Identifying Nanoscale M₁₃ Clusters in the Solid State and Aqueous Solution: Vibrational Spectroscopy and Theoretical Studies

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.