Axially symmetric chemical-shift tensor, powder line shape

Fig. 1. Powder line shapes for an anisotropic chemical shift (a) arbitrary chemical shift tensor (tr, a22 <733).

Fig. 8.6. The powder line shapes of an axially symmetric chemical-shift tensor. (Reproduced with permission from Ref. [26]. 1984 Pergamon Press, Inc.)...

Figure 3.2.10 Schematic representation of theoretical powder line shapes for the chemical-shift tensor, (a) - asymmetric shift anisotropy, (b) axially symmetric shift anisotropy.

Thus the chemical shift will have the same value for the field anywhere in the 2-3 plane but a different value when the field is perpendicular to the plane the chemical-shift tensor is axially symmetric. The chemical shift expected when the field is parallel to the unique axis is labeled a, whereas that expected for the field perpendicular to this axis is Figure 2B demonstrates this averaging and the resultant powder spectrum. Note the characteristic shape vwth the buildup of intensity at <7x- Both a, and individual components of the powder line shape (Seelig, 1978). 

Chemical-shift anisotropy is very sensitive to molecular structure and dynamics. Each nucleus can be pictured as being surrounded by an ellipsoidal chemical-shift field, A, arising from the influences of neighboring spins, as described by Eq. (4). If the molecules in the sample have no preferred orientational order, these tensors will be randomly distributed, and the line-shape is predictable. If the shielding is equivalent in all directions = (5yy = zi, A is spherical), a symmetric peak, like shown that in Fig. 29a, will be observed at qjso, which is defined in Eq. (5). Axial symmetry = Gyy A is, more or less, football-shaped) results in a powder pattern like that shown in Fig. 29b. In this case, the tensor elements may be labeled CTy ) and (g x and Gj ). If there is no symmetry in the chemical-shift field (gxx is a flattened football), then the... 

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