Cross polarization contact times

Fig. 47 Carbon-13 solid state NMR spectrum of camphor (50.309 MHz, cross-polarization contact time 5 ms, spin rate 1650 Hz)...

Figure 3. Cross polarization magnetization for the PIP-cured epoxy under the SL (Hartmann-Hahn) condition. The cross polarization contact time is rcp. The decay corresponds to proton T,p relaxation.

Figure 2. 50.33 MHz 13C NMR spectrum of lime cutin, obtained with cross polarization (contact time 1.5 ms, repetition rate 1.0 s), magic-angle spinning (5.0 kHz), and dipolar decoupling (762/211 = 48 kHz). This spectrum was the result of 6000 accumulations and was processed with a digital line broadening of 20 Hz. Chemical-shift assignments are summarized in Table I. Reproduced from Ref. 7 of the American Chemical Society.

The ratio of the aromatic and C-F carbons in the sample may be determined by varying the F -> cross-polarization contact time. The spectral intensity arising from the ith carbon spins is given by (Mehring 1976)... 

Fig. 26a, b a Experimental static proton decoupled spectra of Qq 4H2O. b Calculated static proton decoupled spectra of C q 4H2O. Cross-polarization contact time (CT) was... 

Magic angle spinning NMR spectra with variable cross polarization contact times were obtained on the intact, non-extracted sediments. The time-dependent spectra reveal subtle differences in organic carbon with depth differences not observed in single contact experiments. Dlpolar-dephased spectra of these same sediments indicate the presence of substantial amounts of substituted aromatic/olefinic carbons which are rapidly altered with depth. 

More definitive conclusions about subtle changes with depth can be drawn from spectra which exploit the time dependence of carbon magnetization. Figure 7a contains plots of cross polarization contact times vs. peak Intensities for a number of different resonances. Clearly, different types of carbon atoms relax at different rates In these sediments. This Is a completely expected result based on previous studies of carbon atoms In model compounds (31) and other geochemical matrices such as coal resins (32). 

Fig. 6.5.1. Plots of peak intensity versus cross-polarization contact time for the GlyGly.

Figure 8 Plots of C signal intensities versus cross-polarization contact time for methanol and dimethyl ether on zeolite HZSM-5. Intensities were normalized by division by the intensity obtained for each species in a direct 90° flip-observe experiment. Note that in this case the cross-polarization signals are less intense than the Bloch decay signals for all choices of the contact time (CT).

The temperature-dependent NMR spectra of the PLA samples collected under the varying temperature from 20 to 80 °C are shown in Fig. 6. Cross polarization-magic angle spinning (CP-MAS) NMR experiments were carried out on a Varian 400 NMR system spectrometer operated at 100.56 MHz for resonance with a cross polarization contact time of 2 ms (Fawcett, 1996). A zirconium oxide rotor of 4 mm diameter was used to acquire the NMR spectra at a spinning rate of 15 kHz. Each sample was packed into a 4 mm cyUnder-type MAS rotor. A set of temperature-dependent NMR spectra were obtained under varying ambient temperature from 20 to 80 °C at every 20 °C step. The heating rate was approximately 10 °C per an hour. 

Exposure of the sample to the atmosphere enables carbon dioxide and water to co-adsorb with the pyridine. The chemical shifts indicate that the carbon dioxide may have reacted to give a carbonate species(, ) whereas, the pyridine spectrum now resembles more closely that of liquid pyridine in that the linewidths are narrower (. 2 ppm) and the intensities are nearly 2 2 l(Figure 4b), The water appears to have altered the surface in such a way as to cause the pyridine to be more loosely bound, or it may be competing with the pyridine for the chemisorbed sites on the surface. However, lengthening the cross-polarization contact time from 1 ms to 3 ms alters the line intensities in favor of the y carbon( Figure 4c) implying that the pyridine maintains a preferential C2 rotation. 

Most of the studies of solid fossil fuels have used the mushroom3c and bullet types,3a made of Kel-F. 1h decoupling fields of about lO-to-14 gauss are most common. In fossil fuel work cross polarization (contact) times of about 1 ms and repetition times of about Is are typically used, although variations of these parameters have been made (see below). Spin-temperature alternation, in which one alternates by 180° the phase of the H spin-locking field and also alternates the... 

NMR spectra were obtained as a function of cross polarization contact time for Hytrel 4056 and Hytrel 7246. Figure 7 shows plots of the signal intensity against contact time for the aliphatic carbons of these two polymers. The strength of the spin lock field was > 20 kHz. 

Figure 7. Plots of intensity vs. cross polarization contact time for the aliphatic carbons of a, Hytrel 4056 and b, Hytrel 7246. Key , -CHtCHtCHf- carbons O, hard segment -OCHs- carbons and A, soft segment -OCHf- carbons. The curve corresponding to the hard segment -OCHt- carbons in b was dispiaced vertically by +20 intensity units.

Figure 4 Schematic rf pulse representation of the slightly modified Goldman-Shen experiment to monitor proton spin diffusion between two domains of dissimilar mobility via high-resolution detection of carbon-13 signals. The dipolar dephasing delay (xi) was fixed at 15 ps, the spin-diffusion mixing period (12) was either 0.01 ps or 10 ms, and the iH-i3C cross-polarization contact time was 100 ps.

FIGURE 51.6. Idealized CP-MAS buildup curves for two components A and B mixed in a 2 1 molar ratio and each with a characteristic resonance frequency. As the cross-polarization contact time increases, the peak intensities increase at different rates and then decay at different rates as a result of relaxation processes. The relative peak intensities for A and B are approximately 3 1 at a 1-ms contact time hut are 1 1 at a 9-ms contact time. 

Fig. 8.21. The change in carbon magnetization with contact time for the cross-polarization experiment. The initial rise is due to the cross-polarization contact time, Tch. and the relaxation decrease is governed by the...

The recommended procedure is to run a series of different delay times using only DD and MAS to check if the cross-polarization is distorting the CP-MAS spectrum. The best method to obtain quantitative areas is to vary the cross-polarization contact time and then to construct a graph of the magnetizations and extrapolate the linear portion to zero contact time [79], as shown in Fig. 8.24. 

If a C resonance line (without dipolar broadening) is not substantially broadened, then cross-polarization discriminates against liquid-like lines. In fact, cross-polarization can be used to distinguish mobile components from rigid components. This ability is illustrated by the study of styrene-butadiene copolymers. For a cross-polarization contact time of 10 ms, only resonances assigned to the butadiene carbons appear, whereas at a contact time of 1.5 ms, both styrene and butadiene signals are observable [79]. 

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