Lowest Ionization Potentials of Some Nitrogen Heterocyclics

The photionization potentials of the highest occupied orbital of pyrazine (9.29 eV), pyrimidine (9.35 eV), pryidazine (8.71 eV), naphthalene (8.15 eV), quinoline (8.62 eV), isoquinoline (8.55 eV), quinoxaline (9.02 eV), cinnoline (8.95 eV), and phthalazine (9.22 eV) are determined. In addition, estimates of the second ionization potentials of cinnoline (∼9.1 eV) and phthalazine (∼9.3 eV) are obtained from the analyses of the spectral characteristics of the photoionization yield curves. A discussion of the observed and estimated values of the ionization potentials in terms of the type of the ionized electron ($\pi$ vs nonbonding) is given. It seems that for all the molecules studied, except pyridazine and phthalazine, the $\pi$ electron has a lower ionization potential than the nonbonding electron. The interchange of the type of the ionized electron in pyridazine and phthalazine is caused mainly by the strong repulsive interaction between the two lone pairs of electrons on the adjacent nitrogen atoms in these molecules as well as by the stabilization of the $\pi$ electrons in these systems.