A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. II. The dimethylacetylene molecule, H3CCCCH3(X(1)A(1g))

The reaction of ground state carbon atoms, C(P-3(j)), with dimethylacetylen e, H3CCCCH3, was studied at three collision energies between 21.2 and 36.9 kJmol(-1) employing the crossed molecular beam approach, Our experiments we re combined with ab initio and RRKM calculations. It is found that the reac tion is barrierless via a loose, early transition stare located at the cent rifugal barrier following indirect scattering dynamics through a,complex. C (P-3(j)) attacks the pi system of the dimethylacetylene molecule to form a dimethylclopropenylidene 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 intermediat es followed by ring closure. The cyclic intermediate ring opens to a linear dimethylpropargylene radical which rotates almost parallel to the total an gular momentum vector J. This complex fragments to atomic hydrogen and a li near 1-methylbutatrienyl radical, H2CCCCCH3(X(2)A"), via a tight exit trans ition state located about 18 kJmol(-1) above the separated products. The ex perimentally determined exothermicity of 190+/-25 kJmol(-1) is in strong ag reement with our calculated data of 180+/-10 kJmol(-1). The explicit verifi cation of the carbon versus hydrogen exchange pathway together with the fir st identification of the H2CCCCCH3 radical represents a third pathway to fo rm chain C5H5 radicals in the reactions of C(P-3(j)) with C4H6 isomers unde r single collision conditions. Previous experiments of atomic carbon with t he 1,3-butadiene isomer verified the formation;,of 1- and 3-vinylpropargyl radicals, HCCCHC2H3(X(2)A"), and H2CCCC2H3(X(2)A"). respectively. In high-d ensity environments; such as combustion flames and circumstellar envelopes of carbon stars, these linear isomers can undergo collision-induced ring cl osure(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 soo t formation and possibly in the production of polycyclic aromatic hydrocarb on molecules in outflow of carbon stars. Likewise, a H atom catalyzed isome rization can interconvert the 3-vinylpropargyl and the 1-methylbutatrienyl radical. (C) 2000 American Institute of Physics. [S0021-9606(00)00438-4].