Isotropic spectra

Shearing of the data is perfonned to obtam isotropic spectra in the FI dimension and to facilitate easy extraction of the ID slices for different peaks. Shearing is a projection of points that lie on a line with a slope equal to the anisotropy axis onto a line that is parallel to the F2 axis [24]- Shearing essentially achieves the same as the split-t experiment or delayed acquisition of the echo. Although sheared spectra may look more attractive, they do not add any extra infomiation and they are certainly not necessary for the extraction of QIS and values. 

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... 

In some systems, when an anisotropic spectrum is detected at low temperatures, increasing the temperature leads to averaging of the principal components of the hyperfine and g tensors, and therefore to isotropic spectra. This is often the situation in polymer systems at temperature above the glass transition temperature, Tg. 

Most of the information content from ESR spectra of organometallic radicals and coordination complexes comes from dilute single-crystal spectra or frozen solution spectra. Nonetheless, there are some bits of information and applications that come uniquely from isotropic spectra, and we discuss those aspects in this chapter. 

Another example comes from the work of Johnson, et a/.18 These workers studied spin labels dissolved in lipid bilayer dispersions of dipalmitoylphos-phatidylcholine and cholesterol (9 1 by weight) in the hope that anisotropic rotational diffusion of the spin label would mimic the motion of the bilayer components. In addition to 5-DS, which is sensitive to rotational motion about the NO bond, they used the steroidal nitroxide 8, which tends to rotate about an axis perpendicular to the N-O bond. ESR measurements were carried out at both 9 and 35 GHz and at temperatures ranging from 30 to 30 °C. Rather different results were obtained with the two spin labels, largely as a result of the different axes of rotation. Because the rotation rates were very slow, ESR spectra appeared as powder patterns rather than isotropic spectra and special methods were needed to extract the motional data. 

The combination of MQMAS and DOR into a single scheme led to the development of the MQDOR method [204,205]. In MQDOR a full averaging of the second-order interaction is achieved in the ti (MQ evolution under DOR condition) and t2 domains (DOR), yielding truly isotropic spectra in both dimensions, where the positions of the lines are given by (75). 

Frydman, L and Harwood, ).S. (1995) Isotropic spectra of half-integer quadrupolar spins from bidimensional magic-angle spinning NMR./. Am. Chem. Soc., 17, 5367-5368. 

The four-coordinate Ni(I) complexes with saturated ligands exhibit anisotropic axial spectra with gg values being greater than g L values. The Ni(II) species of ligand-stabilized radical, which is produced from the reduction of the four-coordinate Ni(II) macrocyclic complexes with conjugated double bonds, exhibit isotropic spectra. The isotropic spectra of Ni11 (mac-) turn to anisotropic axial spectra as the complexes coordinate CO to form five-coordinate Ni(I)-CO adducts (135,137). The anisotropic axial epr spectra of four-coordinate Ni(I) complexes become rhombic as the complexes coordinate extra axial ligand such as CO or acetamide to form five-coordinate Ni(I) species (135, 136). The epr spectral data of some Ni(I) complexes are summarized in Table VII. 

In order to compare primary dynamics with secondary relaxation steps, we depict on the left-hand side of Fig. 15 the anisotropic spectra (a-c), which consist mainly of spectral components with the same linear polarization as directly induced by the pump pulse. On the right-hand side of the figure the corresponding isotropic spectra (d-f) are shown. In the latter spectral components can notably contribute that result from a relaxation process, where the initially orientation of the OH transition dipole is (partially) lost. 

Blinc et al.71 have carried out 207Pb NMR in an electric field. They have observed separately the anisotropic 207Pb spectra of FE like polar clusters and isotropic spectra of spherical glass matrix into which the polar clusters are embedded, confirming the model proposed by Bums and Dacol. They concluded that PMN to be an incipient FE. 

For first-order interactions (dipole-dipole coupling, chemical shift anisotropy, first-order quadrupolar interaction) the expansion in Eq. (23) contains only the zero- and second-rank terms. As shown in Fig. 3, the Legendre polynomial of rank 2 is zero at the magic angle (P2(cosXm)=0 for Xm=54.74°). Therefore, under MAS, and provided that the spinning rate is larger than the anisotropic linewidth, the anisotropic terms of the first-order interactions are averaged to zero and isotropic spectra are obtained. 

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