Nuclear Quadrupole Resonance (NQR) spectroscopy has been known for 70 years. It is suitable for the study of measured (poly)crystalline chemical compounds containing quadrupole nuclei (nuclei with spin I ≥ 1) where the characteristic NQR frequencies represent the fingerprints of these compounds. In several cases, ¹⁴N NQR can distinguish between the polymorphic crystalline phases of active pharmaceutical ingredients (APIs). In order to further stimulate ¹⁴N NQR studies, we review here several results of API polymorphism studies obtained in Ljubljana laboratories: (a) In sulfanilamide, a clear distinction between three known polymorphs (α, β, γ) was demonstrated. (b) In famotidine, the full spectra of all seven different nitrogen positions were measured; two polymorphs were distinguished. (c) In piroxicam, the ¹⁴N NQR data helped in confirming the new polymorphic form V. (d) The compaction pressure in the tablet production of paracetamol, which is connected with linewidth change, can be used to distinguish between producers of paracetamol. We established that paracetamol in the tablets of six different manufacturers can be identified by ¹⁴N NQR linewidth. (e) Finally, in order to get an extremely sensitive ¹⁴N NQR spectrometer, the optical detection of the ¹⁴N NQR signal is mentioned.

The ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ electric field gradient (EFG) and chemical-shift tensors have been determined in KSCN above and below ${\mathit{T}}_{\mathit{c}}$. Whereas all the data below ${\mathit{T}}_{\mathit{c}}$ are compatible with the site symmetry of the ${\mathrm{K}}^{+}$ ions given by the space group ${\mathit{D}}_{2\mathit{h}}^{11}$ (Pbcm) of the low-temperature orthorhombic phase, this is not the case for the EFG data above ${\mathit{T}}_{\mathit{c}}$. The symmetry of the paraelectric phase is broken on the quadrupole-coupling time scale but not on the chemical-shift-tensor time scale, allowing for an interesting insight into the dynamics of the ${\mathrm{SCN}}^{\mathrm{-}}$ head-tail disordering process. The transition is of the order-disorder type for the ${\mathrm{SCN}}^{\mathrm{-}}$ groups and of a displacive type for the ${\mathrm{K}}^{+}$ ions.

The ¹⁷O‐proton nuclear double‐resonance spectra of KH₃(SeO₃)₂ have been measured above and below the ferroelastic transition and the quadrupole coupling has been determined for all three chemically nonequivalent oxygen sites in the unit cell. The ¹⁷O‐proton magnetic dipolar coupling as well as the ¹⁷O quadrupole coupling data clearly show the presence of proton motion between two equilibrium sites in the O(2)–H‐ ‐ ‐O(2) bond which freezes out below T_c. The temperature dependence of ³⁰K quadrupole frequency has been as well determined.

The 1,3,4-thiadiazole derivatives (2-amino-1,3,4-thiadiazole, acetazolamide, sulfamethizole) have been studied experimentally in the solid state by ¹H–¹⁴N NQDR spectroscopy and theoretically by Density Functional Theory (DFT). The specific pattern of the intra and intermolecular interactions in 1,3,4-thiadiazole derivatives is described within the QTAIM (Quantum Theory of Atoms in Molecules)/DFT formalism. The results obtained in this work suggest that considerable differences in the NQR parameters permit differentiation even between specific pure association polymorphic forms and indicate that the stronger hydrogen bonds are accompanied by the larger η and smaller ν₋ and e²Qq/h values. The degree of π-electron delocalization within the 1,3,4-thiadiazole ring and hydrogen bonds is a result of the interplay between the substituents and can be easily observed as a change in NQR parameters at N atoms. In the absence of X-ray data NQR parameters can clarify the details of crystallographic structure revealing information on intermolecular interactions.