A spin-orbit coupling study on the spin inversion processes in the direct methane-to-methanol conversion by FeO+

Possible spin inversion processes in the direct conversion of methane to methanol by the bare FeO+ complex are discussed by means of spin-orbit coupling (SOC) calculations. This reaction proceeds via two transition states (TSs) in the following way; FeO++CH4-->FeO+(CH4)-->[TS1]-->HO-Fe+-CH3-->[TS2]-->Fe+(CH3OH)-->Fe++CH3OH. B3LYP density functional theory calculations show that the potential energies in the quartet and sextet states lie close and involve three crossing seams that can provide a chance of spin-forbidden transition. The spin-forbidden transition leads to a significant decrease in the barrier heights of TS1 and TS2 that correspond to the hydrogen atom abstraction and the methyl shift, respectively. To evaluate the spin-forbidden transition in the reaction pathway, the SOC matrix elements are calculated along the intrinsic reaction coordinate of the reaction. The SOC analysis along the IRC is useful to look at how the FeO+/CH4 reacting system changes its spin multiplicity between the sextet and quartet surfaces. The strength of the SOC between the low-lying quartet state and the sextet state is 133.6 cm(-1) in the reactant complex FeO+(CH4), 21.4 cm(-1) in the hydroxo intermediate HO-Fe+-CH3, and 0.3 cm(-1) in the product complex Fe+(CH3OH). Since the SOC value decreases along the oxidation process, the ease of spin inversion probability is the first crossing seam, the second crossing seam, and the third crossing seam, in this order. (C) 2003 American Institute of Physics.