Non-First-Order Splitting Patterns Strong Coupling

8 NON-FIRST-ORDER SPLITTING PATTERNS STRONG COUPLING 

The simple splitting patterns discussed above appear only when we have weak coupling when the chemical shift difference between two nuclei (expressed in hertz) is much greater than the J coupling between them. In this case, the coupling pattern is symmetric with a maximum of 2n peaks (for n coupled spin Vi nuclei), and the chemical shift is at the exact center of the pattern. When the chemical shift difference in hertz is on the same order 

Let s look at the simple AB system in more detail. Consider that the J coupling is held constant and the chemical shift difference An is gradually reduced (Fig. 2.23). We 

As we continue to reduce the B0 field, moving the Sa position even closer to the 5b position, the two inner lines become very close to each other, but they never meet or cross. The outer lines become so weak that they may appear as tiny bumps, or they may disappear into the noise, depending on the signal-to-noise ratio of the spectrum. At this point of 

Finally, when the two chemical shifts are exactly equal (Av = 0), the two inner lines become one and the two outer lines have zero intensity. The two protons Ha and Hb are chemically equivalent, that is, they have the same chemical shift, and we see no splitting at all. This explains, in a way, why equivalent protons do not split each other they do split each other but the inner lines coincide and the outer lines have zero intensity. Theoretically, the pattern still has four lines, but we only observe one a singlet. The same applies to any number of chemically equivalent protons, for example, a methyl group (CH3). All three protons have the same chemical shift, and in the absence of any other coupling (e.g., CH3O or CH3Cq, where Cq is a quaternary carbon), we will see a singlet with area proportional to three. 

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