High resolution 14N NMR in solids: Single crystals of ammonium hydrogen oxalate hemihydrate

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Abstract
Proton decoupled 14N NMR spectra of ammonium hydrogen oxalate (AHOX) single crystals have been employed to determine the electric field gradient tensor at the 14N sites. Despite the 99.4% abundance of 14N, the spectral lines are narrow (∼300 Hz) and show two inequivalent 14NH4+ ions with (e2qQ/h) =130 kHz, η=0.39 and (e2qQ/h) =350 kHz, η=0.27. On the basis of motional averaging arguments, we surmise that these nonvanishing field gradients arise primarily because of lattice distortions of the N–H distances. The results indicate that it should be relatively easy to obtain 14N spectra in single crystals, and thus to examine a number of phenomena which have heretofore been inaccessible.