Extreme narrowing region reference

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. 

Figure 13 shows a plot of Ti versus Tc for various spectrometer frequencies. At short (rapid tumbling of small molecules, left side of the curve), mV 1 and 1/Ti is linearly related to l/r This is sometimes referred to as the extreme narrowing region of molecular motion. In the extreme narrowing region, Ti is independent of field strength (i.e., the spectrometer resonance frequency) and = 7. ... 

Referring to the right side of Figure 15, if the resonance of Ha is decoupled with a second rf field, the populations of the energy levels separated hy the Ha transitions (red arrows) become equal and are said to be saturated. In the extreme narrowing region 1) the probability of a two-spin... 

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... 

---