Direct detection ofpyridine formation by the reaction of CH (CD) with pyrrole: a ring expansion reaction

Abstract

The reaction of the ground state methylidyne radical CH (X²Π) with pyrrole (C₄H₅N) has been studied in a slow flow tube reactor using Multiplexed Photoionization Mass Spectrometry coupled to quasi-continuous tunable VUV synchrotron radiation at room temperature (295 K) and 363 K, at 4 Torr (533 Pa). Laser photolysis of bromoform (CHBr₃) at 248 nm (KrF excimer laser) is used to produce CH radicals that are free to react with pyrrole molecules in the gaseous mixture. A signal at m/z = 79 (C₅H₅N) is identified as the product of the reaction and resolved from ⁷⁹Br atoms, and the result is consistent with CH addition to pyrrole followed by H-elimination. The photoionization efficiency curve unambiguously identifies m/z = 79 as pyridine. With deuterated methylidyne radicals (CD), the product mass peak is shifted by +1 mass unit, consistent with the formation of C₅H₄DN and identified as deuterated pyridine (d-pyridine). Within detection limits, there is no evidence that the addition intermediate complex undergoes hydrogen scrambling. The results are consistent with a reaction mechanism that proceeds via the direct CH (CD) cycloaddition or insertion into the five-member pyrrole ring, giving rise to ring expansion, followed by H atom elimination from the nitrogen atom in the intermediate to form the resonance stabilized pyridine (d-pyridine) molecule. Implications to interstellar chemistry and planetary atmospheres, in particular Titan, as well as gas-phase combustion processes, are discussed.