Intramolecular hydrogen exchanges during the electron impact‐induced fragmentation of complex alicyclic amines

The three‐step mechanism of formation of the important fragment ion of mass 84 from 3‐dimethylaminosteroids under electron impact is accompanied by intramolecular hydrogen exchanges, as shown by the mass spectra of deuterium labelled derivatives. The hydrogen exchanges are shown to occur in the ring A opened molecular ion, i.e. after the initial C‐3–‐C‐4 bond rupture, and to compete with the final C‐1–‐C‐10 bond rupture. (The intensity of hydrogen‐deuterium exchange reactions depends upon the activation energy of the final rupture.) The electron impact‐induced‐fragmentation of large alicyclic molecules bearing a strong charge stabilizing functional group, such as amines, appears to be the result of a series of displacements of the radical over different sites of the molecular ion through hydrogen (or carbon) rearrangements, with some of the radical sites being in the appropriate position to induce a final rupture.