INDEX chemical shift anisotropy

To acquire better structural information, one has to resort to more advanced solid-state NMR techniques. Here, they will be reviewed by subject rather than in chronological order. First, we shall briefly look at P relaxation. Second, components of the H—> P CP signal will be discussed. Third, much attention will be given to HPO detection by differential H—> P cross-polarization (DCP). Then the P sideband pattern index and chemical shift anisotropy in the study of bone calcification will be briefly reviewed. Finally, a special ADRF-CP technique will be mentioned. 

The conventional, and very convenient, index to describe the random motion associated with thermal processes is the correlation time, r. This index measures the time scale over which noticeable motion occurs. In the limit of fast motion, i.e., short correlation times, such as occur in normal motionally averaged liquids, the well known theory of Bloembergen, Purcell and Pound (BPP) allows calculation of the correlation time when a minimum is observed in a plot of relaxation time (inverse) temperature. However, the motions relevant to the region of a glass-to-rubber transition are definitely not of the fast or motionally averaged variety, so that BPP-type theories are not applicable. Recently, Lee and Tang developed an analytical theory for the slow orientational dynamic behavior of anisotropic ESR hyperfine and fine-structure centers. The theory holds for slow correlation times and is therefore applicable to the onset of polymer chain motions. Lee s theory was generalized to enable calculation of slow motion orientational correlation times from resolved NMR quadrupole spectra, as reported by Lee and Shet and it has now been expressed in terms of resolved NMR chemical shift anisotropy. It is this latter formulation of Lee s theory that shall be used to analyze our experimental results in what follows. The results of the theory are summarized below for the case of axially symmetric chemical shift anisotropy. 

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