Solvent induced variations in nitrogen NMR shieldings of some oxime systems as a test of the Solvaton model of non-specific molecular interactions

Abstract

High precision ¹⁴N NMR measurements are reported for three oxime systems in a variety of solvents. Both hydrogen bonding and solvent polarity effects are found to make significant contributions to the observed range of nitrogen shieldings. For a non-conjugated oxime moiety the solvent induced nitrogen shielding variations are found to be of opposite sign to those found for oximes which have a strongly conjugated carbonyl group. These shielding variations are interpreted in terms of solute–solvent specific and non-specific interactions. The analysis of the shielding data results in the view that, in these two cases of carbonyl group conjugation, there are opposing directions of electronic charge migration as a function of an increase in solvent polarity. There appears to be a change in the preference of the solute hydrogen bond acceptor site from the oxime to the carbonyl moiety in the case of p-benoquinone mono oxime derivatives. Medium polarity effects on the solute nitrogen shielding are adequately accounted for by INDO/S parametrised molecular orbital calculations of nitrogen shieldings as a function of solvent relative permittivity (ε) by means of the Solvaton model. This model describes satisfactorily the effects of a change in solvent polarity on the nitrogen shielding of a variety of nitrogen containing solutes.