Cross-polarization contact carbons

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

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)... 

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). 

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. 

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.

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...

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]. 

Fig. 10.23. Cross-polarization pulse sequence. The high abundance nuclei, such as protons, are first irradiated with a standard 90° pulse to create the initial magnetization. A special pair of spin-locking pulses is applied during a period called the contact time in order to transfer the magnetization from the protons to the low abundance nuclei, such as carbons. Protons are then decoupled from carbons during the acquisition of the carbon signal. In the case of protons and carbons, cross-polarization can enhance the observed carbon signal by as much as four-fold.

Cross-polarization is based on the notion that the vast proton spin system can be tapped to provide some carbon polarization more conveniently than by thermalization with the lattice (7). Advantages are two-fold the carbon signal (from those C nuclei which are indeed in contact with protons) is enhanced and, more importantly, the experiment can be repeated at a rate determined by the proton longitudinal relaxation time Tin, rather than by the carbon T c (I)- There are many variants (7) of crosspolarization and only two common ones are described below (12,20). 

In order to study the molecular aggregation during the volume relaxation of network epoxies, CP/MAS carbon-13 (natural abundance) NMR was utilized. The Hartman-Hahn cross-polarization technique 129) was used with a cross contact time of 1 mi llisecond for transfer of proton polarization to carbon nuclei. The protondecoupling was achieved at the radio frequency of 56.4 MHz. Carbon-13 14.2 MHz spectra were measured in a 1.4 Tesla magnetic field. Room temperature (23 °C) experiments were performed at 54.7° MAS at 1 KHz. The spinner was constructed using an Andrew-type rotor driven by compressed air. 

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