Electronic Structure, Ligand Effects, and Chemical Reactivity of the Ground and Low-Lying Excited Electronic States of NpO3+

Multi-reference and density functional theory calculations are performed for the diatomic and ligated NpO3+ species. The main goal of this study is to provide insights into the stability of the experimentally synthesized N(CH2CH2NR)3NpO (R = SiiPr3) coordination complex and probe its use as a catalyst for the oxidation of methane. The constructed potential energy curves for NpO3+ showed the presence of three different types of minima (Np3+O, Np4+O−, Np5+O2−) depending on the neptunium–oxygen distance. All these minima are higher in energy than the Np2+ + O+ fragments, and the more stable Np5+O2− form is stabilized only due to the presence of the negatively charged -CH2NR− moiety of the ligand. The C–H bond activation of methane was found to be possible only for the first quintet state of the complex which lies about 30 kcal/mol higher than the ground triplet state.