Inversion recovery pulse sequence

The values of the time constants and are important in understanding both internal and overall motional behavior of the sample molecule. values are measured by the inversion recovery pulse sequence ... 

Because the excitation/detection coil is in the x-y plane and the longitudinal component relaxes along the z axis, T cannot be measured directly from an NMR spectrum, but must be obtained using a pulse sequence. The most commonly used pulse sequence to measure T is an inversion recovery pulse sequence (Kemp, 1986). Other commonly used pulse sequences for measuring 7j are given in Ernst et al. (1987). 

FIGURE 31. Typical data set for measurement of the spin-lattice relaxation times of the sp2-hybridized carbon atoms of, 6-carotene at 11.7 T. The chemical shift values are shown across the bottom of the figure. The t-value for each spectrum is the delay time in the inversion-recovery pulse sequence. Reprinted with permission from Reference 49. Copyright (1995) American Chemical Society... 

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...

Relaxation parameters provide valuable information about molecular motions. The spin-lattice relaxation time T is usually determined by the so-called inversion recovery pulse sequence (65). The experiment comprises a set of spectra with different interpulse delays, and Tx is determined by fitting the signal intensities for a given nucleus to Eq. 2, where A and B are constants, x is the respective interpulse delay, and /,is the intensity measured at that delay ... 

FIGURE 4.4 (a) Inversion recovery pulse sequence with inverse gated proton decoupling for 7, measurement. 

EXAMPLE 3.10 In the complex molecule artemisinin (the structure of which can be seen in Chapter 13) each hydrogen exhibits its own signal. Figure 3.23 shows the H spectrum of this compound, obtained by the inversion-recovery pulse sequence. Estimate the value of T, for each signal. 

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... 

FIGURE 8.8 (<2) Partially relaxed 13C NMR spectra from an inversion-recovery pulse sequence. (b) Values of Tj (seconds) for 13C nuclei in n-decanol. (c) Values of Tr for 13C nuclei in phenol. 

Due to efficient spin diffusion, homogeneous crystalline solids present a unique relaxation time, regardless of chemical differences. Then different polymorphs can be differentiated by using pulse sequences able to discriminate substances on the basis of their different relaxation properties. In particular cases it has been possible to extract the individual subspectra from a mixture of polymorphs, by modifying the inversion recovery pulse sequence to decompose the spectra [43]. 

Figure 22 The inversion-recovery pulse sequence used for determination of the Ti relaxation time.

Fig. 21. (a) 23Na NMR absorption spectra for PP0-NaCF3S03 for O/Na ratio 30 at 273 and 353 K. Inset shows 19F absorption spectra at 273 K (b) 23Na spin lattice relaxation spectra for PP0-NaCF3S03 complex (O/Na 30) at 297.5 K using the inversion recovery pulse sequence. The delay time t is indicated for every second spectrum (taken from Ref. 263). 

Other approaches to elimination of the water signal have exploited the difference in relaxation times between water and the compounds of interest, and have the advantage that peaks near the solvent are unaffected. For example, WEFT (Water Eliminated Fourier Transform) NMR uses an inversion recovery pulse sequence (designed for measurement). 

Figure 5.4. H NMR spectrum for W(CO)3(FPr3)2(H2) at —82 °C in toluene-4g. A normal spectrum showing broad resonance for the H2 ligand, which obscures the upheld hydride (Ht) resonance. B spectrum obtained with an inversion-recovery pulse sequence (180-T-90) to null the interfering H2 signal. Reprinted with permission from Kubas et

T, Measurements. Ti values for the mobile domain carbons were measured using a (l80 -t-90 -T) inversion recovery pulse sequence (77) with continuous proton saturation. The optimum 180 pulse width was determined prior to each set of measurements. The time between pulse sequences was 2 s at 21 kG and 3 s at 47 kG. Measurements were generally made for 10-12 values of t which ranged from 0.005 s to T. Both integrated peak intensities and peak heights were used for the analysis of each T) data set equivalent results were obtained in both cases. 

Fig. 8.4 Spin—lattice (T ) inversion recovery pulse sequence. The 180° pulse inverts magnetization, allowing it to recover along the z-axis. The duration of the delay, r, is varied from T to several times the longest expected T relaxation time in the molecule. The resulting.

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