Extreme narrowing region observing

The measurement of correlation times in molten salts and ionic liquids has recently been reviewed [11] (for more recent references refer to Carper et al. [12]). We have measured the spin-lattice relaxation rates l/Tj and nuclear Overhauser factors p in temperature ranges in and outside the extreme narrowing region for the neat ionic liquid [BMIM][PFg], in order to observe the temperature dependence of the spectral density. Subsequently, the models for the description of the reorientation-al dynamics introduced in the theoretical section (Section 4.5.3) were fitted to the experimental relaxation data. The nuclei of the aliphatic chains can be assumed to relax only through the dipolar mechanism. This is in contrast to the aromatic nuclei, which can also relax to some extent through the chemical-shift anisotropy mechanism. The latter mechanism has to be taken into account to fit the models to the experimental relaxation data (cf [1] or [3] for more details). Preliminary results are shown in Figures 4.5-1 and 4.5-2, together with the curves for the fitted functions. 

As shown in Eq. 2, the observed diffusion coefficient should be time-averaged value of and Db. However, the observed value is virtually equal to since values of Db and Pb are so small compared with values of Df j and (1 - Pb). Therefore, observed diffusion coefficient of Na ion in the agar gel mainly represents the motion of Na ion in the extreme narrowing region. 

In both models the rotational shift of the line 5co is the same either in the static limit, where it is equal to zero, or in the case of extreme narrowing where it reaches its maximum value coq. A slight difference in its dependence on tE is observed in the intermediate region only. The experimentally observed density dependence of the shift shown in Fig. 3.5 is in qualitative agreement with theory. 

Application of this technique to measurements of the spectral distribution of tight scattered from a pure SF fluid at its critical point was present by Ford and Benedek The scattering is produced by entropy fluctuations which decay very slowly in the critical region. Therefore the spectrum of the scattered light is extremely narrow (10 - lO cps) and can only be observed by this light beating technique 240a)... 

Deviations from predicted relaxation behavior have been observed for large proteins (3 -7 ), polymers (8y9j and highly associated small molecules (10). Particularly prominent are observations of Ti field dependences and low NOE s within the so-called "extreme spectral narrowing region," where single correlation time models predict field independence of Tp and full NOE s. 

Neglecting the small inflection observed for the A coefficients (Fig. 10c), the relaxation is exponential and can be considered to fall into three regions. In the extreme narrowing limit the relaxation contributions of the c- H and c- c dipolar interactions are essentially additive. Below "10 ° s, the rapid flip-flop Interaction which reflects the first term in... 

Having taken the trouble to see how the relaxation rates in a two-spin system depend on molecular motion, we are now in a position to predict the behaviour of the NOE itself as a function of this motion and of intemuclear separation. Taking the rale constant Eq. (8.4) and substituting these into that for the NOE Eq. ((8.2)) produces the curve presented in Fig. 8.8 for the theoretical variation of the homonuclear NOE as a function of molecular tumbling rates as defined by (where u>o is the spectrometer observation frequency, approximately equal to uii and ujs). Note this is for a two-spin system, which relaxes solely by the dipole-dipole mechanism and as such represents the theoretically maximum possible NOE. The curve has three distinct regions in it, which we shall loosely refer to as the fast, intermediate and slow motion regimes. For those molecules that tumble rapidly in solution (short those in the extreme narrowing limit), the NOE has a... 

Pure rotation spectra can be observed by direct absorption of electromagnetic radiation or by Fourier transform methods, working in the far-IR, millimeter wave or microwave regions of the spectrum, corresponding to wavelengths from 0.05-60 mm (200 to 0.16 cm 6000 to 5 GHz). The lines are extremely narrow, and their positions can also be measured with extreme precision because the source is very stable, so a typical line position of 30 GHz (3 x 10 ° Hz) ean be measurable to zhlOkHz, or one part in 3 million. 

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