Characterization and photochemistry of XCO2 (X = F, NH2, CH3) radicals

The XCO 2 ( X = F, NH 2 , CH 3 ) radicals are present in the Earth atmosphere, where they are produced by the degradation of Volatile Organic Compounds (VOCs), either industrial or natural. Here, we use advanced ab initio methodologies to characterize these species in their ground and electronically excited states. Computations are carried out using the Coupled Clusters, both standard and explicitly correlated versions, and multiconfigurational approaches. Several basis sets were used. Afterward, the geometrical parameters and the total energies were extrapolated to the complete basis set (CBS) limit. We also mapped their potentials along the central bond to have insights on the XCO 2 → X + CO 2 reactions. We thus show that the ground and the lowest electronic excited states are long-lived, for which we provide a set of accurate structural and spectroscopic parameters. The upper electronic states are subject of unimolecular decompositions producing CO 2 and X fragments. Our calculations show that the FCO 2 ( X 2 B 2 ) → F( 2 P) + CO 2 (X 1 Σ g + ) and the CH 3 CO 2 (X 2 A″) → CH 3 (X 2 A″ 2 ) + CO 2 (X 1 Σ g + ) processes require at least 3.5 eV energy to occur, while less energy (of ∼ 2.5 eV) is needed for the NH 2 CO 2 (X 2 A″) → NH 2 (X 2 B 1 ) + CO 2 (X 1 Σ g + ) reaction. The present findings and data are useful to characterize these radicals in the laboratory, in planetary atmospheres and in combustion and to understand their physical chemistry there. Graphical Abstract