Dispersion, of chemical shifts

Bain and co-workers [115] studied the field dependence of 13C linewidths for polyacrylates crosslinked to low levels. The linewidth was directly proportional to field strength, consistent with a significant contribution from a dispersion of chemical shifts arising from new chain structures. In addition the authors used the Delays Alternating with... 

NMR spectroscopy has enormonsly facilitated investigations into structural aspects of complex lignin polymers. Principally dne to its high resolution and the high dispersion of chemical shifts, C NMR has played a major role in the qualitative and qnantitative understanding of lignin structure. Now modern two- and three-dimensional correlative methods are powerful tools for identifying structural units. 

It is cautioned, however, that the use of such a database from solution NMR is not always appropriate for the interpretation of solid-state NMR data, because of a possible source of errors arising from the chemical shift references for the solid and solution NMR studies. Moreover, further larger dispersions of chemical shifts from the solution NMR of globular proteins cannot always be ignored. Such data, however, are not always free from ambiguities in choosing the appropriate chemical shifts of the random coil, which are essential for data collection in solution NMR. These are simply treated as hidden parameters, and not always as necessary parameters for the interpretation of the solid-state NMR, unless NMR data of the fully hydrated samples are concerned. In fact, the chemical shift data available from such databases are in many instances biased as a reference for solid-state NMR because of choice of reference data for both solid and solution NMR. It is advised therefore to utilize the database from the solid state, if one aims to interpret the chemical shift data from solid-state NMR. " ... 

Of the nuclei, H and which both possess nuclear spin and are common to synthetic polymers, C is by far the more sensitive spin probe for polymer NMR studies. NMR spectra suffer neither from a narrow dispersion of chemical shifts (see Figs. 20.8 and 20.9) nor firom extensive homonuclear spin-spin (scalar) coupling, both of which complicate the analyses of H NMR spectra. (See below how two-dimensional observations increase the sensitivity of H NMR to molecular microstructures.) It is the sensitivity of resonance frequencies or chemical shifts, to the microstructures of polymers which makes NMR so useful as a structural probe. We noted in Fig. 20.9 that the methyl carbon resonances observed in the 25 MHz C... 

Frequently, the line width in atactic polymers is determined by an unresolved dispersion of chemical shifts from higher sequence levels. Consequently the linewidth (in Hertz) is proportional to the Bq field, and an increase in Bq may not therefore necessarily improve the resolution. 

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