This Perspective article highlights some of the traditional and non-traditional analytical tools that are presently used to characterize aqueous inorganic nanoscale clusters and polyoxometalate ions. The techniques discussed in this article include nuclear magnetic resonance spectroscopy (NMR), small angle X-ray scattering (SAXS), dynamic and phase analysis light scattering (DLS and PALS), Raman spectroscopy, and quantum mechanical computations (QMC). For each method we briefly describe how it functions and illustrate how these techniques are used to study cluster species in the solid state and in solution through several representative case studies. In addition to highlighting the utility of these techniques, we also discuss limitations of each approach and measures that can be applied to circumvent such limits as it pertains to aqueous inorganic cluster characterization.

Raman spectroscopy, infrared spectroscopy, and quantum mechanical computations were used to characterize and assign observed spectral features, highlight structural characteristics, and investigate the bonding environments of [M13(μ3-OH)6(μ2-OH)18(H2O)24](NO3)15 (M = Al or Ga) nanoscale clusters in the solid phase and aqueous solution. Solid-phase Raman spectroscopy was used to reveal that the metal-oxygen (M-O) symmetric stretch (breathing mode) for the Al13 cluster is observed at 478 cm(-1), whereas this same mode is seen at 464 cm(-1) in the Ga13 cluster. The hydroxide bridges in each cluster are weakly Raman active but show slightly stronger infrared activity. The breathing modes associated with the clusters in the solid state are not clearly visible in aqueous solution. This change in behavior in the solution phase may indicate a symmetry breaking of the cluster or exchange events between protons on the ligands and the protic solvent. Overall, each cluster has several unique vibrational modes in the low wavenumber region (<1500 cm(-1)) that are distinct from the parent nitrate salt and other polymeric species with similar structure, which allows for unambiguous identification of the cluster in solution and solid phases.

[AlxIny(μ3-OH)6(μ-OH)18(H2O)24](NO3)15 hydroxy-aquo clusters (AlxIn13-x) are synthesized through the evaporation of stoichiometrically varied solutions of Al13 and In(NO3)3 using a transmetalation reaction. Several spectroscopic techniques ((1)H NMR, (1)H-diffusion ordered spectroscopy, dynamic light scattering, and Raman) are used to compare AlxIn13-x to its Al13 counterpart. A thin film of aluminum indium oxide was prepared from an Al7In6 cluster ink, showing its utility as a precursor for materials.