Torchia s pulse sequence

Figure 22-(a) shows the DD/MAS spectrum in the resonance range of a-methyl-ene carbon at 0 °C. This spectrum represents the thermal equilibrium state of this sample, because it was obtained by a single pulse sequence with the repetition time of 600 s longer than 5 times the longest Tic in the system. The spectrum (b) is that of the crystalline component, which was obtained with use of Torchia s pulse sequence [53]. In the equilibrium spectrum, the noncrystalline contribution (amorphous plus interfacial) can be seen upfield to the crystalline component. Figure 23 shows the elementary line shapes of the amorphous and crystalline-amorphous interphases that comprise the noncrystalline resonance. 

In an attempt to investigate the phase structure of this sample, the line shape analysis of the CH2 resonance line in the DD/MAS spectrum at 87 °C that is shown in Fig. 25 was examined. The result is shown in Fig. 26-(a). The elementary line shape of the crystalline phase was obtained as the line shape of the longest Tic component by Torchia s pulse sequence [53]. It was a doublet and was represented approximately by two down- and upheld Lorentzians with an intensity ration of 2 1 (Spectrum A shown by dotted line in Fig. 26). Since all methylene carbons in the a-crystalline form of this polymer are equivalent in the intramolecular helical conformation, the origin of the doublet could be attrib-... 

On these systems the C longitudinal relaxation times T of the C5Me5 moieties have been measured by means of the Torchia s pulse sequence [48]. The Ti( C) values obtained for ring carbons have been rationalized on the basis of two relaxation interactions (dipole-dipole and chemical shift anisotropy) modulated by the motions involving the permethylated cyclopentadienyl rings. Interestingly, a qualitative comparison between solution and solid state C relaxation data shows that the same relaxation mechanisms are operative in both physical states. 

Figure 21.10 shows the partially relaxed solid-state NMR spectrum of a P(3HB-co-37%3HP) sample measured by Torchia s pulse sequence [56]. 

Fig. 21.10. 67.9 MHz CP/MAS NMR spectra of P(3HB-co-37%-3HP) measured by Torchia s pulse sequence (t pulse delay time). (Reproduced from Ref. [67] with permission.)...

For the 13C CP/MAS measurement and dipolar-dephasing CP/MAS NMR the experimental conditions were 3 ms contact time, 5 s repetition time, 40 kHz spectral width, and 8 k data points. The l3C chemical shifts were calibrated through the hexamethylbenzene peak (17.3 ppm relative to tetramethylsilane). The l3C relaxation measurements were performed with Torchia s pulse sequence."... 

Figure 6b shows the spectrum of the crystalline component of cotton soaked in H OCwater content=161%), which was obtained by Torchia s pulse sequence(27,28). The delay time between two ir/2 pulses in the pulse sequence was set to be 100 s. As is clearly seen, the spectrum shown in Figure 6b reflects the components corresponding to the downfield sharp lines of C4 and C6 carbons in the whole spectrum shown in Figure 6a. A similar crystalline spectrum was obtained by others(29) using almost the same technique. On the other hand. Figure 6c indicates the spectrum of the noncrystalline component of the cotton cellulose, which was obtained by subtracting the spectrum of the crystalline component shown in Figure 6b from the whole spectrum shown in Figure 6a. This spectrum evidently corresponds to the components associated with the upfield broad resonances of C4 and C6 carbons.

Table 3 summarizes the T, values of some samples they were obtained by Torchia s pulse sequence, which is shown schematically in Fig. 17. It is seen that for all samples examined each carbon contains two distinctly different (long and short) T, values although the shorter of bacterial cellulose could not be discerned except for... 

Figure 11 Temperature dependence of T, for individual carbons of POLG. (A) CH3, (B) a-CH2 and (C) int-CH2 (a and b indicate transition temperatures, m indicates melting point.) For T, measurement in the temperature range -40 to 50 C Torchia s pulse sequence was used, and in the temperature range 50 to 230 C, the inversion-recovery pulse sequence was used. Reproduced with permission of Elsevier Science from Katoh E, Kurosu H, Kuroki S and Ando I (1994) Journal of Molecular Structure 32S 145.

Spin-Lattice Relaxation. In order to determine whether each resonance line comprises a single component, we first measured the spin-lattice relaxation time Tic by the pulse sequence developed by Torchia [53] or by the standard saturation-recovery pulse sequence. The Tic values thus obtained were 2560,263 and 1.7 s for resonance line I and 0.37 s for line II. As reported by several investigators, the line at 33 ppm is associated with three different Tic values [ 17,54,55]. This means that this line is contributed to by three components with different molecular mobilities. However, since each component was represented by a single Lorentzian line shape at 33 ppm, they are all assignable to methylene groups in the orthorhombic crystalline form or in the trans-trans conformation. The component with a Tic of s can be assigned to methylene groups with a some-... 

An example of the effect polymer chain conformation can have on high-resolution solid-state C CPMAS/DD NMR spectra is presented for the semi-crystalline polymer polyethylene (PE) in Figure 5.11. These spectra [20] were recorded with Torchia s CF-T pulse sequence [21], as illustrated, which... 

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