Pyrolysis of furan: Ab initio quantum chemical and kinetic modeling studies

The kinetics of pyrolysis of furan have been investigated theoretically by ab initio quantum chemical techniques and by detailed chemical kinetic mode ling of previously reported experimental results. [Organ, P. P.; Mackle, T. C. J. Chem. Sec., Faraday Trans. 1991, 87, 815.] The kinetic model, contai ning rate constants derived from the ab initio calculations, can satisfacto rily model the species profiles that had been obtained in shock tube experi ments at three initial concentrations of furan. The thermochemistry and rat e parameters of a number of key reactions have been obtained by ab initio c alculations carried out at CASSCF, CASPT2, and G2(MP2) levels of theory. Th e calculations suggest that two parallel processes, initiated by 1,2-H tran sfers that result in the formation of cyclic carbene intermediates and lead to the decomposition products CO + propyne and C2H2 + ketene (as major and minor channels, respectively), are the dominant pathways and enable the qu antitative modeling of the kinetics of furan disappearance and the formatio n of the major products. Direct ring scission in furan, either on a singlet or triplet surface, is found to be much too energetic to contribute to any appreciable degree. No evidence was found for significant participation of a third channel producing HCO + C3H3 H atoms and C3H3 radicals arise essen tially by CH fission of propyne. Hydrogen abstraction from furan by methyl radicals is, however, significant and represents the principal source of me thane in the products.