Determination of the 14N quadrupole coupling tensors and the 13C chemical shielding tensors in a single crystal of L-asparagine monohydrate

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Abstract
The 14N quadrupole coupling tensors for the Nα and Nδ nitrogen nuclei in a single crystal of l-asparagine monohydrate were completely determined using proton enhanced 14N NMR. The quadrupole coupling constants and asymmetry parameters were evaluated to be e2Qq/h=1.152 MHz and η=0.220 for the Nα nitrogen nucleus, and e2Qq/h=−2.677 MHz and η=0.337 for the Nδ nitrogen nucleus. It was found that the direction of the largest absolute principal value of the quadrupole coupling tensor for the Nδ nitrogen nucleus was perpendicular to the NH2 plane (sp2 plane) and that for the Nα nitrogen nucleus was parallel to the Cα–Nα bond. The 13C chemical shielding tensors for all four chemically different nuclei (Cα, Cβ, Cγ, and COO−) were determined and assigned to the molecule in the unit cell on the basis of the 13C–14N dipolar splittings. It turned out that the directions of the most shielded value of the chemical shielding tensors for the Cγ and COO− carbon nuclei were perpendicular to their sp2 plane, and that of the least shielded value of the chemical shielding tensor for the COO− carbon nucleus was parallel to the Cα–COO− bond. The direction of the intermediate value of the Cγ chemical shielding tensor makes an angle of 7.7° with the Cγ–Oδ bond. The lengths of the Cα–Nα and Cγ–Nδ bonds were calculated to be 1.513 and 1.364 Å, respectively, by analyzing the 13C–14N dipolar coupling tenors. These values agree well with those determined by neutron diffraction studies.