Roaming-Mediated CHNH Elimination from the Ionization of Aromatic Ethylamines.

The ionization of aromatic ethylamines by photons or electrons leads to elimination of CHNH fragments, supposedly deriving from the McLafferty rearrangement involving intramolecular γ-hydrogen transfer. Using tryptamine and phenethylamine as examples, the results reported here suggest that the McLafferty mechanism is inadequate for interpreting the observations of CHNH elimination due to much higher calculated appearance energy than experimentally measured values. Furthermore, by considering the roaming-mediated effect, the calculated appearance energy for the elimination of CHNH fragments is reduced and matches well with the experimental results and verifies the existence of roaming-mediated effect. This effect could potentially be extended to explain the general CHNH elimination of aromatic ethylamines. Due to the similar hydrogen transfer to that of the McLafferty mechanism, the roaming-mediated effect was taken into account to suggest a novel mechanism, termed the "roaming-modified McLafferty rearrangement", that explains the observations of CHNH elimination in the ionization of aromatic ethylamines. This is a reasonable modification of the McLafferty rearrangement mechanism.