Solvent Effect in NMR Spectroscopy. II. A Repulsive Interaction between the Benzene Solvent Molecule and the Nitrogen Lone-Pair, and the Effects Induced by the Addition of Protic Substances

The variations in the chemical shifts of proton resonances in imines, aziridines, and oximes, all of which have oriented nitrogen lone-pair electrons, on going from a carbon tetrachloride solution to a benzene solution (“benzene solvent shift”) are determined. Also determined are the changes in the corresponding proton chemical shifts in a benzene solution when protic substances, such as methanol or trifluoroacetic acid, are added (“addition shift”). It is generally found that the upfield benzene-solvent shift is more sensitive to the protons trans to the oriented nitrogen lone-pair than to the cis. This observation indicates that the formation of a benzene-solute collisional complex is most likely to occur at a site as far as possible from the nitrogen lone-pair. The repelling of a benzene solvent molecule by the nitrogen lone-pair may occur due to the repulsive interaction between lone-pair electrons and benzene π-electrons. It is also found that the addition shift is upfield and is larger for the proton trans to the nitrogen lone-pair than for that cis. These general observations are of predictive value in the determination of the orientation of the nitrogen lone-pair electrons and in the assignment of the signals of proton groups in the vicinity of the nitrogen lone-pair.