Rule Ever Really Obeyed

In a linear chain, the n -1-1 Rule is strictly obeyed only if the vicinal proton coupling constants CJ) are exactly the same for every successive pair of carbons. 

Jab Jbc Jcd and so on Condition under which the n -H 1 Rule is strictly obeyed 

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FIGURE 7.34 Construction of a quintet for a methylene group with four neighbors all with identical coupling values. 

Chains of any length can exhibit this phenomenon, whether or not they consist solely of methylene groups. For instance, the spectrum of the protons in the second methylene group of propylbenzene is simulated as follows. The splitting pattern gives a crude sextet, but the second line has a shoulder on the left, and the fourth line shows an unresolved splitting. The other peaks are somewhat broadened. 

6 Measuring Coupling Constants from First-Order Spectra 263 

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