The Curious Case of Pu⁺: Insight on 5f Orbital Activity from Inductively Coupled Plasma Tandem Mass Spectrometry (ICP-MS/MS) Reactions

A comprehensive understanding of when and how 5f orbitals participate in complex chemical bonding is important for a variety of applications. The actinides are unique in that they possess 5f orbitals and can access high oxidation states, which make them attractive for use in catalysis. Fundamental studies of actinide-ligand interactions offer a mechanism to examine the activation of the 5f orbitals so that the selectivity of 5f orbitals can be assessed. A previous study examined the reaction of Pu⁺ + CO₂ and determined that the reaction efficiency is restricted by a barrier, namely, promotion from the Pu⁺ ground-state configuration, 5f⁶7s, to a reactive-state configuration, 5f⁵6d². The present study illustrates the benefit of activation of Pu's 5f orbitals when studying the reaction of Pu⁺ + NO. In this reaction, PuO⁺ forms in an exothermic, barrierless process. The 5f orbitals can and do participate in forming a linear intermediate, [N-Pu-O]⁺, and this drives the exothermic reaction. Understanding the conditions under which 5f orbitals are active in chemical bonding is the key to exploiting the actinides' selective catalytic capabilities.