Powder pattern spectra

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. 6.4.2. chemical shift powder pattern spectra of Boc-Gly-Gly-[ N]Gly-OBz obtained by cross-polarization and H decoupling. (A) Crystalline phase (monoclinic). (B) Microcrystalline phase (triclinic). Dotted lines represent spectral simulations. Asymmetry, n, for the mono-clinic sample is 0.064 while 17 = 0.44 for the triclinic sample. Chemical shift reference is arbitrary. (Reprinted with permission from Hiyama et al. [10].)... 

This raises an important concern about solid-state NMR, the tensor element magnitudes need to be characterized for the nuclear site of interest and, preferably, for the molecule of interest in the conformation of interest. Fortunately, powder pattern spectra can be recorded and the tensor element magnitudes observed directly from the spectra (Fig. 6.4.3(A)). 

Fig. 6.4.3. chemical shift powder pattern spectra of labeled gramicidin A. (A) Trpn gramicidin A—experimental data obtained with cross-polarization and H dipolar decoupling at 20.3 MHz for N. (B) spectral simulation with o-n = 36, saturated solution of NH4N03. (C) [ C,]Leuio-[ N ]Trpn gramicidin A—experiment as in (A) displaying a combination of N chemical shift anisotropy and the — " C dipolar interaction. (D) spectral simulation with the same an values as in (B) and with = 0° and Pu = 106°. 

However, for all of these interactions it is important to have the dynamics effectively modeled. Librational dynamics of significant amplitude can average the tensor elements. Such motions are present even in polycrystalline samples [18]. Shown in Fig. 6.4.5 are powder pattern spectra at 276 K, below the gel to liquid crystalline phase transition that quenches global dynamics, but retains local dynamics [19] and at 143 K, well below the temperature that quenches most librational motions [20]. 

Fig. 6.4.5. N powder pattern spectra of [ N ]Trpi3 gramicidin A in a lipid environment as a function of temperature. (A) at 143 K all significant amplitude motions except for methyl and primary amine groups cease. Samples were fast frozen by plunging thin films into liquid propane.

Fig. 6.4.6. Detailed characterization of local dynamics in an indole side-chain. (A) " N powder pattern spectra as a function of temperature of —Trpn] gramicidin A in fully hydrated...

It is known that polyoxymethylene in the crystalline phase takes the all gauche conformation with a 9/5 helix [21]. However, the amorphous phase has a distribution of gauche and frans-conformations. Figures 7.10 and 7.11 show the CPMAS NMR spectra and powder pattern spectra of polyoxymethylene, respectively [22]. Sample A is a polyoxymethylene single crystal. To produce Sample B, Sample A is heated to 200°C and then quenched in ice water. Sample C is a melt-quenched sample of bulk polyoxymethylene and sample D is a bulk polyoxymethylene heated and cooled at a rate of... 

Fig. 9.7. CP powder pattern spectra of C-C24H48 at ambient temperature [11],...

In order to obtain further information about the molecular motion of the siloxane chain, the Si powder pattern spectra were recorded [40]. Figure 17.32 shows the Si CP NMR powder pattern spectra for the CPTMPS/DMS copolymer obtained without MAS at -90, 25 and 110°C. These powder pattern spectra predominantly come from the TMPS moiety, because the CP efficiency for the DMS moiety is negligibly small when compared with that for the TMPS moiety as seen from the Si CP/MAS NMR experiments. The anisotropy widths (Tu - <7-331 obtained from the tent-like powder pattern spectrum are 57.8, 55.0 and 52.0 ppm at -90, 25 and 110°C, respectively. The anisotropy width decreases as the temperature is increased. This indicates... 

Figure 7.7 shows an example of the energy of the spin sublevels of the triplet state as a function of H for // [a-, H y, and // z [61]. The behaviour of the spin energy levels is orientation-dependent [61], and the fields-for-resonance for the allowed Am = 1 transitions vaiY with the orientation of H relative to the triplet axes. Most ESR and ODMR studies of triplets states in biological molecules [58] and triplet excitons in crystalline semiconductors [55] have consequently been performed on single crystals. All of the samples described in this chapter, however, were polycrystalline or amorphous, and the triplet resonance lineshape is therefore the powder pattern obtained by averaging the spectra over all orientations [62]. Simulated powder pattern spectra for different values of E/D are shown in Figure 7.8 ... 

Figure 2.8 Typical powder-pattern spectra observed in polycrystalline solids Chemical Shift under (a) cubic, (b) axial, and (c) non-axial symmetries (d) Dipolar interaction between two spins 1/2 (I and S) Quadrupolar interaction for spins e) 1 and f) 3/2, considering an EFG with axial symmetry. The zero of the frequency scale in each case corresponds to the frequency associated with isotropic average (as occurring in liquids). The parameter A depends on the anisotropy of the tensors describing each of the interactions.

Structural Studies Based on -Factor Measurements. - Accurate measurements of spin-label magnetic parameters provide important data about the structure of nitroxide radicals. These data are also required for the analysis of the molecular dynamics of spin probes. The high resolution of HF EPR makes possible accurate determination of the principal axis components of the -matrix and y4-tensor from powder pattern spectra eliminating the need to prepare and study single crystals. 

The resulting principal values of the 13C chemical shift tensors of the C60 carbons are 8n = 228 ppm, 822 = 178 ppm, and 833 = -3 ppm. Tycko et al reportet the experimental values 8n = 213 ppm, S22 = 182 ppm, and 833 = 33 ppm obtained from low temperature measurements of a powder pattern spectrum (18). However, the spectra have a low signal to noise ratio and a wide slope so that a larger error for the experimental value can be assumed. The chemical shift anisotropy of 217 ppm corresponds quite well with the spectral range of about 200 ppm reported by Kerkoud et al for low temperature single crystal measurements (19). 

Figure 15.7. The 13C powder pattern spectrum of calcium formate.

As mentioned earlier, the symmetry at Si is slightly distorted octahedral, the site being surrounded by 6 oxide ions. Therefore, Nff at Si and CuH (also in a similar environment are expected to have similar e.s.r. spectra. The spectrum of CuH ion would be the more complex, however, because of hyperfine interaction with the magnetic nuclei Cu and Cu s. In contrast, all but 1-2 % (Ni i) of the Ni nuclei are non-magnetic. The e.s.r. spectrum of a sodium-reduced Ni (5 %)Y sample in which most of the Ni ions are believed to be at Si is shown in fig. 8. This powder-pattern spectrum is characteristic of a system with an axially symmetric -tensor ... 

The powder pattern spectrum of a dipolar coupled nuclear site can be expressed in the following form [33] ... 

Figure 9.7 shows the powder pattern spectrum of C-C24H48 at ambient temperature [11]. As the temperature in the CPMAS probe is thought to be more than 30°C, this is the spectrum above the first transition. The powder... 

The AlP nuclear magnetic resonance spectrum of a corundum or a-AlgOg single crystal is an orientation-dependent quintet arising from the quadrupole moment = - - 0.149 (136-139). O Reilly (140) obtained the Al dispersion mode envelope (141) powder pattern spectrum which results when y-alumina is heated to 1400°, and thereby eonverted to a-AlaOs. He interpreted the line shape in terms of the Redfield modification (142,143) of the Bloch equations. 

Dipolar interaction can lead to an EPR line broadening (Fig. 6). The spectrum of the interacting spins can be treated as the convolution of the non-interacting powder pattern spectrum with a dipolar broadening function which is known as Pake pattern in randomly oriented samples. 

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. 

Figure 15-6. (a) Phoioiuduccd absorption-detected magnetic resonance (ADMR) spectrum of MEH-PPV. HF and FF represents tire half field and full field powder pattern for the triplet (S=l) resonance, respectively, (b) ADMR spectrum ol MEH-PPV/CW, composite film. Both spectra were measured at probe energy 1.35 eV, T=4 K and 3 GHz resonant microwave frequency (reproduced by permission of the American Physical Society from Ref. 1191). 

FIGURE 9.5 CW ENDOR spectrum of 1-carotene radicals, (a) Experimental spectrum of Figure 9.4. (Reported in Wu, Y. et al., Chem. Phys. Lett., 180, 573, 1991.) (b) Simulated ENDOR powder pattern (using linewidth of 0.6MHz) for the sum of radical cation and neutral radicals in 5 3 1 1 ratio. (Reported in Gao, Y. et al., J. Phys. Chem. B, 110, 24750, 2006. With permission.)... 

Early treatments of powder patterns attempted to deal with the spatial distribution of resonant fields by analytical mathematics.9 This approach led to some valuable insights but the algebra is much too complex when non-axial hyperfine matrices are involved. Consider the simplest case a single resonance line without hyperfine structure. The resonant field is given by eqn (4.3). Features in the first derivative spectrum correspond to discontinuities or turning points in the absorption spectrum that arise when dB/dB or dB/dcp are zero ... 

At first glance, it would appear that all orientation dependence should be lost in the spectrum of a randomly oriented sample and that location of the g- and hyperfine-matrix principal axes would be impossible. While it is true that there is no way of obtaining matrix axes relative to molecular axes from a powder pattern, it is frequently possible to find the orientation of a set of matrix axes relative to those of another matrix. 

Biochemical EPR samples are almost always collections of randomly oriented molecules (frozen) aqueous solutions in which each paramagnetic molecule points in a different direction. In order to generate simulations of these powder EPR spectra we have to calculate the individual spectrum for many different orientations and then add these all up to obtain the powder pattern. Numerical procedures that generate sufficient spectra to approximate a powder pattern are collectively known as walking the unit sphere algorithms. Here is the basic procedure ... 

FIGURE 9.2 EPR powder pattern of the [2Fe-2S]1+ cluster in spinach ferredoxin. Trace A shows an attempt to fit the spectrum with the diagonal linewidth Equation 9.1. In trace B the spectrum is fitted with the nondiagonal g-strain Equation 9.18. Trace C shows an experiment in which the spectral features are slightly shifted (solid trace) under the influence of an external hydrostatic stress. (Data replotted from Hagen and Albracht 1982.)... 

Finally, in the very slow time regime, tumbling has become too slow to affect the regular powder spectrum under nonsaturating conditions, however, when, during a regular scan in which the external magnetic field is slowly scanned, an intermediate imposition of the powder pattern is partly saturated, then this saturation can be transferred... 

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