Laboratory study of methyl isocyanate ices under astrophysical conditions

Abstract

Methyl isocyanate has been recently detected in comet 67P/ Churyumov-Gerasimenko (67P/CG) and in the interstellar medium. New physicochemical studies on this species are now necessary as tools for subsequent studies in astrophysics. In this work, infrared spectra of solid CHNCO have been obtained at temperatures of relevance for astronomical environments. The spectra are dominated by a strong, characteristic multiplet feature at 2350-2250 cm, which can be attributed to the antisymmetric stretching of the NCO group. A phase transition from amorphous to crystalline methyl isocyanate is observed at ~ 90 K. The band strengths for the absorptions of CHNCO in ice at 20 K have been measured. Deuterated methyl isocyanate is used to help with the spectral assignment. No X-ray structure has been reported for crystalline CHNCO. Here we advance a tentative theoretical structure, based on Density Functional Theory (DFT) calculations, derived 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. A mixed ice of HO and CHNCO was formed by simultaneous deposition of water and methyl isocyanate at 20 K. The absence of new spectral features indicates that methyl isocyanate and water do not react appreciably at 20 K, but form a stable mixture. The high CHNCO/HO ratio reported for comet 67P/CG, and the characteristic structure of the 2350-2250 cm band, make of it a very good candidate for future astronomical searches.

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

“…In the unprocessed samples the ν a -NCO band, with its characteristic quadruplet structure, stands out at 2400-2200 cm -1 . There are some differences in the peak positions and in the band profiles between the spectra of pure methyl isocyanate and those of methyl isoyanate diluted in water ice, but the characteristic quadruplet structure is present in both cases (see Maté et al 2017 for a discussion on the IR spectroscopy). The ν a -NCO band is the most intense IR absorption of methyl isocyanate.…”

“…The spectra were recorded with a resolution of 2 cm -1 using a HgCdTe (MCT) detector refrigerated with liquid nitrogen. The gas phase composition during deposition was monitored by means of a calibrated quadrupole mass spectrometer (QMS) placed in an adjacent chamber (Maté et al 2017). The base pressure in the vacuum chamber was ~ 0.7-1×10 -8 mbar.…”

“…Ice mixtures of CH 3 NCO and H 2 O, with typical CH 3 NCO/H 2 O ratios in the 4-5 % range, were also co-deposited on the substrate. Separate entrances were used for the H 2 O and CH 3 NCO vapors and the mixture proportion in the ices was estimated from the absorbance ratio of the most intense absorption bands of water: A = 1.9×10 -16 cm molecule -1 for ν-OH (Mastrapa et al 2009), and methyl isocyanate: A= 1.3×10 -16 cm molecule -1 for ν a -NCO (Maté et al 2017).…”

Quasi-symmetry effects in the threshold photoelectron spectrum of methyl isocyanate

“…Methyl isocyanate was prepared with a modified version of the synthesis of Han et al 26 as reported in Maté et al 27 A few grams of the sample were placed in a vial attached to the experiment. The vial was plunged into a chiller set at −40 • C to lower the vapor pressure to around 10 −1 mbar, which was otherwise 280 mbar at 0 • C. This step was necessary to avoid too great a build-up of the sample in the experiment which could lead to polymerization or blockages, but also to limit the consumption of the sample.…”

A Comprehensive Spectral Rotational Analysis of the Interstellar Methyl Isocyanate CH3NCO

“…The parent species of CH 3 NCO has been prepared as previously reported (Maté et al 2017). The syntheses of Han et al (1989) have been modified to prepare CH 3 N 13 CO and 13 CH 3 NCO.…”