Methane C-H Bond Activation by Gas-Phase Th+ and U+: Reaction Mechanisms and Bonding Analysis

Th+ is the only actinide ion that activates exothermically the strong C-H bonds of methane in the gas phase. In contrast, U+, as well as all of the rest of the experimentally studied An(+) cations, Is inert in the reaction with CH4. In this work, the activation of methane by thorium and uranium monocations was investigated using two different density functional theory approaches. The reaction mechanisms and the corresponding potential energy profiles were analyzed in detail. From the formation of the initial ion-molecule adduct, the Th+ + CH4 reaction pathway evolves completely along the doublet spin Surface, whereas that of U+ + CH4 involves solely the quartet bare cation ground spin state. The bonding properties of all of the species involved in the reaction pathways were investigated in terms of diverse analyses including natural bond orbital, atoms in molecules, and electron localization function. The dehydrogenation products (Th+=CH2, and U+=CH2) as well as the last insertion intermediates are characterized by the presence of alpha-agostic geometries.