PNNL

Abstract

The actinides present a unique challenge to chemical theory. From the perspective of classical theory, covalent bonding is driven by the extent of spatial overlap of valence orbitals. Modern theory has expanded assessments of covalency to include considerations of orbital energy degeneracy to assess the extent of orbital energy mixing between metal and ligand valence orbitals. The spatial extension and energies of the 5f orbitals decreases across the actinide series. Actinide-ligand (An-L) bonding has more recently been described by a balance between orbital overlap and orbital energy mixing, where orbital overlap decreases across the series while energy mixing between An 5f and L valence orbitals increases across the series. To test these existing theories, we employed inductively coupled plasma tandem mass spectrometry to examine the kinetic energy dependences of reactions of actinide cations, Th+ - Am+, with methane. This is the first experimental report of the energy dependences of methane activation reactions involving Pa, Np, Pu, and Am and the first experimental determination of transuranic An+-D, An+-CD2, An+-CD3, and An+-CD bond dissociation energies. A comparison of the measured bond energies across the series indicates that An+ 6d orbitals are the dominant contributors in the resulting An+-L (L = D, CD2, CD3, CD) bonds, though a closer examination of the relative reactivities of An+ offers additional support that the intersection of classical and modern theories may occur between Np+ and Pu+ and that spatial extension of An+ 5f orbitals may have a larger impact on An+ reactivity while orbital energy mixing may be more influential in An+ bond formation.