A theoretical investigation on the intermolecular potential curve between ruthenium tetroxide and NOX (X = 1, 2)

Ruthenium tetroxide (RuO4) is one of chemical species of fission products assumed to be released to the environment during a severe accident of nuclear facilities and a target compound to assess the amount produced, reactivity, mobility and release timing. In this article, the NOX (X = 1, 2) adduct formation of RuO4 has been investigated, based on the potential energy curve (PEC) evaluated by UM06, UTPSSh, CASSCF, and CASPT2 methods. At several stationary points, CCSD and LR‐CCSD(T) energies are also computed for a comparison. The PEC shows that there is an activation barrier to form the NOX adduct and that the process is endothermic in terms of free energy. In the system, the electron transfer occurs from NOX to RuO4 when the bond between the nitrogen and oxo ligand is formed. It has been discussed in detail using active orbitals, weight of electron configurations and spin population obtained by CASSCF. The interaction between ruthenium tetroxide and NOX (X = 1, 2) is investigated, which is a fundamental process in the risk assessment of nuclear power and fuel reprocessing plants. The interaction involves the bond formation and electron transfer, which complicates the potential energy evaluation. We try to evaluate the potential energy curve (PEC) by several ab initio molecular‐orbital (MO) calculations and density functional theory methods. The NO‐RuO4 system shows a multiconfigurational character in the bond breaking/forming range on the PEC and that NOX adducts have a strong electron correlation. A highly accurate MO calculation method like CAS perturbation theory second order is required to obtain a reliable PEC for the systems.