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CAN INTERSTELLAR PROPENE (CH3CHCH2) BE FORMED VIA GAS-PHASE REACTIONS?

Propene (CH3CHCH2), detected in the cold core TMC-1, is a surprisingly saturated (H-rich) species for observation in such regions. In a recently proposed gas-phase formation mechanism, interstellar propene is produced from its protonated precursor C3H+7 (CH3CHCH+3) via a dissociative recombination process. The precursor ion C3H+7 is itself produced via two consecutive radiative association reactions involving H2 starting from the isomer of C3H+3 with a linear carbon backbone (CH2CCH+). Initial calculations showed that the radiative association reactions are efficient enough to allow the production of an abundance of propene equal to that observed. However, a combination of experiments and more refined quantum chemical ab initio calculations reported here does not corroborate the initial result. Indeed, from both of these approaches, we have learned that the radiative association reactions leading to protonated propene do not occur efficiently at interstellar temperatures due to activation energy barriers. The result is that propene cannot be produced efficiently by the suggested gas-phase synthetic route. It is still difficult to say, however, that no suitable gas-phase syntheses for propene can occur in cold cores such as TMC-1.