Nitrogen-15 nuclear magnetic resonance spectroscopy. Solvent effects on the 15N chemical shifts of saturated amines and their hydrochlorides

Abstract

Solvent effects were determined for the 15N NMR chemical shifts of some primary, secondary, and tertiary amines along with solvent, concentration, and counterion dependences of the corresponding amine salts. The solvent effects on tertiary-amine nitrogen resonances appear to be dominated by hydrogen bonding to the lone pair, which results in downfield shifts. For primary and secondary amines, the situation is much more complex. Hydrogen bonding to the lone pair is apparently important, as indicated by a linear correlation of shifts produced by going from nonpolar solvents to methanol as solvent and shifts produced by protonation. The very large solvent and counterion effects sometimes observed for the 15N chemical shifts of the conjugate acids of saturated amines seem best explained by ion-pair aggregation in relatively nonpolar solvents like chloroform, and especially with “soft” polarizable anions like iodide. Solvation of the salts in polar solvents or changes of anions expected to cause less ion aggregation result in upfield shifts of the nitrogen resonances, thereby decreasing the magnitude of generally downfield protonation shifts in nonpolar solvents. For some amines, this solvent effect may be so large as to cause protonation of a saturated amine to result in an upfield rather than the usually expected downfield shift.