Nitrogen NMR assessment of interactions between nitroso, cyano and acetyl groups across aliphatic systems

Abstract

AbstractThe nitrogen NMR shieldings (Δσ, equivalent to —Δδ on the frequency scale of chemical shifts) of the NO group in nitrosoalkanes are shown to be a sensitive probe to electron‐withdrawing effects of substituents and to steric strain effects. The former produce high‐field (low‐frequency) shifts, which can amount to about 90 ppm, while the latter result in considerable low‐field (high‐frequency) shifts of the NO nitrogen resonance. The CN nitrogen shieldings show less pronounced low‐field (high‐frequency) shifts under the influence of electron acceptors. The signs and magnitudes of the observed effects are explained in terms of semi‐empirical molecular orbital calculations of the shieldings and molecular mechanics evaluations of strain energies. The overall picture of relationships between the NO and CN nitrogen shieldings and electron charge migration within the molecules concerned is in accord with that obtained in earlier studies of solvent‐induced effects on the nitrogen shieldings in alkyl cyanides and nitro compounds.