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 2 A 2 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 2 B 1 first excited state, only a broad, featureless, weak spectrum is observed near the calculated binding energy. The DFT calculations find a transition state for 2 B 1 electronic symmetry as a result of strong vibronic coupling between the 2 A 2 and 2 B 1 states. The transition state is located very close to a conical intersection of these states. The absence of distinctive features for the 2 B 1 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 Δ acid G 298 (RH) = 351.9 ± 0.4 kcal mol -1 and Δ acid 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.