Static dipolar coupling

Finally, two potential models for the Mo and Fe sites of sulfite oxidase have been prepared in which a hydrotris(3,5-dimethylpyrazolyl)borate complex of oxomolybdenum(V) is covalently attached to one of the phenyl rings of a tetraphenylporphinatoiron(III) derivative in which three phenyl rings carry / -Me groups and the other is either a 2,3- or 3,4-catecholate. Although the metals are not directly bridged in these complexes, they are held at fixed Mo - Fe distances. EPR spectroscopy at 4K shows distance-dependent static dipolar coupling between the Mo and Fe centers. At 77 K the dipolar coupling is modulated by rapid relaxation of the Fe center. Coordination of two A-methylimidazoles to the Fe centers produce S = l/2)Fe-(5 = 1/2)mo coupled systems that are not only... 

The dipolar pattern for polystyrene shown in Figure 3 (left) is not a Pake doublet reflecting only the CH static dipolar coupling (as scaled by WAHUHA irradiation of the protons). 

The isotropic chemical shift, the trace of the chemical shift tensor, is one of the basic NMR parameters often measured for both spin-1/2 and quadrupolar nuclei. The CSA can also be measured in non-cubic environments, such as the n3Cd nuclei experience in the chalcopyrite structure of crystalline CdGeAs2 [141] or CdGeP2 [142], and the 31P nuclei in the latter compound [142], Although the isotropic chemical shift can be measured from the NMR spectrum of a static powder because the CSA is zero in many cases because of cubic symmetry of the lattice, improved resolution is obtained by using MAS to remove dipolar couplings. Two particular areas where the isotropic chemical shifts have proven very informative will now be discussed, semiconductor alloys and semiconductor polytypes. 

Fig. 11.2 Solid-state 13C NMR spectra of a powder sample of U-13C-15N-glycine illustrating the broad NMR resonances in the static sample (a) and the effects of magic-angle spinning at 5 kHz (b) and 10 kHz (c). Both the chemicalshielding tensors and the homonuclear dipolar coupling...

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