Dipolar decoupling

FIGURE 7.4 Schematic representation of the A portion of a powder pattern for an AX system. The solid line refers to nuclei with parallel spin orientation, the dotted line to nuclei with antiparallel spin orientation. Each branch covers a range from 0° to 90°, as indicated, and the two branches cross at 54.7°. 

However, 13C, 15N, and many other low abundance nuclei give only weak signals because of their small magnetic moments, their low abundance, and in most 

In the solid state the restricted molecular motions may cause relaxation times to be relatively long. This means that the experiment cannot be repeated as fast as in solution and a smaller number of accumulations is possible in a given time. The signal can be enhanced by a double resonance technique that is called cross polarization (CP) that solves the problem of slow signal accumulation caused by the long longitudinal relaxation times 71 of heteronuclei in the solid state. The polarization required for the experiment comes from the protons. Thermal equilibrium polarization of the protons is restored with the longitudinal relaxation time of the protons, which is much faster than that of heteronudei. Most commonly, the CP technique is combined with MAS and is denoted CP/MAS [28, 29]. Today this is the predominant method for 13C solid state NMR spectroscopy, but is not restricted only to this isotope. 

The development of new ID and 2D pulse sequences enables the spectroscopist to obtain structure and dynamic information about systems that were previously very hard to study. As an example is reported in Fig. 3.2.13 the 2D spectrum of erythromycin A measured with the FIREMAT (Five p Replicated Magic Angle Turning ) technique [30]. The slow spinning speed of 390 Hz produces a spinning sideband pattern for each peak in one dimension, whereas a multipulse sequence in combination with a special processing method produces isotropic lines in the second dimension. 

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Fig. 7. Nmr spectra of quinine [103-95-0] C2QH24N2O2, acquired on a Bruker 300AMX spectrometer using a Bmker broadband CP/MAS probe, (a) Proton-decoupled spectmm of quinine in CDCl (b) the corresponding spectmm of solid quinine under CP/MAS conditions using high power dipolar decoupling (c) soHd-state spectmm using only MAS and dipolar decoupling, but without cross-polarization and (d) soHd quinine mn using the...

While the combination of the heteronuclear dipolar decoupling and MAS provides a mean to obtain high-resolution isotropic spectra in solids, the serious problem still remains in addition to the relatively small magnetic moment and low natural... 

Fig. 4. A schematic diagram of the 2D PDLF (PELF) method using the BLEW-48 sequence for 1H-1H dipolar decoupling. CP denotes cross polarization. The last two jt/2 pulses of the S spin act as a Z filter. (Reproduced by permission of Elsevier Sicence.)...

Fig. 30.—Comparison of C-N.m.r. Spectra of Bovine, Nasal Cartilage in DsO at 37°. (Presented are spectrum A, obtained by using scalar decoupling with yHj/27r = 3.5 kHz, and spectrum B, by using dipolar decoupling of 65 kHz.)...

Figure 1. Three stages of resolution in a C-I3 spectrum of a cured epoxy. The top spectrum is obtained under conditions appropriate to a liquid-state spectrometer no dipolar decoupling and no magic angle spinning. Dipolar decoupling at 60 kHz is used for the middle spectrum and to that is added magic angle rotation at 2.2 kHz for the bottom figure.

The experimental design was to study both the carbon-13 and proton relaxation as a function of temperature for both polymer and solvent, and to extend these to as high a polymer concentration as the available equipment permitted. Inasmuch as the mechanical properties of polymers can be affected considerably by small amounts of diluents, we would ultimately like to approach the bulk polymer state, where use of strong dipolar decoupling and magic angle spinning are necessary. ... 

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.

However, it is found that a combination of techniques, such as proton dipolar decoupling (removes the dipolar interactions), magic angle spinning (reduces the chemical shift tensor to the isotropic chemical shift value), and cross-polarization (increases the sensitivity of rare spins, like 13C) applied to a solid state material, results in sharp lines for 13C nuclei in the solid state10). Thus, the observation of narrow lines or high resolution NMR in the solid state is possible. 

If i and j are different nuclei (for instance 13C and H) the hetero-nuclear dipolar interaction, which is often strong, can be removed by dipolar decoupling, which consists of irradiation nucleus j (say H) at its resonance frequency while observing nucleus i (say 13C). The time-averaged value of the Hamiltonian is then zero. 

MHz MAS CP DD (magic-angle spinning with cross-polarisation and H dipolar decoupling) 13 C NMR spectra performed at room temperature for the three poly(cycloalkyl methacrylates) (cyclopentyl, cyclohexyl, cyclohep-tyl) are shown in Fig. 8. 

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