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 CH 4 . 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 + +CH 4 reaction pathway evolves completely along the doublet spin surface, whereas that of U + +CH 4 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 + dCH 2 and U + dCH 2 ) as well as the last insertion intermediates are characterized by the presence of R-agostic geometries.

The gas-phase reactions of two dipositive actinide ions, Th(2+) and U(2+), with CH(4), C(2)H(6), and C(3)H(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(2)H(6) and C(3)H(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(2)H(6) effectively rationalize the observed reaction products, ThC(2)H(2)(2+) and UC(2)H(4)(2+). For C(3)H(8) several reaction products were experimentally observed; these and additional potential reaction pathways were computed. The DFT results for the reactions with C(3)H(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.

Two different approaches of density functional theory were used to analyze the C-H and C-C bond activation mechanisms during the reaction of bare Th(+) and U(+) ions with ethane. We report a complete exploration of the potential energy surfaces taking into consideration different spin states. According to B3LYP/SDD computations the double dehydrogenation of C(2)H(6) is thermodynamically favorable only in the case of Th(+). It is shown that the overall C-H and C-C bond activation processes are exothermic in the case of Th(+) and endothermic for U(+). In both cases, the C-C insertion transition state barrier exceeds the energy of the ground state reactants, preventing the observation of these species under thermal conditions.

A density functional theory investigation of the PPh3‐catalyzed formation of amides from benzoic acid was explored. The results confirm the involvement of a phosphonium intermediate that is crucial to activate the carboxylate for nucleophilic acyl substitution.