these isomers, compact three-dimensional structures were also found to be stable at (FeO)n(+) (n ≥ 6). Thus, the two-dimensional and three-dimensional structural isomers coexist for (FeO)n(+) (n = 6-8).

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(-).

Structures and CO-adsorption reactivities of nickel oxide cluster cations were investigated by ion mobility mass spectrometry. The series of Ni n O n−2 + , Ni n O n−1 + and Ni n O n + cluster cations were predominantly observed in a mass spectrum at high ion-injection energy into an ion-drift cell. From the arrival time distributions of Ni n O n + and Ni n O n−1 + in the ion mobility spectrometry, structural transition from two-dimensional (2D) ring to three-dimensional (3D) compact structures was found at n = 5. In addition, 2D and 3D structural isomers were found to coexist for Ni 5 O 5 + , Ni 6 O 5 + and Ni 7 O 6 + . By adding CO gas to buffer gas in the ion-drift cell, Ni 4 O 3 + and Ni 5 O 4 + cluster cations were found to be more reactive for the CO adsorption reactions than Ni 4 O 4 + and Ni 5 O 5 +. Under the pseudo-first-order approximation, rate constants for CO-adsorption were determined to be (8.4 ± 0.7) × 10 −11 cm 3 molecule −1 s −1 for Ni 4 O 3 + and (9.6 ± 0.8) × 10 −11 cm 3 molecule −1 s −1 for Ni 5 O 4 + . These rate constants are 2 orders of magnitude faster than those for Ni 4 O 4 + and Ni 5 O 5 + , which have reported previously. These differences of rate constants can be originated in the structures of the nickel oxide 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.

Structural assignments of gas phase chromium oxide cluster anions, CrO (m = 1-7), have been achieved by comparison between experimental collision cross sections measured by ion mobility mass spectrometry and theoretical collision cross sections of optimized structures by quantum chemical calculations. In the mass spectrum, significant magic behavior between the numbers m and n was not observed for CrO, while wide ranges of compositions were observed around CrO to (CrO) as reported previously. The (CrO) (m = 3-7) ions were assigned to have monocyclic-ring structures for m = 3-5 and bicyclic rings for m = 6 and 7. In addition, gradual structural change from these cyclic structures of (CrO) to three-dimensional structures of CrO was found for m = 4-7. The energy levels of molecular orbitals of a calculated monocyclic structure of CrO were also found to be consistent with previous results of photoelectron spectroscopy, although those of the bicyclic isomer exhibited a different behavior. Moreover, the observation of abundant ions generated by collision induced dissociations at the inlet of the ion drift cell indicates that the larger sized (CrO) (m > 5) series were unstable and easily dissociated to smaller ions.