15N chemical shift tensors in L-histidine hydrochloride monohydrate were determined from orientation-dependent studies of single crystals. In contrast to a previously reported case of a nitrogen in a heterocyclic ring, the imidazole ring nitrogen tensors in the present compound are of moderate size (both about 230 ppm). It is suggested that this is due to the fact that both nitrogens are protonated. The principal components of the tensors are oriented approximately along the molecular symmetry directions of the ring. The least shielded element σ33 lies approximately along the NH bond direction, and the most shielded element ς11 is perpendicular to the plane of the imidazole ring. The NH3+ tensor is rather small (<12 ppm). The data suggest that high-field solution 15N NMR experiments will not be plagued with shift anisotropy broadening, and for similar reasons high-field magic angle spinning spectra of solids will not exhibit inordinate numbers of rotational sidebands. A comparison of the relative sizes of a variety of 15N shift and the corresponding 14N quadrupole tensors is made, and an anomaly with regard to NH3+ groups is pointed out.
ASJC Scopus subject areas
- Colloid and Surface Chemistry