Gas-Phase Reactions of the Bare Th2+ and U2+ Ions with Small Alkanes, CH4, C2H6, and C3H8: Experimental and Theoretical Stu

详细信息    查看全文

• 作者：Emanuela Di Santo ; Marta Santos ; Maria C. Michelini ; Joaquim Mar莽alo ; Nino Russo ; John K. Gibson
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：February 16, 2011
• 年：2011
• 卷：133
• 期：6
• 页码：1955-1970
• 全文大小：1361K
• 年卷期：v.133,no.6(February 16, 2011)
• ISSN：1520-5126

文摘

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鈭抦olecule 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鈭扝 and C鈭扖 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鈭扐n²⁺鈭扝 and C鈭扐n²⁺鈭扖 activated intermediates.