Gas-Phase Reactions of the Bare Th2+ and U2+ Ions with Small Alkanes, CH4, C2H6, and C3H8: Experimental and Theoretical Study of Elementary Organoactinide Chemistry
The gas-phase reactions of two dipositive actinide ions, Th2+ and U2+, with CH4, C2H6, and C3H8 were studied by both experiment and theory. Fourier transform ion cyclotron resonance mass spectrometry was employed to study the bimolecular ion−molecule reactions; the potential energy profiles (PEPs) for the reactions, both observed and nonobserved, were computed by density functional theory (DFT). The experiments revealed that Th2+ reacts with all three alkanes, including CH4 to produce ThCH22+, whereas U2+ reacts with C2H6 and C3H8, with different product distributions than for Th2+. The comparative reactivities of Th2+ and U2+ toward CH4 are well explained by the computed PEPs. The PEPs for the reactions with C2H6 effectively rationalize the observed reaction products, ThC2H22+ and UC2H42+. For C3H8 several reaction products were experimentally observed; these and additional potential reaction pathways were computed. The DFT results for the reactions with C3H8 are consistent with the observed reactions and the different products observed for Th2+ and U2+; however, several exothermic products which emerge from energetically favorable PEPs were not experimentally observed. The comparison between experiment and theory reveals that DFT can effectively exclude unfavorable reaction pathways, due to energetic barriers and/or endothermic products, and can predict energetic differences in similar reaction pathways for different ions. However, and not surprisingly, a simple evaluation of the PEP features is insufficient to reliably exclude energetically favorable pathways. The computed PEPs, which all proceed by insertion, were used to evaluate the relationship between the energetics of the bare Th2+ and U2+ ions and the energies for C−H and C−C activation. It was found that the computed energetics for insertion are entirely consistent with the empirical model which relates insertion efficiency to the energy needed to promote the An2+ ion from its ground state to a prepared divalent state with two non-5f valence electrons (6d2) suitable for bond formation in C−An2+−H and C−An2+−C activated intermediates.