A combined crossed beam and ab initio investigation on the reaction of carbon species with C4H6 isomers. III. 1,2-butadiene, H2CCCH(CH3) (X(1)A(')) -a non-Rice-Ramsperger-Kassel-Marcus system?

Crossed molecular beam experiments were conducted to investigate the reacti on of ground state carbon atoms, C(P-3(j)), with 1,2-butadiene, H2CCCH(CH3) (X (1)A(')), at three collision energies of 20.4 kJ mol-1, 37.9 kJ mol-1, and 48.6 kJ mol-1. Ab initio calculations together with our experimental da ta reveal that the reaction is initiated by a barrier-less addition of the carbon atom to the pi system of the 1,2-butadiene molecule. Dominated by la rge impact parameters, C(P-3(j)) attacks preferentially the C2-C3 double bo nd to form i1 (mechanism 1); to a minor extent, small impact parameters lea d to an addition of atomic carbon to the C1-C2 bond yielding i2 (mechanism 2). Both cyclic intermediates i1 and i2 ring open to triplet methylbutatrie ne complexes i3' ((H2CCCCH)-C-*(CH3)) and i3', ((H2CCCCH)-C-*(CH3)); C-* de notes the attacked carbon atom. i3' is suggested to decay nonstatistically prior to a complete energy randomization via atomic hydrogen loss forming 1 - and 4-methylbutatrienyl CH3CCCCH2 (X (2)A(')) and HCCCCH(CH3) (X (2)A(')) , respectively. The energy randomization in i3' is likely to be complete. T his isomer decomposes via H atom loss to 3-vinylpropargyl, H2CCCC2H3(X (2)A (')), as well as 1- and 4-methylbutatrienyl radicals. In high-density envir onments such as the inner regions of circumstellar envelopes of carbon star s and combustion flames, these linear C5H5 isomers might undergo collision induced isomerization to cyclic structures like the cyclopentadienyl radica l. This isomer is strongly believed to be a key intermediate involved in th e production of polycyclic aromatic hydrocarbon molecules and soot formatio n. These characteristics make the reactions of atomic carbon with C4H6 isom ers compelling candidates to form C5H5 isomers in the outflow of AGB stars and oxygen-deficient hydrocarbon flames. <(C)> 2001 American Institute of P hysics.