Abstract
The MH + ions of N-methylaniline, N,N-dimethylaniline, N-ethylaniline, and N-N-diethylaniline, when prepared by CH 4 chemical ionization, show significant elimination of H 2 in both metastable ion and low energy collision-induced fragmentation. In contrast, the MH + ions of the same species produced by fast atom bombardment (FAB) ionization show no elimination of H 2 in either metastable ion or collision-induced fragmentation. Other metastable ion fragmentation reactions involve loss of CH 3 for the methylanilines and loss of C 2H 4 and C 2H 5 for the ethylanilines. The MH + ions prepared using CH 3OH or (CH 3) 2CO as Brønsted acid reagents also show H 2 elimination whereas protonation using i-C 4H 9NH 2 as reagent produces MH + ions that show essentially no H 2 elimination. It is concluded that FAB ionization leads to the N-protonated species whereas protonation in the gas phase using CH 4, CH 3OH, or (CH 3) 2CO as reagents leads to substantial formation of the ring-protonated tautomer. Protonation of the ring is endothermic for i-C 4H 9NH 3 + as reagent ion. Because N-protonation is thermodynamically favoured, the results provide further evidence for kinetic control in gas-phase Brønsted acid chemical ionization.
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