Stability of CH₃NCO in Astronomical Ices under Energetic Processing: A Laboratory Study

Abstract

Methyl isocyanate (CHNCO) was recently found in hot cores and suggested on comet 67P/CG. The incorporation of this molecule into astrochemical networks requires data on its formation and destruction. In this work, ices of pure CHNCO and of CHNCO(4-5%)/HO mixtures deposited at 20 K were irradiated with a UV D lamp (120-400 nm) and bombarded by 5 keV electrons to mimic the secondary electrons produced by cosmic rays (CRs). The destruction of CHNCO was studied using IR spectroscopy. After processing, the ν-NCO band of CHNCO disappeared and IR bands corresponding to CO, CO, OCN and HCN/CN appeared instead. The products of photon and electron processing were very similar. Destruction cross sections and half-life doses were derived from the measurements. Water ice provides a good shield against UV irradiation (half-life dose of ~ 64 eV molecule for CHNCO in water-ice), but not so good against high-energy electrons (half-life dose ~ 18 eV molecule). It was also found that CHNCO does not react with HO over the 20-200 K temperature range. These results indicate that hypothetical CHNCO in the ices of dense clouds should be stable against UV photons and relatively stable against CRs over the lifetime of a cloud (~ 10 yr), and could sublime in the hot core phase. On the surface of a Kuiper belt object (the original location of comet 67P/CG) the molecule would be swiftly destroyed, both by photons and CRs, but embedded below just 10 μm of water-ice, the molecule could survive for ~ 10 yr.

2-aminooxazole in Astrophysical Environments: IR Spectra and Destruction Cross Sections for Energetic Processing

“…It provides a photon flux of 7.5 x 10 13 photons cm -2 s -1 that is spatially homogeneous over the whole 12 mm diameter exposed surface of the Si substrate (located at 30 mm from the lamp). This flow was estimated from the irradiance of the lamp provided by the manufacturer, integrating in the 120-180 nm wavelength range, as described in Maté et al (2018). The source of high energy electrons is an electron gun built in our laboratory (Maté et al, 2014).…”

Diffusion of CH4in amorphous solid water

“…This experimental setup has been described in detail in previous publications (Maté et al (2018), Gálvez et al (2009), Herrero et al (2010)). It consists of a high vacuum chamber evacuated to a base pressure of 1 x 10 −8 mbar and it is provided with a closed cycle He cryostat.…”

The Simplest, Isolable, Alkynyl Isocyanate HC≡CNCO: Synthesis and Characterization

“…In atmospheric chemistry, isocyanates involve in the combustion of polyurethane and the degradation of pesticides (for example, aldicarb and carbofuran) . In particular, the prototype molecules such as HNCO and CH 3 NCO have been shown to be products of various forms of combustion such as biomass burning, cooking, and cigarette smoking, and they are also interstellar species that have been detected in Sgr B2(N), the hot core of a giant molecular cloud near the galactic center . Therefore, the structures and reactivity of isocyanates such as HNCO and CH 3 NCO have been the focus of numerous experimental and theoretical studies.…”