Atomic and molecular hydrogen elimination in the crossed beam reaction of d1-ethinyl radicals C2D(X 2Σ+) with acetylene, C2H2(X 1Σg+): Dynamics of d1-diacetylene (HCCCCD) and d1-butadiynyl (DCCCC) formation

Abstract

The chemical reaction dynamics to form d1-diacetylene, DCCCCH (X ¹Σ⁺), and the d1-butadiynyl radical, DCCCC, via the reaction of d1-ethinyl, C₂D (X ²Σ⁺), with acetylene, C₂H₂ (X ¹Σ_g⁺), are explored in a crossed molecular beam experiment at an average collision energy of 26.1 kJ mol⁻¹. The experiments show that the reaction follows indirect scattering dynamics via a C₄H₂D intermediate. The calculations confirm that the reaction has no entrance barrier and that it proceeds via an attack of the ethinyl radical on the π electron density of the acetylene molecule. The initially formed trans-1-d-ethinylvinyl-2 (HCCHC₂D) intermediate rearranges to its cis form; the latter is found to fragment predominantly via H atom emission to form d1-diacetylene, HCCCCD (X ¹Σ⁺) and H (²S_1/2) (channel 1). A second involves a [1,2]-H shift in trans-HCCHC₂D to yield a 1-d-ethinylvinyl-1 radical. The latter channel then shows two fragmentation pathways: a molecular hydrogen elimination to form the d1-butadiynyl radical (DCCCC) (channel 2) and an atomic hydrogen loss to yield d1-diacetylene (HCCCCD) (channel 3). Compared to the C₄HD/H products (98–99%), the C₄D/H₂ channel presents only a minor pathway (1–2%). The solid identification of diacetylene under single collision conditions is the first experimental proof of a long-standing hypothesis that the title reaction can synthesize diacetylene in dark, molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as in the atmospheres of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.