Chemical asymmetry parameter

Most valuable chemical information can be extracted from Mbssbauer parameters such as the isomer shift (5), the quadrupole splitting (AEq), the magnetic splitting (AEm), and the asymmetry parameter (n). 

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)...

To calculate the nuclear quadrupole moment from the measured quadrupole splitting, it is necessary to know the electric field gradient, q, at the Te nucleus as well as the asymmetry parameter, rj. These can be calculated in the Townes and Dailey approximation (4) by knowing the chemical bonding in Te. 

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. ...

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) ...

The chemical shielding anisotropy (CSA) tensors for the simple phosphate structures listed in Table I, were calculated using ab initio methods. The isotropic tensor value (aiso), the three principal components (aa), the CSA anisotropy (Act) and asymmetry parameter (rj) were evaluated and are given in Table II. 

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. 

In these studies, the parameters that could provide the most interesting information are likely to be the electric field gradient (nuclear quadrupole coupling constant) at the 33S nucleus and its asymmetry parameter. Indeed, modifications of the lattice structure in different cement matrixes are expected to influence the symmetry of the electronic distribution around the sulphur nucleus more than the chemical environment of sulphur. 

The anisotropy AS, asymmetry parameter i] and isotropic chemical shift <5iso are expressed by the following equations ... 

The Townes and Dailey method thus describes chemical bonding in terms of three parameters ionic character, s-hybridization and multiple bonding, whilst at the most two parameters are experimentally available, ezQqjh and rj. Even in such favourable cases where the extent of multiple bonding can be shown to be zero if it is axially symmetric or determined from the asymmetry parameter if it is not, one is still left with the problem of deciding the extent of either s-hybridization or the percentage ionic character. 

Table 5.6. Oxygen-17 (both bridging, O, and nonbridging, 0 t) nuclear quadrupole coupling constants, asymmetry parameters (iq), and isotropic chemical shifts (8,) for the alkaline earth metasilicates"...

In C NMR spectroscopy, deviations from a Lorentzian lineshape, which is usually obtained in liquids, can be caused by a chemical shift anisotropy (CSA). If a CSA is present, the position of the resonance line depends on the relative orientation of the molecule with respect to the direction of the magnetic field applied (27,22). The superposition of the individual resonance lines results in typical lineshape patterns that can be described by two parameters the chemical shift anisotropy, AS, and the asymmetry parameter, Tj, respectively. In the case of an axially symmetric CSA tensor, i.e., 17 = 0, the relation between the resonance frequency, w, and the orientation of the molecule is given by... 

Fig. 17.9 Fano asymmetry parameter ( q) as a function of distance from the catalyst for (a) annealed and (b) oxidized Si NWs that were in situ boron doped in the lower half and intrinsic in the upper half (With permission from reference [46]. Copyright (2009) by the American Chemical Society)...

Fig. 3 The effect of slow magic-angle spinning. A set of spinning sidebands appears with a centre-band at the isotropic chemical shift and further lines spaced at the spinning frequency. The intensities of the sidebands change with spinning speed with higher-order sidebands (i.e., those further away from the centre-band) becoming less intense as the spinning speed increases. The chemical shift parameters used in the calculation of these sideband patterns are isotropic chemical shift offset 0 Hz chemical shift anisotropy 5 kHz asymmetry 0. Reproduced with permission from [16]...

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