Carbon-13 nuclear magnetic resonance in solid ammonium tartrate

Abstract

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 lightest shielding when H0 is in the carboxyl plane bisecting the C-O bonds. For the hydroxyl 13C, the heaviest shielding occurs with H0 roughly along the C-O bond. The average principal shielding values are, for the carboxyl 13C, σ11 = -110.5, σ22 = -61.7, σ33 = 21.6; and for the hydroxyl 13C, σ11 = 41.9, σ22 = 48.0, σ33 = 74.0 in parts per million (±2) relative to an external reference of liquid benzene. The effects of hydrogen bonding on the tensors are discussed briefly. Copyright © 1974 American Institute of Physics.