AA XX system

Four multiplets between Sh = 7.46 and 9.18 indicate monosubstitution of the pyridine ring, either in the 2- or 3-position but not in the 4-position, since for a 4-substituted pyridine ring an AA XX system would occur. The position of the substituents follows from the eoupling eonstants of the threefold doublet at Sh = 7.46, whose shift is appropriate for a p-proton on the pyridine ring (A). 

The pairs of fluorines in all of these molecules, except those in 1,1-difluoroethene, would also be magnetically equivalent. In order to be magnetically equivalent, nuclei that are chemically equivalent must have identical coupling constants to any other particular nucleus in the molecule, and it can be seen that the two protons in 1,1-difluoroethene do not have the same spatial relationship with respect to a given fluorine substituent. For example, the Fa substituent has a cis relationship to Ha, but a trans relationship to Hb (Scheme 2.30). A spin system such as this one is represented as an AA XX system, which contrasts with the A2X2, A2X, and A2XY systems in Scheme 2.29 wherein both fluorines in each of these systems have identical 2/HF coupling constants. 

These four-spin systems are characterized by two chemical shifts and four coupling constants, Jaa Jbb > Jab. and JAB.. The last two are not equal, leading to magnetic nonequivalence, which occurs for both AA BB and AA XX. A good example of an AA XX system was given in Fig. 6.1. We provide a brief discussion of these spin systems, because they occur frequently. 

The PM3 calculations showed that the twist-boat-chair conformation of dithiocin 307 is more stable by 5.9 kcal mol 1 than the corresponding chair-chair conformation. For the same compound, coupling constants for the AA XX system of the ethylene chain were simulated to be J(AA ) = 10.15 Hz,, /(XX ) OIIz, /(AX) =—14.0Hz, /(AX ) = 9.4Hz... 

Figure 9 (a) S8.88MHz Li NMR spectrum of a mixture of the dimer (Z)) and monomer (M) of ( )-l-lithio-2-(2-lithiophenyl)-l-phenylhex-l-ene (ca 0.3 M in THF/ TMEDA) (b) Li, Li ID INADEQUATE spectnun of the same sample which identifies the signals that belong to the Li, Li AA XX system of the dimer reference 0.1 M LiBr in THF. (Reproduced with permission from ref. [88])... 

An X-ray crystal structure of compound [268a] has been reported independently, (146) and the, 9F shifts and splittings given in this paper agree well with those in Table XV. The triplet spacing in the CF signal, however, was incorrectly attributed to accidental equivalence of the cis-and trans-P-CF couplings, rather than to deceptive simplicity in the AA XX system ... 

The AA XX systems - which are normally easily recognisable from the symmetry of their spectra are poro-disubstituted benzenes such as 4,4 -dimethoxybenzii (5) or a 4-substituted pyridine. 

---