A pictorial view of chemical exchange

Consider, for simplicity, the exchange of a nucleus with spin / between two sites, A and B  

Although the dissociation rate constants and Tg 1 are not known, it is known that, at equilibrium, ta/tb = Pa/Pb- We will give a full description of the effect of variation of Ta (and tb) on the nuclear NMR parameters and describe the relevant equations involved [1,2]. 

When the exchange rate is further increased to the point where it is about equal to the difference Aco in resonance frequency of the two environments, the linewidth is also of the same magnitude. Therefore, the two signals are as broad as their separation and a single broad resonance is observed, extending from o a to b (Fig. 4.1A(5)). 

When the exchange rate becomes faster than A co, then the single resonance, centered at ( a + wb)/2, becomes sharper and sharper. From the NMR point of view, the nuclei are now experiencing an average environment and, as the exchange time decreases further, the linewidth eventually returns to the value of the separate signals in the absence of exchange (Fig. 4.1A(6-9)). 

It has been shown that by increasing r 1 the resonance frequency of each signal passes from g a or b to the average value ( a + o b)/2. This transition is not smooth, and the two signals begin to collapse only for xj values very close to A co. 

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