Mitsumasa Ishiwata

13C and 1H NMR Relaxation of Chloroform in a Nematic Liquid Crystal. II. Dynamical Behavior of the Small Probe Molecule

Longitudinal relaxation of 13 C and 1 H in chloroform dissolved in a nematic liquid crystal has been observed between 25 and 34°C at 89.6 and 400.1 MHz (for 1 H) by the selective and nonselective inversion recovery method. The data can be satisfactorily explained by a combination of fast reorientation of the solute and slow fluctuations in the nematic phase, under consideration of the intramolecular dipole-dipole and external random field interactions together with cross correlations between the two. Local structure fluctuations in the nematic phase have played a substantial role in the spin relaxations. The external random field interaction on the proton and the cross correlation relating this interaction have considerably been dependent on the observing frequency.

Probe Dynamics of Chloroform in a Smectic Liquid Crystal II. Roto-Translational Motion of the Probe

Spectral density data obtained from NMR relaxation have been analyzed for a 13 C- 1 H spin system in 13 C-chloroform dissolved in a smectic liquid crystal. The maximum value and frequency dependence of a component for the magnetic quantum number 0, which corresponds to fluctuation of the dipolar field having a nonzero average in the ordered state, can be successfully explained by the roto-translation coupling model inherent in the smectic phase. Temperature dependence of the orientational order parameter of the probe is discussed on the basis of distribution of the probes in the smectic layer and local variation of the alignment.

Probe Dynamics of Chloroform in a Smectic Liquid Crystal. I. 13C and 1H NMR Relaxation

13 C and 1 H relaxations of 13 C-enriched chloroform dissolved in the smectic A phase of S2 are measured at 90 MHz for 1 H. From various experiments for the coupled spin system together with 1 H-data for the same mol% of chloroform with natural abundance of 13 C, we have separated spectral densities on reorientation of a molecular axis of the chloroform. They are markedly different from those expected from the purely rotational dynamics; namely, only the component for fluctuation of a dipolar field in the direction of the magnetic field, which has a nonzero average in the ordered state, have been fairly large and frequency-dependent. This result is presumably due to the coupling of translational and rotational motions of the solute deeply inherent in the smectic phase.

Double Resonance Saturation Recovery in a Coupled Two-Proton System

The longitudinal relaxation of individual resonance lines in an AX spin system in dichloroacetaldehyde was observed by the saturation-recovery method under double resonance. A second radio-frequency field continuously stirred either of two lines of the proton different from the one to which the observed line belonged. This stirring field was not too strong to result in any appreciable splitting of the observed line. It was found that the recovery from saturation could be accelerated by the stirring field when the stirred line was progressively connected to the observed. The acceleration occurred only for an appropriate range of the amplitude of the stirring field. A brief account of this phenomenon is given in terms of a nutational motion of the stirred spin.

13C and 1H NMR relaxation of chloroform dissolved in a nematic liquid crystal. I, Cross correlation between intramolecular and intnermolecular dipole-dipole interactions

13 C and 1 H relaxation times of a coupled 13 C- 1 H spin system dissolved in a nematic phase have been measured at 400 MHz (for 1 H) by the selective and nonselective inversion recovery method. The relaxation times of doublet lines of each spin are different from each other. Especially, when a 1 H line is inverted selectively, the relaxation time of the 13 C line connected progressively to the inverted one is fairly different depending on which line of the 1 H is inverted. Master equations for longitudinal relaxation are derived under the assumption that in addition to two relaxation mechanisms of the intramolecular dipole-dipole and external random field interactions, interference effects between the two might be nonnegligible in the oriented phase. The differences in the relaxation times have been explained by this interference effect.

Double Resonance Saturation Recovery in 2,6-Lutidine

The time variation of the longitudinal magnetization of an individual A-proton line in an AB 2 -proton system in 2,6-lutidine was observed by the saturation-recovery method under continuous irradiation of a second radio-frequency field to certain particular lines of the B protons. The stirring field was so weak that the spin-tickling effect caused no detectable splitting of the observed line. The dependence of the recovery time T H 2 on the strength H 2 of the stirring field differs according to the combination of the observed and the stirred lines; as H 2 increases, the T H 2 for regressively connected lines increases monotonically, that for unconnected lines hardly changes, and that for progressively connected lines decreases at first and then increases.

Double resonance inversion recovery in a heteronuclear two spin system

For a heteronuclear coupled AX spin system, recovery after inversion of the A spin lines in A-{X} double resonance spectra is observed under a spin tickling condition. Oscillations in the A lines after inversion are found to decay more rapidly than expected and depend on inhomogeneity of an applied static field. After the initial oscillating stage, the recovery of the tickling spectra can considerably be accelerated by an irradiation field which is applied near a resonance of an X line. A simple mathematical method is presented for calculating the double resonance recovery process under the inhomogeneity of the static field. Such an inhomogeneity modifies a recovery time near the resonance. Experimental results for 13 C-enriched formic acid are reproduced by numerical computation. Acceleration of the recovery is explained in terms of a saturation effect of the irradiated line.

Theory of Longitudinal Relaxation of an AX Spin System under Double Resonance.I.Density Matrix Formulation of Saturation Recovery

Concrete equations of motion describing longitudinal relaxation of an AX system under irradiation of a selective rf field are worked out on the basis of Blochs density matrix theory. These equations are composed of thermal relaxation and coherent motion of the stirred spin, which determine dynamical characteristics of the spin system. Explicit equations of motion are obtained also for the magnetizations of another spin observed in the saturation-recovery experiment under double resonance. When the stirring field is not too strong to cause any appreciable splitting of the observed line, the recovery of the observed line can be described in terms of several exponential decay terms together with a pair of oscillating terms corresponding to the coherent nutation.

Selective Inversion of Spin-Tickling Spectrain a 13C-1H Spin System

Selective spin inversion usually produces transfer of the longitudinal magnetization in a coupled spin system. Under irradiation of a stirring-field, however, transverse ones and multiple quantum coherences can also be simultaneously transferred in this process. For a coupled 13 C- 1 H spin system under irradiation resonant with one of the 1 H lines, 13 C tickling spectra are observed immediately after inversion of one component of the 13 C-polarizations. Spectra thus obtained are considerably different from a simple selectively-inverted pattern of the steady-state ones. These changes depend on amplitude and frequency of the rf field, frequency and phase of the inversion pulse and inhomogeneity of the static magnetic field. The most prominent change in these spectra can be explained by transfer of zero quantum coherence, which is made in the differential form into each line of the spectra.

Order Parameters and Distribution in the Smectic Layer of Probe Molecules Dissolved in Ferroelectric Liquid Crystalline Mixtures

For a few types of relatively small probe molecules with different sizes and geometrical anisotropies, alignment-induced 13 C chemical shifts or spacings in the multiplet due to the residual dipola...