Experimental and Theoretical Approach to Probe the Aquatic Speciation of Transuranic (Neptunyl) ion in Presence of Two Omnipresent Organic Moieties

Abstract Pyrazines are omnipresent in nature and have their occurrence in plants, microbes, food supplies, marine arenas. The present studies aimed at aquatic speciation of the neptunyl ion (NpO2+) with two pyrazine compounds namely pyrazine monocarboxylic acid (PMC) and pyrazine dicarboxylic acid (PDC). Absorption spectrophotometry was used to probe the stability, speciation and spectral properties for the complexation process. NpO2+ forms a more stable complex with PMC than PDC for 1:1 (ML), while for 1:2 (ML2) the opposite trend is observed. The extent of shift in λmax, which is also an indicator for the strength of complexation, reflected similar trends for the complexation process. Isothermal titration calorimetry was employed to determine the enthalpies of complex formation, which is found to be endothermic. The complexation process is entropy driven. Linear free energy correlations were established to retrieve the coordination modes of the complexes. The variation in peak potentials (the cyclic voltammograms) with change in pH and metal to ligand ratio were explored to understand redox speciation, electron transfer kinetics and Eh-pH characteristics for the interaction of NpO2+ with pyrazine carboxylate ligands. Density functional theory calculations were employed to optimize the geometries and to calculate the bond distances and partial charges on key atoms of the optimized geometries. The theoretical calculations helped to reveal the contributions from two different configurations of the same geometry towards the optical absorption. The bond distances and partial charges estimated theoretically helped to understand the aqueous interactions at the molecular level.