NITROGEN NMR SHIELDINGS OF NITROALKANES AS A STRUCTURAL AND CONFORMATIONAL PROBE

Nitrogen NMR shieldings (Delta sigma = -delta on the frequency scale o f chemical shifts), measured by high-precision N-14 NMR and bulk susce ptibility corrected, of 20 nitroalkanes in dilute solutions in cyclohe xane and in acetone are reported, including vic-dinitro, gem-dinitro a nd trinitro structures and tetranitromethane. The data obtained for cy clohexane solutions yield an excellent linear correlation with theoret ical shieldings calculated for the corresponding isolated molecules by the semi-empirical TNDO/2 method. The NO2 nitrogen shieldings of nitr oalkanes are shown to depend linearly on the net charges of the corres ponding nitro groups, each taken as a whole, but there is no correlati on with the atomic charges of the nitrogen atoms concerned. Thus, the nitrogen shieldings seem to provide a measure of electron-acceptor or electron-donor strength of the substituted alkyl system bound directly to the nitro group. Solvent polarity effects on the shieldings are di scussed. The nitro group nitrogen shieldings are shown to be generally sensitive to conformational effects, particularly in sterically hinde red molecules, and to provide insight into conformational or rotamer e quilibria in nitroalkane systems. Ab;initio calculations of the shield ings of some simple nitroalkanes, using the Hartree-Fock/GIAO approach , are reported. They indicate that the semi-empirical method employed, combined with the experimental data, yields results which are compara ble to those obtained by the ab initio method, as far as relative shie ldings are concerned.