The propene molecule C₃H₆, aka propylene, has been identified in spectra recorded toward TMC-1 by
N. Marcelino, J. Cernicharo, M. Agúndez, E. Roueff, M. Gerin, J. Martín-Pintado, R. Mauersberger, and C. Thum,
Discovery of Interstellar Propylene (CH₂CHCH₃): Missing Links in Interstellar Gas-Phase Chemistry
Astrophys. J. 665, L127–L130 (2007).
Six rotational transitions each split into two A and E components due to the CH₃ interal rotation were observed plus one A component for which the E line is overlapped. The observation of this molecule in TMC-1 was rather surprising because apparently noone in the community considered such an almost saturated molecule to be likely in this source – especially because of thus far missing routes to this molecule. Even more remarkable is the detection because of the high abundance; a column density of 4 × 10¹³ cm^-2 was derived; comparable to the abundance of c-C₃H₂.

M. Agúndez, J. Cernicharo, and M. Guélin
reported the detection of propene by the two methyl internal rotor states of the 5₀₅ – 4₀₄ transition near 86.65 GHz with the IRAM 30 m telescope toward 4 out of 5 dark clouds (Lupus-1a, L1495B, L1521F, and Serpens South 1a) in their report on
Discovery of Interstellar Ketenyl (HCCO), a Surprisingly Abundant Radical
Astron. Astrophys. 577, Art. No. L5 (2015).
Emission of up to three more rotational transitions were also detected.

Contributor(s): H. S. P. Müller; 08, 2007; 05, 2015