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 b ut not at the nitrogen atom, as determined by deuterium labeling and n eutralization-reionization mass spectrometry. Proton affinities in pyr role 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 neutralizat ion of protonated pyrrole generates bound radicals that in part dissoc iate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom fro m C-2-and C-3-protonated pyrrole cations is preceded by proton migrati on in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cati on. Proton affinities in imidazole are calculated as 941, 804, 791, 79 1 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. Vert ical neutralization of N-3-protonated imidazole is accompanied by larg e Franck-Condon effects which deposit on average 183 kJ mol(-1) vibrat ional energy in the radicals formed. The radicals dissociate unimolecu larly by loss of hydrogen atom, which involves both direct N-H bond cl eavage and isomerization to the more stable C-2 H-isomer. Potential en ergy barriers to isomerizations and dissociations in protonated pyrrol e and imidazole isomers and their radicals were investigated by ab ini tio calculations.