1 Reaction Dynamics of Carbon-Bearing Radicals in Circumstellar Envelopes of Carbon Stars

Crossed molecular beams experiments on dicarbon molecules, C2(X1Σg+/a3Πu), with unsaturated hydrocarbons acetylene (C2H2(X1Σg+)), ethylene (C2H4(X1Ag)), methylacetylene (CH3CCH(X1A1)), and allene (H2CCCH2 (X1A1)) were carried out at 18 collision energies between 10.6 and 50.3 kJmol-1 utilizing a universal crossed beams machine to untangle the reaction dynamics forming hydrogen defi-cient hydrocarbon radicals in circumstellar envelopes of carbons stars and in cold molecular clouds. We find that all reactions proceed without entrance barrier through indirect (complex forming) scatte-ring dynamics. Each bimolecular collision is initiated by an addition of the dicarbon molecule to the pi 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 1,3-butadiynyl [C4H(X2Σ+) HCCCC], 1-butene-3-yne-2-yl [i-C4H3(X2A’) H2CCCCH], 2,4-pentadiy-nyl-1 [C5H3(X2B1) HCCCCCH2], and 1,4-pentadiynyl-3 radical [C5H3(X2B1) HCCCHCCH] unde