Gauche terminal substituents

Let us complete our discussion of the conformational connection between the microstructures and NMR spectra of polymers, which is provided by the conformationally sensitive y-gauche effect, by considering the nonequivalent values for e isopropyl methyl carbons in several branched alkanes [8,20, 21] as presented in Table 2.5. Even though the isopropyl methyl carbons in each alkane have the same a-, fi-, and y-substituents, we note in column 2 that the observed nonequivalence progressively decreases as the number of carbons separating the terminal isopropyl group from the asymmetric center is increased. This behavior can be understood [22] if we focus on the source of the nonequivalent values observed for the isopropyl methyl carbons in 2,4-dimethylhexane (2,4-DMH). 

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