Steric and Electronic Effects on ^(15)N Chemical Shifts of Saturated Aliphatic Amines and Their Hydrochlorides

Abstract

Natural-abundance ^(15)N NMR chemical shifts of saturated aliphatic primary, secondary, and tertiary amines and their hydrochlorides have been measured in different solvents. Good linear correlations of these ^(15)N shifts with the ^(13)C shifts of carbons in the same positions in the corresponding hydrocarbon analogues were only found for the primary compounds. The degree of correlation between ^(l5)N and ^(13)C shifts decreased successively with secondary to tertiary amines and their hydrochlorides. Despite this, sets of solvent-dependent additive shift parameters were derived which give reasonably satisfactory agreement between calculated and experimental shifts of all of the amines and hydrochlorides. Some of the substituent-induced shifts appear to be conformational effects and can be compared with substituent effects observed previously for cyclic amines. The generally downfield protonation shifts could also be correlated with empirical substituent parameters.

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