Formation and Evolution of H₂C₃O^+• Radical Cations: A Computational and Matrix Isolation Study

The family of isomeric H₂C₃O^+• radical cations is of great interest for physical organic chemistry and chemistry occurring in extraterrestrial media. In this work, we have experimentally examined a unique synthetic route to the generation of H₂C₃O^+• from the C₂H₂···CO intermolecular complex and also considered the relative stability and monomolecular transformations of the H₂C₃O^+• isomers through high-level ab initio calculations. The structures, energetics, harmonic frequencies, hyperfine coupling constants, and isomerization pathways for several of the most important H₂C₃O^+• isomers were calculated at the UCCSD(T) level of theory. The complementary FTIR and EPR studies in argon matrices at 5 K have demonstrated that the ionized C₂H₂···CO complex transforms into the E-HCCHCO^+• isomer, and this latter species is supposed to be the key intermediate in further chemical transformations, providing a remarkable piece of evidence for kinetic control in low-temperature chemistry. Photolysis of this species at λ = 410-465 nm results in its transformation to the thermodynamically most stable H₂CCCO^+• isomer. Possible implications of the results and potentiality of the proposed synthetic strategy to the preparation of highly reactive organic radical cations are discussed.