Photochemical Reaction Mechanism of Intramolecular H Transfer Reaction of H₂C₃O⁺⋅ Radical Cation

The photochemical reaction mechanism underlying the intramolecular H-transfer of the H₂C₃O⁺⋅ radical cation to the H₂CCCO⁺⋅ methylene ketene cation was elucidated using time-dependent density functional theory and high-level ab initio methods. Once the D₁ state of H₂C₃O⁺⋅ is populated, the reaction proceeds to form an intermediate (IM) in the D₁ state (IM4_D1). The molecular structure of the conical intersection (CI) was optimized using a multiconfigurational ab initio method. The CI is readily accessible because it lies slightly above the IM4_D1 in energy. In addition, the gradient difference vector of the CI is almost parallel to the intramolecular H-transfer reaction coordinate. Once the vibration mode of IM4_D1 which is parallel to the reaction coordinate is populated, the degeneracy of the CI is readily lifted and H₂CCCO⁺⋅ was formed via a relaxation pathway in the D₀ state. Our calculated results clearly describe the photochemical intramolecular H transfer reaction reported in a recent study.