Chemical dynamics of the CH(X2Π) + C2H4(X1A1g), CH(X2Π) + C2D4(X1A1g), and CD(X2Π) + C2H4(X1A1g) reactions studied under single collision conditions

Abstract

The crossed beam reactions of the methylidyne radical with ethylene (CH(X²Π) + C₂H₄(X¹A_1g)), methylidyne with D4-ethylene (CH(X²Π) + C₂D₄(X¹A_1g)), and D1-methylidyne with ethylene (CD(X²Π) + C₂H₄(X¹A_1g)) were conducted at nominal collision energies of 17–18 kJ mol⁻¹ to untangle the chemical dynamics involved in the formation of distinct C₃H₄ isomers methylacetylene (CH₃CCH), allene (H₂CCCH₂), and cyclopropene (c-C₃H₄) viaC₃H₅ intermediates. By tracing the atomic hydrogen and deuterium loss pathways, our experimental data suggest indirect scattering dynamics and an initial addition of the (D1)-methylidyne radical to the carbon–carbon double bond of the (D4)-ethylene reactant forming a cyclopropyl radical intermediate (c-C₃H₅/c-C₃D₄H/c-C₃H₄D). The latter was found to ring-open to the allyl radical (H₂CCHCH₂/D₂CCHCD₂/H₂CCDCH₂). This intermediate was found to be long lived with life times of at least five times its rotational period and decomposed via atomic hydrogen/deuterium loss from the central carbon atom (C2) to form allenevia a rather loose exit transition state in an overall strongly exoergic reaction. Based on the experiments with partially deuterated reactants, no compelling evidence could be provided to support the formation of the cyclopropene and methylacetylene isomers under single collision conditions. Likewise, hydrogen/deuterium shifts in the allyl radical intermediates or an initial insertion of the (D1)-methylidyne radical into the carbon–hydrogen/deuterium bond of the (D4)-ethylene reactant were found to be—if at all—of minor importance. Our experiments propose that in hydrocarbon-rich atmospheres of planets and their moons such as Saturn's satellite Titan, the reaction of methylidyne radicals should lead predominantly to the hitherto elusive allene molecule in these reducing environments.