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 [C 12 ], ThC 12 , and UC 12 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 [C 12 ], ThC 12 , and UC 12 compounds have a planar structure with C 6 h , D 12 h , and D 12 h symmetry, respectively. [C 12 ] has a hyperconjugation structure containing alternating single and double bonds. The significant stabilization when forming AnC 12 predominantly comes from the electrostatic interaction between An 4+ and [C 12 ] 4- and also from a certain degree of orbital interaction between the An 5f6d7s valence shell and [C 12 ] 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 [C 12 ] and the An 5f ϕ AO, thus leading to an unprecedented electronic configuration of d 1 f 1 for U in UC 12 . 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 AnC 12 in future experiments.