Spin-flip reaction of Re + CH4--a relativistic density functional theory investigation

To explore the reaction mechanisms of methane dehydrogenation by gas-phase Re atom, the sextet, quartet, and doublet potential energy surfaces have been performed using density functional theory (DFT) and zero-order regular approximation relativistic corrections at the PW91/TZ2P level. The minimum energy reaction path is found to proceed through the following steps: (6)Re + CH(4) --> ReCH(4) ((6)1) --> H(3)CReH ((4)2) --> (4)TS2/3 --> H(2)CReH(2) ((4)3) --> (2)TS3/4 --> HCReH(3) ((2)4). Also, the reaction path involves the spin inversion twice in the different reaction steps. To better understand the spin inversion processes, the low energy crossing point is determined with the help of the density functional fractional occupation number approach. The first spin inversion, from the sextet state to the quartet state, makes the activation of the 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 186.1 to 24.2 kJ/mol. The overall reaction is calculated to be exothermic by 149.8 kJ/mol, and the final products in three spin states are investigated by NBO analysis, to compare the Re-C bonds and the C-H bonds.