The thermal decomposition of pyrrole: an ab initio quantum chemical study of the potential energy surface associated with the hydrogen cyanide plus propyne channel

An ab initio quantum chemical study of the thermal decomposition of pyrrole to hydrogen cyanide and propyne is reported. Using CASSCF, CASPT2 and G2(MP2) methods three distinct pathways are investigated. The crucial step in each is a 1,5 sigmatropic shift that results in the concerted decomposition of an imine intermediate. The lowest energy pathway proceeds via a cyclic carbene, which, by ring opening produces the imine. The critical energy of the reaction is estimated as 75.6 kcal mol−1 which is consistent with the experimental activation energy of 74.1±3 kcal mol−1.