Synthesizing and characterizing simple molecular compounds containing the Np(V)O2+ or Np(VI)O22+ cation is vital for understanding speciation and chemical behavior of high valent actinide elements in solution. We are particularly interested in how intermolecular interactions, like actinyl-actinyl interactions (also referred to as cation-cation interactions), influences the spectroscopic signals and chemical properties of the neptunyl cation under aqueous conditions. In this study, we focused on evaluating the spectral features observed within the Raman spectra of neptunyl complexes to understand the activation of combination modes and the influence of these bands on the bonding within Np(V) solid-state compounds and related solution phases. The crystalline materials were characterized by single crystal X-ray Diffraction and Raman spectroscopy. Solid-state Raman spectroscopy showed an unusually high number of vibrations bands in the spectral window of interest (600-900 cm-1) and atypical positions for the neptunyl v1 band. Similar spectral features were observed in solution phase during evaporation of the aqueous solutions achieved high enough concentrations of Np(V) to support these interactions. Computational studies were also performed to highlight the appearance of combination modes and aid in the assignment of a strongly red-shifted neptunyl stretch.

Spectroscopic characterization of actinyl-actinyl interactions in aqueous solutions and solid-state materials

ACS Fall 2021

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