Simple Spin Coupling

Since chemical shift increments are approximately additive, it is possible to calculate the ring proton shifts in polysubstituted benzene rings from the monosubsti-tuted values in Appendix Chart D.l. The chemical shift increments for the ring protons of m-diacetylbenzene, for example, are calculated as follows. 

41 and 4.42. Finally, the spin coupling pattern for each isomer would be distinctive, as will become evident at the end of Section 4.8 and on study of Appendix F. Obviously, proton NMR spectrometry is a powerful tool for elucidating aromatic substitution patterns—as is carbon-13 NMR (see Chapter 5). Two-dimensional NMR spectrometry offers another powerful tool (see Chapter 6). 

FIGURE 4.23. Spin coupling between two protons with very different chemical shifts. 

Geminal coupling Vicinal coupling 2 bonds(V) 3 bonds(V) 

Suppose that two vicinal protons are in very different chemical environments from one another, as in the com-OR CR, 

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Kaupp, M., Malkina, O.L., Malkin, V.G. and Pyykkd, P. (1998) How do spin-orbit induced heavy-atom effects on NMR chemical shifts function Validation of a simple analogy to spin-spin coupling by DFT calculations on some iodo compounds. Chemistry - A European Journal, 4, 118-126. 

The F—H spin-spin coupling constants of these compounds remain much the same as those of the simple alkenes. 

Because of spin spin coupling, most nmr spectra do not show simple single peaks but rather groups of peaks that tend to cluster about certain 8 values. To see how this coupling arises we examine the molecular fragment... 

However, all three particles in this equation are fermions with intrinsic spins S = jh. Therefore, we cannot balance the angular momentum in the reaction as written. The spins of the proton and the electron can be coupled to 0 or 1 ft and can also have relative angular momenta with any integral value from the emission process. This simple spin algebra will never yield the half-integral value on the left-hand side of the equation. Another fermion must be present among the products. 

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