We have analyzed the possibilities of dynamically polarizing proton spin system via the quadrupole 14N spin system. The increase of the proton magnetization was calculated. The proton polarization rate was found to be related to the transition probabilities between the 14N NQR and 1H NMR energy levels. The experiments performed in 1,3,5-triazine confirmed the results of the theoretical analysis. We proposed a new double resonance technique for the measurement of NQR frequencies of the order of 100 kHz and lower.
The complete 14N NQR spectra have been measured in the two polymorphic crystalline phases of the molecular complex isonicotinamide oxalic acid (2:1) by nuclear quadrupole double resonance. The observed NQR parameters have been assigned to the two nitrogen positions (ring and amide) in a molecule and analyzed in a model, where it is assumed that an additional electric charge on the nitrogen atom changes the NQR parameters. The model suggests that this additional electric charge is negative so that the N H O hydrogen bond seem to be partially ionic, of the type N...H-O.
The 1,3,4-thiadiazole derivatives 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 suggest that considerable differences in the NQR parameters permit differentiation even between specific pure association polymorphic forms and indicate that the stronger hydrogen bonds.
We have demonstrated the detection of the N-14 nuclear quadrupole resonance signal in an experimental setup, where the sample occupies the space between the capacitor plates instead of the coil as usual. The new technique may be useful fro the detection of the NQR signal from thin surface layers.
Nitrogen atoms are present in a number of solid explosives and illicit substances. The nuclear quadrupole resonance (NQR) spectra and spin–lattice relaxation of the nitrogen atomic nucleus 14N can be used to characterize these compounds and to distinguish between possible crystal polymorphs. After the characteristic 14N NQR frequencies and spin–lattice relaxation rates in a compound are determined, NQR can be used to detect this compounds and, in case of crystal polymorphs, also to determine the method of preparation. The 14N NQR frequencies and spin–lattice relaxation rates are measured either by pulse NQR or by nuclear quadrupole double resonance (NQDR) based on magnetic field cycling. Here, we discuss several 1H–14N NQDR techniques which can be used to measure the 14N NQR frequencies and spin–lattice relaxation rates under various experimental conditions. Some characteristic applications of these techniques are presented and discussed in details.