Radiation-Induced Defects in Uranyl Trinitrate Solids.

Actinides are inherently radioactive; thus, ionizing radiation is emitted by these elements can have profound effects on its surrounding chemical environment through the formation of free radical species. While previous work has noted that the presence of free radicals in the system impacts the redox state of the actinides, there is little atomistic understanding of how these metal cations interact with free radicals. Herein, we explore the effects of radiation (UV and γ) on three U(VI) trinitrate complexes, M[UO2(NO3)3] (where M=K+, Rb+, Cs+), and their respective nitrate salts in the solid state via electron paramagnetic resonance (EPR) and Raman spectroscopy paired with Density Functional Theory (DFT) methods. We find that the alkali salts form nitrate radicals under UV and γ irradiation, but also note the presence of additional degradation products. M[UO2(NO3)3] solids also form nitrate radicals and additional DFT calculations indicate the species corresponds to a change from the bidentate bound nitrate anion into a monodentate NO3 • radical. Computational studies also highlight the need to include the second sphere coordination environment around the [UO2(NO3)3]0,1 species to gain agreement between the experimental and predicted EPR signatures.

Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State.

Neptunium can exist in multiple oxidation states, including the rare and poorly understood heptavalent form. In this work, we monitored the formation of heptavalent neptunium [Np(VII)O4(OH)2]3- during ozonolysis of aqueous MOH (M=Li, Na, K) solutions using a combined experimental and theoretical approach. All experimental reactions were closely monitored via absorption and vibrational spectroscopy to follow both the oxidation state and the speciation of neptunium guided by the calculated vibrational frequencies for various neptunium species. The mechanism of the reaction partly involves oxidative dissolution of transient Np(VI) oxide/hydroxide solid phases, the identity of which are dependent on the co-precipitating counter-cation Li+/Na+/K+. Additional calculations suggest that the most favorable energetic pathway occurs through the reaction of a [Np(V)O2(OH)4]3- with the hydroxide radical to form [Np(VI)O2(OH)4]2-, followed by an additional oxidation with HO⋅ to create [Np(VII)O4(OH)2]3-.

Systematic Study of Solid-State U(VI) Photoreactivity: Long-Lived Radicalization and Electron Transfer in Uranyl Tetrachloride.

Reported are the syntheses, structural characterizations, and luminescence properties of three novel [UO2Cl4]2- bearing compounds containing substituted 1,1'-dialkyl-4,4'-bipyridinum dications (i.e., viologens). These compounds undergo photoinduced luminescence quenching upon exposure to UV radiation. This reactivity is concurrent with two phenomena: radicalization of the uranyl tetrachloride anion and photoelectron transfer to the viologen which constitutes the formal transfer of one electron from [UO2Cl4]2- to the viologen species. This behavior is elucidated using electron paramagnetic resonance (EPR) spectroscopy and further probed through a series of characterization and computational techniques including Rehm-Weller analysis, time-dependent density functional theory (TD-DFT), and density of states (DOS). This work provides a systematic study of the photoreactivity of the uranyl unit in the solid state, an under-described aspect of fundamental uranyl chemistry.

Superoxide Radicals in Uranyl Peroxide Solids: Lasting Signatures Identified by Electron Paramagnetic Resonance Spectroscopy.

U(VI) peroxide phases (studtite and meta-studtite) are found throughout the nuclear fuel cycle and exist as corrosion products in high radiation fields. Peroxides are part of a family of reactive oxygen species (ROS) that include hydroperoxyl and superoxide species and are produced during alpha radiolysis of water. While U(VI) peroxides have been thoroughly investigated, the incorporation and stability of ROS species within studtite have not been validated. In the current study, electron paramagnetic resonance (EPR) spectroscopy was used to identify the presence of free radicals within a series of U(VI) peroxide samples containing depleted, highly enriched, and natural uranium. Density functional theory calculations indicated that the predicted EPR signals matched well with a superoxide (O2 -⋅) species incorporated into the studtite structure, confirming the presence of ROS in the material. Further analysis of samples that were synthesized between 1945 and 2023 indicated that there is a correlation between the radical signal and the product of specific activity multiplied by age of the sample.

Off-the-shelf thermosalience of anthracene-9-thiocarboxamide.

We demonstrate thermosalient behavior in anthracene-9-thiocarboxamide. Upon cooling, the crystalline material spontaneously fractures and jumps. Strong anisotropic thermal expansion precedes thermosalience, and the combination of hydrogen bonds and weaker interlayer interactions affords the macroscopic response. By incorporating structural moieties from different classes of thermosalient solids and using an underexplored supramolecular synthon, a dynamic, multi-functional material is achieved.

Synthesis, characterization, and density functional theory investigation of (CH6N3)2[NpO2Cl3] and Rb[NpO2Cl2(H2O)] chain structures.

The actinyl tetrachloro complex [An(V/VI)O2Cl4]2-/3- tends to form discrete molecular units in both solution and solid state materials, but related aquachloro complexes have been observed as both discrete coordination compounds and 1-D chain topologies. Subtle differences in the inner sphere coordination significantly influence the formation of structural topologies in the actinyl chloride system, but the exact reasoning for these variations has not been delineated. In the current study, we present the synthesis, structural characterization, and vibrational analysis of two 1-D neptunyl(V) chain compounds: (CH6N3)2[NpO2Cl3] (Np-Gua) and Rb[NpO2Cl2(H2O)] (Np-Rb). Bonding and non-covalent interactions (NCIs) in the systems were evaluated using periodic Density Functional Theory (DFT) to link these properties to related phases. We observed ∼6.5% and ∼3.9% weakening of NpO bonds in Np-Gua and Np-Rb compared to the reference Cs3[NpO2Cl4]. NCI analysis distinguished specific assembly modes, where Np-Gua was connected via hydrogen bonding (N-H⋯Cleq and N-H⋯Oyl) and Np-Rb contained both cation interactions (Rb+⋯Oyl and Rb+⋯Cleq) and hydrogen bonding (Oeq-H⋯Oyl) networks. Thermodynamically viable formation pathways for both compounds were explored using DFT methodology. The [NpO2Cl4](aq)3- and [NpO2Cl3(H2O)](aq)2- substructures were identified as precursors to Np-Gua and [NpO2Cl3(H2O)](aq)2- and [NpO2Cl2(H2O)2](aq)- were isolated as the primary building units of Np-Rb. Finally, we utilized DFT to analyze the vibrational modes for Np-Gua and Np-Rb, where we found evidence of the NpO bond weakening within the Np(V) chain structures compared to [NpO2Cl4]3-.