Intrinsic reaction coordinate analysis of the conversion of methane to methanol by an iron-oxo species: A study of crossing seams of potential energysurfaces

Crossing seams between the potential energy surfaces and possible spin inve rsion processes for the direct conversion of methane to methanol by the bar e FeO+ species are discussed by means of the intrinsic reaction coordinate (IRC) approach. There are three crossing seams between the sextet and the q uartet potential energy surfaces, and spin inversion should occur twice in the entrance and the exit channels; FeO+((6)Sigma(+)) + CH4((1)A(1)) --> OF e+(CH4)((6)A) --> TS1((4)A') --> HO-Fe+-CH3((4)A) --> TS2 ((4)A) --> Fe+(CH 3OH)((4)A) --> Fe+(D-6) + CH3OH((1)A'). The first crossing seam exists in p rior to TS1, a four-centered transition state for the cleavage of a C-H bon d of methane. This crossing seam is the most important aspect in this react ion pathway because the molecular system should change its spin multiplicit y from the sextet state to the quartet state near this crossing region, lea ding to a significant decrease in the barrier height of TS1 from 31.1 to 22 .1 kcal/mol at the B3LYP level of density functional theory. The second cro ssing seam occurs in the vicinity of the hydroxy intermediate (HO-Fe+-Ch(3) ), but this crossing seam would not play a significant role because the qua rtet IRC valley always lies below the sextet one in this region of reaction coordinate and accordingly the molecular system would preferentially move on the quartet potential energy surface. The third crossing seam exists in the exit channel in which the elimination of methanol occurs from the produ ct complex. This crossing seam will again lead to spin inversion from the q uartet to the sextet state, by which the elimination energy can be decrease d from 57.2 to 37.4 kcal/mol in the FeO+/CH4 system. (C) 1999 American Inst itute of Physics. [S0021-9606(99)01926-1].