Methyl isocyanate ices. Solid-phase infrared spectroscopy and energetic processing of a newly discovered astrophysical molecule

Abstract

Methyl isocyanate has been recently detected at the surface of comet 67P/ Churyumov-Gerasimenko (67P/CG) [1] and in the gas-phase of interstellar medium [2-5]. It is believed that the molecule forms through ice chemistry. Till now this molecule was usually neglected in astrochemical networks. New physicochemical studies on this species are now necessary as tools for subsequent studies in astrophysics. In this work, infrared spectra of solid CH3NCO have been obtained at temperatures of relevance for astronomical environments (down to 20 K). The spectra are dominated by a strong, multiplet feature at 2350-2250 cm-1, due to the antisymmetric stretching of the NCO group (va-NCO). A phase transition from amorphous to crystalline methyl isocyanate is observed at ¿ 90 K. The band strengths for the absorptions of CH3NCO in ice at 20 K have been measured. No X-ray structure has been reported for crystalline CH3NCO. Here we advance a tentative theoretical structure, based on Density Functional Theory (DFT) calculations, derived by taking as a starting point the crystal of isocyanic acid. A harmonic theoretical spectrum is calculated then for the proposed structure, and compared with the experimental data [6]. The stability of CH3NCO under UV irradiation and cosmic ray (CR) bombardment was investigated in our laboratory. A D2 lamp was used to generate UV photons (¿ 7.6 eV) ; and high energy electrons (5 keV) were employed to study the effects of cosmic rays. Samples of CH3NCO and of CH3NCO diluted in H2O ice, formed by simultaneous deposition of water and methyl isocyanate at 20 K, were subjected either to electron bombardment or UV irradiation. Heating of the ices leads to variations in the va-NCO band profile but not to an appreciable depletion of CH3NCO until H2O sublimation (beyond T=150 K) and thus rules out hydrolysis in the ice. UV irradiation and electron bombardment at 20 K lead to the disapperance of CH3NCO and to the formation of CO, CO2 and OCN- in the processed ices. The decay of the CH3NCO concentration as a function of photon or electron fluence follows a first order kinetics. Destruction cross sections are in the 10-19 cm2 range for UV photons and in the 10-16-10-15 cm2 range for electrons. A preliminary evaluation of these results indicates that cosmic rays are more efficient than UV photons for the destruction of this molecule in the ices of dense clouds and UV radiation is more efficient at the surface of Kuiper belt objects.