Dehydrogenation of methane by gas-phase Th, Th+, and Th2+ : theoretical insights into actinide chemistry

Abstract

Unrestricted density functional theory calculations have been carried out to investigate the reactivity of Th, Th+, and Th2+ toward the methane dehydrogenation process. A close description of the reaction mechanisms together with the analysis of the electronic factors offer insights into the reactivity of the thorium species. All possible spin states of the metal centers were considered as well as the effect of spin−orbit interactions on the transition-state barrier heights. The three reactions investigated are found to be exothermic, with the best thermochemical conditions observed for Th2+ around 105 kJ mol−1. The Th+ + CH4 reaction is found to be kinetically more favorable than that for the neutral Th atom. The DFT results indicate a direct participation of 5f electrons/orbitals in the reactivity of thorium species. The presence of electrons in 5f orbitals has an important effect on the insertion activation barrier, providing an electrostatic repulsion toward the closed-shell methane. The NBO results show that 5f orbitals play an important role in the overall strengths of the Th−C and Th−H chemical bonds, favoring thermochemical conditions of these reactions.