A relationship between some long-range 2H/1H isotope shifts in 13C nuclear magnetic resonance and long-range spin–spin coupling constants for toluene, ethylbenzene, and isopropylbenzene

A very good correlation exists between long-range spin–spin coupling constants involving the α-protons in toluene, ethylbenzene, and cumene, and the changes in the chemical shifts of the para and α carbons caused by deuterium substitution at the α carbon. The coupling constants depend on sin2 θ, where θ is the angle by which the α C—H bond twists out of the ring plane, and can be used to find the expectation value of sin2 θ. Consequently the observed correlation provides quantitative support for the hyperconjugative model employed by Wesener and Günther.