Thermochemistry and Electronic Structure of the Pyrrolyl Radical

The 364-nm photoelectron spectrum of pyrrolide anion, prepared by deprotonation of pyrrole, has been measured. The electron affinity (EA) of pyrrolyl radical has been determined to be 2.145 ± 0.010 eV. Harmonic vibrational frequencies of 925 ± 65, 1012 ± 25, and 1464 ± 20 cm-1 are observed in the spectrum of the 2A2 ground state of pyrrolyl. This spectrum is well reproduced by Franck−Condon fitting on the basis of the optimized geometries and the vibrational frequencies of the anion and the radical obtained at the B3LYP/6-311++G(d,p) level of density functional theory (DFT). The observed vibrational modes involve large displacements along the ring coordinates. While the Franck−Condon analysis also predicts a very similar spectrum for the 2B1 first excited state, only a broad, featureless, weak spectrum is observed near the calculated binding energy. The DFT calculations find a transition state for 2B1 electronic symmetry as a result of strong vibronic coupling between the 2A2 and 2B1 states. The transition state is located very close to a conical intersection of these states. The absence of distinctive features for the 2B1 transition state in the spectrum arises from the associated lifetime broadening. Using the EA of pyrrolyl together with the N−H bond dissociation energy (BDE) of pyrrole recently determined by Ashfold, the gas-phase acidity of pyrrole is Δacid G 298(RH) = 351.9 ± 0.4 kcal mol-1 and Δacid H 298(RH) = 359.4 ± 0.4 kcal mol-1. The gas-phase acidity of pyrrole was also independently determined relative to methanethiol using a tandem flowing afterglow-selected ion flow tube. These measurements now provide a much more accurate set of benchmark acidities for pyrrole and methanethiol, a frequently employed reference acid.