Other Examples of Reversible Complexation Chemical Shift Reagents

7 OTHER EXAMPLES OF REVERSIBLE COMPLEXATION CHEMICAL SHIFT REAGENTS 

In Section 9.9 we discussed how the appearance of a spin-coupled NMR spectrum is determined by the ratio of Av (the difference in chemical shifts between the coupling nuclei) to J (the coupling constant they share). For the spectrum to exhibit first-order multiplet intensities (Pascal s triangle Section 8.5), the value of Av//has to be at least 10. Smaller values of Av/J lead to progressively greater complications due to second-order effects. 

Although it is possible, in principle, to analyze a second-order spectrum through computer simulation (Section 9.10), it would be easier to interpret such a spectrum if we could somehow force it into a first-order appearance. Since the magnitude of J is determined solely by the molecule s structure (and is field independent Section 8.2), the only way we can increase Av/7 is to increase Av. One way to accomplish this is to use a spectrometer with a higher field strength and operating frequency (Section 9.9), but such an instrument may not be readily accessible to us when we need it. In many cases there is a relatively simple way to accomplish the same thing  

An example of PSR-induced shifts can be seen in the 80-MHz H spectrum of dibutyl ether  

For example, the 60-MHz H spectrum of cyclopropane derivative 10-10 dissolved in carbon tetrachloride exhibits only one sharp signal at 8 1.45 ppm,9 indicating that the two sets of hydrogens (six methyl and two methylene) are accidentally equivalent (Section 4.4)  

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