Theoretical study of thorium and uranium tetracarbide molecules

The electronic structure and ground-state molecular properties of neutral Th and U tetracarbides were investigated by relativistic multireference calculations using complete active space self-consistent field/multiconfiguration second-order perturbation theory (CASSCF/CASPT2) with an all-electron basis set as well as by density functional theory (B3LYP) in conjunction with relativistic pseudopotentials. The former calculations were extended with complete active space state interaction (CASSI) calculations accounting for spin-orbit coupling. The four carbon atoms can be arranged in various forms around the actinide atom to result in ten structural isomers found altogether in our study. The electronic ground state in both molecules corresponds to a planar fan-type structure of C_2v symmetry, in which the actinide atom is connected to a bent C₄ moiety. Three structures arise from various arrangements of C₂ moieties around the Th and U, whereas six isomers contain a C₃ moiety and a lone C bonded to the actinide. The latter CAnC₃-type structures are very high-energy structures (over 200 kJ mol^-1 with respect to the ground state). The bonding characteristics in the most relevant structures were analysed on the basis of the valence molecular orbitals and the natural bond orbital method. We evaluated and interpreted the vibrational and electronic spectra. The present computed data for UC₄ suggest a revision of the assignment of a previously observed IR absorption band to a different isomer. (Copyright copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)