Hydroxyacetonitrile (HOCH_2CN) as a precursor for formylcyanide (CHOCN), ketenimine (CH_2CNH), and cyanogen (NCCN) in astrophysical conditions

摘要

The reactivity in astrophysical environments can be investigated in the laboratory through experimental simulations, which provide understanding of the formation of specific molecules detected in the solid phase or in the gas phase of these environments. In this context, the most complex molecules are generally suggested to form at the surface of interstellar grains and to be released into the gas phase through thermal or non-thermal desorption, where they can be detected through rotational spectroscopy. Here, we focus our experiments on the photochemistry of hydroxyacetonitrile (HOCH_2CN), whose formation has been shown to compete with aminomethanol (NH _2CH_2OH), a glycine precursor, through the Strecker synthesis. Aims. We present the first experimental investigation of the ultraviolet (UV) photochemistry of hydroxyacetonitrile (HOCH_2CN) as a pure solid or diluted in water ice. Methods. We used Fourier transform infrared (FT-IR) spectroscopy to characterize photoproducts of hydroxyacetonitrile (HOCH_2CN) and to determine the different photodegradation pathways of this compound. To improve the photoproduct identifications, irradiations of hydroxyacetonitrile ~(14)N and ~(15)N isotopologues were performed, coupled with theoretical calculations. Results. We demonstrate that the photochemistry of pure hydroxyacetonitrile (HOCH_2CN) (σphoto = 5.7 ± 1.0 × 10~(-20) photon s~(-1) cm~2) under the influence of UV photons, or diluted in water ice (σphoto = 8.6 ± 1.0 × 10~(-20) photon s~(-1) cm~2), leads to the formation of formylcyanide (CHOCN), ketenimine (CH_2CNH), formaldehyde (CH_2O), hydrogen cyanide (HCN), carbon monoxyde (CO), and carbon dioxyde (CO _2); the presence of water increases its photodegradation rate. Furthermore, because hydroxyacetonitrile is more highly refractory than water, our results suggest that in astrophysical environments, hydroxyacetonitrile can be formed on icy grains from formaldehyde and hydrogen cyanide, and can be subsequently photodegradated in the water ice, or irradiated as a pure solid at the surface of dry grains after water desorption. As some of the hydroxyacetonitrile photochemistry products are detected in protostellar cores (e.g. formylcyanide or ketenimine), this compound maybe considered as one of the possible sources of these molecules at the grain surface in fairly cold regions. These photoproducts can then be released in the gas phase in a warmer region.