Formation and Evolution of H 2 C 3 O +• Radical Cations: A Computational and Matrix Isolation Study.

The family of isomeric H 2 C 3 O +• 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 H 2 C 3 O +• from the C 2 H 2 ···CO intermolecular complex and also considered the relative stability and monomolecular transformations of the H 2 C 3 O +• isomers through high-level ab initio calculations. The structures, energetics, harmonic frequencies, hyperfine coupling constants, and isomerization pathways for several of the most important H 2 C 3 O +• 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 C 2 H 2 ···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 H 2 CCCO +• isomer. Possible implications of the results and potentiality of the proposed synthetic strategy to the preparation of highly reactive organic radical cations are discussed.