Isotropic Dimension

Advantages. Compared to DOR, a small rotor can be used allowing relatively fast spiiming speeds high RF powers can be attained and if the coil is moved with the rotor a good filling factor can be obtained. In the isotropic dimension high-resolution spectra are produced and the second dimension retains the anisotropic information. 

The final 2D spectra obtained by shearing or by using the split-/, pulse sequence can benefit from a proper scaling of the isotropic and anisotropic dimensions in order to facilitate comparisons between various experiments [171, 176-179]. In our opinion, the most convenient way to reference such 2D spectra is to define a ppm scale using, in the isotropic dimension, an apparent Larmor frequency depending on the given experiment and defined as... 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

Figure 4.39 AI MQMAS NMR of a steamed projection along the isotropic dimension. MTW zeolite powder. The spectmm was MQMAS clearly shows that the middle re...

Figure 6.25. O MAS NMR spectrum of the zeolite stilbite, showing at right the projections of the isotropic dimension of the 3QMAS spectra of the original isotopically-enriched sample, and after back-reaction with isotopically normal water vapour. From Stebbins et al. (1999) by permission of the Mineralogical Society of America.

Figure 10.16. NMR spectra of LiNbO.-j. A. MAS NMR spectra acquired at 14.1 T (spinning speed 18 kHz) tind 9.4 T (spinning sf)eed 25 kHz). B. DAS NMR spectrum also showing the ID projection of the isotropic dimension. C. Triple-quantum MAS NMR spectrum also showing the ID projection of the isotropic dimension. D. Pure-phase 2D nutation spectrum also showing the ID projection of the nutation dimension. From Prasad et al. (2001) by permission of the copyright owner.

A rigorous examination of the various MQ MAS sequences has been carried out with reference to sensitivity enhancement in the isotropic dimension and the lineshapes of the corresponding MAS peaks in the anisotropic dimension. An echo efficiency parameter has been defined as an indicator of the performance aspects of the various sequences. A consequence of the systematic analysis has been the combination of a spin-lock pulse for excitation of MQ coherences and an amplitude-modulated pulse for their conversion into observable single-quantum coherences. This approach has resulted in an improved performance over other sequences with respect to both the anisotropic lineshapes and the isotropic intensities. 

Orientation in crosslinked elastomers primarily reflects the configurational entropy and intramolecular conformational energy of the chains. However, as first shown by deuterium NMR experiments on silicone rubber (Deloche and Samulski, 1981 Sotta et al., 1987), unattached probe molecules and chains become oriented by virtue of their presence in a deformed network. This nematic coupling effect is brought about intermolecular interactions (excluded volume interactions and anisotropic forces) which can cause nematic coupling (Zemel and Roland, 1992a Tassin et al., 1990). The orientation is only locally effective, so it makes a negligible conttibution to the stress (Doi and Watanabe, 1991), and the chains retain their isotropic dimensions (Sotta et al., 1987). 

MQMAS of for bio-organic solids was shown by Wn et al. The observed spectral resolution in the isotropic dimension is nearly at the snb-ppm level, which approaches the intrinsic resolution limit determined primarily by qnadrupole relaxation. 

Figure 6 static and MAS (11 kHz) NMR spectra of kyanite, black and green (dark gray in the print version) traces, respectively, and their crystal structure. The bottom spectrum (blue (black in the print version) trace) is a triple-quantum sky-line projection (isotropic dimension) of the Al 3Q/MAS NMR spectrum of kyanite. All spectra were measured at 11.7 T. 

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