Ionisation energies of pyridine N-oxides determined by photoelectron spectroscopy

Abstract

High resolution HeI photoelectron spectra of twelve pyridine N-oxides are presented. The exceptional separation of the first four, or more, bands throughout the series allows assignments of these in relation to the proposed scheme for pyridine N-oxide. These are discussed in view of the vibrational detail discernible, variations in the relative intensities of the bands with HeII and HeI radiation, especially in the chloro derivatives, and substitution trends. The correlation of the assignments affords a probe for substituent effects on the ground and first two excited states from the well defined changes in the corresponding ionisation energies. The latter are considered in terms of dipole perturbations due to the substituents. The linear regressions observed for the para-substituted derivatives between the first three ionisation energies and dipole moments, basicities and reactivity constants are discussed.