GAS-PHASE PROTONATION OF PYRIDINE - A VARIABLE-TIME NEUTRALIZATION-REIONIZATION AND AB-INITIO STUDY OF PYRIDINIUM RADICALS

Gas-phase protonation of pyridine with CH3NH3+, NH4+, t-C4H9+, H3O+ an d CH5+ under thermal conditions was studied by variable-time neutraliz ation-reionization mass spectrometry and ab initio calculations. N-Pro tonation was found to occur exclusively for CH3NH3+ through H3O+ and p redominantly for CH5+. The calculated MP2/6-311G(2d,p) energies gave t he proton affinities of N, C-2, C-3 and C-4 in pyridine as 924, 658, 6 86 and 637 kJ mol(-1), respectively, which were in good agreement with previous experimental and theoretical results. Vertical neutralizatio n of the N-protonated isomer (1H(+)) was accompanied by moderate Pranc k-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 sta te. A substantial fraction of-stable 1H(.) was detected in the spectra , which depended on the precursor ion internal energy. Deuterium label ing showed a specific loss of the N-bound hydrogen or deuterium in the radicals. The specificity increased with increasing internal energy i n the radicals and decreasing contribution of ion dissociations follow ing 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 endot hermic and was accompanied by rearrangements that partially scrambled the ring hydrogens.