Directed Gas-Phase Formation of The Propargyl Family of Resonance-Stabilized Radicals in The Reactions of Ground-State Carbon Atoms (C; ³ P j ) with Butene Isomers (C ₄ H ₈ ): Dimethylpropargyl and Ethylpropargyl

The propargyl radical (C 3 H 3 ) is the simplest resonance-stabilized free radical (RSFR), but how does stepwise methyl substitution in the alkene reactant affect its dynamics of their formation? We report a crossed molecular beam study of the reactions of atomic carbon (C, 3 P j ) with four butene isomers (C 4 H 8 ) under single-collision conditions at a collision energy of 28 ± 2 kJ mol –1 . Barrierless addition of atomic carbon to the alkene C═C bond triggers ring opening to substituted triplet allenes─a de facto insertion mechanism─followed by unimolecular decomposition via atomic hydrogen (H), methyl (CH 3 ), or ethyl (C 2 H 5 ) loss, yielding a family of propargyl-type RSFRs. RRKM calculations reveal that the branching ratios are highly sensitive to the alkene structure. While the methyl loss channel, affording 1-methylpropargyl, dominates for 2-butenes (80–90%), the predicted hydrogen-atom loss channel (≈10%), leading to 1,3-dimethylpropargyl is identified experimentally by comparison with theoretical energetics. For isobutene, near-equal competition is observed, with the reaction producing 3-methylpropargyl (≈50%) and 1,1-dimethylpropargyl (≈40%), along with 2-vinylallyl (≈5%), whose formation is supported by experimental data. Most notably, the reaction with 1-butene uniquely favors an enthalpically driven hydrogen shift, eventually producing 1-vinylallyl (≈38%), which is assigned based on the excellent agreement between the measured and calculated reaction exothermicity. Rapid entropically favored fragmentation channels yield ≈40% of propargyl-type species (propargyl, 1- and 3-ethylpropargyls), slightly outcompeting the allyl-type product. These results establish a systematic progression from C 2 H 4 via C 3 H 6 to C 4 H 8, where the increasing alkyl substitution unlocks new fragmentation channels, providing a versatile gas-phase route to alkylated RSFRs─key intermediates in the growth of methylated and ethylated PAHs and aliphatic chains in combustion and cold interstellar environments (molecular clouds).