P-31-NMR STUDY OF POWDER AND SINGLE-CRYSTAL SAMPLES OF AMMONIUM DIHYDROGEN PHOSPHATE - EFFECT OF HOMONUCLEAR DIPOLAR COUPLING

A P-31 NMR study of powder and single-crystal samples of ammonium dihydrogen phosphate (ADP), NH4H2PO4, was undertaken in order to resolve the apparent discrepancies in the literature concerning the local symmetry at the phosphorus nucleus of the H2PO4 moiety. In two independent P-31 NMR studies of solid dihydrogen phosphates, evidence has been presented which indicates that the phosphorus chemical shift tensor of ADP is nonaxially symmetric, in contrast to expectations based on the accepted crystal structure from X-ray and neutron diffraction experiments. In analyzing the single-crystal P-31 NMR results presented here, it is essential to consider the influence of homonuclear P-31-P-31 dipolar interactions. The analysis indicates that the phosphorus chemical shift tensor is axially symmetric, delta11 = delta22 = 12 ppm and delta33 = -20 ppm, consistent with the expectations based on the known crystal structure. The unique component of the phosphorus chemical shift tensor is oriented along the c axis of the tetragonal crystal. The present study also demonstrates the dangers of analyzing magic-angle spinning (MAS) NMR spectra of powder samples if the nuclei under consideration are strongly dipolar coupled to neighboring spins.