13C–13C dipolar spectra in solids

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Abstract
A single crystalNMR study has been made of the 13C–13C dipolar interaction in oxalic acid dihydrate (HOX) and diammonium oxalate monohydrate (DAOX). The results show that indirect contributions to the interaction are negligibly small so that the dipolar tensor provides information on the C–C bond length and orientation in the crystal lattice. In HOX the relative orientations of the principal components of the chemical shift and dipolar tensors confirm the unusual orientation of the shift tensor components with respect to the molecular coordinate system. Because of the nonplanar structure of the oxalate ion in DAOX, the ratio between the chemical shift difference of the two 13C atoms in one molecule and the dipolar splitting can be varied by a suitable choice of crystal orientation. As a consequence, the 13C–13C spectra exhibit A2 and AB patterns.