14N nuclear magnetic resonance: amines and ammonium ions

The 14N nuclear magnetic resonance (n.m.r.) signals of alkylamines and ammonium ions are shifted to lower magnetic fields with the increasing number of alkyl groups at the carbon atom directly bonded to the nitrogen atom, and at the nitrogen atom itself. The resonance of an ammonium ion always occurs at a field lower than that for the corresponding amine. It seems that high-lying excited states, not only those corresponding to lowest-wavelength transitions, are important for an explanation of the observed resonance shifts. A downfield shift of the 14N resonance and a substantial increase in signal width, as compared with the spectra of neat amines, are observed for aqueous solutions of amines. There is a very characteristic chemical shift for the R—CH2—NH2 molecules (R = alkyl, phenyl, vinyl) in the 14N n.m.r. spectra.