VUV photoionisation of free azabenzenes: Pyridine, pyrazine, pyrimidine, pyridazine and s-triazine

Photoionisation mass spectrometry was used to obtain the fragmentation pathways of pyridine, pyridazine, pyrimidine, pyrazine and s-triazine molecules upon absorption of 23.0, 15.7 and 13.8 eV synchrotron photons. The ionic fragments observed vary from molecule to molecule, however C₂H₂⁺, HCN⁺and HCNH⁺ are common to all five molecules at the three photon energies. Furthermore, the presence of C₂H₂N₂⁺, C₃H₃N⁺ and C₄H₄⁺ in the spectra of some of the molecules suggests dissociation pathways via loss of HCN moieties. The respective parent cations, m/q=79, 80 and 81 have a greater yield at low photon energies when compared to the most intense fragment peak in each spectra. We recorded two of the fragment cation yields, as well as the parent photoion yield curves of pyridine, pyridazine, and pyrimidine in the 8–30 eV range. The formation of abundant cation fragments show a strong propensity of the molecules for dissociation after the absorption of VUV photons higher than 14 eV. The differences in relative fragment yields from molecule to molecule, and when changing the excitation energy, suggest significant bond rearrangements and nuclear motion during the dissociation time. Thus, bond cleavage is dependent on the photon energy deposited in the molecule and on intramolecular reactivity. With the aid of photoion yield curves and energy estimations we have assigned major peaks in the spectra and discussed their fragmentation pathways.