The Role of Residual Charges in the Interaction between NpO 2 + and Th 4+ Cations: Spectrophotometric and Computational Studies

Spectrophotometric investigations have been carried out in order to understand the interaction of NpO 2 + (8.5 × 10 –5 M ) with Th 4+ under varying conditions of nitric acid (1–6 M HNO 3 ), thorium concentrations (up to 0.86 M ), and temperature (293–343 K). These studies show the appearance of the signature peak of Np IV (ca. 964 nm) in addition to those responsible for the NpO 2 + species (980 nm) and the NpO 2 + –Th 4+ complex (ca. 994 nm). The intensity of the Np IV peak increases with increasing temperature, while that of the Np V peak decreases. The Th interaction is noticed at higher Th IV concentrations, which explains the reason for neglecting the Np V –Np IV interactions. The variation of the stability constant data with temperature suggests both exothermic (at low acidity) and endothermic nature of interactions. Density functional theory (DFT) calculations have shown that the NpO 2 + –Th 4+ interaction is stronger than that of the NpO 2 + –UO 2 2+ one, which has been attributed to high positive charge on the Th 4+ ions.