Electron impact ionization succeeded by a twofold 1,5-H-shift causes fission
of the double bond in stilbenes with a β-aminoethyl-(1) or a β-phenylethyl-(4)
sidechain in o-position leading to ions at m/z 278 and m/z 253, respectively.
Deuteriation and analysis of metastable ions reveal formation of indene cations at
m/z 175 after benzylic cleavage of the side chains in 1 and 4. The McLafferty
fragment of 1 cyclizes to an indanyl radial-cation which is the precursor of the
dimethoxybenzyl cation at m/z 151.