Structural Characterization of Molybdenum Oxide Nanoclusters Using Ion Mobility Spectrometry Mass Spectrometry and Infrared Action Spectroscopy

Polyoxometalate clusters possess unique catalytic and electromagnetic properties. The structure and function of polyoxometalates is dictated by complex oligomerization processes, which in turn depend on the solution conditions. In this work, small gas-phase polyoxomolybdate nanoclusters n = 1-8, and Mon017,1, n = 2-8) were investigated after nanoelectrospray of an acidified solution of ammonium heptamolybdate heptahydrate by ion mobility spectrometry mass spectrometry (IMS MS), infrared multiple photon dissociation (IRMPD) spectroscopy, and infrared action spectroscopy in helium nano droplets. The spectra and collision cross sections obtained were matched to predictions from density-functional theory (DFT) to unravel the structural progression of nanoclusters with increasing size. For doubly charged clusters, transitions among chain (n = 2-3), ring (n = 4-5), and compact (n > 6) structures are observed in IMS MS and IR spectroscopy experiments, in agreement with low-energy structures from DFT calculations. For singly charged clusters, reduced Coulombic repulsion and hydrogen bonding interactions are found to strongly influence the most stable cluster structure. Notably, a noncovalent ring structure is observed for HMo3010, stabilized by a strong intramolecular hydrogen bond, and a compact structure is observed for HMo5016, in contrast to the ring structure favored for Mo5021e.