Secondary Equilibrium Isotope Effects on CS

One of the main trademarks of isotope effects on CS in tautomeric systems is the unusually large effects often of both signs and often far away from the site of isotope substitution. This characteristic is clearly different from that of the intrinsic isotope effects given above. The reason for this is the change in chemical equilibrium upon isotope substitution. However, in any situation a thorough knowledge about intrinsic isotope effects is necessary. 

From these two equations, the ratio can easily be obtained. However, this is complex unless the equilibrium part is much larger than the intrinsic part If this is the case, we obtain the very simple ratio AC — 1(XD)/AC — 3(XD) = (3CSH — 3C = S)/ SC = O — 3COH). For an illustration of this principle, see later in this section. 

One way of analyzing equilibrium systems is by the equation given by Bordner et al. [37]  

With this graph, the mole fraction a = [B]/[A] can be determined assuming that the mole fraction is less than 0.5 or larger than 0.5. It should, of course, also be noticed that the change in the equilibrium isotope effects is small for mole fractions close to 0.25 and 0.77. In order to take advantage of this curve, temperature experiments are very useful to determine the mole fraction. 

Deuterium isotope on cbemical sbifts is very suitable for the investigation of equilibrium isotope effects because of the large CS difference between the C-N and the C=N nitrogen. The chemical shift difference is estimated to be 100-140 ppm [40]. 

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