Gas-Phase Methane Activation by the Ac+-Pu+ Ions: Theoretical Insights into the Role of 5f Electrons/Orbitals in Early Actinide Chemistry

Density functional theory (DFT) calculations have been performed to investigate the reactivity of early actinide ions (Ac+-Pu+) toward methane C-H bond activation. The first step of the methane dehydrogenation process, corresponding to an oxidative insertion, was studied for all ground and excited spin electronic states of these actinide ions. We find that Pu+7D (5f(6)7s(1)) may react endoergonically with methane, whereas exoergonic reactions are observed for the other actinide ions investigated. The activation barriers are computed to be higher than 20 kcal mol(-1), except for Th+ F-4 6d(2)7s(1) and U+ I-4 5f(3)7s(2) ions, for which an effectively barrierless process (Delta G(double dagger) < 2.5 kcal mol(-1)) was predicted for Th+, while small values (Delta G(double dagger) < 15 kcal mol(-1)) were computed for U+. The analyses of results indicate a direct participation of 5f electrons and 5f orbitals in the reactivity of the early actinide ions. While the 5f electrons give rise to a repulsive electrostatic interaction with the closed-shell methane, increasing the size of the activation barrier, a strong participation of 5f orbitals in the actinide chemical bonds makes the thermochemical conditions of the insertion process unfavorable. Th+ is observed to be the most efficient actinide ion toward methane C-H bond activation. We find four salient factors responsible for this effectiveness: (i) its F-4 [Rn]6d(2)7s(1) ground electron configuration, (ii) an "early" transition structure in the insertion process, (iii) a proper 5f orbital mix with the 6d7s valence shell, leading to enhanced strengths of the Th-H and Th-C bonds, and (iv) the absence of electrons in 5f orbitals.