A rapid determination of nuclear quadrupole resonance frequencies using field-cycling magnetic resonance and frequency modulated RF excitations.

A modification of Slusher-Hahn's double resonance technique is described and experimentally tested. It is based on application of multiple frequency sweeps and can be used for a rapid location of nuclear quadrupole resonance (NQR) frequencies. The resolution of the present technique is relatively low but, when the NQR frequencies are located, it is easy to use either the Slusher-Hahn's technique or pulse NQR to determine the NQR frequencies with a higher precision.

¹H-²H cross-relaxation study in a partially deuterated nematic liquid crystal.

A detailed study of the cross-relaxation effects between the ¹H and ²H spins systems is presented in the nematic phase of a 5-cyanobiphenyl (5CB) liquid crystal, partially deuterated at α position (5CB-αd2). The proton spin-lattice relaxation time was measured at a frequency range from 5 kHz to 100 MHz at a temperature 5 K below the nematic-isotropic phase transition. In the low frequency domain, the spin-lattice relaxation rate (T1⁻¹) dispersion clearly differs from that of the fully protonated 5CB homologue. At two distinct frequencies, T1⁻¹ presents two distinct local maxima and for low frequencies T1⁻¹ presents a stronger frequency dependence when compared with what is observed for 5CB. The T1⁻¹ dispersion obtained for 5CB-αd2 for frequencies above 60 kHz was interpreted in terms of the relaxation mechanisms usually accepted to interpret the spin-lattice relaxation in nematic phases in general and 5CB in particular. For lower frequencies it was necessary to consider cross-relaxation contributions between the proton and deuterium reservoirs. A detailed model interpretation of the deuterium quadrupolar dips with respect to the proton-spin relaxation is presented. The analysis of the quadrupolar relaxation independently confirms that the order director fluctuations is the dominant mechanism of proton relaxation in the low frequency domain.

Fast field-cycling NMR relaxometry study of chiral and nonchiral nematic liquid crystals.

We present a proton NMR relaxometry study of the molecular dynamics in three liquid crystalline systems: 4'-n-pentyl-4-cyanobiphenyl (5CB), (S)-4'-(3-methylpentyl)-4-cyanobiphenyl (5CB*), and a 12% weight mixture of 5CB* in 5CB. The proton spin-lattice relaxation time (T1) was measured as a function of temperature and Larmor frequency in the isotropic, nematic, chiral nematic (N*), and smectic A phases of these liquid crystalline systems. A unified relaxation model was used to analyze the molecular dynamics, considering local molecular rotations/reorientations, translational self-diffusion, and collective motions as the relaxation mechanisms that contribute most effectively to the T1(-1) relaxation. Additionally, in the chiral nematic phase a fourth relaxation mechanism associated with the rotations induced by the translational diffusion along the helical axis (RMTD) was included in the model. All experimental results were consistently analyzed taking into account the physical parameters known for 5CB. The global analysis of the experimental results shows that the RMTDs are associated with the pitch value measured for the N* phases and that its contribution to the T1(-1) dispersion is observed at low frequencies. The T1(-1) dispersion in the smectic A phase of 5CB* is strongly dominated by the layer undulations relaxation mechanism over a broad frequency range from the low kilohertz regime to tens of megahertz. It was the first time such behavior was observed in a low molecular weight liquid crystalline system.

Liquid crystal 8CB in random porous glass: NMR relaxometry study of molecular diffusion and director fluctuations.

We present the measurements of the proton spin-lattice relaxation time T1 of liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB) confined into randomly oriented approximately 15 nm pores of untreated porous glass. In the low kilohertz range the spin-lattice relaxation rate in the nanoconfined 8CB is about ten times larger than in the bulk. We show that the increase is mainly due to molecular reorientations mediated by translational displacements (RMTD). In the paranematic phase the power law describing the RMTD dispersion, (T1(-1))RMTD proportional, omega(-p), is well characterized by the exponent p=0.5+/-0.06 and suggests an equipartition of diffusion modes with different wavelengths. The largest distance related to the decay of the orientational correlation function is about twice the diameter of the cavity. The situation is different in the nematic phase, where the orientational correlation is eventually lost at approximately 60 nm in the direction along the pore, a distance corresponding roughly to the length of a pore segment in the glassy matrix. The exponent p is between 0.65 and 0.9, depending on the temperature, which implies that in the nematic phase long wavelength modes are relatively more important--a consequence of the uniform director field along the pore. These observations are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. We point out that in strongly confined liquid crystals the proton NMR relaxometry does not provide the evidence of director fluctuations correlated over micrometer distances as was suggested earlier. The local translational diffusion of molecules within the cavities is found about as fast as in bulk.

Dynamics of the pinned modulation wave in incommensurate bis (4-chlorophenyl) sulfone (BCPS).

We show that both the anomalously huge resonance-frequency dependence of the (35)Cl nuclear quadrupole resonance (NQR) spin-lattice relaxation time in BCPS, reported here for the first time, and its anomalous temperature dependence can be explained by large-scale fluctuations of the pinned modulation wave instead of small-scale fluctuations (phasons and amplitudons). The results were obtained by measuring the laboratory (T(1Q)) and rotating frame (T(1Q,rho)) (35)Cl relaxation times. This is the first time that an effective resonance frequency dependence of the spin-lattice relaxation rate was measured in pure NQR.

Surface-induced order and diffusion in 5CB liquid crystal confined to porous glass.

Liquid crystals confined into small cavities are known to have a weak orientational order even above the nematic-isotropic transition temperature. The surface-induced order and molecular dynamics in this temperature range are studied with the aid of deuteron NMR spectra, spin relaxation times T(1) and T(2,) proton dipolar-correlation effect, and direct measurements of the effective diffusion coefficient for the liquid crystal 5CB confined to controlled-pore glasses. Our results show that an arrangement of molecules parallel to the wall is induced by local molecular interactions between the liquid crystal and solid, resulting in a weak and temperature independent surface order parameter, S(0) approximately 0.02 +/- 0.01. There is no indication of a significant slowing-down of molecular diffusion at the wall, neither rotational nor translational. In cavities of nanometer size, where the nematic order evolves gradually upon cooling, a broadening of the NMR linewidths due to dynamic effects should be taken into account.

Modeling the static fringe field of superconducting magnets.

The resonance frequency-space and the frequency gradient-space relations are evaluated analytically for the static fringe magnetic field of superconducting magnets used in the NMR diffusion measurements. The model takes into account the actual design of the high-homogeneity magnet coil system that consists of the main coil and the cryoshim coils and enables a precise calibration of the on-axis magnetic field gradient and the resonance frequency inside and outside of the superconducting coil.

Cross-section areas of calf muscles in athletes of different sports.

Differences in cross-section areas of calf structures were studied in different sportsmen groups and controls: sprinters, long-distance runners, free-climbers, and sport non active persons. The cross-section areas of calf tissues were established by magnetic resonance imaging. There were no statistically significant differences between the groups in single muscle areas. The only statistically significant difference was found in subcutaneous fat between sportsmen and non active persons. Results show a general adaptive tendency of the muscle to greater activity, without exhibiting sport-group specificity differences.