Slanting Multiplets and Second-Order Strong Coupling Effects

A3X3 spin system, where A3 represents the three magnetically equivalent hydrogens and X3 the three magnetically equivalent fluorines. Of course, if bond rotation were stopped, the system would become an AA A XX X system, because the magnetic equivalence would have been lost. 

But since para (five-bond) couplings (A-B and A -B) are essentially zero, we can regard the spin system as two equivalent (and superimposed) ABX systems.  

9 SLANTING MULTIPLETS AND SECOND-ORDER (STRONG COUPLING) EFFECTS 

All of the coupling and multiplets we described in Chapter 8 result from what is called first order (or weak) coupling. Now we will introduce a complication. 

Why is the asymmetry very slight in the higher field spectrum and much more pronounced in the lower field spectrum To answer that question, we need to examine the quantity A 8v/7, where A 8v is the difference in chemical shift (measured in hertz Section 5.1) between the two multiplets and y (in hertz) is the absolute value (magnitude without sign) of the coupling constant they share. 

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From the above example, you can infer that the extent to which the intensity ratios depart from first-order (Pascal s triangle) expectations is a function of A 8v/7. If this ratio is large (A 5v/7 > 10), we describe the spin system as weakly coupled, and the resulting multiplets will exhibit essentially first-order intensity ratios (as do the triplets and quartets in Figures 8.1 and 8.13). But as A 8v/J decreases, second-order effects (multiplet slanting and even the appearance of extra lines) become increasingly apparent. Such a spin system is said to be strongly coupled. 

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