Studies in mass spectrometry. Part XXXIII. Hydrogen scrambling in methylpyridines and quinoline

Abstract

Deuterium-labelling of 2-methylpyridine and 4-methylpyridine suggests that at all beam energies complete (or almost complete) hydrogen scrambling occurs in the molecular ions before expulsion of HCN. In contrast, at high beam energies loss of CH₃ largely occurs before hydrogen scrambling, but at energies close to the appearance potential a substantial fraction of M⁺–CH₃ ions are formed after hydrogen scrambling. The results are compatible with a relatively low energy of activation and low frequency factor for hydrogen scrambling, with the loss of HCN at 70 eV occuring on average from molecular ions with longer lifetimes than the molecular ions which are the precursors of M⁺–CH₃ ions. Quinoline molecular ions which expel HCN after approximately 5–15 µsec. have undergone complete hydrogen scrambling.