Site of protonation of N-alkylanilines

The MH+ ions of N-methylaniline, N,N-dimethylaniline, N-ethylaniline, and N -N-diethylaniline, when prepared by CH, chemical ionization, show significa nt elimination of H-2, in both metastable ion and low energy collision-indu ced fragmentation. In contrast, the MH+ ions of the same species produced b y fast atom bombardment (FAB) ionization show no elimination of H-2, in eit her metastable ion or collision-induced fragmentation. Other metastable ion fragmentation reactions involve loss of CH, for the methylanilines and los s of C2H4 and C2H5 for the ethylanilines. The MH+ ions prepared using CH,OH or (CH3)(2)CO as Br phi nsted acid reagents also show H-2 elimination wher eas protonation using i-C4H9NH2 as reagent produces MH+ ions that show esse ntially no ii, elimination. It is concluded that FAB ionization leads to th e N-protonated species whereas protonation in the gas phase using CH4, CH3O H, or (CH3)(2)CO as reagents leads to substantial formation of the ring-pro tonated tautomer. Protonation of the ring is endothermic for i-C4H9NH3+ as reagent ion. Because N-protonation is thermodynamically favoured, the resul ts provide further evidence for kinetic control in gas-phase Br phi nsted a cid chemical ionization. (Int J Mass Spectrom 195/196 (2000) 33-43) (C) 200 0 Elsevier Science B.V.