First detection of C<sub>2</sub>H<sub>5</sub>NCO in the ISM and search of other isocyanates towards the G+0.693-0.027 molecular cloud
L. F. Rodríguez-Almeida, V. M. Rivilla, I. Jiménez-Serra, M. Melosso, L. Colzi, S. Zeng, B. Tercero, P. de Vicente, S. Martín, M. A. Requena-Torres, F. Rico-Villas, J. Martín-Pintado
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 2 NCO + ), and methyl isocyanate (CH 3 NCO). 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 3 NCO. Aims. After CH 3 NCO, the next most complex isocyanates are ethyl isocyanate (C 2 H 5 NCO) and vinyl isocyanate (C 2 H 3 NCO). Their detection in the ISM would enhance our understanding of the formation of these compounds in space. Methods. We have searched for C 2 H 5 NCO, H 2 NCO + , C 2 H 3 NCO, 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 2 H 5 NCO and H 2 NCO + 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 3 NCO/C 2 H 5 NCO = 8 ± 1 is obtained; therefore, the relative abundance determined for HNCO:CH 3 NCO:C 2 H 5 NCO is 1:1/55:1/447, which implies a decrease by more than one order of magnitude, going progressively from HNCO to CH 3 NCO and to C 2 H 5 NCO. This is similar to what has been found for alcohols and thiols, for example, and suggests that C 2 H 5 NCO is likely formed on the surface of dust grains. In addition, we have obtained column density ratios of HNCO/NCO > 269, HNCO/H 2 NCO + ∼ 2100, and C 2 H 3 NCO/C 2 H 5 NCO < 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.