Static polycrystalline spectra

Recently, Frydman et al. successfully analysed the Co static powder spectra of a series of cobalamins and cobaloximes measured at 4,7, 7.1 and 11.8 TP As pointed out by the authors, there were considerable changes in the Co quadrupole coupling constants depending on the crystallization conditions of the cobalamins, whereas the Co chemical shift tensors remained fairly constant. It was suggested that the different behaviour of the EFG and chemical shift tensors is due to a different dependence of these tensors on an uncharacterized structural rearrangement when the cobalamins were recrystallized. Furthermore, the close resemblance between the Co NMR data of cobalamins and cobaloximes illustrates that cobaloximes are much better model compounds of cobalamins than cobaltoporphyrins. In a study of polyammonium cobalticyanide supercomplexes, Zhou et al. combined the use 

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. 

Numerical simulation. Lineshape simulations are useful for spectra with well-defined singularities. Initial parameters for lineshape simulations may be obtained from the published NMR data of similar systems. Alternatively, moment analysis of the spectra could produce a useful initial guess. Since the degree of freedom involved in the spectral analysis of a Co spectrum is large, 

RF pulse and spinning speed effects/ (iv) When the second-order quadrupolar interaction is much smaller than the chemical shift anisotropy, the quadrupolar parameters can be estimated by monitoring the frequency shift of the centre band as a function of the Bo field and the chemical shift anisotropy can be determined by Herzfeld—Berger sideband analysis/ We shall discuss the applicabihty of these methods to Co systems in turn. 

The first strategy may be useful for a Co system with smaller than 1.2 MHz if we assume the bandwidth of a commercial MAS probe to be 0.5 MHz. It seems that La[Co(CN)6] is a good candidate for this approach. Also, it has long been known that the cobalt site of K2Na[Co(N02)6] has cubic site symmetry. However, no MAS studies have yet been done for these compounds. 

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FIGURE 7.10 2H (D) NMR spectrum of polycrystalline alanine-d3 at three temperatures. The line shape variation results from reorientation of the CD3 group, (a) At 123 K the powder pattern represents a nearly static CD3. (b) At 177 K, the line shape is distorted by motional averaging, (c) At 293 K, motion is fast enough to produce an undistorted axially symmetric averaged powder pattern. Spectra courtesy of Dennis A.Torchia (National Institutes of Health). 

Fig. 2 NMR spectrum of rotating and static sample of polycrystalline zinc phosphide Zn3P2- The two lines are attributed to crystallographically inequivalent phosphorus sites in the tetragonal unit cell. Reproduced with permission from [15]...

Figure 10 Static and MAS O spectra of amorphous Si02 and polycrystalline (C H ), SiOH obtained at 67.8 MHz. (a) H-decoupled static spectrum of Si02 without CP 108 scans, (b) H-decoupled MAS spectrum 100 scans, 7.6 kHz spinning sp>eed ( indicates spinning sidebands), (c) H—> 0 static spectrum of SiO 200 scans, 0.1 ms contact time, (d) H 0 mas spectrum of Si02, 200 scans, 0.1 ms contact time, (e) H-decoupled static spiectrum of (CfiHs) SiOH without CP 500 scans, (f) H-decoupled MAS spectrum of (CfiHs) SiOH 800 scans, 4.0 kHz spinning speed. All spectra were obtained using a 2 s recycle time. (From Ref. 26.)...

Figure 40 Static and MAS spectra of polycrystalline (Bu4N)iMo207 (a) H- Mo spectrum with 3477 scans, 5-s recycle delay, and Lorentzian line broadening of 500 Hz (b) H-decoupled Bloch decay spectrum with the same line broadening and number of transients as in (a) (c) 4.7 kHz MAS spectrum with H decoupling, 6970 scans, 5-s recycle delay, and 50 Hz of line broadening. (From Ref. 31.)...

Fig. 98. Left The Gaussian-broadened Gaussian relaxation fimction (bottom) (explanation see text). ZF pSR asymmetry spectrum in polycrystalline CeCuo2Nio.jSn at 0.08K (top). The dashed line is a fit of the static Gaussian Kubo-Toyabe relaxation function. The solid line is a fit of the static Gaussian-broadened Gaussian function. From Noakes and Kalvius (1997). Right The minimum polarization achieved by the Monte Carlo RCMMV static ZF muon spin relaxation functions, as a function of the reciprocal of the moment-magnitude correlation length, in units of the magnetic-ion nearest-neighbor separation in the model lattice. The horizontal line represents the 1/3 asymptote, above which the minimum polarization cannot rise. The dashed line is a...

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