| Abstract: | For studying the adiabatic and nonadiabatic mechanisms of the ClO (X 2Π) + ClO (X 2Π) → ClOOCl → 2Cl (2 P u) + O2 (X 3Σ g −) reaction (1) and the ClO (X 2Π) + ClO (X 2Π) → ClOOCl → Cl2 (X 1Σ g +) + O2 (X 3Σ g −) reaction (2), we calculated, by partial geometry optimizations under the C2 constraint, the O–O and O–Cl dissociation potential energy curves (PECs) from the five low-lying states of ClOOCl at the CASPT2 level. The CASSCF minimum-energy crossing point (MECP) between the potential energy surfaces of the 1 1A ground state [correlating with the product of reaction (1)] and the 1 3B state [correlating with the product of reaction (2)] states was also determined. Based on the CAS calculation results (PECs, energies, and spin–orbit coupling at the MECP), we predict that reaction (1) occurs along pathway 1: ClO (X 2Π) + ClO (X 2Π) → ClOOCl (1 1A) → 2Cl (2 P u) + O2 (X 3Σ g −) and that reaction (2) occurs along pathway 2: ClO (X 2Π) + ClO (X 2Π) → ClOOCl (1 1A) → 1 1A/1 3B MECP (142.4 cm−1) → ClOOCl (1 3B) → Cl2 (X 1Σ g +) + O2 (X 3Σ g −). The needed energies (relative to the reactant) for pathways 1 and 2 are predicted to be 5.3 and 11.1 kcal/mol, respectively, which indicates that reaction (1) is more favorable than reaction (2). The present work supports the traditional photochemical model for ozone degradation: ClOOCl (1 1A), formed by two ClO (X 2Π), can directly produce O2 plus two Cl atoms. |
