A Theoretical Study of Pyrolysis Mechanisms of Pyrrole

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Density functional theory and ab initio calculations were carried out to investigate the pyrolysis mechanisms of pyrrole. All equilibrium and transition state structures of the proposed reaction channels were fully optimized by the density functional B3LYP method using the 6-31G(d,p) basis set. Relative energies were evaluated at the QCISD(T)/6-311G(d,p) level of theory. In addition to the mechanism proposed in experimental studies, alternative unimolecular pathways for the formation of cis-crotonitrile and allyl cyanide, major nitrogen-containing isomerization products, were proposed and investigated. The results suggest that a mechanism proposed in the present study is more likely responsible for the formation of allyl cyanide. For the formation of cis-crotonitrile, a mechanism proposed in the present study should also be competitive, especially under low-pressure conditions. Although extensive calculations were carried out, we failed to identify a unimolecular decomposition pathway generating HCN, another major nitrogen-containing pyrolysis product, with an activation barrier close to the experimental value.