Nitrogen-15 magnetic resonance spectroscopy. XIV. Natural-abundance nitrogen-15 chemical shifts of amines

It has been found possible to measure the shifts of a variety of amines to an accuracy of 0.2 ppm with the isotope at its natural-abundance level. Successful observation of these resonances with our existing equipment is dependent on a favorable nuclear Overhauser effect arising from irradiation of the protons. Chemical exchange of protons directly bound to the amine nitrogen can reduce the signal intensity sufficiently to make detection of the ^(15)N resonance difficult or impossible. The ^(15)N shifts generally correlate linearly with the ^(13)C shifts of alkanes derived by replacing the amine nitrogen with an appropriately substituted carbon. Detailed examination leads to deshielding α and β substituent shift parameters, a shielding y effect, and possible deshielding associated with hydrogen bonding. Effects of constraints in geometrically rigid systems as well as incorporation of the nitrogen into six-, five-, and three-membered rings are discussed and compared with corresponding ^(13)C shifts. Parallel trends were found for all cases. Natural-abundance ^(15)N spectroscopy is thus a practicable, if far from routine, means of structural analysis.