Spectroscopic characterization of the complex of vinyl radical and carbon dioxide: Matrix isolation and ab initio...

We report on the preparation and vibrational characterization of the C₂H₃⋯CO₂ complex, the first example of a stable intermolecular complex involving vinyl radicals. This complex was prepared in Ar and Kr matrices using UV photolysis of propiolic acid (HC₃OOH) and subsequent thermal mobilization of H atoms. This preparation procedure provides vinyl radicals formed exclusively as a complex with CO₂, without the presence of either CO₂ or C₂H₃ monomers. The absorption bands corresponding to the ν₅(C₂H₃), ν₇(C₂H₃), ν₈(C₂H₃), ν₂(CO₂), and ν₃(CO₂) modes of the C₂H₃⋯CO₂ complex were detected experimentally. The calculations at the UCCSD(T)/L2a level of theory predict two structures of the C₂H₃⋯CO₂ complex with C_s and C₁ symmetries and interaction energies of -1.92 and -5.19 kJ mol^-1. The harmonic vibrational frequencies of these structures were calculated at the same level of theory. The structural assignment of the experimental species is not straightforward because of rather small complexation-induced shifts and matrix-site splitting of the bands (for both complex and monomers). We conclude that the C₁ structure is the most probable candidate for the experimental C₂H₃⋯CO₂ complex based on the significant splitting of the bending vibration of CO₂ and on the energetic and structural considerations.