Th doped carbon clusters ThCn (n = 1–7): Stability and bonding natures

Distinguishing the Geometric and Electronic Structures of Actinide Carbides AnxC8 (An = Th, U; x = 2, 3) through Exchange Interactions

Global-minimum optimizations combined with relativistic quantum chemistry calculations have been performed to characterize the ground-state stable structures of four titled compounds and to analyze the bonding properties. Th2C8 was identified as being a ThC4-Th(C2)2 structure, U2C8 has been found to favor the U-U(C8) structure, and both Th3C8 and U3C8 adopt the (AnC3)2-(AnC2) structure. Then, the wave function analyses reveal that the interactions between the Th 7s-based orbital and the σg molecular orbital of the C2 unit compensate for the excitation energy of 7s16d1 → 6d2 and lead to the stabilization of two Th(IV)s in the ThC4-Th(C2)2 structure. It also reveals that the U species exhibit magnetic exchange coupling behavior in UxC8, for instance, as seen in the direct interaction of U2C8 and the superexchange pathway of U3C8, which effectively stabilizes their low-spin states. This interpretation indicates that the geometric and electronic structures of AnxC8 species are largely influenced by the local magnetic moment and spin correlation.

12-Membered Ring Carbides with Stabilization of Actinide Atoms

Actinide (Th and U) carbides as the potential nuclear fuels in nuclear reactors require basic research in order to understand the thermodynamic stability and performance of these substances. Here we report the structural characterization and bonding analyses of [C₁₂], ThC₁₂, and UC₁₂ clusters via a global-minimum search combined with relativistic quantum chemistry calculations to elucidate the stability and bonding nature of An-C bonds. We predict that these [C₁₂], ThC₁₂, and UC₁₂ compounds have a planar structure with C_6h, D_12h, and D_12h symmetry, respectively. [C₁₂] has a hyperconjugation structure containing alternating single and double bonds. The significant stabilization when forming AnC₁₂ predominantly comes from the electrostatic interaction between An⁴⁺ and [C₁₂]^4- and also from a certain degree of orbital interaction between the An 5f6d7s valence shell and [C₁₂] π orbitals. The covalent character of the An-C bonds exhibits a direct in-plane σ-type overlap of the C 2p-derived MOs of [C₁₂] and the An 5f_ϕ AO, thus leading to an unprecedented electronic configuration of d¹f¹ for U in UC₁₂. Our results present an example of the novel properties that can be expected for actinide compounds and would provide the knowledge required to obtain novel structures of AnC₁₂ in future experiments.