Composite Single-Reference and Explicitly Correlated Multireference Calculations on the Mechanism of Pyrrole Pyrolysis

Abstract

The pyrolysis of pyrrole has previously been studied in shock-tube experiments and in jet-stirred reactors. The results have been interpreted with the aid of density functional and ab initio electronic-structure calculations, in combination with kinetic models. The present work has sought to reinvestigate the mechanism and kinetics using more sophisticated electronic structure calculations and a multiwell master equation kinetic model. The intent has been not only to probe the reaction starting with pyrrole at high temperature and pressure but also to use the same mechanistic network to examine the reaction of allyl radical with •CN at low temperature and pressure. The question being addressed is whether the high exothermicity of this radical combination reaction could drive formation of pyrrole in extraterrestrial environments, such as the interstellar medium or in planetary atmospheres. The results reveal that two new intermediates, not considered in previous mechanisms, play important roles in the overall chemistry. Furthermore, two previously proposed mechanistic steps are found not to be viable. The master-equation analysis reveals that only traces of pyrrole would be expected to form from allyl + •CN, despite there being enough excess kinetic energy to overcome all barriers to its formation. An explanation for this conclusion is offered.