We present advances on the study of a nuclear magnetic resonance technique in the rotating frame. The method, based on the spin-lock experiment, allows to get information on the residual local fields in molecular organized materials. The magnetization after a certain lock-time is studied as a function of the radio-frequency field atenuation. The obtained dispersion curve, within a frequency interval ranging from a few kHz to about 50 kHz, contains information about the residual local field along the external magnetic field. The interpretation of the results is consistent and comparable with those obtained by other methods.

Se presentan avances del estudio de una técnica experimental de resonancia magnética nuclear en el sistema rotante. Esta técnica, basada en el experimento de anclaje de espín, permite obtener informacion sobre la magnitud de los campos locales residuales en materiales con organización molecular. La magnetización, luego de un cierto tiempo de anclaje, es estudiada en función de la atenuación del campo de radiofrecuencia. La curva de dispersión obtenida, en un intervalo desde unos pocos kHz hasta aproximadamente 50 kHz, contiene información de la componente residual del campo local a lo largo de la dirección establecida por el campo magnético externo. El análisis de los resultados arroja valores consistentes y comparables con aquéllos obtenidos mediante otros métodos.

Nuclear Quadrupole Resonance studies in Nematic and Smectic C thermotropic liquid crystalline mesophases of p-heptyl azoxyanizole are reported. The local molecular microstructure is modeled from NQR spectra. In the Smectic C mesophase, the NQR data is explained by assuming two features: i) biaxiality in the electric field gradients at the sites occupied by 14N nuclei, and ii) the elemental unit cell of the mesophase is composed by two HpAb molecules. These bimolecular units coexist together with single molecules and the relative proportions change with temperature, the quantity of individual molecules and the relative proportions change with temperature, the quantity of individual molecules being predominant as the temperature approaches the Nematic phase transition. At the lowest temperatures in the Nematic mesophase the NQR spectrum behaves similarly to the corresponding at low temperatures in the SmC mesophase; this means that we can propose again the existence of a huge number of bimolecular and biaxial unit cells. At higher temperatures the transition to single molecular units is deduced from NQR spectra. That transition is characterized by the one describing the passage from a partially disordered molecular system -the system is conserving some local anisotropic properties of symmetry from the crystalline solid state- to a partially ordered system that resembles an oriented liquid.