More About the ABX System Deceptive Simplicity and Virtual Coupling

12 MORE ABOUT THE ABX SYSTEM DECEPTIVE SIMPLICITY AND VIRTUAL COUPLING 

Before we leave the topic of second-order effects, we revisit the three-spin system previously introduced in Example 9.15. There we established that if the chemical shifts of the three nuclei in a structure such as 9-3 are sufficiently different (i.e., an AMX system), the spectrum will consist of 12 lines 3 (first-order) doublets of doublets, each one centered at the appropriate chemical shift and exhibiting line spacings equal to the appropriate coupling constants. 

Let us now consider the consequences of changing some of the relationships between the nuclei. Suppose we were to make two of the nuclei magnetically equivalent to give an A2X system. The resulting spectrum is determined by just three parameters the chemical shifts of nuclei A and X and the coupling constant between them (Ax) any coupling between the two equivalent A nuclei would not affect the appearance 

TABLE 9.7 Simulated Spectra for IVo ABX Spectra Differing Only in the Sign of One J Value  

Position (Hz) Intensity Apparent J Position (Hz) Intensity Apparent J 

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