title: Gas-phase protonation of pyridine. A variable-time neutralization-reionization and Ab initio study of pyridinium radicals.
abstract: Gas-phase protonation of pyridine with CH3NH3+, NH4+, t-C4H9+, H3O+ and CH5+ under thermal conditions was studied by variable-time neutralization-reionization mass spectrometry and ab initio calculations. N-Protonation was found to occur exclusively for CH3NH3+ through H3O+ and predominantly for CH5+. The calculated MP2/6-311G(2d,p) energies gave the proton affinities of N, C-2, C-3 and C-4 in pyridine as 924, 658, 686 and 637 kJ mol-1, respectively, which were in good agreement with previous experimental and theoretical results. Vertical neutralization of the N-protonated isomer (1H+) was accompanied by moderate Franck-Condon effects that deposited 20-21 kJ mol-1 in the 1H-pyridinium radicals (1H) formed. 1H was calculated by UMP2/6-311G(2d,p) and B3LYP/6-311G(2d,p) to be a bound species in its ground electronic state. A substantial fraction of stable 1H was detected in the spectra, which depended on the precursor ion internal energy. Deuterium labeling showed a specific loss of the N-bound hydrogen or deuterium in the radicals. The specificity increased with increasing internal energy in the radicals and decreasing contribution of ion dissociations following reionization. Variable-time measurements established specific loss of the N-bound deuterium also in dissociating low-energy 1D. Loss of hydrogen from 1H+ cations following reionization was highly endothermic and was accompanied by rearrangements that partially scrambled the ring hydrogens.
authors.affiliation: Department of Chemistry, University of Washington, Seattle 98195-1700, USA.
authors.forename: V Q
authors.initials: VQ
authors.lastname: Nguyen
authors.forename: F
authors.initials: F
authors.lastname: Turecek
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D005740:Gases / Q000378:metabolism; D013058:Mass Spectrometry; D015394:Molecular Structure; D011522:Protons; D011725:Pyridines / Q000378:metabolism*
chemical_list: D005740:Gases; D011522:Protons; D011725:Pyridines; C023666:pyridine