Fragmentation reactions of alkylphenylammonium ions

Abstract

The fragmentation reactions of a variety of alkylphenylammonium ions, C6H5NH3 −nRn+ (n ≥ 1, R = CH3, C2H5, i-C3H7, n-C4H9) were studied by energy-resolved mass spectrometry. Ionization was by fast atom bombardment (FAB) or electrospray ionization. Energy-resolved fragmentation data were obtained by low-energy collision-induced dissociation (CID) in the quadrupole cell of a hybrid sector/quadrupole instrument following FAB ionization and by cone-voltage CID in the interface region of the electrospray/quadrupole instrument. A comparison of the two methods of obtaining energy-resolved data showed that very similar results are obtained by the two methods. The fragmentation reactions of the alkylphenylammonium ions are rationalized in terms of competitive formation of an [R+—NC6H5H3−nRn−1] complex or a [C6H5H3−nRn−1N+·—·R] complex. The former complex fragments by internal proton transfer to yield C6H5H3 −nRn −1NH+ and [R −H] whereas the latter complex fragments to form C6H5H3 −nRn −1N+ and an alkyl radical. Alkane elimination, which is very prominent for tetraalkylammonium ions, most likely involves sequential elimination of an alkyl radical and either an H atom or an alkyl radical for the phenyl-substituted ammonium ions. Copyright © 1999 John Wiley & Sons, Ltd.