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

The oxidative additions of CH₄ 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 CH₄. It involves the transition state and/or the activated complex. The activation energies of CH₄ with the triplet Pt(³D; 5d9s1), singlet Pt(¹ S; 5d¹⁰), anion Pt^-(²S; 5d¹⁰6s¹), Pt^-(²P; 5d¹⁰6p¹), and cation Pt⁺(²D; 5d⁹) are 102, 59, 75, 41, and 52 kcal mol^- respectively. Further, there is a possibility for the excited state of the Pt^- + CH₄ 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)(CH₃) is 29 kcal mol^- more stable than the dissociation limit of the excited Pt^- + CH₄ system. This suggests the possibility of C-H activation by photoexcited Pt^-. In the Pt^- (H)(CH₃) 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.