The NMR spectra of cyclic nitrones. 3. Effect of protonation and a hydrogen bond on the chemical shifts in the13c NMR spectra of derivatives of 3-imidazoline 3-oxide

Abstract

The effect of solvents on the13C NMR spectra of the cyclic nitrones of 3-imidazoline 3-oxide derivatives was studied. It was shown that in solvents capable of forming hydrogen bonds with the N-oxide group the signal for the nitrone carbon atom is shifted downfield by 1.5–2.5 ppm for solutions in chloroform and 5–9 ppm for solutions in methanol. The size of the shift depends on the substituent at position 1 of the imidazoline ring and decreases with increase in its accepting character. Analogous effects are observed during the formation of an intramolecular hydrogen bond with the oxygen atom of the nitrone group. During protonation of the nitrone group the downfield shift of the signal for the nitrone carbon atom amounts to 30–39 ppm.