The Rotational Spectrum of Anti-Ethylamine (CH₃CH₂NH₂) from 10 to 270 GHz: A Laboratory Study and Astronomical Search in Sgr B2(N)

The pure rotational spectrum of the lowest energy (anti-) conformer of ethylamine (CH₃CH₂NH₂) has been measured in the frequency range of 10-270 GHz. The spectrum was recorded using both millimeter-wave absorption spectroscopy and Fourier transform microwave (FTMW) techniques. Ten rotational transitions of this molecule were recorded in the frequency range of 10-40 GHz using FTMW methods, resulting in the assignment of 53 quadrupole-resolved hyperfine lines; in the millimeter-wave region (48-270 GHz), nearly 600 transitions were assigned to the ground (anti-) state. The amine group in CH₃CH₂NH₂ undergoes inversion, resulting in a doubling that is frequently small and most apparent in the low-frequency K-doubling transitions. In addition, seemingly random rotational levels of this molecule were found to be significantly perturbed. The cause of these perturbations is presently uncertain, but torsion-rotation interactions with the higher lying gauche conformers seem to be a likely explanation. An astronomical search was conducted for ethylamine toward Sgr B2(N) using the Kitt Peak 12 m antenna and the Sub-Millimeter Telescope (SMT) of the Arizona Radio Observatory. Frequencies of 70 favorable rotational transitions were observed in this search, which covered the range 68-263 GHz. Ethylamine was not conclusively detected in Sgr B2(N), with an upper limit to the column density of (1–8) × 10¹³ cm⁻² with f(CH₃CH₂NH₂/H₂) ∼ (0.3–3) × 10⁻¹¹, assuming a rotational temperature of 50-220 K. These observations indicate a gas-phase CH₃CH₂NH₂/CH₃NH₂ ratio of <0.001-0.01, in contrast to the nearly equal ratio suggested by the acid hydrolysis of cometary solids from the Stardust mission.