Overcoming Long 7 Cross Polarization

A potential problem in obtaining solid-state NMR spectra involves the longitudinal relaxation times which tend to be long, thus compromising sensitivity. To overcome this, cross-polarization from abundant nuclei, such as H, to the dilute spin, S, under observation may be employed. Recycle delays are then determined by the Ti of rather than of S. In addition, the magnetization of S can be increased up to a maximum of 7h/7s-... 

Si NMR studies of solutions are difficult because of the long spin-lattice relaxation times of the nucleus and its negative nuclear Overhauser enhancement. The 29Si-1H dipole-dipole relaxation is inefficient because in most compounds the intemuclear distance is large. Fortunately, the problem of relaxation can often be overcome by resorting to cross-polarization (see Section II,E). 

Solid state 13C-NMR spectra of humic substances are usually obtained by cross polarization techniques (15). In this method, relaxation occurs at almost the proton T rate, rather than the carbon Ti rate which can be slower by a factor of 100. Hence, in principle, the problems of long carbon T. s in solution are overcome by obtaining the spectrum in the solid state. However, solid state Tis are... 

Because has a low natural abundance, NMR measurements on this nucleus require si al averaging. The spin lattice relaxation time, Ti, determines how rapidly a single-pulse I R experiment can be repeated. The rule of thumb is to repeat the experiment every 3Ti to 57 i so that >95% of the equilibrium signal intensity is established between pulses. If we suppose that the T of a C nucleus is 2 min, the single-pulse experiment would be repeated every 6 to 10 min. If 1000 signals must be accumulated to obtain a reasonable signal-to-noise (S/N) ratio, the total experiment time would be between 100 and 167 h. Fortunately, cross-polarization overcomes the problems of the long T s and consequent limited sensitivity. 

Dilute nuclei, such as and are more difficult to observe than abundant nuclei, such as H or particularly when they also have a low gyromagnetic ratio. However, the dilute and abundant nuclei are often in close proximity, and coupled via the dipolar interaction. Cross-polarization (CP) exploits this interaction to observe dilute nuclei, at the same time overcoming two serious problems often encountered in solid-state NMR (i) because of a very small population difference in the polarized sample, NMR actually observes very few dilute spins and consequently the sensitivity of the experiment is low (ii) spin-lattice relaxation times of spin- nuclei in solids are often very long, so that long delays are required between experiments and the spectral signal-to-noise ratio is poor. 

The problem of long relaxation times of nuclei in solids has been overcome by utilizing cross-polarization (CP) and proton enhanced (PE) spectroscopy. By this process the C spin system transfers its energy to the spin system. By relaxation of the protons, the energy of the C nuclei can then be transferred to the lattice. Since the spin-lattice relaxation times are... 

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