Intrinsic reaction coordinate analysis of the activation of CH4 by molybdenum atoms: A density functional theory study of the crossing seams of the potential energy surfaces

Abstract

Density functional theory (DFT) calculations were performed to investigate the quintet, triplet, and singlet potential energy surfaces associated with the C-H activation of methane by laser-ablated molybdenum (Mo) atoms recently observed experimentally by Andrews and co-workers. The present computational study aims to better understand the nature of the reaction mechanisms for C-H activation by Mo atoms. The processes for activation of methane by the excited Mo atoms appear to produce CH 3-MoH, CH 2=MoH 2, and CH≡MoH 3 complexes. The crossing seams between the potential energy surfaces and possible spin inversion processes for the direct conversion of methane to a high oxidation state transition metal complex that contains a carbon-metal double or triple bond are examined using the intrinsic reaction coordinate (IRC) approach. The minimum energy reaction pathway is found to involve spin inversion three times in different reaction steps. In total, three spin states (quintet, triplet, and singlet) are involved in going from the entrance channel to the exit channel: 5Mo + CH 4 → CH 3 - MoH ( 51) → CH 2=MoH 2 ( 32) → CH≡MoH 3 ( 13). The first crossing seam exists prior to TS 1-2, a three-centered transition state for the H-transfer of complex 1 to form a double carbon-metal bond complex 2. This crossing seam is a key aspect in the reaction pathway because the molecular system should change its spin multiplicity from the quintet state to the triplet state near this crossing region, which leads to a significant decrease in the barrier height of TS 1-2 from 51.0 to 33.8 kcal mol -1 at the B3LYP level of theory. The second crossing seam between the quintet potential energy surface (PES) and the singlet PES is found not to play a significant role because the triplet potential energy surface lies significantly below the quintet and singlet potential energy surfaces. Accordingly, the molecular system would preferentially move on the triplet potential energy surfaces before encountering the second seam. The crossing seam between the triplet exit channel and the singlet exit channel can take place, in which complex 13 containing a triple carbon-metal bond is formed via an H-transfer process. This crossing seam will again lead to a spin inversion from the triplet to the singlet state, and this leads to a large decrease in the height of the reaction barrier from 40.8 to 17.2 kcal mol -1. © 2008 American Chemical Society.