The oxygen-18 isotope shift in nitrogen-15 NMR spectroscopy. 3. Effects of structure and solvent

Abstract

Various /sup 15/N, /sup 18/O-labeled compounds were synthesized and the /sup 18/O-induced isotope shifts on their /sup 15/N NMR spectra were measured in order to examine the effects of structural changes on the magnitudes of the shifts. The measured shifts vary greatly, ranging from 0.027 ppm for a nitrile oxide to 0.159 ppm for an isoxazole. The large isotope shifts of isoxazoles compared to oximes and isoxazoline are attributed to the aromatic nature and shorter N-O bond lengths of the isoxazoles. The nature of substituents in the para position of some aromatic aldehyde and ketone oximes affects the magnitude of the /sup 18/O-induced shift. Intramolecular hydrogen bonding significantly decreases the magnitude of the isotope shift in oximes. A study of the effect of solvents on the isotope shifts of three oximes is made. A significant decrease in the isotope shift is observed in solvents having an electron donor atom such as oxygen or nitrogen. This decrease in the magnitude of the isotope shift is ascribed to the formation of hydrogen bonds between the oxime hydroxyl proton and the oxygen or nitrogen of the solvent. Possible applications of the /sup 18/O isotope in /sup 15/N NMR are briefly discussed.