Perfluoro effects in 15N nuclear magnetic resonance spectroscopy. Part 1. Aryl and N-heteroaryl azides

Abstract

¹⁵ N N.m.r. spectroscopic results are reported for a range of covalent azides RN_αN_βN_γ(where R is perfluoroaryl, perfluoro- or chlorofluoro-pyridyl or -pyrimidinyl, or trimethylsilyl), and compared with literature values for related azides. The N_α and N_γ chemical shifts vary over a range of 80–100 p.p.m., and correlate with weak long-wavelength bands in the electronic spectrum that arise from magnetically active transitions (n→π* and π→π′*), the N_α,γ lines moving downfield as the bands move to lower energies. The N_β chemical shift, however, varies rather little from its value in the azide anion. The N_{α, γ} chemical shifts are sensitive to the known inductive and conjugative influences of the R group. Perfluorination increases the shielding, by increasing the energy of the paramagnetic circulation; the perfluoro-effect diminishing with distance from the fluoro-substituent, and with the extent to which n→π* circulations are important in the deshielding. Thus the patterns of chemical shifts are explained by the relationships of the frontier orbitals for rotation of charge in the magnetic field.