Reaction dynamics of carbon-bearing radicals in circumstellar envelopes of carbon stars

Abstract

Crossed molecular beams experiments on dicarbon molecules, C₂(X¹Σ⁺_g/a³Π_u), with unsaturated hydrocarbons acetylene (C₂H₂(X¹Σ⁺_g)), ethylene (C₂H₄(X¹A_g)), methylacetylene (CH₃CCH(X¹A₁)), and allene (H₂CCCH₂ (X¹A₁)) were carried out at 18 collision energies between 10.6 and 50.3 kJ mol⁻¹ utilizing a universal crossed beams machine to untangle the reaction dynamics forming hydrogen deficient hydrocarbon radicals in circumstellar envelopes of carbons stars and in cold molecular clouds. We find that all reactions proceed without the entrance barrier through indirect (complex forming) scattering dynamics. Each bimolecular collision is initiated by an addition of the dicarbon molecule to the π bond of the unsaturated hydrocarbon molecule yielding initially acyclic (triplet) and three- or four-membered cyclic collision complexes (triplet and singlet surface). On the singlet surface, the cyclic structures isomerize to form eventually diacetylene (HCCCCH; C₂/C₂H₂), butatriene (H₂CCCCH₂; C₂/C₂H₄), methyldiacetylene (CH₃CCCCH; C₂/CH₃CCH), and pentatetraene (H₂CCCCCH₂; C₂/H₂CCCH₂) intermediates. The latter were found to decompose via atomic hydrogen loss yielding the buta-1,3-diynyl [C₄H(X²Σ⁺) HCCCC], 1-butene-3-yne-2-yl [i-C₄H₃(X²A′) H₂CCCCH], penta-2,4-diynyl-1 [C₅H₃(X²B₁) HCCCCCH₂], and penta-1,4-diynyl-3 radical [C₅H₃(X²B₁) HCCCHCCH] under single collision conditions. The underlying characteristics of these dicarbon versus atomic hydrogen replacement pathways (indirect scattering dynamics; no entrance barrier; isomerization barriers below the energy of the separated reactants; exoergic reactions) suggest the enormous potential of the dicarbon plus unsaturated hydrocarbon reaction class to form highly hydrogen-deficient carbonaceous molecules in cold molecular clouds and in circumstellar envelopes of carbon stars. The studies therefore present an important advancement in establishing a comprehensive database of reaction intermediates and products involved in bimolecular collisions of dicarbon molecules with unsaturated hydrocarbons which can be utilized in refined astrochemical models and also in future searches of hitherto unidentified interstellar molecules. Implications of these experiments to understand related combustion processes are also addressed.