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

¹ Department of Chemistry “Giacomo Ciamician”, University of Bologna, Via Selmi 2, Bologna, Italy
e-mail: sonia.melandri@unibo.it
² Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir Km. 4, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: vrivilla@cab.inta-csic.es
³ INAF-Osservatorio Astrofisico di Arcetri, Florence, Italy

Received: 20 December 2021
Accepted: 2 February 2022

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.