Hydrogenation of pyrrole

The reaction of hydrogen atoms (H) with pyrrole (C4H4NH) in solid para-hydrogen (p-H2) matrices at 3.2 K has been studied by infrared spectroscopy. Upon reaction of the H atoms with pyrrole in p-H2, a new series of lines appeared in the infrared spectrum, and based on secondary photolysis, it was determined that the majority of the new lines belong to two distinct chemical species; these lines are designated as set A and set B. According to quantum-chemical calculations performed at the B3PW91/6-311++G(2d,2p) level, the most likely reactions to occur under low temperature conditions in solid p-H2 are the addition of an H atom to carbon 2 or 3 of C4H4NH to produce the corresponding hydrogen-atom addition radicals (HC4H4NH•). When the lines in sets A and B are compared to the scaled harmonic and anharmonic vibrational infrared stick spectra of these two radicals, the best agreement for set A is with the radical produced by the addition to carbon 3 (2,3-dihydropyrrol-2-yl radical, 3-HC4H4NH•), and the best agreement for set B is with the radical produced by addition to carbon 2 (2,3-dihydropyrrol-3-yl radical, 2-HC4H4NH•). The ratio of the 2-HC4H4NH•to 3-HC4H4NH•radicals is estimated to be 4-5:1, consistent with the smaller predicted barrier height for the H-atom addition to C2. In addition to the assignments of the 2,3-dihydropyrrol-2-yl and 2,3-dihydropyrrol-3-yl radicals, a series of lines that appear upon 455-nm photolysis have been assigned to 1,3-pyrrolenine (2-HC4H4N).

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in (J. Chem. Phys. 153, 164302 (2020)) and may be found at (https://doi.org/10.1063/5.0024495).