NITROGEN NMR ASSESSMENT OF INTERACTIONS BETWEEN NITROSO, CYANO AND ACETYL GROUPS ACROSS ALIPHATIC SYSTEMS

The nitrogen NMR shieldings (Delta sigma, equivalent to Delta delta on the frequency scale of chemical shifts) of the NO group in nitrosoalk anes 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 o f the NO nitrogen resonance. The CN nitrogen shieldings show less pron ounced low-field (high-frequency) shifts under the influence of electr on accepters. The signs and magnitudes of the observed effects are exp lained in terms of semi-empirical molecular orbital calculations of th e shieldings and molecular mechanics evaluations of strain energies. T he overall picture of relationships between the NO and CN nitrogen shi eldings and electron charge migration within the molecules concerned i s in accord with that obtained in earlier studies of solvent-induced e ffects on the nitrogen shieldings in alkyl cyanides and nitro compound s.