Protonation sites in gaseous pyrrole and imidazole: a neutralization-reionization and ab initio study.

Mild gas-phase acids C4H9+ and NH4+ 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.