RADICAL-LIKE BEHAVIOR OF MANGANESE OXIDE CATION IN ITS GAS-PHASE REACTIONS WITH DIHYDROGEN AND ALKANES [Review]

摘要

The gas-phase ion-molecule reactivity of MnO+ with dihydrogen and small alkanes has been examined by using Fourier transform ion cyclotron resonance mass spectrometry MnO+ was produced from the reaction of laser-desorbed Mn+* with N2O. Thermalized MnO+ reacts very efficiently (k(R) = 2.3 x 10(-10) cm(3) molecule(-1) s(-1)) with H-2 to eliminate either a H-. radical or H2O from the collision complex; both reactions commence with H-atom abstraction. Similarly, H-atom abstraction predominates in the reaction of MnO+ with methane (k(R) = 5.1 x 10(-10) cm(3) molecule(-1) s(-1)) with a small fraction of methanol being formed. In the reaction of MnO+ with ethane also beta-hydrogen transfer to yield water and ethene as neutral products takes place, and for alkanes larger than ethane, formation of MnOH(olefin)+ products is also observed; the latter reaction is suggested to occur through an initial hydrogen-abstraction process followed by metal-induced C-C bond cleavage. For the MnO+/propane system, ligand exchange reactions were applied to characterize the product formed via CH3. loss, which exhibits a (C2H4)MnOH+ structure rather than that of an alkoxide, i.e., Mn(OC2H5)(+). In order to evaluate the electronic ground state of MnO+ and MnOH+, ab initio MO calculations have been performed. At the CASPT2D level of theory, MnO+ exhibits a (5) Sigma(+) ground state with a very closely spaced 5 Pi state. For MnO+ the computed bond dissociation energy (BDE) of 65 kcal/mol compares well with the experimental figure of 68 kcal/mol. For MnOH+ the calculations reveal a (6)A(') ground state with BDE(Mn+-OH) = 74 kcal/mol; the experimental value amounts to 81 +/- 4 kcal/mol. [References: 102]