A combined crossed beam and ab initio investigation on the reaction of carbon species with C 4 H 6 isomers . II . The dimethylacetylene molecule , H 3 CCCCH 3 „ X

The reaction of ground state carbon atoms, C~Pj), with dimethylacetylene, H3CCCCH3 , was studied at three collision energies between 21.2 and 36.9 kJmol employing the crossed molecular beam approach. Our experiments were combined with ab initio and RRKM calculations. It is found that the reaction is barrierless via a loose, early transition state located at the centrifugal barrier following indirect scattering dynamics through a complex. C~Pj) attacks the p system of the dimethylacetylene molecule to form a dimethylcyclopropenylidene intermediate either in one step via an addition to C1 and C2 of the acetylenic bond or through an addition to only one carbon atom to give a short-lived cis/trans dimethylpropenediylidene intermediates followed by ring closure. The cyclic intermediate ring opens to a linear dimethylpropargylene radical which rotates almost parallel to the total angular momentum vector J. This complex fragments to atomic hydrogen and a linear 1-methylbutatrienyl radical, H2CCCCCH3(X A9), via a tight exit transition state located about 18 kJmol above the separated products. The experimentally determined exothermicity of 190625 kJmol is in strong agreement with our calculated data of 180610 kJmol. The explicit verification of the carbon versus hydrogen exchange pathway together with the first identification of the H2CCCCCH3 radical represents a third pathway to form chain C5H5 radicals in the reactions of C~Pj) with C4H6 isomers under single collision conditions. Previous experiments of atomic carbon with the 1,3-butadiene isomer verified the formation of 1and 3-vinylpropargyl radicals, HCCCHC2H3(X A9), and H2CCCC2H3(X A9), respectively. In high-density environments such as combustion flames and circumstellar envelopes of carbon stars, these linear isomers can undergo collision-induced ring closure~s! and/or H atom migration~s! which can lead to the cyclopentadienyl radical. The latter is thought to be a crucial reactive intermediate in soot formation and possibly in the production of polycyclic aromatic hydrocarbon molecules in outflow of carbon stars. Likewise, a H atom catalyzed isomerization can interconvert the 3-vinylpropargyl and the 1-methylbutatrienyl radical. © 2000 American Institute of Physics. @S0021-9606~00!00438-4#