The pyrolysis of cyclopentadiene: quantum chemical and kinetic modelling studies of the acetylene plus propyne/allene decomposition channels

The thermal decomposition of cyclopentadiene to acetylene plus propyne or a llene was studied using ab initio CASSCF, CASPT2, G2(MP2) as well as densit y functional methods. Three distinct reaction pathways were explored that i nvolve 1,2-hydrogen transfers, yielding cyclic carbenes or allyl vinylidene as initial intermediates. After ring opening or a second 1,2-hydrogen tran sfer, the resulting open chain isomers dissociate via 1,5 sigmatropic shift s or by CC bond fission. From the computed CASSCF/CASPT2 data the appropria te RRKM rate constants for these reactions were computed and incorporated i nto a kinetic model containing the appropriate parameters for the conventio nal model of acetylene production via cyclopentadienyl, c-C5H5. The modelli ng studies, although not achieving quantitative agreement with experiment w ithout some adjustment of the ab initio derived kinetic parameters, demonst rate that the proposed additional mechanism may be a major source of acetyl ene under a range of shock tube conditions. Consequently, the rate constant for the decomposition of c-C5H5 to C2H2 + C3H3 cannot be derived directly from existing experimental data.