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.