An Experimental and Theoretical Study of Pyrrole Pyrolysis with Tunable Synchrotron VUV Photoionization and Molecular-Beam Mass Spectrometry

The Pyrolysis of pyrrole (6.46% pyrrole in argon) has been performed with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS) technique. The experiment was carried out over the temperature range of 1260-1710 K at a pressure of 267 Pa. About 30 intermediates have been identified by near-threshold measurements of photoionization mass spectra, and the corresponding mole fractions versus temperatures have been obtained. Moreover, the isomers of some pyrolysis products have been identified by the measurement of photoionization efficiency spectrum. The major products are H-2, C2H2, HCN, C3H4 (propyne), and C2H3N (acetonitrile). Meanwhile, some new intermediates, such as C4H4N (cyanoallyl radical) and C2H2N (cyanomethyl radical), have been determined. The major pyrolysis channels have been provided with the high-level ab initio G3B3 calculation and are well consistent with the experimental observation. The formation pathway of HCN via the cyclic carbene tautomer has been proved to be the lowest formation pathway, which is in accordance with previous theoretical work. The potential pathways of the early formed C4H4N species together with their subsequent consumption to C2H2N and C2H2 have been discussed in detail. Also, the formation pathways of the major products of C2H3N and C2H2 have been investigated as well.