Theoretical study on the kinetic behavior of Np(<sc>vi</sc>) reduction by hydroxylamine and its derivatives: substituent effect

Spent fuel reprocessing entails controlling the valence state of Np and its routing in the plutonium-uranium reduction extraction (PUREX) process. Hydroxylamine (HA) and its derivatives are effective salt-free reductants that can reduce Np(vi) to Np(v) without its further reduction. Experimentally, hydroxylamine, N-methylhydroxylamine (MHA) and N,N-dimethylhydroxylamine (DMHA) reduce Np(vi) at different reaction rates. To investigate the impact of methyl substitution on the Np(vi) reduction mechanism, we theoretically studied the Np(vi) reduction reaction by HA, MHA and DMHA. It was observed that the reduction of Np(vi) involves hydrogen atom transfer from these reductants. The two steps for Np(vi) reduction by HA occurre via hydrogen transfer. Alternatively, Np(vi) reduction by both MHA and DMHA initially proceede via hydrogen atom transfer, followed by an outer-sphere electron transfer mechanism. The rate-determining step for MHA and DMHA is the first Np(vi) reduction step, and the energy barrier for DMHA is lower than that for MHA, which are 6.2 and 7.7 kcal mol-1, respectively. So the reaction rate for the reduction of Np(vi) by DMHA is faster than that by MHA due to the influence of the methyl group, which is consistent with the experimental results. Finally, we analyzed the bonding evolution using the quantum theory of atoms in molecules (QTAIM), interaction region indicator (IRI), Mayer bond order (MBO), localized molecular orbitals (LMO) and spin density. This study presents kinetic insights into the effect of methyl substitution on the reduction of Np(vi) by hydroxylamine, providing an in-depth understanding of Np(vi) reduction by hydroxylamine derivatives in spent fuel reprocessing.