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Abstract

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 HMo_nO_3n+1^1-, n = 1--8, and Mo_nO_3n+1^2-, 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 nanodroplets. 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, a transition between chain (n = 2--3), ring (n = 4--5), and compact (n ≥ 6) structures is observed in IM--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 HMo₃O₁₀^1-, stabilized by a strong intramolecular hydrogen bond, and a compact structure is observed for HMo₅O₁₆^1-, in contrast to the ring structure favored for Mo₅O₁₆^2-.