Heterogeneously broadened line

However, low-temperature spectroscopy reveals that the chromophore absorption spectrum is heterogeneously broadened, and Frauenfelder has shown that there is a functional link between the spectral line position and the recombination rate. Agmon and Hopfield pointed out that the protein myoglobin is photolyzed into a strained configuration and must undergo conformational relaxation to reach the true deoxy configuration, and that the recombination rate is slowest in the fully relaxed deoxy... 

It is difficult to give an exact limit because the impact of thickness broadening depends on the intrinsic width of experimental lines [31], which often exceeds the natural width 2r at by 0.05—0.1 mm s for Fe as studied in inorganic chemistry. This inhomogeneous broadening, which is due to heterogeneity and strain in the sample, causes a reduction of the effective thickness. Rancourt et ai. have treated this feature in detail for iron minerals [32]. 

Since its first description in 1971 [35], gel-phase NMR was applied to peptide chemistry by Manatt and coworkers [36, 37], These authors used 13C NMR to determine the extent of chloromethylation of crosslinked polymers and 19F NMR to monitor protection-deprotection reactions. These two nuclei are the most commonly used in these types of studies, mainly because of their significant chemical shift dispersion, which can alleviate in part the resolution loss due to the non ideal linewidth obtained in the gel state. Apart from restricted molecular motion, that shortens T2 because of an efficient transverse relaxation, other sources of line-broadening derive from magnetic susceptibility variations within the sample (due to the physical heterogeneity of the system) and residual dipolar couplings. 

MAS NMR comes from an old observation (172) that spinning a heterogeneous NMR sample at an angle of 54.736° reduces the line broadening of solid/swollen... 

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