Dipolar decoupled magic angle spinning

An extensive study was conducted on the effect of chemical and structural aspects of zeolites on the fire performance of the intumescent system, ammonium polyphosphate-pentaerythritol (APP-PER), where a marked improvement of the fire-retardant properties within different polymeric matrices has been observed.56 58 The synergistic mechanism of zeolite 4A with the intumescent materials was investigated using solid-state NMR. Chemical analysis combined with cross-polarization dipolar-decoupled magic-angle spinning NMR revealed that the materials resulting from the thermal treatment of the APP-PER and APP-PER/4A systems were formed by carbonaceous and phosphocarbonaceous species, and that the zeolite enhances the stability of the phosphocarbonaceous species. 

Instruments are now available commercially that incorporate dipolar decoupling, magic angle spinning, and cross polarization, thus making possible the acquisition of high-resolution C spectra from solids. Figure 19-30, which shows spectra for crystalline adaman-tane collected under various conditions, illustrates the power of these instruments. 

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. 

The modem methods of taking NMR spectra involve the use of very short radio frequency pulses (of variable duration from 1 to 200 ms) instead of a continuous signal as in older NMR. This requires full automation of the test, the Fourier transform analysis, data storage and multiple scan capability. With the scalar (low power, ca. 4 kHz) and dipolar (about 45 kHz) decoupling, magic angle spinning and cross polarization methods one can obtain spectra of solid samples with resolution similar to those known for liquids. The spectra provide precise information on the... 

Fig. 8 Schematic diagrams for the following pulse sequences (A) single pulse excitation/magic-angle spinning, (B) total suppression of sidebands, and (C) delayed decoupling, or dipolar dephasing.

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.

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