Reorientation isotropic

Well-shaped crystals of free 1-boraadamantane 5 can be obtained by vacuum sublimation. However, all attempts to determine the structure of the compound were unsuccessful. This is because the solid 1-boraadamantane at ambient temperature exists as a plastic meso-phase with isotropic reorientation of the molecule and the ///era-phase was found to transform into organized crystals only below — 90 °C <1989IZV483>. 

In order to extract some more information from the csa contribution to relaxation times, the next step is to switch to a molecular frame (x,y,z) where the shielding tensor is diagonal (x, y, z is called the Principal Axis System i.e., PAS). Owing to the properties reported in (44), the relevant calculations include the transformation of gzz into g x, yy, and g z involving, for the calculation of spectral densities, the correlation function of squares of trigonometric functions such as cos20(t)cos20(O) (see the previous section and more importantly Eq. (29) for the definition of the normalized spectral density J((d)). They yield for an isotropic reorientation (the molecule is supposed to behave as a sphere)... 

The line broadening caused by partial motional narrowing can be distinguished from that due to isotropic reorientation at a reduced rate by appropriate magic angle spinning experiments. 

Several attempts have been made to interpret the relaxation data of polysaccharides by employing a variety of dynamic models suitable for local chain motions of synthetic polymers. One motional model that has been applied to polysaccharides133135136 is the log 2) model72 (Eq. A-6 in the Appendix), which assumes that isotropic reorientation is characterized by a skewed distribution of correlation times, with tails toward longer correlation times (Fig. 13). The distribution func-... 

In view of these results for binary glasses composed of molecules with different masses respectively different Tg, one may speculate that the isotropic reorientation of the small molecules results from translational diffusion in an essentially rigid glassy matrix formed by the large molecules, i.e., the diffusional process may be probed by 2H NMR via rotational-translational coupling.183 Finally, we note that the stimulated-echo technique was also applied to study the main relaxation in... 

Minimum Number of Steps n, to Reach Convergence of the C.F. Describing the Absorption of a Molecule in a Tr let Spin State with Isotropic Reorientation (q — max i —... 

Figure 9.11. - C NMR spectra of polycrystalline Ceo at various temperatures, showing motional narrowing of the NMR line due to rapid isotropic reorientation of the molecules above 140 K. The static CSA powder pattern lineshape was calculated using principal values of 213,182 and 33 ppm.

The relaxation times have been interpreted on the basis of a large-step isotropic reorientation model. The results show that in methylamine the internal reorientations of the methyl group are fast compared to the overall molecular rotations, whereas in dimethylamine they are of comparable magnitude and in trimethylamine the overall molecular motions are the faster. (171)... 

The isotropic reorientation of the hydrated complex has the same T value as that found for sugars by relaxation. Below 270 K, no dynamic equilibrium between bulk and hydrate water is detected the correlation times diverge and values are no longer identical. 

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