Isomerization of pyrrole. Quantum chemical calculations and kinetic modeling

Density functional theory (DFT) calculations including configuration interaction (CI) were carried out to investigate the pathways of the unimolecular isomerizations of pyrrole. Vibrational frequencies calculated with the DFT method were used to estimate transition-state theory frequency factors. The potentional energy surface of the overall isomerization of pyrrole is composed of pyrrolenine, two biradical intermediates, and five transition states, in addition to pyrrole and its stable isomers. The first step of the isomerization is a fast transition from pyrrole to pyrrolenine. These two species reach a state of equilibrium that is maintained during the entire process. There is no direct path that leads from pyrrole to its stable isomers, which are produced only from pyrrolenine. Two biradical intermediates that are very similar in their structure and energetics and that isomerize to one another by a low-barrier rotation are involved in the process of pyrrole isomerization. One intermediate forms only cis-crotonitrile, and the other intermediate forms only vinylacetonitrile. These two biradical intermediates and several transition states have resonance structures. There is no direct route that leads from pyrrolenine to trans-crotonitrile. The latter is formed from the cis isomer by cis → trans isomerization. RRKM calculations were carried out to transfer values of A_∞ and E_∞ of the rate constants of two high-barrier steps in the isomerizations to A and E corresponding to the experimental conditions. Kinetic modeling, which uses the calculated rate constants, gives a very good agreement between the calculated and the experimental yields of the isomerization products.