SolventInduced Effects on the Nitrogen NMR Shieldings of Some Nitrosobenzene Systems

Abstract

High-precision 14N NMR measurements of solvent-induced shielding variations are reported for some nitrosobenzene systems. These variations are shown to result from a combination of three major factors, solvent to solute hydrogen bonding where the solute nitrogen lone pair electrons are involved, solvent polarity and interactions between the electron-deficient benzene ring of the nitrosobenzenes and basic centres in the solvent molecules. The last of these three factors produces nitrogen deshielding of the nitroso group, and in the present work this interaction was found to be the largest of its type so far observed. Consequently, this implies that, in nitroso aromatic compounds, the benzene ring shows a remarkable deficit of electronic charge. The former two factors produce an increase in nitroso nitrogen shielding, thus indicating a strong electron-withdrawing effect of the nitroso group which is consistent with previous observations. INDO/S parameterized molecular orbital calculations of solute nitrogen shieldings, incorporating the Solvaton model of non-specific solute–solvent interactions, predict that the nitrogen shielding will increase as the polarity of the medium increases. This is in very good agreement with the observation that the nitroso nitrogen shielding analysis yields a large and positive value for the s term which describes the influence of solvent polarity/polarizability on the shielding variation as a function of solvent. For the 0.2 M solutions studied at 35°C, a significant amount of the dimeric isodioxy form is only observed for o-nitrosotoluene. © 1997 by John Wiley & Sons, Ltd.