A DFT study on the reaction mechanism of SrO+CH4

The reaction mechanism of SrO + CH4 has been investigated on a singlet spin state potential energy surface at the B3LYP/SDD, AUG-cc-pVQZ//B3LYP/SDD, 6-311+G(2df,2p) levels of theory. Initially, the reactants give two molecule-molecule complexes: (i) a collinear C-H approach to O-end of the Sr-O bond forming SrOCH4 molecular complex with C-3v symmetry, and (ii) a side-on to side-on approach of C-H bond to Sr-O forming OSrCH4 molecular complex with C-1 symmetry. From SrOCH4, the SrOH+CH3 radicals are formed by the direct abstraction of a hydrogen atom from CH4 moiety. From OSrCH4, the gas-phase methane to methanol conversion by SrO is suggested to involve the SrO insertion into a C-H bond of CH4 to produce the hydroxy and methoxy intermediates, HOSrCH3 and HSrOCH3, and the reaction pathway via the hydroxy intermediate ( HOSrCH3) is energetically more favorable than the other one via the methoxy intermediate ( HSrOCH3). These channels to form (SrOH + CH3) and (Sr + CH3OH) are expected to compete with each other, and the formation of methyl radical via the direct Sr-C cleavage from HOSrCH3 is energetically more preferable. On the other hand, the intermediates HSrOCH3 and HOSrCH3 are predicted to be the energetically preferred configuration in the reaction of Sr + CH3OH, which is precisely the reverse reaction of methane hydroxylation. For the reaction of MO (M = Mg, Ca, Sr) with CH4, the two main reaction pathways, methyl formation via a-TS1 and hydroxyl intermediate formation via b-TS1, are expected to compete with each other, and the preference of methyl formation pathway via a-TS1 would follow the sequence: MgO > CaO > SrO.