A2B2 spin system

It may not be easy to distinguish AA BB and A2B2 spin systems when J/S becomes relatively small. 

The bicyclic phosphazene V forms colorless crystals, mp 184 to 185°. The compound is readily soluble in water, chloroform, and dichioromethane and is fairly soluble in hot benzene and moderately soluble in hot petroleum. The IR spectrum of the compound shows a split band at 1183 and 1218 cm" , attributable to a ring-P=N stretching vibration. The H NMR spectrum is discussed in the literature. The P H NMR spectrum is that of an A2B2 spin system (8a = 18.6, 8b = 15.3, Vab = 40.9 Hz). 

P- H n.m.r. spectra is shown in Fig. 8. Two isomers are apparent, which give rise to A2B2 and A MX spin-systems respectively, the former being favoured particularly at low temperatures. Only one isomer is evident with the N-methyl analogue (57)... 

The two systems shown in Figures 5.32 and 5.33 are, then, AB2 AvU < 10) and AX2 AvlJ> 10) in one case, and A2B2 (Av/J < 10) and A2X2 Av/J > 10) in the other. We will leave discussion of these types of spin systems to more advanced texts, such as those in the reference list at the end of this chapter. Remember that situations occasionally arise that cannot possibly be interpreted with only a limited knowledge. 

Figure 9 Illustration of second-order coupling for (a) A2X2 coupling system, (b) when approaches the chemical shift difference befween fwo coupled groups of spins to form an A2M2 system, and (c) it finally becomes an A2B2 system.

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