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 hydrogen bonding, influences the spectroscopic signals and chemical properties of the neptunyl cation under aqueous conditions. In this study, we focused on the neptunyl tetrachloride species, [NpO2Cl4]2-/3-, which offers a simple molecular complex to explore with cations that engage in charge-assisted hydrogen bonding (piperazinium, pyridinium, morpholinium). Room-temperature evaporation experiments resulted in the crystallization of the [Np(V)O2Cl4]3- unit with piperazinium and morphonlinium counter ions that exhibited hydrogen bonding interactions with the neptunyl oxo group. These 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 (650-900 cm-1) and atypical positions for the neptunyl v1 band, which we relate to the second sphere coordination (H-bonding). Computational studies highlight the appearance of combination modes and aid in the assignment of a strongly red-shifted neptunyl stretch. Influence of the counter cations on the neptunyl unit in solution was further evaluated with Raman titration studies and electrochemical analysis. Spectroscopic results confirm the oxidation state of neptunium in the solution to be stabilized in the pentavalent state, but indicate a shift in redox potentials for Np(V)/Np(VI) in solutions containing either morpholine, piperazine, or pyridine.

Chemical behavior of the Np(V)O2Cl4 complex: Characterization by structural, spectroscopic, electrochemical and theoretical approaches

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.