Abstract The isomeric pairs [CH 3 CN]+ and [CH 2 CNH]+ and [CH 3 NC]+ and [CH 2 NCH]+ have been established as stable, noninterconverting structures. The conclusion derives from studies of collision induced decomposition spectra. The same conclusion pertains for the ions [CH 3 CH 2 CN]+ and [CH 3 CHCNH]+, and for [NCCH 2 CH 2 CN]+,[HNCCHCH 2 CN]+ and [HNCCHCHCNH]+. The energy barrier of a [1, 3]-hydrogen shift, a possible ...