Ab initio quantum chemical and kinetic modeling study of the pyrolysis kinetics of pyrrole

The kinetics of pyrolysis of pyrrole have been investigated theoretically b y ab initio quantum chemical techniques and by detailed chemical kinetic mo deling of previously reported experimental results. [Mackie, J. C.; Colket, M. B.; Nelson, P. F.; Esler, hi. Int. J. Chern. Kinet. 1991, 23, 733.] The overall kinetics can be successfully modeled by a 117 step kinetic model t hat gives good agreement with temperature profiles of major products and al so provides an acceptable fit for minor products. The thermochemistry and r ate 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. Several reaction pathways were investigated. The major product, KCN, arises principally from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of this carbene leads to an allenic imine precursor of HCN and propyne. This is the decomposition pathway of lo west energy. Pyrolysis is preceded by the facile tautomerization of pyrrole to 2H-pyrrolenine. The latter can undergo CN fission to form an open chain biradical species, which is the precursor of the butenenitrile isomeric pr oducts, cis- and trans-crotononitrile and allyl cyanide. The biradical can also undergo facile H-fission to form cyanoallyl radical, which is an impor tant precursor of acetylene, acetonitrile, and acrylonitrile. H-2 also aris es principally from H-fission of the biradical.