Substituent effects of halopyridines

The electronic structures of a number of halopyridines were measured by HeI photoelectron (PE) spectroscopy. Combined the empirical arguments and theoretical methods, the observed PE bands were interpreted. The careful analysis of measured π-orbital and halogen lone pair ionization energies enable us to describe substituent effects in terms of inductive, resonance, and spin-orbit coupling interactions. The Natural Population Analysis (NPA) has been also used to illuminate the substituent effects between halogen atoms with the parent ring.