Cross polarization repetition rates

The problems involved in quantitative analysis using NMR spectroscopy, have been discussed by several authors and it is evident that it still causes a lot of problems as especially pointed out by Hays55 in his excellent review on the subject. Thus in liquid state NMR spectroscopy the quantitative estimation of atoms and groups involves the use of normal analytical method. In the case of solid state NMR spectroscopy, however, the application of the cross-polarization technique results in signal enhancements and allows repetition rates faster than those allowed by the carbon C-13 Tl. Therefore, the distortion of relative spectral intensities must always be considered a possibility, and hence quantitative spectra will not always be obtained. 

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.

Fig. 8. MIONP enhancement of nuclear spin polarization in a polycrystalline sample of pentacene doped naphthalene/ (a) Build-up curve for the enhanced spin polarization, with an MIONP repetition rate of 50 Hz (determined by the laser) the time constant for the build-up was 3730 s. TCP is an acronym for integrated cross-polarization - the authors term for their variant of the ISE. (b) 13.6 MHz single-shot H NMR spectra, taken with (top) and without bottom) MIONP enhancement (respectively, acquired at 100 K and room temperature). The top spectrum was obtained with a 10,000 s MIONP build-up time. The enormous sensitivity enhancement obtained with ONP is clear from comparing the two spectra. (Figures courtesy of K. Takeda and T. Terao, reprinted from Ref. [71], copyright 2001, with permission from Elsevier.)...

The long signal accumulation times required by the low repetition rate for C nuclei with long Tis in solids can be circumvented by transferring polarization from abundant H nuclear spins with short Ti s to the nuclei. The repetition rates for signal averaging are now determined by the shorter HJiS (ms-s), because energy is transferred from the proton to the carbon nuclei, a process termed cross-polarization (CP) [38]. 

Hartmann and Hahn [39] showed that CP can be achieved when two rf fields Bin and B c = 4Bih are simultaneously applied. Jn/jc = 4, so, when B c = 4Bm energy is transferred between them, or they are cross-polarized, because Tc ic = Th ih (which is called the Hartmann-Hahn match of heteronuclear rotating-frame frequencies). In the CP experiment C nuclei obtain their spin polarization from H nuclei, so, not only do the shorter proton spin-lattice relaxation times determine the repetition rate of... 

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