Dipolar decoupling, solid sample

Apart from MAS, another popular method for obtaining high-resolution NMR spectra is the efficient decoupling of dipolar interactions in solid samples. Section 3 is devoted to progresses in this area, which has witnessed renewed interests in the past few years. 

Figure 8. Pulse diagrams for solid sample NMR experiments a. 1h-13c cross polarization with dipolar decoupling b. WAHUHA - H dipolar decoupling c. T p relaxation sequence (time period, T, is varied) d. Tj relaxation sequence (time period, t, is varied).

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... 

The spin-lattice relaxation time for in solids is very long (several minutes). Since the nuclei have to relax before another excitation pulse can be sent, this requires hours of instrument time in order to collect a spectrum of reasonable intensity. A pulse technique called cross-polarization can be used to reduce this effect by having the protons interact with the carbon nuclei, causing them to relax more rapidly. FTNMR systems for sohd samples include the hardware and software to produce narrow line spectra from solid samples in a reasonable amount of time using high-power dipolar decoupling, MAS, and cross-polarization. 

The efficient relaxation of quadrupolar nuclei in solution generally results in self-decoupling, such that quadrupolar effects are generally not observed in NMR spectra of spin-1/2 nuclei. Self-decoupling may also be a factor in NMR spectra of solid samples this often manifests itself as an asymmetric line shape. By decreasing the temperature, the Ti relaxation time of the quadrupolar nucleus may increase sufficiently to allow acquisition of NMR spectra where the effects of residual dipolar coupling are sufficiently resolved. ... 

Solid state CP/MAS NMR spectra were recorded with a 75 MHz Chemagnetics spectrometer. The standard pulse sequence for cross polarization with bilevel dipolar decoupling was employed with a contact time and recycle delay of 1 ms and 3 s, respectively. Chemical shifts were referenced to hexamethylbenzene at 17.4 ppm. Additional details concerning sample preparation and NMR studies can be found in a forthcoming paper. ... 

Special probes have been developed for solid-state NMR that automatically position the sample at the magic angle. Modern instruments with MAS make the analysis of solid samples by NMR a routine analytical procedure. MAS, combined with two RF pulse techniques called cross-polarization and dipolar decoupling (discussed in Section 3.6.4), permits the use of the low-abundance nuclei C and Si to analyze insoluble materials by NMR, including highly cross-linked polymers, glasses, ceramics, and minerals. 

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