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

VUV photochemistry of cyclopropenone (c-C3H2O): formation rate and abundance ratios of propynal (HCCCHO) and propadienone (CH2CCO)

The distribution of isomeric species in the interstellar medium cannot be directly related to their relative energetic stabilities but more to their mechanisms of formation and evolution. The abundances of the three isomers of C3H2O, cyclopropenone, propynal and propadienone, are an example among many other interstellar species wherein kinetic effects control their presence in astrophysical regions. To date, only propynal and cyclopropenone, the two less stable isomers of propadienone, have been detected in the interstellar medium. In this work, we examined the vacuum ultraviolet (VUV) photochemistry of cyclopropenone (c-C3H2O), an unsaturated ketone and aromatic compound, which is the most thermodynamically unstable form of the three isomers of C3H2O. Using Fourier transform infrared spectroscopy and mass spectrometry to probe the photoproducts, we investigated the photolysis of cyclopropenone ice and cyclopropenone trapped in a neon matrix. We showed that the VUV photolysis of c-C3H2O ice at 10 K led mainly to CO and C2H2 fragments, in addition to propynal HCCCHO and propadienone CH2CCO. The distribution of the two isomers depended on the irradiation time. Within a short irradiation time, HCCCHO was formed in twice the abundance of the most stable isomer CH2CCO, while upon increasing the irradiation time, the same abundances were observed, showing that equilibrium was reached between the two isomers. The photolysis of c-C3H2O isolated in the neon matrix allowed the characterization of HCCCO and CCCO as reaction intermediates, resulting from the VUV-induced dehydrogenation of cyclopropenone, which are the precursors of propynal and propadienone through the CCCO + 2H reaction.

Radiation-Induced Transformations of an Isolated C6H6···HCN Complex: Possible Way to Synthesis of Interstellar Benzonitrile

The recent detection of benzonitrile (C6H5CN) in the interstellar medium is one of the most fascinating discoveries in astrochemistry and molecular astrophysics. However, the mechanism of its formation in interstellar ices remains unclear. Here, we report the first evidence for the direct synthesis of benzonitrile through the radiation-induced transformations of an isolated C6H6···HCN complex in inert rigid media at cryogenic temperature (4.5 K), as monitored by Fourier transform infrared (FTIR) spectroscopy. The complex was prepared in a solid krypton matrix and characterized by the experimentally observed complexation-induced shifts in the FTIR spectra on the basis of comparison with the results of ab initio calculations. The formation of benzonitrile was revealed through the observation of its three fundamentals and partially confirmed by experiments with deuterated benzene. Presumably, C6H5CN results from the dehydrogenation of complex excited states followed by prompt radical-radical recombination within the matrix cage. The proposed route may be relevant to the formation of C6H5CN both in the bulky astrophysical ices and on the surface of interstellar dust grains.

Astrochemically Relevant Radicals and Radical-Molecule Complexes: A New Insight from Matrix Isolation

The reactive open-shell species play a very important role in the radiation-induced molecular evolution occurring in the cold areas of space and presumably leading to the formation of biologically relevant molecules. This review presents an insight into the mechanism of such processes coming from matrix isolation studies with a main focus on the experimental and theoretical studies performed in the author's laboratory during the past decade. The radicals and radical cations produced from astrochemically relevant molecules were characterized by Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopy. Small organic radicals containing C, O, and N atoms are considered in view of their possible role in the formation of complex organic molecules (COMs) in space, and a comparison with earlier results is given. In addition, the radical-molecule complexes generated from isolated intermolecular complexes in matrices are discussed in connection with their model significance as the building blocks for COMs formed under the conditions of extremely restricted molecular mobility at cryogenic temperatures.

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

This paper addresses the basic question of the impact of a chemically inert environment on the radiation-induced transformations of isolated organic molecules in icy media at cryogenic temperatures with possible implications for astrochemical issues. The radicals produced by X-ray irradiation of isolated ethanol molecules (C₂H₅OH and CH₃CD₂OH) in solid argon and xenon matrices at 7 K were characterized by electron paramagnetic resonance (EPR) spectroscopy. It was shown that methyl (CH₃˙) and formyl (HCO˙) radicals resulting from the C-C bond cleavage were mainly produced in the case of solid argon, which was attributed to the significant role of "hot" ionic fragmentation and inefficient energy dissipation to medium. In contrast, irradiation in xenon results in the predominant formation of α-hydroxyethyl radicals (CH₃˙CHOH or CH₃˙CDOH(D) in the cases of C₂H₅OH and CH₃CD₂OH, respectively). Remarkably, the experiments with selectively deuterated ethanol provide strong indirect evidence for the primary formation of ethoxy (CH₃CD₂O˙) radicals due to O-H bond cleavage, which convert to the α-hydroxyethyl radicals due to isomerization occurring at 7 K. The α-hydroxyethyl radicals adopt a specific rigid conformation with a non-rotating methyl group at low temperatures, which is an unusual effect for neutral CH₃˙CHX species, and exhibit free rotation in solid xenon only at ca. 65 K.