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Gas‐phase Protonation of Pyridine. A Variable‐time Neutralization–Reionization and Ab Initio Study of Pyridinium Radicals
Author(s) -
Nguyen Viet Q.,
Tureček František
Publication year - 1997
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/(sici)1096-9888(199701)32:1<55::aid-jms447>3.0.co;2-m
Subject(s) - chemistry , protonation , radical , deuterium , pyridine , pyridinium , reionization , proton affinity , endothermic process , ion , photochemistry , medicinal chemistry , ionization , organic chemistry , physics , adsorption , quantum mechanics
Gas‐phase protonation of pyridine with CH 3 NH 3 + , NH 4 + , t ‐C 4 H 9 + , H 3 O + and CH 5 + under thermal conditions was studied by variable‐time neutralization–reionization mass spectrometry and ab initio calculations. N‐Protonation was found to occur exclusively for CH 3 NH 3 + through H 3 O + and predominantly for CH 5 + . 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. © 1997 by John Wiley & Sons, Ltd.