Theoretical study on the methane activation reactions by Pt, Pt +, and Pt − atoms

Abstract

The oxidative additions of CH 4 to the ground and excited states of Pt, Pt −, and Pt + species are studied by the symmetry-adapted cluster (SAC) and SAC-CI methods. The reaction path is examined by calculating the Hellmann-Feynman forces acting on C and H atoms of CH 4. It involves the transition state and/or the activated complex. The activation energies of CH 4 with the triplet Pt( 3D; 5d9s1), singlet Pt( 1 S; 5d 10), anion Pt −( 2S; 5d 106s 1), Pt −( 2P; 5d 106p 1), and cation Pt +( 2D; 5d 9) are 102, 59, 75, 41, and 52 kcal mol − respectively. Further, there is a possibility for the excited state of the Pt − + CH 4 system that the reaction proceeds with lower activation energy by relaxing onto the ground state curve along the reaction process. The activated complex Pt −(H)(CH 3) is 29 kcal mol − more stable than the dissociation limit of the excited Pt − + CH 4 system. This suggests the possibility of C-H activation by photoexcited Pt −. In the Pt − (H)(CH 3) complex, both bent and linear forms are possible; the two forms transform through an energy barrier of 22 kcal mol −1. In the Pt and Pt + complexes, only the bent forms are stable.