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A strategy aiming at the determination of the chemical shielding tensor (CST) in the liquid state is described based on the quantum theory calculations and multifield NMR relaxation measurements. It requires the characterization of molecular tumbling by using either analytical spectral densities or spectral density mapping.

The carbon-13 chemical shielding tensor in the liquid state of hexafluorobenzene which has a complicated spin system may be determined using simple experimental nuclear magnetic resonance methods combined with quantum chemistry calculations. Modifications of the electronic distribution around the carbon nuclei in hexafluorobenzene when going from pure solid to pure liquid or solutions were attributed to the particular arrangement in the solid phase.

Pulsed field gradient NMR diffusion experiments can, in principle, lead to the “diffusive diffraction” phenomenon. In practice, its observation by gradients of the static magnetic field is difficult in real systems because they involve internal gradients due to the static magnetic field (necessary for polarizing nuclear spins). This latter drawback can be circumvented by using gradients of the radio-frequency (rf) field (the other magnetic field used in any NMR experiment). For the first time, by means of rf gradients, a so-called diffusive diffraction peak has been observed in a real porous system and its position provides a value of the mean distance between pores; this can be further complemented by the mean pore size determined from the dependence of the apparent diffusion coefficient with respect to the diffusion interval. [ABSTRACT FROM AUTHOR]

Several theoretical and experimental studies are conducted to explain the reorientational dynamics of fullerene [C.sub.70] in various aromatic solvents. The compound is shown to exhibit anisotropic behavior at the lowest and the highest temperatures.

This paper deals with the complete interpretation of the heteronuclear Overhauser effect spectroscopy (HOESY) two dimensional nuclear magnetic resonance (NMR) experiment. This experiment aims at the determination of cross relaxation rates (having their origin in dipolar interactions) between two different nuclear species, here proton and carbon-13. Extra correlation peaks observed in the 2D map of the system under investigation (micellized sodium octanoate) are shown to arise from remote dipolar interactions and not from strong (J) coupling effects. Several experiments have been performed at different mixing times (during which the nuclear Overhauser effect builds up), allowing the observation of the whole carbon-13 magnetization evolution. Their analysis yields cross-relaxation rates, refined with respect to those derived from initial behavior. However, this requires inclusion of the effect of spin diffusion, within the proton spin system, in the data treatment. A novel approach is proposed which models the multiexponential character by means of effective proton relaxation parameters. All the results of this study are consistent with data obtained by other methods [nuclear Overhauser effect (NOE) factor determinations and frequency dependent longitudinal 13C relaxation times). [ABSTRACT FROM AUTHOR]

The X-filtered HOESY experiment is used for identifying the intermolecular contacts between small- and medium-sized molecules (micelles) in homomolecular systems. The micelles formed by the combination of identical molecules produce cross peaks between H(super 1) and C(super 13) nuclei, and may have both intra- and intermolecular contributions. The experiment reveals that the protons and the carbons belonging to the same and different CH2 units have a strong intramolecular cross-relaxation.

A previous study of C70 in deuterated benzenes generated evidence suggesting C70 exhibited unique reorientational behavior depending on its environment. We present a comparison of the dynamic behavior of this fullerene, in the solid and solution phases, to explore any unique features between these two phases. The effective correlation times, τeff C , of C70 in the solid state are 2 to 3 times longer than in solution. In the solid state, a noticeable decrease in all the carbons' correlation times is seen between 293 K to 303 K; suggesting a transition from isotropic to anisotropic reorientational behavior at this temperature change. Although C70 in solution experiences van derWaals type interactions, these interactions are not strong enough to slow the solution-state motion below what is observed in the solid state. All observed differences in the diffusion constants, DX and DZ, in solution are smaller than in the solid state suggesting a lower energy of activation between these twomodes of reorientation in the liquid phase. A small-step diffusion "like" condition appears to be thermally generated in the solid phase at 323 K. [ABSTRACT FROM AUTHOR]

Taking advantage of the fact that α,α,2,6 tetrachlorotoluene possesses only one symmetry element (the aromatic ring plane), it proved possible to measure seven different [sup 13]C-[sup 1]H crossrelaxation rates which enable one to determine the three rotation-diffusion coefficients (D[sub xx], D[sub yy], D[sub zz]), in addition to the orientation of the relevant principal axis system (PAS) with respect to a chosen molecular axis system. It turns out that molecular reorientation is strongly anisotropic and that the rotation-diffusion PAS cannot be directly correlated with electrical molecular properties. [ABSTRACT FROM AUTHOR]