Gas-Phase Formation of 1-Methylcyclopropene and 3-Methylcyclopropene via the Reaction of the Methylidyne Radical (CH; X 2 Π) with Propylene (CH 3 CHCH 2 ; X 1 A')
The crossed molecular beam reactions of the methylidyne radical (CH; X 2 Π) with propylene (CH 3 CHCH 2 ; X 1 A') along with (partially) substituted reactants were conducted at collision energies of 19.3 kJ mol –1 . Combining our experimental data with ab initio electronic structure and statistical calculations, the methylidyne radical is revealed to add barrierlessly to the carbon–carbon double bond of propylene reactant resulting in a cyclic doublet C 4 H 7 intermediate with a lifetime longer than its rotation period. These adducts undergo a nonstatistical unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming the cyclic products 1-methylcyclopropene and 3-methylcyclopropene with overall reaction exoergicities of 168 ± 25 kJ mol –1 . These C 4 H 6 isomers are predicted to exist even in low-temperature environments such as cold molecular clouds like TMC-1, since the reaction is barrierless and exoergic, all transition states are below the energy of the separated reactants, and both the methylidyne radical (CH; X 2 Π) and propylene reactant were detected in cold molecular clouds such as TMC-1.