A DFT STUDY ON THE REACTION MECHANISM OF SrO + CH4

Abstract

The reaction mechanism of SrO + CH 4 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 SrOCH 4 molecular complex with C 3v symmetry, and (ii) a side-on to side-on approach of C – H bond to Sr – O forming OSrCH 4 molecular complex with C 1 symmetry. From SrOCH 4, the SrOH + CH 3 radicals are formed by the direct abstraction of a hydrogen atom from CH 4 moiety. From OSrCH 4, the gas-phase methane to methanol conversion by SrO is suggested to involve the SrO insertion into a C – H bond of CH 4 to produce the hydroxy and methoxy intermediates, HOSrCH 3 and HSrOCH 3, and the reaction pathway via the hydroxy intermediate ( HOSrCH 3) is energetically more favorable than the other one via the methoxy intermediate ( HSrOCH 3). These channels to form ( SrOH + CH 3) and ( Sr + CH 3 OH ) are expected to compete with each other, and the formation of methyl radical via the direct Sr – C cleavage from HOSrCH 3 is energetically more preferable. On the other hand, the intermediates HSrOCH 3 and HOSrCH 3 are predicted to be the energetically preferred configuration in the reaction of Sr + CH 3 OH , which is precisely the reverse reaction of methane hydroxylation. For the reaction of MO ( M = Mg , Ca , Sr ) with CH 4, 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 .