Determination of the 14N quadrupole coupling tensor and the 13C chemical shielding tensors in a single crystal of l-serine monohydrate

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Abstract
The 14N quadrupole coupling tensor for the NH3+ nitrogen nucleus in a single crystal of l-serine monohydrate was completely determined. The quadrupole coupling constant, and the asymmetry parameter, were evaluated to be e2Qq/h=1.069 MHz and η=0.214 at room temperature by measuring the angular variation of the proton-enhanced 14N NMR spectra about the three experimental axes. It was found that the direction of the largest principal value of the 14N quadrupole coupling tensor is at an angle of 3.5° to the direction of the C–N bond. The sign of the 14N quadrupole coupling constant was determined to be positive by analyzing the 13C–14N dipolar splitting pattern in the 13C NMR signal of the Cα carbon. The 13C chemical shielding tensors for three chemically different carbon nuclei, namely COO−, Cα, and Cβ were determined and assigned to the molecule in the unit cell by inspecting the 13C–14N dipolar interaction rather than relying only on the local symmetry. The most shielded direction of the COO− carbon is perpendicular to the O–C–O plane, and the least shielded direction is parallel to the Cα–C bond direction. It was found that the most shielded direction for the Cβ carbon is almost parallel to the Cβ–O bond direction.