2021A&A...654L...1R

Context. Little is known about the chemistry of isocyanates (compounds with the functional group R-N=C=O) in the interstellar medium (ISM), as only four of them have been detected so far: isocyanate radical (NCO), isocyanic acid (HNCO), N-protonated isocyanic acid (H₂NCO⁺), and methyl isocyanate (CH₃NCO). The molecular cloud G+0.693-0.027, located in the Galactic Centre, represents an excellent candidate to search for new isocyanates since it exhibits high abundances of the simplest ones, HNCO and CH₃NCO.
Aims. After CH₃NCO, the next most complex isocyanates are ethyl isocyanate (C₂H₅NCO) and vinyl isocyanate (C₂H₃NCO). Their detection in the ISM would enhance our understanding of the formation of these compounds in space.
Methods. We have searched for C₂H₅NCO, H₂NCO⁺, C₂H₃NCO, and cyanogen isocyanate (NCNCO) in a sensitive unbiased spectral survey carried out in the 2 mm and 7 mm radio windows using the IRAM 30m and Yebes 40m radio telescopes, respectively.
Results. We have detected C₂H₅NCO and H₂NCO⁺ towards G+0.693-0.027 (the former for the first time in the ISM) with molecular abundances of (4.7-7.3) x 10^–11 and (1.0-1.5) x 10^–11, respectively. A ratio of CH₃NCO/C₂H₅NCO = 8±1 is obtained; therefore, the relative abundance determined for HNCO:CH₃NCO:C₂H₅NCO is 1:1/55:1/447, which implies a decrease by more than one order of magnitude, going progressively from HNCO to CH₃NCO and to C₂H₅NCO. This is similar to what has been found for alcohols and thiols, for example, and suggests that C₂H₅NCO is likely formed on the surface of dust grains. In addition, we have obtained column density ratios of HNCO/NCO > 269, HNCO/H₂NCO⁺∼2100, and C₂H₃NCO/C₂H₅NCO < 4. A comparison of the methyl/ethyl ratios for isocyanates (-NCO), alcohols (-OH), formiates (HCOO-), nitriles (-CN), and thiols (-SH) is performed and shows that ethyl derivatives may be formed more efficiently for the N-bearing molecules than for the O- and S-bearing molecules.