Dipolar coupling lineshapes

For this reason, the study in question19 examined a sample of high-density polyethylene that was iso topically labelled with, 3C so as to produce isolated 13C spin pairs. Static 13C powder lineshapes were then observed as a function of temperature. Analysis of these by lineshape simulation shows that, indeed, the polyethylene chains do undergo 180° chain flips. The static lineshapes in this case result from the sum of chemical shift anisotropy and dipolar coupling. However, the chemical shift anisotropy is known and, as mentioned previously,... 

In the review period there have been several papers on 2H lineshape analysis. One of the most important is that discussing the validity of Markov models for modelling molecular dynamics6 discussed previously. Another important paper discusses the effect of dipolar coupling on 2H lineshapes, with particular emphasis on ND and ND2 groups.34 The effect of any dipolar... 

A special case occurs if the solid contains nuclear spin pairs that are sufficiently far apart from each other such that intrapair coupling dominates interpair coupling. In this situation, instead of a featureless broad lineshape, we observe a well-defined powder pattern known as a Pake doublet (Figure 15.15). Since the dipolar coupling between two nuclei... 

Both the heteronuclear and the homonuclear dipolar coupling depend on the orientation of the inter-nuclear vector and the inverse cube of the distance, and often give rise to broad lines in powders. However the characteristic lineshape has features, particularly singularities. For an isolated pair of spin- /2 nuclei a pair of strong singularities... 

The dipolar coupling is also central to many schemes for magnetisation transfer between nuclei using double resonance techniques such as cross-polarisation. The data become more difficult to unravel when a nucleus experiences numerous different dipolar interactions at the same time. Even an increase to just three spins causes the lineshape... 

The work of Hong et al.15 discusses the theoretical analysis of the dipolar lineshapes in the limit of rapid, anisotropic motion. The analysis assumes a motionally averaged dipolar coupling tensor, i.e. the average of the tensor over all the sites/orientations visited during the course of the motion, which has an effective anisotropy AD and asymmetry //>... 

Fig. 11. The LG-CP experiment of Fig. 9(a) applied to - C dipolar couplings in polycarbonate.15 (a) The full two-dimensional spectrum, (b) The LG-CP dipolar lineshape for the quaternary 13C (carbon 4), which as expected shows only very small - C dipolar couplings (as evidenced by the lack of observable splitting of the lineshape). (c) The LG-CP dipolar lineshape for carbon 3. (d) The best-fit simulation to the lineshape in (c). The motion used in this simulation (fast limit) assumes that the phenyl ring containing the carbon reorients about the local twofold axis of the ring through an angle of 120°, with the reorientation axis wobbling with an amplitude of 20°.

Fig. 14. The results of the constant time J-WISE experiment of Fig. 13(b) applied to L-alanine.18 (a) The full two-dimensional spectrum showing 111 lineshapes (with sidebands due to dipolar coupling), (b) Slices through the f dimension...

For proton-driven spin diffusion, the most important difference to abundant high-y spin systems is that the main contribution to the zero-quantum line-width is no longer the homonuclear dipolar coupling, but rather, the hetero-nuclear dipolar coupling to protons or other abundant high-y spins in the sample. Because the zero-quantum lineshape is largely independent of the size of the homonuclear dipolar couplings and, therefore, independent of the... 

Dipolar coupled lines in solids, especially in single crystals, are usually homogeneous, too, but in a different way. In this case, there are separate contributions to different parts of the line but they cannot be so identified because the coupling between the separate parts of the line is so good. The basic lineshape approximates a Gaussian and its width is totally unrelated to 1/T except that it must equal or exceed it. 

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