Micro- and millimeter-wave spectra of five conformers of cysteamine and their interstellar search

Abstract

Context. Cysteamine (NH2CH2CH2SH), a molecule of potential astrobiological interest, has not yet been detected in the interstellar medium. Furthermore, the sulfur- stituted isomer of ethanolamine (or 2-aminoethanol) has been recently detected in the molecular cloud G+0.693–0.027. Aims. In order to conduct a new interstellar search for cysteamine in the molecular cloud G+0.693–0.027, its pure rotational spectrum needs to be investigated in the laboratory. Methods. A pulsed-jet Fourier transform microwave spectrometer and a Stark-modulated free-jet millimeter-wave absorption spectrometer were used to measure the purely rotational spectrum of cysteamine in the range of 6.5-18 GHz (46.12–16.66 mm) and 59.6–120.0 GHz (5.03–2.72 mm), respectively. We used a deep spectral line survey toward the molecular cloud G+0.693-0.027 obtained with the IRAM 30 m and Yebes 40 m radiotelescopes to search for cysteamine. Results. We assigned 815 rotational transition lines of five conformers (gGt, gGg, g′Gg, g′Gg′, and g'Gt) to fit the rotational constants, quartic centrifugal distortion constants, and the 14N nuclear quadrupole coupling constants. For four conformers (gGt, gGg, g′Gg, and g′Gg′), the 34S isotopologs were observed, and for two of them (gGg and g'Gg), the 13C and 15N isotopolog spectra were also detected; all in natural abundance. The five conformers of cysteamine were not detected toward the G+0.693–0.027 molecular cloud. We derived upper limits for their molecular abundances compared to molecular hydrogen of <(0.2–1.3) × 10–10. The relative abundances with respect to the oxygen counterpart ethanolamine, previously detected toward this cloud, are NH2CH2CH2OH/NH2CH2CH2SH > 0.8–5.3.