Title: “Gas Phase Reactions of Actinide Cations with Small Molecules”

Abstract:

The actinides (An) test the understanding of chemical theory because several competing electronic structure effects converge. An chemistry can be divided into 1) reactivity (observed reaction rates) and 2) bonding. A central question to explore in An research is the role and extent of 5f orbital participation in An reactivity and bonding. Most of the An are highly radioactive, so a primary obstacle in understanding An chemistry has been the limited availability of experimental observations for transuranic species. Gas phase studies offer an ideal route to examine the intrinsic chemical behavior of the actinides because perturbations such as solvent effects are absent thereby enabling the observation of the fundamental interaction between the An and ligand. Experimental gas phase measurement also offer a direct comparison to theoretical models so that the accuracy and validity of the model can be assessed and improved. Energy dependent studies of the An+ + NO reactions illustrate the utility of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) to elucidate An trends. A comparison of the reaction efficiencies of the oxidation reaction indicates that the 6d orbitals play a pivotal role in bond activation; however, the 5f orbitals heavily influence the observed reaction rate even if they are not appreciably involved in forming the AnO+ bond. An examination of the minor product, AnN+, bond dissociation energy trend also confirms the importance of the 6d orbitals in An chemistry, although a possible deviation in the observed trend may suggest a difference in bonding between the early and later An. Further reactions of AnH+ + O2/CO2 (An = Th and U) also point to a difference in bonding between the early and late An.

Bio: 

Richard Cox began his career in chemistry as an undergraduate research assistant developing a uranium-selective probe, an experience that sparked a lasting interest in the chemistry of radionuclides. He pursued this passion through graduate work with Peter Armentrout at the University of Utah, where he studied the reactions of thorium and samarium cations with small molecules. His work provided new insights into the gas-phase chemistry of actinides, including a predictive model for key thermodynamic values of several actinide species.

Currently, Richard is a member of the Nuclear Chemistry and Engineering group at Pacific Northwest National Laboratory. At PNNL, he operates a custom-built mass spectrometer designed for the quantification of ultra-trace helium concentrations in solid and gas samples, with applications in retrospective dosimetry.