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

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 C_α carbon. The ¹³C chemical shielding tensors for three chemically different carbon nuclei, namely COO⁻, C_α, and C_β 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 C_α–C bond direction. It was found that the most shielded direction for the C_β carbon is almost parallel to the C_β–O bond direction.