Carbon-13 nuclear magnetic resonance in solid ammonium tartrate

Chemical shielding tensors have been determined for 13C in single crystals of ammonium D‐tartrate using proton‐enhanced NMR. The unit cell contains two molecules, and in some orientations eight lines are resolved from the eight magnetically inequivalent carbon nuclei. Details of the experimental approach, equipment, and analysis are provided, and results are presented for three determinations on two single crystals. These indicate a precision of ±2 ppm in the determination of the principal values and ±3° in the orientation of the principal axes for the carboxyl groups, with lower precision for the hydroxyl groups. The orientations of the principal axes of the tensors are related to the crystallographic (a, b, c*) axes by employing orientation data from x‐ray precession work in this study. The most plausible assignment of the tensors to particular 13C nuclei in the unit cell indicates that the heaviest shielding for 13C in the carboxyl groups occurs when H0 is perpendicular to the carboxyl plane and the li...