Partially relaxed spectra

Fig. 13. Calculated 2H solid echo spectra for log-Gaussian distributions of correlation times of different widths. Note the differences of the line shapes for fully relaxed and partially relaxed spectra. The centre of the distribution of correlation times is given as a normalized exchange rate a0 = 1/3tc. For deuterons in aliphatic C—H bonds the conversion factor is approximately 4.10s sec-1...

Fig. 21. Deuteron NMR spectra for the phenyl groups in polycarbonate (amorphous film, drawn from solution at room temperature, Mw 30000, Mw/M = 1 -8). Left column fully relaxed spectra. Right column partially relaxed spectra corresponding to the mobile groups only...

The most popular, and also a very accurate, experimental method for measuring nonselective spin-lattice relaxation-rates is the inversion recovery (180°-r-90°-AT-PD)NT pulse sequence. Here, t is the variable parameter, the little t between pulses, AT is the acquisition time, PD is the pulse delay, set such that AT-I- PD s 5 x T, and NT is the total number of transients required for an acceptable signal-to-noise ratio. Sequential application of a series of two-pulse sequences, each using a different pulsespacing, t, gives a series of partially relaxed spectra. Values of Rj can... 

Fig. 41 Experimental and calculated 2H spectra of phenyl deuterated BPA-PC at various temperatures a fully relaxed spectra and b partially relaxed spectra (from [36])...

In order to determine the content of this noncrystalline line further, we examined in more detail the behavior of the spin-lattice relaxation. Figure 5 shows the partially relaxed spectra in the course of the inversion recovery pulse sequence (180°-t-90°-FIDdd-10s)i2o with varying x values. The magnetization that was recovered for 10 s in the z direction was turned to negative z direction by 180° pulse and the magnetization recovered in z direction for varying x was measured in the xy plane under H DD. The spectra at different steps of the longitudinal relaxation were obtained by Fourier transform and are shown in Fig. 5. In these spectra the contribution from the crystalline components with Tic s of2,560 and 263 s are eliminated due to the lack of time for recovery at each pulse sequence. Therefore, we observed preferentially the relaxation process of the noncrys-... 

Fig. 5. Partially relaxed spectra of the bulk polyethylene with Mv of 3.0 x 106, taken by inversion recovery pulse sequence (180°-T-90°-10s)i2o with different relaxation times x s...

FIG. 7. The normal (lower) and partially relaxed spectra of 5a-cholestan-3-one in the region around 36 ppm. 

Even without the effects of cross correlation, the assumption of a single correlation time for a given relaxation mechanism is usually oversimplified, as we indicated in Section 8.1. Figure 8.8a shows partially relaxed spectra resulting from an inversion-recovery pulse sequence (Section 2.9) for the 13C spins in a small organic molecule. It is apparent that the values of Tx are somewhat different, because the null times vary from one carbon atom to another. From the analysis of similar partially relaxed spectra, the values of T, (13C) shown in Fig. 8.8b and c have been obtained for phenol and n-decanol. Both molecules tumble rapidly in... 

Figures 2.13, 9.1, and 9.2 demonstrate the formation of an echo following a tt pulse. Application of additional tt pulses can be used to form a train of echoes. It is clear that the dephasing of magnetizations following an echo is of the same form as the initial dephasing during the FID and that application of a second tt pulse at 3T causes a second echo at 4t, etc. The envelope formed by the echo peaks decays according to the real T2, rather than T2, and Fourier transform of each echo provides a set of partially relaxed spectra, from which T2 of each line may be determined. (Carr and Purcell first recognized the value of such a long sequence of TT pulses,104 and their names are usually used to depict the method, but the technique that we described for the spin echo in Chapter 2 and that discussed here include a refinement by Meiboom and Gill,105 as discussed later.)...

The analysis of the partially relaxed spectra allows a unique assignment of the values to the... 

Fig. 6. Partially relaxed spectra of the bulk polyethylene with My of 3 X 10 , taken by pulse sequence II with = 3.5 s.A,B, C,D,E,F, G, and H show the transversally relaxed spectra for It = 0.5,20, 60,100,140,500,2000, and 4000 ps, respectively...

Next, we examined the partially relaxed spectra in the transverse direction. As was explained in the theoretical sections, the relaxation of magnetization of macromolecules whose only nuclei possessing spin are and H is predominantly conducted by the time-fluctuation of the dipole-dipole interaction between and neighbouring H nuclei. However, the mechanism is somewhat different depending on the direction of the relaxation. The longitudinal relaxation is carried out by the o>c and ct>c cuh frequency components (see eqn (33)) of the fluctuation arising from random reorientation of chemically bonded internuclear vectors via... 

Fig. 11. The pulse sequence for the partially relaxed spectra in the transverse...

Chemical shifts (in ppm) were evaluated with respect to DSS in D2O. Proton spin-lattice relaxation times (T ) were measured at 100 MHz by the inversion-recovery pulse sequence t-180 -T-90°, being the delay between the perturbing (180°) and the monitoring (90°) pulses, and a long delay time between the application of successive pulse sequences (>> 5 T] ). In order to minimize the problems arising from the limited dynamic range of the computer, the partially relaxed spectra were obtained under double precision conditions. The accuracy of T values was estimated to be around 10%. 

A series of partially relaxed spectra obtained by the inversion recovery pulse sequence with increasing delay time t (in seconds) is shown as stack runs in Fig. 2.31 (282). At first, all peaks are negative they then gradually become positive with T, protons with shorter Tj becoming positive earlier than those with longer... 

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