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 (HMonO3n+11–, n = 1–8, and MonO3n+12–, 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, 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 HMo3O101–, stabilized by a strong intramolecular hydrogen bond, and a compact
structure is observed for HMo5O161–, in contrast to the ring structure
favored for Mo5O162–.