Experimental and first principles insights into high valent actinide chemistry

Harindu Rajapaksha

doi:10.25820/etd.008433

Actinides such as uranium and neptunium play central roles in nuclear fuel cycles, waste storage, and reprocessing technologies. Their accessible multiple oxidation states, complex coordination chemistry, and sensitivity to radiolysis make it difficult to predict their long-term behavior under conditions relevant to spent fuel management and legacy nuclear waste. This dissertation addresses that challenge by examining how the primary coordination sphere, the secondary coordination sphere, and radiation-generated reactive intermediates together govern the chemistry of high-valent uranium and neptunium. By integrating first-principles density functional theory with synthesis, X-ray diffraction, vibrational spectroscopy, calorimetry, and Electron Paramagnetic Resonance (EPR), the work establishes a molecular-level framework for understanding actinide bonding, thermodynamics, spectroscopy, and radiation-driven transformations in chemically complex environments. A central theme of the dissertation is that secondary-sphere interactions can exert a decisive influence on actinide properties. Using uranyl and neptunyl tetrachloride solids as model systems, the work demonstrates that hydrogen bonding and actinyl–cation interactions measurably perturb both thermodynamic stability and vibrational response (Chapter 3 and 4). In uranyl materials, detailed analysis of the three-dimensional hydrogen-bonding network showed that participation of the axial oxo atoms in hydrogen bonding is associated with systematic red-shifts in the uranyl symmetric and antisymmetric stretching modes. In neptunyl(VI) systems, the stability of hydrogen-bonded phases was found to correlate with cumulative hydrogen-bond strength, whereas phases dominated by cation interactions correlated with the total electrostatic attraction between the neptunyl unit and surrounding counterions. Across both uranyl and neptunyl families, these results establish that formation enthalpy and vibrational behavior cannot be rationalized solely from the immediate coordination environment. Rather, they emerge from extended secondary-sphere interaction networks that propagate through the crystal lattice. This finding significantly broadens the conceptual framework for actinide solid-state chemistry by demonstrating that secondary coordination is not a subtle structural feature, but a governing chemical variable. The dissertation further shows that modest perturbations to the primary coordination sphere can have system-wide consequences for bonding and lattice energetics (Chapter 5 and 6). In hydrated uranyl chloride phases containing the [UO2 Cl_4(H_2O)]^(2-) unit, coordination of a single additional water molecule to the equatorial plane strengthens the axial U=O bond relative to [UO_2 Cl_4 ]^(2-), producing blue-shifted uranyl stretching frequencies and reducing the Lewis basicity of the oxo groups. In Np(V) chloride systems, one-dimensional chain structures assembled through bridging chloride ligands were shown to retain neptunyl bond lengths comparable to those of discrete molecular analogues, yet bond-order analysis revealed measurable weakening of the Np=O interaction upon chain formation. These chains are stabilized by a combination of charge-assisted hydrogen bonding and cation-mediated contacts, reinforcing the broader conclusion that actinide properties are governed by cooperative effects spanning both primary and secondary coordination spheres. A second major contribution of the dissertation is the demonstration that ionizing radiation can drive chemically specific transformations in actinide solids (Chapter 7 and 8). Rather than functioning merely as a source of nonspecific structural damage, radiation was shown to generate radicals and reactive oxygen species that directly alter actinide coordination environments. In uranyl nitrate solids, irradiation produced nitrate-centered radical defects with characteristic EPR signatures, and the stabilization of these species depended strongly on the surrounding alkali cation environment. In related uranyl chloride systems, lattice water was found to undergo radiolysis to produce peroxide- and superoxide-derived intermediates capable of displacing equatorial chloride ligands and generating uranyl chloro-peroxide and chloro-superoxide species. In hydrated phases {〖(K or Rb)〗_2[UO_2 Cl_4]·2H_2O}, these reactive oxygen species initiated the formation of new uranium-containing alteration phases, demonstrating that water radiolysis can reshape primary coordination even in the solid state. Collectively, these studies provide a mechanistic view of how hydration, local composition, and radiation exposure interact to control the evolution of irradiated actinide materials over time. The dissertation also advances the chemistry of heptavalent neptunium, Np(VII), a highly oxidized and comparatively rare actinide species relevant to strongly alkaline and oxidizing waste environments such as those encountered in legacy tank waste (Chapter 9). Combined computational and experimental studies revealed that protonation of Np(VII) proceeds stepwise, with increasing protonation progressively changing both spectroscopic behavior and susceptibility to reduction. Thermodynamic analysis showed that protonation of the [NpO_4(OH)_2 ]^(3-) unit makes reduction to Np(VI) increasingly favorable, particularly in the presence of reactive oxygen species such as peroxide, hydroperoxide, and superoxide. At the same time, experimental studies showed that Np(VII) can persist closer to neutral conditions than had been generally assumed, both in solution and in the solid state. Raman and optical measurements further demonstrated that protonation produces systematic spectral shifts, providing chemically meaningful markers for tracking Np(VII) speciation across a wide pH range. These findings advance current understanding of neptunium behavior in alkaline waste streams by clarifying how pH, redox chemistry, and radiolysis products jointly govern the persistence and transformation of this unusual oxidation state. A final major theme concerns actinide peroxides and their role as hosts for reactive oxygen species (Chapter 10). Direct comparison of isostructural uranyl and neptunyl triperoxide complexes revealed subtle but chemically meaningful differences in bonding and vibrational behavior between the uranium and neptunium systems, including axial bond elongation and red-shifted vibrational features in the neptunyl analogue. More significantly, solid-state spectroscopy, solution spin-trapping experiments, and density functional theory together provided evidence that actinyl peroxide environments can stabilize superoxide. In the neptunyl system, this work provided the first direct evidence for stabilization of superoxide in a transuranic peroxide compound. Computational analysis further indicated that the relevant U(VI) and Np(VI) superoxide-containing species have comparable thermodynamic stability within the lattice, supporting the conclusion that actinyl triperoxide frameworks can serve as chemically realistic hosts for oxygen radical species under oxidative and radiolytic conditions. This discovery expands the known chemistry of actinides and identifies a new mechanistic dimension through which reactive oxygen species may influence nuclear materials chemistry. Taken together, the dissertation makes both fundamental and applied contributions of substantial significance. Fundamentally, it advances actinide chemistry by demonstrating that the behavior of high-valent uranium and neptunium emerges from the interplay of electronic structure, local coordination, extended a, hydration, redox chemistry, and radical reactivity. The work shows that oxidation-state assignments and nearest-neighbor bonding arguments alone are insufficient to explain the properties of these systems; instead, a broader system-level framework is required. Practically, these insights provide a stronger molecular basis for predicting actinide behavior during reprocessing, storage, corrosion, and long-term disposal of nuclear materials. In this way, the dissertation contributes not only new compounds, measurements, and mechanistic discoveries, but also a more predictive and conceptually unified framework for understanding actinide chemistry in environments central to nuclear waste management and stewardship.

Experimental and first principles insights into high valent actinide chemistry

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