Discovery of interstellar propylene (CH2CHCH3):: Missing links in interstellar gas-phase chemistry

We report the discovery of propylene (also called propene, CH2CHCH3) with the IRAM 30 m radio telescope toward the dark cloud TMC-1. Propylene is the most saturated hydrocarbon ever detected in space through radio astronomical techniques. In spite of its weak dipole moment, six doublets (A and E species) plus another line from the A species have been observed with main-beam temperatures above 20 mK. The derived total column density of propylene is 4 x 10(13) cm(-2), which corresponds to an abundance relative to H-2 of 4 x 10(-9), i.e., comparable to that of other well-known and abundant hydrocarbons in this cloud, such as c-C3H2. Although this isomer of C3H6 could play an important role in interstellar chemistry, it has been ignored by previous chemical models of dark clouds as there seems to be no obvious formation pathway in gas phase. The discovery of this species in a dark cloud indicates that a thorough analysis of the completeness of gas-phase chemistry has to be done.