Spectra static powder

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. 

Representative spectral patterns are shown in Fig. 10.20 for a static powder sample. The three spectra show the impact that the degree of chemical shift symmetry in space has on the observed spectrum. 

Fig. 7. I3C spectra of iPB (form I). (a) SUPER cross-sections at the six peak maxima of the MAS spectrum in (b). The splittings of the CH2 resonances in (b) have been attributed to packing effects.91 (c) Simulation of the static powder pattern shown in (d), based on the experimental deconvolution in (a). The underlying six powder patterns are also shown the CH2 patterns were scaled up by a factor of two for clarity. Adapted from ref. 28. Reproduced with permission of Academic Press.

In the case of MAS, the spectra typically display a number of spinning sidebands spaced by the spinning frequency. The intensities of the spinning sidebands approximately represent the intensity of the static powder spectrum and hence the overall envelope of the sideband intensities represents the powder line shape. While analytical solutions have also been derived for the intensity of the sidebands as a function of the spinning frequency and anisotropic shielding parameters, it is typically much easier to determine these interaction parameters from numerical simulations - again considering that this method directly takes the experimental errors and spectral noise into account. 

Any motion on the fast or intermediate timescale changes the appearance of the static powder spectrum, as shown in Fig. 6.2.2 on the left. When the orientation of the C—bond gets rapidly averaged around a symmetry axis (with Tc I/Aj q), the spectrum retains its axially symmetric powder lineshape but its width is narrowed by a geometric factor. Given that 8 is the new effective angle between the motional symmetry axis and Bq, Equation... 

Figure Bl.12.1. (a) Energy level diagram for an 7 = nucleus showing the effects of the Zeeman interaction and first- and second-order quadrupolar effect. The resulting spectra show static powder spectra for (b) first-order perturbation for all transitions and (c) second-order broadening of the central transition, (d) The MAS spectrum for the central transition. ...

These could consist of a whole range of Mo(VI) and Mo(IV) mixed oxygen/ sulfur compounds. There is no evidence for the presence of MoOi, except in one case. Simulation of the static powder lineshapes allowed the deconvolution of the various components, thus yielding values of quadrupole parameters as well as relative intensity data. From the spikelet experiments it was ascertained that all species are present as both static adsorbed and dynamically active phases. The Mo spectrum of the used catalyst shows the presence of the tetrahedral molybdenum-oxo species, along with a much reduced MoSt resonance (relative to the fresh system), perhaps suggesting that MoSt is the active site in the... 

In contrast to the SNMR method, it is crucial to measure an undistorted polycrystaUine static spectrum in order to have a reUable spectral analysis when the effects of finite RF pulses and detection bandwidth are ignored in the numerical simulation. Since Co static powder spectra in general have well-defined singularities and therefore are suitable for lineshape simulation, the issue of spectral distortion seems to be particularly important for Co NMR practitioners. In this section we will give a brief account of lineshape analysis, including experimental considerations and precautions for numerical simulations. 

General Description of Spectra. Figure 5(a-c) shows the static C NMR solid state spectra of Hytrel 4056, 7246, and polyfbutylene terephthalate) obtained with cross polarization and dipolar decoupling. Each spectrum contains a broad carbonyl/aromatic resonance which extends from ca. 250 ppm upheld into the aliphatic region, an —OCHi— peak centered at ca. 70 ppm, and a peak due to —CH2 HiCH2— carbons at ca. 29 ppm. The intensity differences in the aliphatic region are due to relative differences in the amounts of hard and soft segments in the polymers. Other than these intensity differences, little information is availaUe directly from these static powder spectra. 

Figure 5. Proton-enhanced static powder spectra of a, Hytrel 4056 b, Hytrel 7246 and c, polybutylene terephthalate, compared with the respective calculated spectra of the carbonyl and aromatic regions (d-f). The experimental spectra were obtained at ambient temperature on 0.3 g of sample. Each spectrum represents 8192 accumulations using a 0.5 ms contact time and a 3 s recycle delay. The calculated spectra were obtained using the values listed in Table III.

F . 9.4 Simulated static powder spectrum first-order quadrupolar coupling of a spin / = 1 (with added noise) for the anisotropic broaden- nucleus, e. g., H. (Reproduced by permission of ing due either to a dipolar coupling between the SocieU Italiana di Fisica from (5).) an isolated pair of spin / =1/2 nuclei or to the... 

Figure lA. Line narrowing by the MAS technique of the 270 MHz NMR spectrum of a dehydrated HY zeolite, (a) static powder spectrum (b) MAS spectrum at 2.5 kHz rotation frequency ( sealed glass ampoule) (20). 

Figure 1. Deuteron NMR line shapes. Top static powder pattern for a macroscopically disordered sample in the absence of molecular motions (77 = 0 case). Middle , motionally narrowed powder pattern for a macroscopically disordered mesophase with axially symmetric molecular motions (( ]) = 0). The ratio of the peak splittings in the top and middle spectra defines the microscopic order parameter. Bottom , doublet spectrum of a macroscopically aligned mesophase. If the microscopic order parameter is known, the angle between the director and the magnetic field can be obtained from the splitting.

Another example of the use of NMR to discriminate between motional modes involves the solid-solid phase transition of nonadecane. The transition involves a change from orthorhombic to hexagonal packing of the alkane. The orthorhombic phase gives rise to a static powder spectrum (Figure 6A) Transition to a... 

Figure 6.9 Left Experimental Al MAS NMR spectra (7.1 T) of the central and satellite transitions for AICI3-6H20 obtained without spinning and with spinning speeds of Vr = 12.0 kHz and 2.0 kHz. Right simulations of the full static-powder spectrum along with subspectra of the central transition (Cen), the inner (In) and outer (Out) satellite transitions and the MAS spectrum including all transitions acquired with Vr = 2.0 kHz. The vertical expansion factors are indicated at the right-hand side. The spectra were simulated with the parameters 5 so = 0.0 ppm, Cq = 0.116 MHz and tIq = 0.0, as determined from the experimental spectra.

This compound has two crystallographically distinct vanadium sites. While the static spectrum is a superposition of two powder patterns of the kind shown in Figure 3, MAS leads to well-resolved sharp resonances. Weak peaks denoted by asterisks are spinning sidebands due to the quadrupolar interaction. 

As a consequence of the small quadrupole moment of Li, the quadrupolar interaction in solid state NMR spectra is much smaller for Li than for Li. This has been used to advantage for the determination of the Li chemical shift anisotropy from the Li static solid state powder spectrum of 2,4,6-tris(isopropyl)phenyllithium (see below) . Applying MAS up to 10 kHz, the CSA contributions to the lineshape can be completely ehminated in most Li spectra of organolithium compounds. If the measurement of the quadrupolar... 

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