Line chemical shift anisotropy

Fig. 2.2 Behaviour of the longitudinal (bold lines) and transverse (dotted lines) chemical shift anisotropy relaxation times (CSA) and T CCSA) for two field strengths (2.114 and 11.74 T) and two values of the chemical shift anisotropy (100 and 500 ppm) (axial symmetry - isotropic reorientation)...

Chemical shift anisotropy 5.5,5 xxr yy zz Line-shape analysis MAS-sidebands Coordination symmetry... 

Figure 3 Characteristic solid state NMR line shapes, dominated by the chemical shift anisotropy. The spatial distribution of the chemical shift is assumed to be spherically symmetric (a), axially symmetric (b), and completely asymmetric (c). The top trace shows theoretical line shapes, while the bottom trace shows rear spectra influenced by broadening effects due to dipole-dipole couplings.

Here, ojr is the rate of spinner rotation. I is the proton spin number, 8 is the chemical shift anisotropy (CSA) and q is the asymmetric parameter of the CSA tensor. Thus, the line broadening occurs when an incoherent fluctuation frequency is very close to the coherent amplitude of proton decoupling monotonously decreased values without such interference in Figure 1. 

Fig. 2 Experimental analysis of the chemical shift anisotropy of high-silica ZSM-5 zeolite, (a) 29Si MAS NMR and (b) extracted CSA lineshapes from a two-dimensional CSA recoupling sequence dashed lines are simulated lineshapes. Adapted with permission from [79]. Copyright 2008 American Chemical Society...

Solids—Many polymers are either soluble or insoluble. NMR of solids generally give broad lines because of the effects of dipolar coupling between nuclei and the effect of chemical shift anisotropy (CSA). Both of these effects are greatly reduced for polymers in solution and allow for decent spectra of soluble polymers in solution. 

Figures 15 and 16 show the temperature dependence of the chemical shift for 80% deuterated KD2PO4 and RbH2P04. Obviously, iso varies significantly with temperature in the paraelectric phase and shows a clear break at Tc of 202 K for DKDP and 147 K for RbH2P04, respectively. The shift exhibits a distinct discontinuity at Tc while the line width shows an abrupt increase below Tc, in agreement with the (close to) first-order nature of the phase transition, and an anticipated pronounced distortion in the PO4 moiety. The cause of the line width increase below Tc has yet to be explained, but is at least partly due to a lack of the increased spinning speed to average out the enhanced chemical shift anisotropy below Tc.

Measurements of the static 13C line shape or sideband intensities of acetone on many solid acids at room temperature underestimate the chemical shift anisotropy due to motion, but the principal components of the chemical shift tensor can be accurately measured at reduced temperature. Table V reports these data for acetone on a wide variety of Brpnsted and Lewis acids (43, 45) note that the largest contribution to the isotropic shift is <5n The shift induced by A1C13 and other Lewis acids is rationalized by... 

Standard (gel-phase) NMR spectra of polymers usually show significant line broadening, mainly because of chemical shift anisotropy and dipolar coupling [98], Only nuclei with strong chemical shift dispersion, e.g. 13C [99-106], 15N [107], 19F [108-112], and 31P [113] give sufficiently resolved gel-phase NMR spectra. The resolution of... 

It should be noted that the activation energies for motional processes in the same crosslinked polymer gel calculated from line widths in 13C NMR spectra are higher than those calculated from 1H NMR spectra under the magic angle conditions (residual line width). These findings indicate that 13C line widths (which are of the order of 10 Hz) are probably more affected by sample inhomogeneity and by relatively small residual chemical shift anisotropies 162). 

The carbon-proton dipolar interaction and the chemical shift anisotropies broaden the lines in solid state 13C NMR spectra. The major effect arises from the dipolar coupling of the carbon nuclei with neighboring protons homonuclear dipolar couplings between two adjacent 13C nuclei are neglegible because of their low natural abundance. The large magnitude of dipolar 13C— H coupling (up to 40 kHz) results in broad and structureless proton-coupled 13C NMR absorptions. 

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) ...

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