A2B spin system

From the foregoing discussion, it is evident that a variety of isomers are possible for unsymmetrically substituted cyclophosphines P NMR spectra are informative for distinguishing between these isomers. For example, two isomers of the cyclic trimers cyclo-(PR)2(PR ) are observed. The symmetrical isomer with identical substituents on the same side of the ring exhibits an A2B spin system, whereas the asymmetric isomer gives rise to an ABC spin system (Figure 11.5) (see Section 3.4.3). 

Thus the first case examined (Figure 4.23) is an AX spin system. The second case (Fig. 4.25) is an AB spin system, and the third case we examined (Fig. 4.27) is an A2X spin system. As AiV7 decreases, the A2X spin system approaches an A2B spin system, and the simple first-order splitting of the A2X spin system becomes more complex. 

---