Perfluoro effects in 15N nuclear magnetic resonance spectroscopy. Part 2.—π-Fluoro and π-chloro effects in azabenzenes

Abstract

The nuclear magnetic shielding of nitrogen in pyridine, pyrimidine and s-triazine is increased considerably on fluorination and to a lesser extent on chlorination of the ring, the “π-fluoro effects” corresponding to those observed in electronic and photoelectron spectroscopy. Halogeno-substitution stabilises the σ orbital manifold relative to the π, which is little affected because of opposed σ and π inductive effects. Thus the nitrogen shielding tends to decrease with decrease in ΔE(n_N→π^*) and in the ionisation energy of the highest n_N orbital. Increasing 1,3-aza-substitution in hydrogeno-, perchloro- or perfluoro-pyridines increases the nitrogen shielding, but nitrogen is deshielded by 1,4-aza-substitution and deshielded strongly by 1,2-aza-substitution, with increase in overlap of the n_N orbitals, raising the h.o.m.o. (highest occupied molecular orbital), changes in the π^* l.u.m.o. (lowest unoccupied molecular orbital) being estimated from literature values of the electron affinities or the ab initio SCF orbital energies.