Peculiar chemical bonding between thorium and a carbon hexagon in carbon nanomaterials

Abstract

We explore an unusual nature of chemical bonding of the thorium atom with a ring of six carbon atoms (hexagon) in novel carbon materials. Our ab initio calculations of Th-based metallofullerenes (Th@C60, Th@C20) and Th bound to benzene or coronene at the Hartree-Fock level with the second order perturbation (MP2) correction accounting for the van der Waals interactions, demonstrate that the optimal position of the thorium atom is where it faces the center of a hexagon and is located at a distance of 2.01-2.07 A from the center. For Th encapsulated in C60 it is found at 2.01 A, whereas the other local energy minima are shifted to larger energies (0.22 eV and higher). Inside C60 the highest local minimum at 1.17 eV is observed when Th faces the center of the five member carbon ring (pentagon). Based on our calculations for Th with benzene and coronene where the global minimum for Th corresponds to its position at 2.05 A (benzene) or 2.02 A (coronene) from the hexagon center, we conclude that a well pronounced minimum is likely to present in graphene and in a single wall carbon nanotube. The ground state of Th is singlet, other high spin states (triplet and quintet) lie higher in energy (> 1.62 eV). We discuss a potential use of the carbon nanomaterials with the 229Th isotope having the nuclear transition of the optical range, for metrological purposes.