Protonation Sites in Gaseous Pyrrole and Imidazole: A Neutralization–Reionization and ab initio Study

Mild gas‐phase acids C 4 H 9 + and NH 4 + protonate pyrrole at C‐2 and C‐3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization–reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6–311G(2d, p) as 866, 845 and 786 kJ mol ‐1 for protonation at C‐2, C‐3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C‐2‐and C‐3‐protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N‐3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N‐3, C‐4, C‐2, C‐5 and N‐1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N‐3‐protonated imidazole is accompanied by large Franck–Condon effects which deposit on average 183 kJ mol ‐1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N‐H bond cleavage and isomerization to the more stable C‐2 H‐isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.