Tentative theoretical structure and infrared spectra of methyl isocyanate ice.

Abstract

The prebiotic molecule methyl isocyanate (CH3NCO) was recently detected at the surface of comet 67P/CG [1] and shortly afterwards in the gas-phase of the interstellar medium [2,3]. Its estimated abundance with respect to water was found to be very high (1.3%) in the comet. The crystal structure of CH3NCO is unknown. In order to make a prediction, we first carried out theoretical calculations at Density Functional Theory (DFT) level on the known structure of isocyanic acid, HNCO. We achieved a correct reproduction of the crystalline structure of this molecule. Then we replaced the H atom by a CH3 group, optimized the structure and predicted the infrared spectrum of solid CH3NCO. We have recorded in our lab experimental spectra of this species and of CD3NCO at temperatures of relevance for astronomical environments. Comparison between theoretical and experimental spectra leads us to propose a tentative structure for the CH3NCO crystal.