The aim of the article was to demonstarte the efficiency of WURST pulses with the QCPMG pulse sequence. WURST pulses are shaped pulses wich allow a greater excitation bandwidth than conventional rectangular pulses at same power levels. QCPMG is a pulse sequence which creates the "spin-lock" effect, that is a slow decay of magnetization. This slow decay allows to substantially increase the S/N ratio compared to a single pulse sequence. We have here demonstrated that the "spin-lock" effect created by WURST pulses is essentially the same as for rectangular pulses, although WURST pulses are quite different in nature. This result is very important for applied use, as RF power is usually resource hungry; requires a lot of space and power.
14N nuclear quadrupole resonance frequencies have been measured in solid 2-pyridone, 3-hydroxypyridine, and 4-pyridone by 1H-14N nuclear quadrupole double resonance. Two slightly nonequivalent nitrogen positions are observed in solid 3-hydroxypyridine, whereas only one nitrogen position has been observed in 2-pyridone and 4-pyridone within the experimental resolution. Rather low 14N quadrupole coupling constants in pyridones are the consequence of the delocalization of the nitrogen lone pair electrons in the aromatic rings. Two different compounds have been obtained by crystallization of 4-pyridone from ethanol in a normal and in a dry atmosphere. The compound obtained in the dry atmosphere is identical to the commercial sample. The compound obtained in the normal atmosphere cannot be converted to the commercial polymorph by melting. It is thus not a polymorph of anhydrous 4-pyridone. The temperature coefficient of the 14N quadrupole coupling constant is negative in 3-hydroxypyridine and positive in 2- and 4-pyridone. Therefore, in 3-hydroxypyridine, molecular librations dominate the temperature variation of the quadrupole coupling constant, whereas in 2- and 4-pyridone, the changes in the hydrogen bonding interactions with temperature seem to give the dominant effect.
Substituted imidazole and benzimidazole often form polar hydrogen-bonded chains in the solid state. 2-Methylbenzimidazole is known to be a high-temperature organic ferroelectric. We have measured temperature dependences of 14N nuclear quadrupole resonance (NQR) frequencies and proton T1 in 2-methylbenzimidazole and 5,6-dimethylbenzimidazole. The NQR frequencies are assigned to amino and imino nitrogen positions. The NQR data exclude the possibility of proton two-site exchange in an N–H···N hydrogen bond. The activation energies for the methyl group hindered rotation are determined in both compounds. The present 14N NQR data are compared to the published 14N NQR data in solid substituted and coordinated imidazoles, in substituted benzimidazoles and in imidazole in the gas phase. A linear correlation between the two in-plane principal values of the quadrupole coupling tensor and the out-of-plane principal value of the quadrupole coupling tensor is observed in imidazole, ranging from the amino nitrogen position to the imino nitrogen position. Transition from the amino to the imino nitrogen position is determined on the correlation diagram. The correlation diagram can be used to quantitatively observe the asymmetry of the N–H···N hydrogen bond. A similar correlation diagram is proposed also for substituted benzimidazoles. The magnitudes of the principal values of the 14N quadrupole coupling tensor in ferroelectric 2-methylbenzimidazole show that the macroscopic ferroelectric ordering has in this compound a minor effect on the asymmetry of the N–H···N hydrogen bonds.