Fragmentation of 1,4,5,8-tetraazaphenanthrenes upon electron impact: A new example of structural integrity of gas phase heteroaromatic cations

1,4,5,8-Tetraazaphenanthrene and some derivatives have been studied by mass and ion kinetic energy spectrometries. Deuterium labelling experiments show an unexpected structural integrity of the gas phase molecular ions which eliminate H(D)CN, even near threshold energies (field-free region reactions). The loss of N(1(8)) predominates largely over the loss of N(4(5)); the consecutive losses of H(D)CN are also highly specific. A similar specificty is observed for the chlorinated and methylated isomers. The atoms of the homocycle are not involved in the first steps of the fragmentation as indicated by the behaviour of 9-chloro-and 9-methyl-tetraazaphenanthrenes.