Chemical shift asymmetry

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

The relatively poor resolution of the XPS systems has lead to an extensive use of deconvolution techniques in order to prove the presence of shifted core levels of low intensity in the presence of unshifted levels (thin oxide layers on metal substrates). Deconvolution techniques should be used only in those cases where the presence of multi components is shown up by a shoulder in the intensity distribution. Interpretation of asymmetric peaks in terms of chemical shifts can be misleading in some cases because the asymmetry may change due to a change of the electron population at the Fermi level as was demonstrated for the metallic oxide Ir02 [23, 24],... 

This mechanism arises from the asymmetry of the shielding tensor. Let us recall that the chemical shift in NMR has its origin in the screening (shielding) of the static magnetic field B0 by the electronic distribution at... 

Fig. 12 C -detected C CSA patterns of the SHPrP109 i22 fibril sample. The upper and lower traces correspond to the experimental and simulated spectra, respectively. Simulations correspond to the evolution of a one-spin system under the ROCSA sequence. The only variables are the chemical shift anisotropy and the asymmetry parameter. A Gaussian window function of 400 Hz was applied to the simulated spectmm before the Fourier transformation. (Figure and caption adapted from [164], Copyright (2007), with permission from Elsevier)...

Here, ak is the isotropic chemical shift referenced in ppm from the carrier frequency co0, SkSA is the anisotropy and tfk SA the asymmetry of the chemical-shielding tensor, here also expressed in ppm. Note that for heteronuclear cases different reference frequencies co0 are chosen for different nuclei (doubly rotating frame of reference). The two Euler angles ak and pk describe the orientation of the chemical-shielding tensor with respect to the laboratory-fixed frame of reference. The anisotropy dkSA defines the width and the asymmetry t]kSA the shape of the powder line shape (see Fig. 11.1a). 

Fig. 16 Temperature dependence of the P isotropic chemical shift (top) and line width (bottom) for RbH2P04 (left panel). The right panel shows the temperature-dependent asymmetry of the normalized proton potential (solid lines) in comparison to the normalized data shown in the right panel of Fig. 16 [25]...

Alternatively, in the so-called Haeberlen notation, the convention is <5zz- isol > l xx- isol > YY- iso / where the capital letters refer to the principal components and the isotropic chemical shift has a definition similar to Equation 1. The anisotropy (A5) and asymmetry ( ) are then defined as ... 

Figure 1 Simulations of CT lineshapes corresponding to (A) static and (B) MAS experiments, for different values of the asymmetry parameter rjq.The positions of the isotropic chemical shift (c5iso) and some well-defined singularities are shown, in terms of the parameter A, defined in Equation 8. All simulations were done with the DMFIT software. ...

Figure 14. 2SNa chemical shift data for Nal in H20-ethyl-enediamine (en) (45). — theoretical curve (Equation 74) with n = 4, K = 2.3, k = 0.8. The asymmetry occurs at the wrong end to account for by bidentate solvate formation by ethylenediamine... 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

Exercise 9-39 Show how one can use the asymmetry of the line intensities of the 60-MHz proton spectrum in Figure 9-45 to show which groups of lines are interconnected by spin-spin coupling. Write structural formulas for the compounds involved that fit the observed splitting patterns and chemical shifts. 

From the results shown in Table 1, the following conclusions can be drawn out Ci1 All the spectra correspond to non axially symmetric 31P tensors, (ii) the [dn - 033] difference ranges from 206 to 263 ppm and is definitely larger than the chemical shift difference observed in the liquid phase between a phosphonite and the corresponding thioxo-phosphonate, (iii) a linear relationship appears when the asymmetry parameter n is plotted against the intracyclic O-P-O bond angle a. 

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