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

Abstract

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.

An EPR study on the radiolysis of isolated ethanol molecules in solid argon and xenon: matrix control of radiation-induced generation of radicals in cryogenic media

“…Freon-11 (CFCl 3 , 99.8%, Khimprom, Volgograd, Russia) was used as an electron scavenger. According to previous experience, 25,34 this scavenger is particularly convenient for the EPR studies of radical cations and products of their reactions because it yields broad and relatively weak signals of fluorinated radicals, which do not Paper PCCP mask relatively sharp signals from organic species in noble gas matrices.…”

“…The applications of EPR to the matrix isolation studies of radiation-induced transformations relevant to astrochemistry are still rather limited, 25 nevertheless, one may recall some examples of the effective use of this technique in the astrochemical context, as a complementary tool to FTIR spectroscopy. 33,34 It should be stressed that radical chemistry is supposed to be very important in cold “synthetic” astrochemistry occurring under the natural processing of irradiated interstellar ices.…”

Photochemical Reaction Mechanism of Intramolecular H Transfer Reaction of H2C3O+⋅ Radical Cation

“…The spin-density distributions of IM1 E and IM3 calculated using UCAM-B3LYP-D3BJ/def2-TZVPP are identical to that calcu-lated using UCCSD(T)/L3a_3 in a recent study. [4] Finally, the HFC constants of the IM1 E and IM3 were calculated using UCAM-B3LYP-D3BJ/def2-TZVPP, and the calculated HFC constants were listed in Table S3 in the Supporting Information. As shown in Table S3, the HFC constants of IM1 E and IM3 using UCAM-B3LYP-D3BJ/def2-TZVPP are in reasonable agreement with the experimental values reported in a recent study.…”

“…[4] The intense absorption of the C=O stretching vibration in the Fourier transform infrared spectrum indicated that the H 2 C 3 O + * radical cation was E-HCCHCO + * . [4] In that study, high-level quantum chemical methods, such as coupled-cluster singles and doubles with perturbative triples (CCSD(T)) with large basis sets up to the complete basis set (CBS) limit, were used to elucidate the reaction mechanism of the thermal reaction. [4] Subsequent blue light irradiation (410-465 nm) of E-HCCHCO + * led to its isomerization to H 2 CCCO + * (methylene ketene cation); [4] essentially, an intramolecular H transfer reaction occurred.…”

“…[4] In that study, high-level quantum chemical methods, such as coupled-cluster singles and doubles with perturbative triples (CCSD(T)) with large basis sets up to the complete basis set (CBS) limit, were used to elucidate the reaction mechanism of the thermal reaction. [4] Subsequent blue light irradiation (410-465 nm) of E-HCCHCO + * led to its isomerization to H 2 CCCO + * (methylene ketene cation); [4] essentially, an intramolecular H transfer reaction occurred. Such a reaction should overcome a high reaction barrier because the strong CÀ H bond must be broken.…”