Exchange processes, electron spin resonance

Electron spin resonance (ESR) data suggest that solution in pyridine does not significantly enhance the content of free radicals in humic substances. Pyridine does, however, significantly enhance their nitrogen contents (Table 5), especially in the cases of the fulvic acids, as was found also for extracts with EDA. Sorption, by ion-exchange and possibly by charge-transfer processes, might provide plausible explanations for the enrichments. 

Electron spin resonance (ESR) has been used to measure the rates of polymer adsorption and exchange [63 ]. Similarly to NMR, the method is based on a mobility criterion to measure the bound polymer fraction. However, it has the advantage that it can be readily carried out in systems rich in H such as water and that it has a much higher sensitivity. A major drawback of ESR is that the polymer must be spin labeled and again the label can influence the conformation and the adsorbed amount at the surface. For this method, only sufficiently concentrated dispersions can be used. For that reason it is only useful for relatively slow processes in which geometrical factors determining transport are of less importance. 

The contact chemical shift measurements establish both the magnitude and the sign of the coupling constant. In brief, this approach depends upon favorably rapid ligand exchange and electron relaxation rates (114,115). When these processes are rapid the nmr resonances of the amine ligand are shifted but the lines are not unduly broadened. The extent of the shift depends on the pi spin... 

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