| Abstract
| - 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 (H 2NCO +), and methyl isocyanate (CH 3NCO). 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 CH 3NCO. Aims. After CH 3NCO, the next most complex isocyanates are ethyl isocyanate (C 2H 5NCO) and vinyl isocyanate (C 2H 3NCO). Their detection in the ISM would enhance our understanding of the formation of these compounds in space. Methods. We have searched for C 2H 5NCO, H 2NCO +, C 2H 3NCO, 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 C 2H 5NCO and H 2NCO + towards G+0.693-0.027 (the former for the first time in the ISM) with molecular abundances of (4.7-7.3) × 10 −11 and (1.0-1.5) × 10 −11, respectively. A ratio of CH 3NCO/C 2H 5NCO = 8 ± 1 is obtained; therefore, the relative abundance determined for HNCO:CH 3NCO:C 2H 5NCO is 1:1/55:1/447, which implies a decrease by more than one order of magnitude, going progressively from HNCO to CH 3NCO and to C 2H 5NCO. This is similar to what has been found for alcohols and thiols, for example, and suggests that C 2H 5NCO is likely formed on the surface of dust grains. In addition, we have obtained column density ratios of HNCO/NCO > 269, HNCO/H 2NCO + ∼ 2100, and C 2H 3NCO/C 2H 5NCO < 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.
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