Theoretical investigation of NpC, NpC2 and NpC4 molecules

The electronic structure and geometrical parameters of ground and low-lying excited states of neptunium mono-, di- and tetracarbides were investigated by relativistic CASSCF/CASPT2 multireference calculations with an all-electron basis set as well as with density functional theory (B3LYP) in conjunction with relativistic pseudopotentials. The CASPT2 calculations were extended with complete active space state interaction calculations in order to include spin–orbit coupling and calculate the absorption electronic spectra. In case of the actinide monocarbides, vibrational frequencies were calculated both for the lowest-energy spin-free and spin–orbit states. For the di- and tetracarbides infrared and Raman spectral characteristics were obtained using DFT calculations. For NpC2, two structures were investigated: the symmetric triangular (Np-C2) structure including an acetylide moiety which was found to be the ground state and the symmetric linear (CNpC) structure. In case of NpC4, fourteen different structures were investigated. The ground state was found to be a planar fan-like structure where neptunium is connected to a bent C4 moiety. The found structures agree well with the previous results on other actinide carbides (Th, U, Pu, Am). The bonding characteristics of the low-energy structures were analysed on the basis of valence molecular orbitals.