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 modeling of previously reported experimental results. [Organ, P. P.; Mackie, J. C. J. Chem. Soc., Faraday Trans. 1991, 87, 815.] The kinetic model, containing rate constants derived from the ab initio calculations, can satisfactorily model the species profiles that had been obtained in shock tube experiments at three initial concentrations of furan. The thermochemistry and rate parameters of a number of key reactions have been obtained by ab initio calculations carried out at CASSCF, CASPT2, and G2(MP2) levels of theory. The calculations suggest that two parallel processes, initiated by 1,2-H transfers 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 quantitative modeling of the kinetics of furan disappearance and the formation 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 essentially by CH fission of propyne. Hydrogen abstraction from furan by methyl radicals is, however, significant and represents the principal source of methane in the products.