Density functional study on the mechanisms of the reactions of gas-phase OsOn+ (n=1-4) with methane

The sextet, quartet, and doublet potential energy surfaces (PESs) of the reactions of methane with gas-phase OsOn+ (n=1-4) have been explored via density functional calculations to investigate the mechanisms of these reactions. For the reactions of OsOn+ (n = 1-2) with methane, the minimum energy reaction paths are found to involve two spin inversions in the entrance and exit channels. Specifically, they are most likely to proceed through the following steps : (OsO+)-Os-4 + CH4 --> (OOsCH4+)-O-2 --> (OOs)-O-2(H)CH3+ --> (OOsH)-O-2(H)-(CH2)(+) --> (OOs)-O-4(CH2)(+) + H-2; and (OsO2+)-Os-4 + CH4 --> O-2(O)OsCH4+ --> O-2(O)Os(H)CH3+ --> O-2-(HO)Os(CH3)(+) --> O-2(HO)Os(H)(CH2)(+) --> (OOs)-O-2(H2O)(CH2)(+) --> (OOs)-O-4(CH2)(+) + H2O, respectively. Along the minimum energy pathway the exothermicity of the overall reaction would be 17.3 kcal/mol for OsO+ + CH4 --> OOs(CH2)(+) + H-2, and 24.0 kcal/mol for OsO2+ + CH4 --> OOS(CH2)(+) + H2O. For the reaction of OsO3+ with methane, it is found to be kinetically unfavorable at room temperature because the intermediate generated by the activation of the first C-H bond has a significantly positive Gibbs free energy relative to the reactants. For OsO4+, our calculations suggest that it can readily react with methane on the doublet PES by a direct hydrogen atom abstraction process, and the whole reaction is exothermic by 31.8 kcal/mol. These results agree with the experimental observation that OsOn+ (n = 1-2, 4) can readily activate methane but OsO3+ cannot.