Gas-Phase Reactions of the Bare Th~(2+) and U~(2+) Ions with Small Alkanes, CH_4, C_2H_6, and C_3H_8: Experimental and Theoretical Study of Elementary Organoactinide Chemistry

摘要

The gas-phase reactions of two dipositive actinide ions, Th~(2+) and U~(2+), with CH_4, C_2H_6, and C_3H_8 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 Th~(2+) reacts with all three alkanes, including CH_4 to produce ThCH_2~(2+), whereas U~(2+) reacts with C_2H_6 and C_3H_8, with different product distributions than for Th~(2+). The comparative reactivities of Th~(2+) and U~(2+) toward CH_4 are well explained by the computed PEPs. The PEPs for the reactions with C_2H_6 effectively rationalize the observed reaction products, ThC_2H_2~(2+) and UC_2H_4~(2+). For C_3H_8 several reaction products were experimentally observed; these and additional potential reaction pathways were computed. The DFT results for the reactions with C_3H_8 are consistent with the observed reactions and the different products observed for Th~(2+) and U~(2+); 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 Th~(2+) and U~(2+) 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 An~(2+) ion from its ground state to a prepared divalent state with two non-5f valence electrons (6d~2) suitable for bond formation in C-An~(2+)-H and C~An~(2+)-C activated intermediates.