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.
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