Dehydrogenation of methane by gas-phase Os+: A density functional study

Density functional calculations have been performed to explore the sextet, quartet, and doublet potential energy surfaces of methane dehydrogenation by gas-phase Os+ for understanding the reaction mechanism. The minimum energy reaction path is found to involve the spin inversion three times in the different reaction steps. Totally, three spin states (sextet, quartet, and doublet) are involved in the whole reaction. Specifically, the reaction is most likely to proceed through the following steps: Os-6(+) + CH4 --> OsCH4+ ((6)1) --> HOsCH3 ((4)2) --> HOsH(CH2)(+) ((2)3) --> (H-2)Os(CH2)(+) ((4)4) --> Os(CH2)(+) ((4)5) + H-2. The overall reaction is calculated to be exothermic by 4.2 kcal/mol, which is in good agreement with the available experimental results. The first spin inversion, from the sextet state to the quartet state, makes the activation of the first C-H bond energetically spontaneous. The second transition from the quartet state to the doublet state facilitates the cleavage of the second C-H bond, lowering the barrier from 25.9 kcal/mol to 16.1 kcal/mol. The third spin inversion occurs from the doublet state to the quartet state in the reductive elimination step of H-2, and this spin inversion leads to a decrease in the barrier height from 41.0 to 30.4 kcal/mol.