PNNL

Abstract

The role of 5f-electrons in actinide (An) reactivity and An – ligand (L) bond formation remains an area of interest. To assess the contribution of 5f electrons in An+ reactivity, reactions between An+ (An+ = Th+, U+, Np+, Pu+, Am+) and NO were explored using inductively coupled plasma tandem mass spectrometry (ICP-MS/MS). The energy dependences of the AnN+ reaction cross sections were modeled to provide reaction threshold energies used to derive experimental AnN+ bond dissociation energies (BDEs). The experimental AnN+ BDEs for An+ = U+ – Am+ obtained in this work are compared to new relativistic coupled cluster calculations with overall good agreement in the observed trends. The observed reaction rates for An+ + NO ? AnO+ + N correlate with the promotion energy of An+ to the first excited state with two unpaired 6d-electrons, Ep(6d2), indicating that the crossing point between the reaction surface evolving from the ground state reactants and the surface evolving from this excited state, which leads to the ground state products, heavily influences the reaction rate. Additionally, previous studies corroborated by theoretical calculations suggest that linear An+ centered species form when the 5f orbitals contribute to bonding. Calculations presented here further support involvement of the 5f -electrons in bonding to form the linear NAnO+ intermediate. Formation of the linear NAnO+ intermediate appears to facilitate An+ reactivity, resulting in an increased reaction efficiency compared to other reactions where the f orbitals do not participate along the reaction surface.