ARE THE STERIC EFFECTS ON THE C-13-NMR CHEMICAL-SHIFTS OF HYDROCARBON POLYMERS REALLY LONG-RANGE

It is demonstrated that the stereosequence-dependent 13C-NMR chemical shifts observed in hydrocarbon polymers can be satisfactorily understood on the basis of the γ effect. Carbon atoms separated by three bonds and in a gauche arrangement, or conformation, are more shielded by ca. 5 ppm than when in the trans arrangement. The probability of finding the two carbon atoms in the gauche conformation, and therefore the probability of the γ effect, depends on longer range stereoregularity, but it is the interaction between carbons separated by three bonds (γ), and not four (δ), five (ϵ), or six (ξ) bonds, which is responsible for the shielding. © 1979, American Chemical Society. All rights reserved.