title: First principles insights into material and molecular properties
title: environmental and actinide chemistry applications
titleType: Subtitle
Creators: name: Augustine, Logan J.
Publisher: name: University of Iowa
Year: 2023
Subjects: public abstract

description: The value of theory and modeling in chemistry is in its ability to provide insight into the atomistic and electronic effects which control material and molecular properties, beyond what can be observed and measured experimentally. The behavior of materials and molecules is controlled by the formation and breaking of chemical bonds, making it desirable to study these systems through the principles of quantum mechanics. In this thesis, Density Functional Theory (DFT) is used to investigate the geometric, bonding, energetic, and vibrational properties of distinct systems, primarily in relation to environmental and actinide chemistry. DFT has been shown to be a reliable method for the prediction and understanding of materials observed in a vast range of scientific fields. Computationally, these systems are often reduced to smaller, simpler structures, therefore it is important to understand how results can be directly related to experimental observations and measurements. Thus, the work presented here is highly collaborative with experimentalist to (1) help validate new DFT and thermodynamic methodology for understanding reactions occurring in the environment and (2) provide insight into complex vibrational signatures observed in actinide chemistry. Overall, this shows the value and ultimate need for combined theoretical and experimental studies to further our understanding and continue to grow the fields of environmental and actinide chemistry.
description: Environmental processes are predominantly controlled by diverse acid-base, ligand-exchange, and redox reactions occurring along the mineral-water interface, and a complete understanding of their underlying thermodynamics allows for prediction and insight into how systems behave with each other. Studying these reactions computationally often entails a hydroxylated metal oxide surface represented as a 2D periodic slab, with additional molecules binding to the facet. But one of the biggest pitfalls in the theoretical calculation of reaction energies is the proper description of the aqueous and redox energetics involved during the process. In this thesis, a DFT + Thermodynamics methodology is presented which looks to recover these energetics through a step-wise approach. The utility of the method is to design thermodynamic cycles which breakdown the overall reaction into elementary steps calculated either purely through DFT or tabulated experimental data. Since DFT is reliable in predicting properties of solid and gaseous materials, the first step references all aqueous species in the overall reaction to their standard state. The second step then applies experimental data with additional Nernst-based corrections to determine the energetic change associated with transitioning from the standard state to aqueous phase. Overall, the approach develops a modeling framework that can consider atomistic and electronic factors which may control adsorption behavior, while also reliably predicting aqueous reaction thermodynamics observed at the macroscopic level. Here, such an approach is applied to investigate the adsorption behavior of oxyanions (arsenate, phosphate, and sulfate) and organics (oxalate and pyrocatechol) along the surfaces of alumina and hematite, with results matching well with experimental observations. The robustness of the methodology is then displayed in its ability to accurately predict formation energies of uranyl hybrid materials.
description: Additionally, actinide elements are becoming increasingly important as they have found applications in numerous scientific fields including energy, weapons, waste treatment, forensics, and medicine. Therefore, exploring their chemistry and understanding the complexities of their f-orbitals will lead to continued growth in these areas. Under aqueous, oxidative environments, the early actinides (uranium-americium) form into the distinct actinyl cation, [An(V/VI)O2]+/2+ and understanding the behavior of this linear trans-dioxo cation can be particularly important for separations and reprocessing technologies. Strong, covalent bonding between the actinide 5f/6d orbitals and oxygen 2p orbitals directs much of the interactions with the actinyl to occur along its equatorial plane, but weakening of the An=Oyl bond can lead to increased intermolecular interactions with the oxo groups. Both equatorial binding and intermolecular interactions with the actinyl cation results in red or blue-shifting and the activation of new vibrational signatures involving its ν1 and ν3 modes. Therefore, Raman and IR spectroscopy are desirable methods to study these effects. In this thesis, combined theoretical and experimental studies were performed to investigate the formation of unique neptunyl oligomers built through neptunyl-neptunyl interactions. DFT was used to predict the formation of dimer, trimer, and tetramer oligomers at high concentrations, while additional dimer structures could be induced through ligand complexation at low concentrations. Upon oligomerization, DFT showed new vibrational bands were activated involving concerted ν1 and ν3 motion on each neptunyl, which was further validated through experimental Raman spectroscopy. In the last study, the bonding along the equatorial plane of the neptunyl cation was influenced through the incorporation of electron-withdrawing and electron-donating groups on carboxylate and polypyridine ligands. Results from the work can have direct applications in future separation and reprocessing techniques involving neptunium and concepts of ligand design.