The structures of nitronate salts in solution as determined by 13C and 1H n.m.r. spectroscopy

Abstract

The ¹³C and ¹H n.m.r. spectra of a series of nitroalkanes and their nitronate anions in the solvents dimethyl sulphoxide and methanol are recorded and assigned. In methanol, the resonance of the α-carbon atom of nitronates appears at ca. 10 p.p.m. to lower field than in dimethyl sulphoxide. This observation is explained by postulating a solvent-dependence of charge distribution in nitronate anions, such that in methanol, hydrogen-bonding causes a higher electron density to reside on the oxygen atoms of the anions. This supports a similar suggestion previously made using other spectroscopic techniques. The influence of temperature and the nature of the metallic counterion on the ¹³C n.m.r. spectra of the nitronates is described. A deshielding effect of the nitronate grouping on neighbouring protons is used to deduce the preferred conformation of nitronate anions. The axial conformation is favoured for 2-methyl- and 2-phenyl-cyclohexane nitronate anion.