Ethanimine: Interstellar Imine Detected in Sagittarius B2

Ethanimine (CH₃CH=NH) is a reactive organonitrogen compound that readily polymerizes, yet it has been observed in the interstellar medium, notably toward the Sagittarius B2 (Sgr B2) cloud near the Galactic Center [1]. Laboratory studies show it exists as E‑ and Z‑stereoisomers and as a tautomeric pair with ethenamine, but the imine form dominates under typical conditions [2].

Key takeaways

• Ethanimine is an imine formed by condensation of acetaldehyde and ammonia, rapidly polymerizing to a trimer [1].
• The molecule has E and Z stereoisomers; the E‑isomer is more stable by ~2.7 kJ mol⁻¹ and has been firmly detected in Sgr B2(N) [2].
• Rotational transitions near 140 GHz enable radio telescopes such as the Green Bank Telescope to identify ethanimine in hot cores of interstellar clouds [1][3].
• Laboratory synthesis uses gas‑phase pyrolysis of ethylamine or low‑temperature ice analog experiments, producing transient ethanimine for spectroscopic study [2].
• Detection of ethanimine supports models of nitrogen‑bearing organic chemistry in star‑forming regions and informs the inventory of prebiotic molecules in space [3].

Interstellar detection and spectroscopic signature

Observations with the Green Bank Telescope’s PRIMOS survey matched laboratory broadband rotational spectra of ethanimine to unidentified lines in the Sgr B2(N) region [3]. Both the E‑ and Z‑isomers showed transitions between 8 and 130 GHz, with the E‑isomer’s strongest lines around 140 GHz, a frequency range accessible to radio facilities operating from 1–300 GHz [1]. The rotational constants measured for the E‑isomer (A = 53120.565 MHz, B = 9782.7713 MHz, C = 8697.0236 MHz) provide precise predictions that confirm its presence in the hot core environments of Sgr B2(N) and Sgr B2(M) [2].

Laboratory characterization and tautomerism

In the laboratory, ethanimine is generated by condensing acetaldehyde with ammonia under anhydrous conditions, or by pyrolyzing ethylamine at 800–1000 K, yielding both stereoisomers for spectroscopic analysis [2]. Quantum‑chemical calculations indicate the E‑isomer’s C=N bond length is about 1.272 Å and the molecule adopts Cs symmetry, with the heavy atoms lying in a plane [2]. The compound’s tautomeric partner, ethenamine (CH₂=CHNH₂), is less stable by roughly 14 kJ mol⁻¹, making the imine form predominant in both laboratory and interstellar settings [2].

Why it matters

The identification of ethanimine in Sgr B2 adds to the growing list of nitrogen‑containing organics detected in star‑forming regions, highlighting pathways that could lead to more complex prebiotic molecules. Its detection relies on precise rotational spectroscopy, bridging laboratory chemistry and astronomical observation. Future surveys targeting similar rotational transitions may reveal additional imine species, refining our understanding of interstellar chemistry and the molecular inventory that seeds nascent planetary systems.