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 allene was studied using ab initio CASSCF, CASPT2, G2(MP2) as well as density functional methods. Three distinct reaction pathways were explored that involve 1,2-hydrogen transfers, yielding cyclic carbenes or allyl vinylidene as initial intermediates. After ring opening or a second 1,2-hydrogen transfer, the resulting open chain isomers dissociate ia 1,5 sigmatropic shifts or by CC bond fission. From the computed CASSCF/CASPT2 data the appropriate RRKM rate constants for these reactions were computed and incorporated into a kinetic model containing the appropriate parameters for the conventional model of acetylene production ia cyclopentadienyl, c-C5H5. The modelling studies, although not achieving quantitative agreement with experiment without some adjustment of the ab initio derived kinetic parameters, demonstrate that the proposed additional mechanism may be a major source of acetylene 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.